Encyclopedia of Biodiversity, Revised Edition

By Stanley Rice

Praise for the previous edition:

"...make[s] high-level scientific concepts accessible to secondary students."—Library Journal

"...clearly written and well organized..."—School Library Journal

"Fulfilling educational benchmarks identified by the National Academy of Sciences, this encyclopedia is an excellent choice for both public and academic libraries. Recommended."—Choice

"...a thorough and informative work...provide[s] accessible information...There is simply no other work that compares to this...High-school and public libraries will welcome such a well-researched title..."—Booklist

"The text is suitable for high school students but advanced enough for adult readers, too...presents important biodiversity topics...a handy overview for term papers and class presentations."—Library Journal

Biodiversity and ecology are founded in evolutionary science. In order to understand why species of organisms occupy different parts of the world, it is important to comprehend how they evolved.

Encyclopedia of Biodiversity, Revised Edition examines this evolutionary framework with the help of more than 150 entries and five essays averaging at least 2,000 words each. High school teachers can use these entries—grouped by topic—to meet many of the science education goals established by the National Academy of Sciences. Written by a leading expert in the field, this comprehensive, full-color encyclopedia makes information about groups of organisms (from bacteria to mammals) and about ecological concepts and processes (such as biogeography and ecological succession) clearly and readily available to students and the general public. Tables at the end of each entry have a consistent structure, allowing readers to see how environmental conditions and biodiversity have changed through evolutionary time. 

Entries include:

• Acid rain and fog
• Biodiversity in the Jurassic period
• Darwin's finches
• Galápagos Islands
• Peter and Rosemary Grant
• Life in bogs
• Natural selection
• Population genetics
• Seedless plants
• Tropical rainforests and deforestation
• Alfred Russel Wallace.

• Language: English
• Publisher: Facts On File
• Release date: Jun 1, 2020
• ISBN: 9781438195926

Book preview

title

Encyclopedia of Biodiversity, Revised Edition

Copyright © 2020 by Stanley A. Rice

All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For more information, contact:

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New York NY 10001

ISBN 978-1-4381-9592-6

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Contents

Entries

adaptation

adaptive radiation

allometry (biodiversity)

archaea

archaea (biodiversity)

avoidance (biodiversity)

bacteria (biodiversity)

Bates, Henry Walter

biodiversity and acid rain and fog

biodiversity and agriculture

biodiversity and cultural diversity

biodiversity and decomposers

biodiversity and ecological disturbances

biodiversity and habitat fragmentation

biodiversity and herbivory

biodiversity and insects

biodiversity and lichens

biodiversity and mimicry 

biodiversity and parasitism

biodiversity and pollination

biodiversity and pollution

biodiversity and predation

biodiversity and seed dispersal

biodiversity and the Cambrian explosion

biodiversity and the Galápagos Islands

biodiversity and the greenhouse effect

biodiversity and the hydrological cycle

biodiversity and the nitrogen cycle

biodiversity in Darwin's finches

biodiversity in deep oceans

biodiversity in the Cambrian period

biodiversity in the Carboniferous period

biodiversity in the Cretaceous period

biodiversity in the Devonian period

biodiversity in the Jurassic period

biodiversity in the Neogene period

biodiversity in the Ordovician period

biodiversity in the Paleogene period

biodiversity in the Permian period

biodiversity in the Quaternary period

biodiversity in the Silurian period

biodiversity in the Triassic period

biodiversity of amphibians

biodiversity of animals

biodiversity of arthropods

biodiversity of bacteria

biodiversity of birds

biodiversity of bogs

biodiversity of boreal and subalpine forests

biodiversity of carnivorous plants

biodiversity of cloud forests

biodiversity of conifers

biodiversity of cool deserts

biodiversity of crustaceans

biodiversity of dicots

biodiversity of fishes

biodiversity of flowering plants

biodiversity of fungi

biodiversity of grasslands and savannas

biodiversity of living fossils

biodiversity of mammals

biodiversity of marshes and swamps

biodiversity of monocots

biodiversity of polar deserts

biodiversity of protists

biodiversity of reptiles

biodiversity of rivers, streams, ponds, and lakes

biodiversity of seed plants

biodiversity of seedless plants

biodiversity of shrublands

biodiversity of spiders

biodiversity of temperate coniferous forests

biodiversity of temperate deciduous forests

biodiversity of temperate dry forests

biodiversity of temperate rain forests

biodiversity of the tundra

biodiversity of tropical seasonal forests

biodiversity of warm deserts

biodiversity on continental shelves

biodiversity on coral reefs

biogeochemical cycles

biogeochemical cycles (biodiversity)

biogeography

biological classification

biomagnification

biomimicry

biophilia

bioremediation

carbon cycle

Carson, Rachel Louise

cladistics

class (taxonomic)

cloning extinct animals

cloud forests

coevolution

colony collapse disorder

commensalism

concept of biodiversity

conservation biology

convergence

Cretaceous extinction

Darwin, Charles

decomposition (biodiversity)

dinosaurs

disjunct species

diversity of life history

DNA studies of biodiversity

domain (taxonomic)

ecological communities

ecological community

ecological competition

ecological competition

ecological disturbance

ecological succession

ecological succession

ecology

ecosystem services

Ecosystem Services: Public or Private Costs?

ecosystems

ecotone (biodiversity)

Ediacaran organisms

endangered species

endemic species

endosymbionts in insects

environmental ethics

environmental psychology

epigenetics (biodiversity)

estuary (biodiversity)

eukaryotes

evolution of resistance

evolutionary fitness

exaptation

extinction

family (taxonomic)

federal government role in conservation

fishes

fossilization

fossils and fossilization

founder effect and genetic drift

fungi (biodiversity)

Gaia hypothesis

geological era

geological period

Ghosts of Biodiversity Past

Grant, Peter and Rosemary

greenhouse effect

homeotherm

Humboldt, Alexander von

hybridization

hybridization (biodiversity)

hydrothermal vents (biodiversity)

ice ages

If Humans Vanished …

importance of biodiversity to agriculture

importance of biodiversity to medicine

inbreeding (biodiversity)

integrated pest management (IPM)

invasive species

island biogeography

Jackson, Wes

Janzen, Daniel H.

keystone species

Leopold, Aldo

Life of Earth: Portrait of a Beautiful, Middle-Aged, Stressed-Out Planet

Linnaeus, Carolus

Maathai, Wangari

Margulis, Lynn

mass extinctions

mesophication

microbiome sequencing 

microbiomes

Muir, John

mutualism

mutualism (biodiversity)

natural selection

natural systems agriculture

natural systems agriculture

order (taxonomic)

organic agriculture

patterns of biodiversity

Permian extinction

phenotypic plasticity (biodiversity)

phylum (taxonomic)

Pinchot, Gifford

Pleistocene extinction

polyculture (biodiversity)

population biology

population genetics

primates

productivity (biodiversity)

protists

quantification of biodiversity

reef (biodiversity)

relictual population

restoration ecology

root nodules

sexual selection

speciation

spider

subalpine forest

symbiogenesis

symbiogenesis

Thoreau, Henry David

tolerance (biodiversity)

tropical rain forests and deforestation

Wallace, Alfred Russel

Why are there so many Species?

Wilson, Edward O.

Entries

adaptation

Adaptation is the fit of an organism to its environment, which allows successful survival and reproduction. The environment of an organism includes other organisms of the same and different species. For any physical or biological environment, many different adaptations are possible, and this has resulted in exuberant biodiversity on Earth.

Characteristics that are not Genetically Based

Physiologists often use the word adaptation to describe environmentally induced characteristics rather than genetically based ones. These physiological adjustments can occur on the following three different levels:

Physiological response occurs when an organism makes immediate physiological or behavioral changes to its environment. On a hot day, the stomata (pores) of a leaf may open. This allows water to evaporate from the leaf, making it cooler through the process of transpiration. The evaporation of water may also cool a mammal on a hot day (perspiration). The mammal may also respond behaviorally by seeking shade and by exposing its skin surface areas, allowing heat to diffuse into the air. In contrast, on a cold night, the mammal curls into a ball, reducing its heat loss. These responses typically do not involve changes in gene expression. Instead, the molecules and structures that are already present behave differently under the changed conditions.

Acclimatization occurs when an individual organism adjusts its physical characteristics. Consider again the example of a leaf on a hot day. The leaf cells accumulate or manufacture molecules, which cause water to diffuse into them. The influx of water makes the leaf more resistant to wilting. Or consider humans at high elevations. When humans live at high elevations for a week or more, their bone marrow begins to manufacture more red blood cells, which compensates for the lower availability of oxygen at high elevations. These are not immediate responses; they require the genetically controlled manufacture of new materials. In animals, such changes can occur also as a result of behavioral changes (e.g., muscle building).

Phenotypic plasticity occurs when individual organisms adjust to their environments as they develop. The leaves of plants that grow in cool, shady, moist conditions are often larger than those of plants that grow in warm, bright, dry conditions, even if all the plants are genetically identical, and the plants that grow in warm, bright, dry conditions have relatively larger root systems. This is illustrated by an experiment with the weed Abutilon theophrasti. The characteristics of both kinds of leaf result from the same genes, but the genes are expressed differently in each group of plants. Or consider humans who live from childhood at high elevations. These people may develop larger lungs than they would have had they lived at sea level; larger lungs compensate for the lower oxygen availability at high elevations.

Plasticity is rarely reversible and rarely changes after development of an organ or organism is complete. Although leaves can change the amount of dissolved substances in their cells, they cannot change their size once their development is complete. That is, acclimatization is reversible and plasticity, in this case, is not. The red blood cell count can change, but lung size cannot after childhood. In this case also, acclimatization is reversible and plasticity is not. As a result of plasticity, various cells or tissues of an organism can respond differently to the same environmental conditions. Plasticity and acclimatization are responsible for much of the phenotypic variability observed in natural environments.

Not all environmentally induced differences among individuals in a population are plasticity; in some cases, they may result simply from patterns of growth. As herbaceous plants grow, they accumulate stems and roots but shed their old leaves. A bigger plant will therefore have a relatively lesser amount of leaf material. Plants that grow in the shade are smaller and have relatively more leaf material than plants that grow in bright sunlight. The greater amount of leaf material is not necessarily plasticity; it may be due solely to the smaller size of the shade plants.

Plasticity can be passed down from one generation to another. For example, a large, healthy plant may produce not only more seeds but bigger seeds that contain more nutrients. These seeds may grow into plants that have different characteristics than those that grow from the seeds of smaller plants, even if they are genetically identical. Research by the botanists Laura Galloway and Julie Etterson showed that in populations of a forest floor wildflower, plants that had grown in deeper shade produced seeds that grew into plants that had superior growth in deeper shade compared to seeds from plants that had grown in patches of sunlight. Such intergenerational plasticity may result from epigenetics, in which the expression of the gene, rather than the gene itself, changes, and this change is passed on to the next generation.

Plants that develop in the shade grow taller and produce smaller root systems and more leaf area than plants that develop in full sunlight. Plants that develop in dry conditions grow shorter and produce larger root systems and less leaf area than plants that develop in moist conditions. This is an example of phenotypic plasticity, which is sometimes called adaptation, but which does not involve genetic changes. The numbers come from an experiment by the author using the weed Abutilon theophrasti. The three treatments are compared at the overall average weight of 854 mg.

Source: Infobase Learning.

Characteristics that are Genetically Based

Evolutionary and conservation biologists restrict the use of the word adaptation to the genetic results of natural selection and sexual selection. Therefore, not all genetic variation is adaptation. There are several sources of genetically based variation that are not produced by selection. Some of the genetic differences among individuals, populations, and species may have resulted from historical accidents that affected the course of evolution. The most common genes may have been the ones that got lucky, rather than being selected. Genetic drift and the founder effect are examples of historical accidents. The genes that are most common in a population or a species may simply be those that were most common when the species almost became extinct or when it was first founded by a small number of individuals. In both cases, the species experienced a genetic bottleneck in which, by chance, much of the genetic variation was lost. Consider a population of flowering plants on an island. All the flowers might be red not because natural selection favored red flowers on the island but because the only seeds that reached the island in the original population were those of red-flowered individuals. Populations of many endangered species have genetic characteristics that resulted from genetic bottlenecks.

Exaptations are genetically based traits that may have originally evolved because they performed one function but then were available for another function. This means that selection may not have acted directly on the trait being considered; the trait may have been a side effect of selection. A frequently cited example of an exaptation is feathers. The earliest birds apparently used feathers as insulation. After feathers had evolved as insulation, natural selection acted upon them as flight structures. After that, sexual selection acted upon them as colorful adornments. Feathers, therefore, can be considered adaptations for insulation and exaptations for flight and sexual adornment.

It can be difficult to distinguish adaptations from exaptations. The classic example of an adaptation that has appeared in numerous popular science books and textbooks for more than a century is the neck of the giraffe. To most observers, it would appear obvious that long necks are an adaptation that allows giraffes to feed at the tops of trees, but if this is so, why do female giraffes have shorter necks than male giraffes, and why are giraffes so frequently observed actually bending down to eat leaves? Observations of giraffes in the wild have shown that males with longer necks prevail in male-to-male competition, and females choose the males with longer necks. Now that giraffes have long necks, they can use them for eating leaves from treetops, but that may not have been the original reason that long necks evolved in these animals. The long neck of the giraffe appears to be an adaptation to combat and an exaptation for reaching leaves at the tops of trees.

Giraffes walk together in Etosha National Park.

Source: Corbis.

Some characteristics of organisms are structurally inevitable. Large plants must have relatively thick trunks, and large animals must have relatively thick legs. Structurally inevitable features are called allometric characteristics and are not considered to be adaptations.

In most cases, an adaptation does not have just a single function. The glands within an animal's epidermis have several functions, each of which may have provided a separate evolutionary advantage. Some of the glands produce sweat, which allows the animal to become cooler as the sweat evaporates. Sweat also contains dissolved molecules. The body of the animal uses sweat as one of its methods of disposing of excess or toxic materials. Molecules in sweat can also serve as chemical communication between animals. Mammary glands are modified sweat glands. Sweat glands, therefore, are an adaptation with at least four different functions.

Adaptations are never perfect. Natural and sexual selection produce adaptations that are only as good as they need to be. One example is the digestive system of the panda. The immediate ancestors of pandas were carnivores, but pandas are herbivores, living exclusively on leaves. Pandas have intestines that are better suited to a carnivore. In well-adapted herbivores, such as sheep, the intestines are up to 35 times as long as the body, while in well-adapted carnivores, the intestines are much shorter, only four to seven times as long as the body. With the help of bacteria, longer intestines allow herbivores more time to digest coarse plant materials, such as cellulose. Meat, in contrast, requires less digestive breakdown. Pandas have intestines that are in the carnivore, not the herbivore, range. The panda's digestive system is, therefore, not well adapted to its function. Modern evolutionary scientists attribute this to the recent evolutionary shift from meat to leaves in the diet of the panda's ancestors: There has not been time for better adaptation in this case.

Adaptation and Biodiversity

Adaptation is the reason there is such an astonishingly large number of species. If there were only one perfect adaptation for every situation, such as a dry climate, then there would be very few species. All plants in a dry climate would look more or less the same, and one species would dominate that climate. However, this is far from the case. There are many possible plant adaptations to a dry climate. Some plants, such as mesquite shrubs, grow very deep roots to draw water up from the water table. They also have small leaves, which diffuse heat into the air more quickly than do large leaves. In this way, the small-leaved shrubs can keep their leaves cool without much transpiration. Some plants do both of these things. Others, such as cactuses, have shallow roots, but they store water in their succulent tissues. Still others, such as the spring ephemeral wildflowers of the desert, have neither of these adaptations. Instead, they germinate and grow rapidly during the brief rainy season, surviving the long drought in the form of seeds. A similar diversity of adaptation can be seen in desert animals, some of which, such as kangaroo mice, function with very little water, while others, such as desert frogs, remain dormant in the soil between periods of rain. Throughout evolutionary history, various evolutionary lineages have responded to environmental circumstances in many different and equally successful ways.

Coevolution also produces many different adaptations and promotes biodiversity. Coevolution occurs when the evolution of one type of organism influences the evolution of another; they adapt to one another. When flowering plants evolved near the beginning of the Cretaceous period, they began to depend upon many different kinds of pollination. Some depended upon the wind, but many engaged the services of animals such as insects to carry their pollen. This began an explosive diversification of flowering plant species, each evolving the ability to attract a different insect species. Meanwhile, the insects were also evolving numerous different and equally successful adaptations that allowed them to obtain food from the flowers they pollinated. Evolutionary diversification in flowering plants and pollinators reinforced each other. The adaptations that have resulted from coevolution have been the major source of biodiversity on Earth.

Further Information

Dawkins, Richard. The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe Without Design. New York: Norton, 1996.

Galloway, Laura F., and Julie R. Etterson. Transgenerational plasticity is adaptive in the wild. Science 318 (2007): 1,134–1,136.

Gould, Stephen Jay, and Elisabeth Vrba. "Exaptation—a missing term in the science of form. Paleobiology 8 (1982): 4–15.

Rice, Stanley A., and Fakhri A. Bazzaz. "Plasticity to light conditions in Abutilon theophrasti: comparing phenotypes at a common weight." Oecologia 78 (1989): 502–507.

Entry Author: Rice, Stanley A.

adaptive radiation

Adaptive radiation is the evolution of species from a single, ancestral population. Throughout the history of life, many species have become extinct either by natural selection or simply bad luck, while others have produced many new species by adaptive radiation from the surviving species. Adaptive radiation can result either from the dispersal of a species to a new location, after which the founding population undergoes radiation, or the survival of remnant populations of a once widespread species, after which the remnants undergo radiation. The net result has been a steady increase in biodiversity through evolutionary time. Adaptive radiation occurs because a single ancestral population separates into distinct populations that do not interbreed, thus allowing separate directions of evolution to occur in each.

The world is full of numerous examples of adaptive radiation. Nearly every genus that contains more than one species or any family that contains more than one genus can be considered an example. Two examples follow.

First, consider the white oaks, part of the genus and subgenus Quercus. At one time, probably before 80 million years ago, there was a single species of white oaks that may have lived in what is now East Asia. In the late Mesozoic and Cenozoic eras, warm conditions were widespread on Earth. Oaks spread throughout the Northern Hemisphere. Three of the locations in which they are now found are Europe, eastern North America, and California.

North America then separated from Eurasia, new mountain ranges such as the Rocky Mountains arose, and conditions became cooler and drier during the latter part of the Cenozoic era. These events separated the white oaks of Europe, eastern North America, and California. Separate species evolved in each of these locations, including Quercus robur (English oak) in Europe, Quercus macrocarpa (bur oak) in eastern North America, and Quercus douglasii (blue oak) in California. Furthermore, in each of these locations, cooler conditions allowed the evolution of the deciduous pattern of leaf production in some areas, while warmer conditions encouraged the evolution of the evergreen pattern in other areas. The previously mentioned species are all deciduous. Quercus suber (cork oak) is an evergreen white oak of Europe; Quercus virginiana (live oak) is the evergreen white oak of eastern North America; and Quercus dumosa (scrub oak) is an evergreen white oak of California. In all three areas, both deciduous and evergreen (live) oaks evolved (with some exceptions). These oak species and many others constitute an example of adaptive radiation within the white oak subgenus.

Second, while the adaptive radiation of oaks occurred over the course of about 80 million years, the adaptive radiation of the approximately 500 species of cichlid fishes of Lakes Malawi, Tanganyika, and Victoria in Africa has occurred in just the last few thousand years. From what was probably a single ancestral species, the cichlids in Lake Victoria have radiated into such different species as fish with heavy jaws that crush mollusks; slender, swift fish that eat plankton; small fish that eat parasites off the skins of larger cichlid fishes; large fish that eat other fishes; and fish with sharp teeth that scrape algae off rocks. There is very little genetic variation among these species, so rapid and recent has been their adaptive radiation.

Species of white oaks have evolved separately in Europe, eastern North America, and California. Also, evergreen and deciduous species of white oaks have evolved separately in these three locations. All of these oaks evolved (radiated) from a single ancestral species.

Source: Infobase Learning.

The Hawaiian archipelago also provides an example of adaptive radiation in both time and space. The Hawaiian archipelago, on the Pacific Plate, sits over a plume of lava that rises from the mantle, known as a hot spot. Volcanic eruptions occur at the site of the hot spot. The Pacific Plate has moved northwest, while the hot spot has remained stationary. The oldest islands in the Hawaiian archipelago are toward the northwest, and the youngest toward the southeast. The oldest islands have now eroded and are beneath sea level. The oldest major islands still in the Hawaiian archipelago are Niihau and Kauai, which were formed about 5 million years ago. They have no active volcanoes anymore. The islands of Oahu, Molokai, Lanai, and Maui are of intermediate age. The youngest island, Hawaii, is less than 1 million years old and still has active volcanic eruptions. Plant and animal species have dispersed to the islands, primarily from the southwest, and species unique to the Hawaiian archipelago have evolved. Species have also dispersed from older islands to younger islands once the younger islands became suitable for life, radiating into new species on each island. Adaptive radiation has occurred in space (unique species on each island) and in time (from older to younger islands) in the Hawaiian archipelago.

Perhaps the most famous adaptive radiation has been the evolution of Darwin's finches in the Galápagos Islands. A single ancestral species, which may have come from the South American mainland, has radiated out into 13 species in four genera. Finches in three of the genera live in trees. Some of them have some amazing adaptations, such as the species that uses sticks to pry insects out of holes in branches. The largest genus, with six species, consists of finches that hop on the ground and eat mostly seeds.

Sometimes, speciation occurs as one species coevolves in the presence of another. However, the presence of other species can preclude some directions of speciation. On the Galápagos Islands, predators are largely absent, and some of the finches have figured out a way to drink blood from the legs of boobies (large marine birds) by pecking them. Also, woodpeckers are absent, and some of the finches, as mentioned above, use sticks to get insects out of holes. In a habitat already filled with efficient predators and woodpeckers, the amateurish adaptations displayed by the blood-drinking and insect-prying finches would probably not have succeeded. Therefore, adaptive radiation can occur rapidly when a population invades a new habitat—if, that is, the population is adapted to the conditions of the new habitat and if the new habitat does not have potential competitors.

Release from competition is the reason adaptive radiation has occurred most spectacularly in the history of Earth after large disasters. During the Mesozoic era, mammals were few in number and low in diversity. Their constant body temperatures made them successful nocturnal animals. The dinosaurs, meanwhile, ruled the daytime. After the Cretaceous extinction, which cleared away the dinosaurs, mammals underwent an explosive adaptive radiation. Large mammals did not exist before the Cenozoic era; any mammal species that began to evolve a large size would have been unsuccessful in competition with large dinosaurs, but many huge mammals evolved in the early Cenozoic. Along with the big mammals, many of the modern categories of mammals, including whales, primates, and bats, evolved. Mammals have continued to have adaptive radiations, but none quite as spectacular as the adaptive radiation that occurred early in the Cenozoic era.

Within species, the adaptive radiation of subspecies and varieties continues. These represent possible future species. The preservation of biodiversity requires not just the protection of species but of subspecies and varieties as well.

Further Information

Gillespie, Rosemary G. The ecology and evolution of Hawaiian spider communities. American Scientist 93 (2005): 122–131.

Grant, Peter R., and B. Rosemary Grant. How and Why Species Multiply: The Radiation of Darwin's Finches. Princeton, N.J.: Princeton University Press, 2007.

Schilthuizen, Menno. Frogs, Flies, and Dandelions: The Making of Species. New York: Oxford University Press, 2001.

Schluter, Dolph. The Ecology of Adaptive Radiation. New York: Oxford University Press, 2000.

Entry Author: Rice, Stanley A.

allometry (biodiversity)

differences between organisms due to their different sizes, which are necessary to allow structural support or the exchange of matter and energy with the physical environment

Entry Author: Rice, Stanley A.

archaea

Also known as: archea

Formerly known as archaebacteria, the archaea, or archeans, are single-celled bacteria-like organisms, most of which live in extreme conditions of temperature, salt, or acidity. Most of them have been discovered only recently, because they live in conditions in which scientists did not expect to find any organisms. Their superficial resemblance to bacteria also contributed to their uniqueness being overlooked for many decades. Among their similarities to bacteria are the following:

Like bacteria, archaeans have simple shapes, usually spheres, rods, or spirals.

Like most bacteria, most archaeans have cell walls. In both bacteria and archaeans, there are some types that lack cell walls (Mycoplasma in bacteria, Thermoplasma in archaeans) and have irregular shapes.

Like bacteria, archaeans have DNA but no nucleus; both are prokaryotes.

Like many bacteria, many archaeans can form colonies.

Both bacteria and archaeans are very small, often 10 times smaller than eukaryotic cells.

Bacteria can be aerobic (grow in the presence of oxygen gas) or anaerobic. Most archaeans are anaerobic, although some Sulfolobus archaeans can tolerate oxygen.

However, archaeans are distinct from bacteria in some fundamental ways, such as the following:

Bacterial cell walls consist largely of peptidoglycan; archaean cell walls do not.

Cell membranes of all cells consist largely of phospholipids, but these are different in bacteria and archaeans. In bacteria, the phospholipids have ester linkages, while archaean cell membranes have phospholipids with ether linkages instead. The fatty chain of the bacterial phospholipid is a fatty acid, while in archaeans it is an isoprene. Eukaryotic cell membrane phospholipids are similar to those of bacteria, not archaeans.

Cell membranes in bacteria (and eukaryotes) are double layered, while in some archaeans they form a single layer.

Some archaeans have shapes not found in bacteria, such as flat, square forms and extremely narrow, needlelike forms.

Archaean DNA is associated with histone proteins, like the chromosomes of eukaryotic cells; the DNA of bacteria is not.

Archaeans have enzymes associated with the transcription of RNA and translation of proteins that more closely resemble those of eukaryotic cells than of bacteria.

Both bacteria and archaeans often have flagella, whiplike structures that allow them to swim, but the flagella of bacteria and archaeans appear to have evolved separately, the former from secretory proteins, the latter from hairlike extensions called pili.

Both bacteria and archaeans have many of the same ways of obtaining metabolic energy. For example, some species in both groups obtain energy from sunlight or obtain carbon from inorganic sources such as carbon dioxide. Some bacteria (such as cyanobacteria) can do both, but there are no archaeans that can do both. Some types of metabolism, such as the production of methane gas, are unique to archaeans.

These differences, plus the fact that archaeans have very different DNA sequences from those of bacteria (up to 15 percent of archaean genes encode proteins not known in bacteria), justify the classification of the Archaea as one of the three principal domains of life, equally distinct from bacteria and Eucarya (eukaryotes).

Many of the organisms that live in extreme conditions are archaeans. Most of them live in conditions of extreme acidity, temperature, salinity, or pressure that biologists at one time considered impossible for organisms to endure. Pyrolobus fumarii, for example, lives in deep ocean vents where the water reaches 235°F (113°C). Water boils at 212°F (100°C) at the air pressure of sea level, but the pressure at the bottom of the ocean allows superheated water to exist, in which this archaean lives. However, the most primitive lineages of bacteria, represented by Aquifex and Thermotoga, thrive under extreme conditions as well. The earliest bacteria, like the earliest archaeans, must therefore have evolved under extreme conditions on the early Earth.

The extreme conditions in which most archaeans live are not necessarily distant from the places where most eukaryotes reside. Many archaeans live in animal intestines (under extreme anoxic, basic, or acidic conditions), where they digest cellulose and other molecules. Some intestinal archaeans are commensalistic, neither beneficial nor harmful. An example is Methanobrevibacter smithii, a methane-producing archaean that lives in human intestines. Not all archaeans, however, live in extreme conditions. For example, archaeans are a major component of marine plankton. No archaeans are known to cause diseases in eukaryotes.

The evolutionary scientist Carl Woese began making nucleic acid comparisons among many different species in the 1970s. At that time, all bacteria were lumped together into one category. Woese found that certain bacteria that lived in extreme conditions had nucleic acid sequences as different from those of other bacteria as they were from those of eukaryotes. It was from this discovery that evolutionary scientists began distinguishing the archaebacteria (as they were then known) as a distinct branch of life from the eukaryotes and the eubacteria.

DNA studies since the time of Woese's original work have continued to confirm the uniqueness of archaeans. The complete genome of Methanococcus janaschii was published in 1996. Only 11 to 17 percent of its genome matched that of known eubacteria, and more than half of its genes are unknown in either bacteria or eukaryotes. Moreover, some of the DNA similarity between archaeans and bacteria may be due to horizontal gene transfer, in which bacteria exchange small segments of DNA. Although most horizontal gene transfer occurs between bacteria closely related to one another, exchange between archaeans and bacteria has occurred frequently during the history of life.

The major groups of archaebacteria are recognized by many evolutionary scientists on the basis of DNA sequences:

Euryarcheota include the methanogens and the halophiles. Methanogens convert hydrogen gas (H2) and carbon dioxide gas (CO2) into methane gas (CH4). They live in marshes and in the intestinal tracts of animals such as cows and humans. They are a principal source of methane in the atmosphere. Halophiles live in extremely salty conditions such as the Dead Sea and the Great Salt Lake. Halophiles may have evolved more recently than other archaebacteria. First, they obtain their carbon from organic molecules produced by the decay of other organisms, which implies that they evolved after these other organisms were already in existence. Second, they use a molecule that resembles the rhodopsin visual pigment in the vertebrate eye in order to produce energy from sunlight. Third, DNA analyses place halophiles outward toward the branches of the archaebacterial lineage, rather than near the primitive base of the lineage.

Crenarcheota include thermophiles that live in very hot environments, but some crenarcheotes live in soil and water at moderate temperatures.

Korarcheota, Nanoarcheota, and Thaumarcheota have been identified mainly by their DNA sequences, and little is yet known about them.

The earliest cell fossils, such as those found in the 3.5 billion-year-old Apex chert, were of types that, if they lived today, would be classified as bacteria or archaeans. Throughout the Archaean era, the entire world was prokaryotic. Throughout the Proterozoic, the world was mostly microbial, shared between prokaryotes and eukaryotes. Only in the last 540 million years (the last 14 percent of Earth history since the beginning of the Archaean era) have multicellular organisms been numerous. The evolutionary biologist Stephen Jay Gould pointed out that in every age of Earth history, bacteria and archaeans have been the modal type of organism.

As an examination of the differences between bacteria and archaeans reveals, the modern eukaryotic cell (such as a human cell) resembles bacteria, while the modern eukaryotic nucleus resembles archaeans. This constitutes evidence for the theory, increasingly accepted among evolutionary scientists, that the eukaryotic cell came into existence when a small archaean penetrated a larger bacterium and formed a mutualistic association, followed by the uptake of bacterial DNA by the archaean, which became the nucleus. If this is true, then archaeans are not an evolutionary sideshow or mere leftover from the distant past, but are the partial ancestors of all complex life on Earth, including humans.

Further Information

Baker, B. J., et al. Lineages of acidophilic Archaea revealed by community genomic analysis. Science 314 (2006): 1,933–1,935.

Barns, S. M., et al. Perspectives on archaeal diversity, thermophily and monophyly from environmental rRNA sequences. Proceedings of the National Academy of Sciences USA 93 (1996): 9,188–9,193.

Brocks, J. J., et al. Archean molecular fossils and the early rise of eukaryotes. Science 285 (1999): 1,0331,036.

Cox, C. J., et al. The archaebacterial origin of eukaryotes. Proceedings of the National Academy of Sciences USA 105 (2008): 20,356–20,361.

DeLong, E. F. Archaea in coastal marine environments. Proceedings of the National Academy of Sciences USA 89 (1992): 5,685–5,689.

Eppley, J. M., et al. "Genetic exchange across a species boundary in the archaeal genus Ferroplasma." Genetics 177 (2007): 407–416.

Friend, Tim. The Third Domain: The Untold Story of Archaea and the Future of Biotechnology. Washington, D.C.: Joseph Henry Press, 2007.

Howland, John L. The Surprising Archaea: Discovering Another Domain of Life. Oxford: Oxford University Press, 2000.

Huber, H., et al. A new phylum of Archaea represented by a nanosized hyperthermophilic symbiont. Nature 417 (2002): 63–67.

Nelson, K. E., et al. "Evidence for lateral gene transfer between Archaea and bacteria from genome sequence of Thermotoga maritima." Nature 399 (1999): 323–329.

Rappé, M. S., and S. J. Giovannoni. The uncultured microbial majority. Annual Reviews of Microbiology 57 (2003): 369–394.

Thauer, R. K. A fifth pathway of carbon fixation. Science 318 (2007): 1,732–1,733.

Walsby, A. E. A square bacterium. Nature 283 (1980): 69–71.

Whitman, W. B., et al. Prokaryotes: The unseen majority. Proceedings of the National Academy of Sciences USA 95 (1998): 6,578–6,583.

Woese, Carl R., and George E. Fox. Phylogenetic structure of the prokaryotic domain: The primary kingdoms. Proceedings of the National Academy of Sciences USA 74 (1977): 5,088–5,090.

———, O. Kandler, and M. L. Wheelis. Towards a natural system of organisms: Proposal for the domains Archaea, Bacteria, and Eucarya. Proceedings of the National Academy of Sciences USA 87 (1990): 4,576–4,579.

Entry Author: Rice, Stanley A.

archaea (biodiversity)

prokaryotes with unique cellular characteristics and that are usually extremophiles

Entry Author: Rice, Stanley A.

avoidance (biodiversity)

a method of acclimatization, plasticity, or adaptation that allows organisms to maintain their physiological processes by producing internal conditions that are more favorable to their function than are the external conditions

Entry Author: Rice, Stanley A.

bacteria (biodiversity)

prokaryotes with unique cellular characteristics and that are usually not extremophiles

Entry Author: Rice, Stanley A.

Bates, Henry Walter

(b. 1825–d. 1892)

naturalist

Born on February 8, 1825, Henry Walter Bates was a British naturalist who specialized in collecting and studying insects. His insights were very important to the modern understanding of biodiversity and coevolution.

Bates befriended another British naturalist, Alfred Russel Wallace. In 1848, Bates and Wallace traveled together to the Amazon rain forest of South America, where both of them extensively collected insects and other animals. Wallace returned to England and then traveled to Indonesia, but Bates remained in the Amazon rain forest for 14 years.

While studying Amazonian insects, Bates discovered that certain nonpoisonous species of heliconid butterflies had wing coloration patterns that resembled those of poisonous heliconid butterfly species. Bates considered this to be mimicry, which afforded protection to the nonpoisonous butterflies without the cost of producing the poison. Since Bates's initial discovery, numerous other examples of mimicry have been found. This type of mimicry is now called Batesian mimicry, in contrast to Mullerian mimicry, in which numerous prey species converge upon a common set of warning coloration patterns. Bates's discovery of mimicry contributed significantly to Charles Darwin's work on evolution. Darwin cited Bates's discovery in later editions of The Origin of Species. Bates and Darwin both maintained that such mimicry could not have resulted from the use and disuse of the structures involved and must have resulted from natural selection.

Bates returned to England in 1862. He published an account of his work in the rain forest, The Naturalist on the River Amazons, in 1863. Bates continued scientific research and was elected a fellow of the Royal Society in 1881. He died on February 16, 1892.

Further Information

Beddall, Barbara G. Wallace and Bates in the Tropics: An Introduction to the Theory of Natural Selection, Based on the Writings of Alfred Russel Wallace and Henry Walter Bates. London: Macmillan, 1969.

Entry Author: Rice, Stanley A.

biodiversity and acid rain and fog

Acid rain and fog, also called acid precipitation, occur when air pollution acidifies the water droplets in clouds and the resulting acid comes in contact with objects and organisms. The major air pollutants that cause acid precipitation are nitric acid (HNO3) and sulfuric acid (H2SO4), which are the dissolved forms of nitrous and nitric oxide and sulfur dioxide gases, although natural sources of pollutants include wetlands and volcanoes. Sulfur dioxide is particularly common in the smoke from power plants that burn high-sulfur coal. Many power plants now have scrubbers that remove sulfur dioxide from the effluent before it is released into the atmosphere. In some cases, rain downwind from industrial areas has been as acidic as orange juice or vinegar. The record pH is 2.4. (Pure water is pH 7, and changes in pH represent orders of magnitude of difference in acidity.)

Acid rain and fog can be considered the unintended consequence of the control of particulate air pollution (soot) in the second half of the 20th century. Soot helps to neutralize some of the acid. By producing less soot, power plants caused more acid-producing compounds. Moreover, by using taller smokestacks, power plants displaced air pollution away from cities and into layers of air that brought acid rain and fog into the mountains.

Acid dissolves limestone. Before the limitation of air pollution in cities, acid rain was steadily dissolving limestone statues and architecture. Acid rain has also been reported to damage the finish on automobiles. The major impact of acid precipitation has been on biodiversity, as in these examples.

Acid rain has the potential to cause direct damage to plants by harming the waxy cuticles of leaves and penetrating into the photosynthetic tissues. This kind of damage has not occurred very extensively, however, because only the first few minutes of a rain event are significantly acidic; thereafter, rain that is less acidic rinses much of the previously deposited acid away before it can damage plants.

Acid rain has the potential to remove nutrients from soils. The negatively charged surfaces of soil clay particles attract and hold positively charged ions such as potassium and magnesium, which are plant nutrients. Hydrogen ions (which are the cause of acidity) displace the plant nutrients, thus reducing the fertility of the soil. Moreover, some toxic ions such as lead and aluminum, normally scarce in soil water, may become common if the water is acidified. This does not occur, however, in soils derived from limestone, as the limestone will neutralize the acidic rain.

Acid rain has the potential to accumulate in lakes, thereby reducing the growth of algae and making less food available for the entire food chain. Acidity can directly affect some kinds of fish, particularly their eggs. Trout are especially sensitive to acidified water. This does not occur in lakes fed by groundwater that filters through limestone.

Acid precipitation has been most harmful in the form of acid fog. Acidified droplets in clouds settle on the leaves of plants, especially coniferous trees of mountains, and remain for long periods of time. Often, these trees die. Extensive areas of forest in Europe (such as the forests of Switzerland and the Black Forest of Germany) have suffered from acid fog. In many cases, acid fog is an indirect cause of death. Much of the subalpine fir forest near Mount Mitchell, at the summit of the Appalachians, has died from disease introduced by woolly adelgid bugs (Adelges tsugae). The firs normally resist the bugs—unless they are weakened by acid fog.

Acid precipitation was identified as a major problem during the 1970s. Congress passed the Acid Deposition Act in 1980, which established a 10-year study. The Reagan administration convened the Acid Rain Peer Review Panel of the National Academy of Sciences. They then insisted that a scientist notorious for his outspoken antienvironmental views, S. Fred Singer, be included on the panel. Singer's fame arose from his work on nuclear weapons three decades earlier, and he did not have recognized expertise in the study of acid pollution. He claimed, without adequate study, that controlling the acid rain problem would devastate the American economy and cause utility prices to increase immensely. The panel, with the exception of Singer, concluded that acid rain was a serious problem and that utilities and industry should reduce sulfur emissions. The Reagan administration's Office of Science and Technology Policy, however, altered the executive summary of the report and focused on Singer's viewpoint as the major conclusion of the panel. As a result, meaningful action on acid rain was delayed until the administration of George H. W. Bush. Congress passed amendments to the Clean Air Act in 1990, establishing a system of cap and trade to encourage industrial polluters to limit their emissions. Corporations could comply with the law by reducing emissions or by buying pollution credits from other corporations that produced fewer emissions than those permitted by law. The economic devastation predicted by Singer did not occur, and the price of electricity even decreased slightly. Singer continues his attacks on environmental issues, presently focusing on global warming.

A sign in Kansas boasts of the efficiency of modern agriculture. However, modern agriculture, being utterly dependent on fossil fuels, is very inefficient in its use of energy.

Source: Stanley A. Rice.

Although sulfur dioxide emissions have decreased by 40 percent since the law took effect and acid rain has decreased 65 percent since 1976, it remains a serious problem. The ecosystem ecologist Gene Likens indicated in 2009 that the forests of northeastern North America, damaged by acid rain, have not yet begun to grow back. The cap and trade system apparently worked, but not well enough to reverse the problem.

Acid precipitation is also an international problem, as the air pollution of one country (such as the United States or Germany) can cause the death of forests in another (such as in Canada or Switzerland). The control of air pollution, particularly of sulfur dioxide, has reduced the impact of acid precipitation since the 1980s. Due to pressure from industry, the U.S. government has been reluctant to call attention to acid precipitation. For example, National Park Service displays in Shenandoah National Park attributed the death of subalpine fir trees to the adelgid bugs, not mentioning the effects of acid fog.

Further Information

Galloway, J. N., et al. 1987. Acid rain: a comparison of China, United States and a remote area. Science 236 (1987): 1,559–1,562.

Kizlinski, M. L., et al. Direct and indirect ecosystem consequences of an invasive pest on forests dominated by eastern hemlock. Journal of Biogeography 29 (2002): 1,489–1,503.

Likens, G. E., and F. H. Bormann. Acid rain: A serious regional environmental problem. Science 184 (1974): 1,176–1,179.

———, et al. Long-term effects of acid rain: Response and recovery of a forest ecosystem. Science 272 (1996): 244–246.

———, and Jerry F. Franklin. Ecosystem thinking in the northern forests—and beyond. BioScience 59 (2009): 511–513.

Menon, Manoj, et al. Effects of heavy metal soil pollution and acid rain on growth and water use efficiency of a young model forest ecosystem. Plant and Soil 297 (2007): 171–183.

Mphepya, J. N., et al. Precipitation chemistry and wet deposition in Kruger National Park, South Africa. Journal of Atmospheric Chemistry 53 (2006): 169–183.

Oreskes, Naomi, and Erik M. Conway. Sowing the seeds of doubt: Acid rain. Chapter 3 in Merchants of Doubt: How a Handful of Scientists Obscured the Truth on Issues from Tobacco Smoke to Global Warming. New York: Bloomsbury, 2010.

Entry Author: Rice, Stanley A.

biodiversity and agriculture

Agriculture is the deliberate cultivation of plants or fungi by animals. Cultivation here refers to planting, care, and harvest. Agricultural plants and fungi are usually very different from wild plants and fungi; they have characteristics that allow them to be adapted to the agricultural system in which they live, useful to the animals that cultivate them, and poorly adapted to living in the wild. Agriculture is an example of mutualism, which confers benefits on both the agricultural plant or fungus and the animal that cultivates it. In agriculture, crops and the animals that cultivate them are mutually dependent. Agriculture has been, perhaps, the most significant component of human cultural evolution. More broadly defined, agriculture may include the care and use of livestock animals by other animals. Agriculture interacts with biodiversity in two ways: First, it has an impact upon biodiversity, as explained below, and second, it depends upon wild biodiversity as a source of new crops or genes, as explained in another entry.

Evolution of Agriculture

Several species of ants carry out activities that bear striking parallels to human agriculture. Leaf-cutter ants (genera Atta and Acromyrmex) cultivate gardens of fungus (family Lepiotaceae). Massive foraging parties of leaf-cutter ants gather pieces of leaves from many species of tropical plants and carry them back to their nests. Leaf-cutter ants are the single most important source of herbivory in rain forest areas where they are abundant. The ants do not eat the leaves, which contain many toxins. Instead, they chew them up into compost, on which a kind of fungus grows and breaks down the toxins. The fungus grows nowhere else except in the mounds of leaf-cutter ants; when the ants disperse, they take fungus tissue with them. The ants eat fungus tissue and almost nothing else. Beneficial bacteria that grow on the bodies of the ants produce chemicals that inhibit the growth of other bacteria in the compost. For this reason, some biologists consider these and other ants to be a promising source of new antibiotics. Because the ants deliberately prepare compost for the fungus and because of the mutual dependence of ants and fungus upon each other, the ant-fungus relationship can be considered an example of agriculture.

Several species of ants in the seasonal tropics of Central America live on and in trees of the genus Acacia. The ants do not eat the leaves of the acacia. Instead, they consume nectar that is produced by glands on the stems (not in the flowers) of the trees, and they eat globules of protein and fat, called Beltian bodies, that grow on the tips of immature leaves. The ants chew out the insides of the acacias' unusually large thorns and live inside the thorns. In some cases, experimental manipulation has shown the ants to be dependent upon the acacias for survival. The ants attack and kill other insects and drive away larger animals that attempt to feed on the acacias. When vines or other plants begin to grow in the immediate vicinity of the acacias, the ants chew them down. In most cases, the acacias are completely dependent upon the ants; when the ants are experimentally removed, vines overgrow the acacias, and animals browse the leaves heavily. The acacias remain green during the dry season, when most of the other trees lose their leaves, but these green targets go undisturbed by herbivores because of their protective ant army. Because the ants weed out other plants from the vicinity of the acacias and defend their crops and because of the mutual dependence of ant and acacia, the ant-acacia relationship can be considered an example of agriculture. Ant partnerships with other kinds of trees in the rain forests of Africa have evolved independently of the ant-acacia partnership.

As natural selection favored the characteristics that allowed the partnerships of ants with fungi and ants with acacias, hybridization between these species and others closely related to them was unsuccessful. The result was the evolution of new species of fungi, acacias, and ants. In this way, agriculture has increased biodiversity.

As the above examples illustrate, when agriculture evolved in the human species about 10,000 years ago, it was not the first time that agriculture had evolved on the planet. Hereafter, agriculture will refer to human agriculture.

Some scholars once believed that agriculture was invented by a brilliant man in a tribal society of hunter-gatherers. Other scholars pointed out that since women gathered most of the plant materials, agriculture was probably invented by a woman. Both the brilliant man theory and the brilliant woman theory are incorrect, however, because agriculture could not have been invented in a single step by anyone. It had to evolve. Unmodified wild plants are unsuitable for agriculture. The following are four reasons for this.

The seeds of most wild plant species are dormant when they are mature; that is, when planted, the seeds will not germinate. Many require a period of exposure to cool, moist conditions before the inhibitors within the plant embryo break down and the seed germinates. If a brilliant man or woman planted the first agricultural seed from a wild plant, it would not have grown, and the person would have rightly concluded that agriculture was not a good idea.

The seeds of many wild grains shatter or fall off the stem as soon as they are mature. Since natural selection generally favors seeds that disperse to new locations, shattering is beneficial to the plant, but it is extremely inconvenient for a human harvester.

The seeds of many wild plant species contain toxins.

The seeds of many wild grains are small.

Furthermore, the advantages of primitive agriculture would not have been immediately apparent to intelligent hunter-gatherers. Agriculture requires intense labor. Modern hunter-gatherers often barely eke out an existence in marginal habitats such as the Kalahari Desert of Africa or the Great Outback of Australia, but these are the habitats to which tribes and nations with more advanced tools have driven them. Before agriculture, many tribes lived in rich habitats in which hunting and gathering in many cases provided a comfortable level of existence. For these reasons also, agriculture had to evolve gradually.

Agriculture originated separately in several parts of the world: at least twice in the New World (Central America and South America), in Mesopotamia, and in China. Agriculture may have had several separate origins in each of these areas, as well as in New Guinea. The only inhabited part of the world in which agriculture did not evolve was Europe; European agriculture was imported from the Middle East. Scientists and historians know that agriculture originated separately in these locations because the staple crops were different in each. The domesticated sumpweeds and sunflowers grown by inhabitants of North America were replaced by Mexican maize and beans in pre-Columbian times.

Agriculture began earliest (about 10,000 years ago) in the Middle East, especially in the Tigris-Euphrates floodplain of Mesopotamia. It began there first apparently because there were many wild species of plants that were almost suited for agriculture. Of the 56 species of wild grains that have large seeds, 32 grow in the Middle East. These wild grains needed little evolutionary transformation to become crops. The transformation from gathering to agriculture would have been a relatively quick and easy process in the Middle East.

At first, the gatherers unconsciously transformed wild into domesticated plants. Within each species of grain (such as wild wheat and wild barley), the largest seeds, the seeds that shattered the least, and the ones that tasted the best were what people preferentially gathered. The gatherers also would take grain seeds with them when they traveled and also may have cast them onto the ground in new locations. In this way, the seeds with the least dormancy were the ones most frequently chosen by the gatherers. Small, shattering, less palatable, dormant grains (the wild type) were thus gradually transformed, by unconscious natural selection and by deliberate artificial selection, into large, nonshattering, palatable, nondormant grains similar to today's crops. A similar process occurred with other wild food plants.

This same process occurred in all the other places where agriculture originated, but it took longer. There were fewer suitable wild food plants in Mexico and even fewer in the Andes. It took longer for wild teosinte to evolve into maize, as a greater evolutionary transformation was needed; it took even longer for poisonous wild potatoes to evolve into edible ones.

Meanwhile, the human population was growing. By 10,000 years ago, all available favorable habitats were occupied by humans. Hunters and gatherers were much less free to move to a new location when resources became scarce. About the same time, the weather became cooler and drier in the Middle East. Hunting and gathering became a much less desirable way of life. When certain tribes then tried deliberate cultivation, it was worth the extra work, and the plants were suitable.

Once agriculture had evolved, societies that depended upon it could not easily revert to hunting and gathering for several reasons:

Agriculture allowed greater food production and greater population growth. A large population could not revert to hunting and gathering. This is obvious today, for the Earth cannot support billions of hunters and gatherers, but it was also true in all local regions in which agriculture evolved thousands of years ago.

Since agriculture allowed greater food production, it was no longer necessary for nearly all tribal members to participate in food procurement. Farmers raised enough food for everybody, which allowed other people to become soldiers and priests. A hunting, gathering tribe was ill equipped to fight an agricultural tribe with a dedicated army. A world trapped in agriculture was now trapped into war. Agriculture allowed the rise of religious and governmental hierarchies as well as armies.

With the evolution of agriculture, productive farmland became valuable. People settled into cities because they had to stay in one place at least long enough for one harvest. Agriculture promoted the rise of civilization. Civilization rose earlier in Mesopotamia than in other places because agriculture began earlier there. With civilization came advanced technology. Because they needed to defend particular tracts of territory, the armies now had a lot more to fight about. The cultural groups that developed agriculture first were the first to be civilized and to have advanced technology that allowed them to conquer the cultural groups in which this process had not progressed as far. The biologist Jared Diamond explains that this is why Europeans conquered America and drove natives onto reservations, rather than Native Americans conquering Europe and driving Europeans into remote corners of the Alps and Pyrenees.

The evolution of agriculture is a perfect illustration that evolution does not operate for the good of the species. Agriculture evolved in the human species because it provided an advantage to some individuals within human societies. Agriculture did not improve the average health of human beings. In fact, the average life span in early agricultural societies was shorter than that in contemporaneous hunter-gatherer societies. This occurred for two reasons:

Diseases spread more rapidly in cities, in which people were trapped with one another's wastes, garbage, and germs.

Agriculture actually decreased the quality of human nutrition by making people dependent upon a few crop plants rather than a diversity of wild foods. In particular, the human body evolved under conditions in which ascorbic acid (vitamin C) was readily available from wild fruits. When entire populations became dependent upon crops with little vitamin C, scurvy became a way of life.

Herding began when people started to manage herds of wild animal species that were most amenable to survival and reproduction under captivity. Unconscious natural selection and then conscious artificial selection resulted in the evolution of livestock species such as the cow, which evolved in western Eurasia and Southeast Asia from two distinct wild species. Once again, the earlier development of herding in the Middle East than in other areas occurred because many wild animal species of the Middle East such as goats and sheep were amenable to herding, whereas wild animal species such as deer in North America were not. Livestock animals provided high-quality food (meat and milk), often by consuming wild foods that humans could not eat. This was especially true of goats. Pigs, on the other hand, eat many of the same foods as humans. This competition between pigs and humans for food may be one reason that pigs are considered undesirable (unclean) by some cultures in the arid regions of the Middle East. As with agriculture, herding caused a narrowing of the food diversity base, from many wild animal species to a few livestock species.

Effects of Agriculture on Biodiversity of Natural Systems

Agriculture has had several effects upon biodiversity, as indicated by the following examples.

Escape of agricultural species into natural ecosystems. The result of agriculture has been the evolution of many new species of plants and animals that are adapted to human care and often cannot survive in the wild. In this sense, agriculture has created new biodiversity, even though crop species do not grow well in the wild. For example, since wheat (Triticum aestivum) and maize (Zea mays) do not have seed dormancy and do not shatter their seeds, they can usually survive only where they have been planted. However, crop species have contributed, in a largely negative way, to natural biodiversity because wild relatives of cultivated plants are frequent invaders of natural ecosystems. Examples include Johnson grass (Sorghum halepense), a relative of cultivated sorghum (S. bicolor), and wild oats (Avena fatua and A. barbata), relatives of cultivated oats (A. sativa). Hybridization between crops and wild relatives can produce new, aggressive strains of weeds. The ecologists Norm Ellstrand and Christina Schierenbeck have listed 28 examples. The new weeds are particularly hard to control since they so closely resemble the crops they infest. This same process may endanger some of the wild relatives also. Some varieties of wild maize in Mexico have been essentially hybridized out of existence.

The use of GMO (genetically modified organism) crops became widespread in the 1990s. For some crops, such as soybean and cotton, most of the acreage consists of GMO breeds. Many GMO crops have wild relatives with which they can interbreed. For example, cultivated mustards such as canola (domesticated Brassica napus and B. campestris) can exchange pollen with wild mustard species. This raises the possibility that modern agriculture may introduce genetic changes, such as herbicide resistance or the production of compounds that are toxic to insect herbivores, into some wild populations. Wild, weedy mustards with leaves that are toxic to insects might undergo population explosions as a result of release from herbivore damage, and the herbivores might suffer a population crash. Such genetic pollution has occurred, but not yet on a large scale.

Numerous domesticated animals have escaped into the wild. Livestock animals, though they have also been artificially selected to live under artificial conditions, can thrive under