Introduction to evolution

This article is intended as a generally accessible introduction to the subject. For the main encyclopedia article, please see Evolution.

To see a brief description of evolution in simpler language, visit the Simple Wikipedia article on evolution.

  Evolution is the accumulation of changes through succeeding generations of organisms that accounts for biological diversity as well as the emergence of new species. Since the origin of life, evolution has transformed the earliest life forms (the common ancestors of all living things) into the estimated 1.75 million different species living today.^[1] Evolutionary biology, the study of evolution, has provided a clear understanding of the processes that accounts for the variety of organisms, both present and extinct. The current understanding of evolutionary biology has advanced far beyond the foundation established by Charles Darwin in the late 1800's. Evolution has hence drawn support from the research of individuals in many scientific fields. For example, Gregor Mendel's work with plants explained the hereditary patterns of genetics which led to a clearer understanding of the mechanisms of inheritance.^[2] The discovery of the structure of DNA combined with advances in the field of population genetics provided insight into the source of variations in organisms. This information led to explanations on how new species develop from ancestral forms, an important component of evolution known as speciation. The theory of evolution serves as the foundation for much of the research conducted in biology, including molecular biology, paleontology, and taxonomy. Supported by a large quantity of reliable scientific evidence, evolution is supported by 99.9% of the scientific community.^[3]

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Darwin's idea: evolution by natural selection

In 1859, Charles Darwin published the first full-fledged theory of evolution by natural selection in On the Origin of Species. In part, Darwin was motivated to publish the book by a letter he received from Alfred Wallace who ask him to review a manuscript that contained a theory of natural selection that was essentially identical to his own. Both Wallace and Darwin viewed the history of life like a tree, each fork in the tree’s limbs representing a shared ancestry. The tips of the limbs represented modern species and the branches represented the common ancestors shared amongst species. To explain these relationships, Darwin contended that all living things were related and descended from a few forms, or even from a single common ancestor. He called this process "descent with modification".^[4]

Darwin's explanation of the mechanism of evolution relied on his theory of natural selection, a theory developed from the following observations:^[5]

1. If all the individuals of a species reproduced successfully, the population of that species would increase exponentially.
2. Except for seasonal changes, populations tend to remain stable in size.
3. Environmental resources are limited.
4. The traits found in a population vary greatly. No two individuals in a given species are exactly alike.
5. Many of the variations found in a population can be passed on to offspring.

From these observations, Darwin deduced that the production of more offspring than the environment can support leads to a struggle for existence, with only a small percentage of individuals surviving in each generation. He explained that the chance for surviving this struggle is not random, but depends on how well-adapted each individual is to its environment. Well-adapted, or "fit" individuals are more likely to leave a greater number of offspring than their less well-adapted competitors. Darwin concluded that the unequal ability of individuals to survive and reproduce leads to gradual changes in the population. Traits which help the organism survive and reproduce will accumulate over generations. Those traits that inhibit survival and reproduction are lost. Darwin used the term natural selection to describe this process.^[6]

The theory of natural selection was based on observations of variations in animals and plants. Darwin observed a reciprocal relationship between orchids and insects, ensuring successful pollination of the plant. He noted that orchids have developed a variety of elaborate structures to attract insects, which guarantee that the pollen sticks to their bodies and is transported to the female orchid. Despite the appearance of design, the flower parts in the orchid evolved from ordinary parts that usually perform different functions. Darwin proposed that the orchids do not represent the work of an ideal engineer, but that they were “rigged” from pre-existing parts.^[7]

Darwin's view that life can be depicted as a tree and thus that all living things are related by common ancestry generated some controversy at the time because humans did not receive special consideration. Though Darwin did not make this explicit at first, his friend and supporter Thomas Henry Huxley soon presented evidence that humans and apes shared a common ancestor. The popular press of the day misinterpreted this as a claim that humans were descended from monkeys.

The Source of Variation

Darwin’s theory of natural selection laid the groundwork for modern evolutionary theory, and his experiments and observations showed that heritable variations occurred within populations and were controlled by natural selection. However, he lacked an explanation for the source of these variations. Like many of his predecessors, Darwin mistakenly thought that heritable traits were a product of use and disuse, and that characteristics acquired during an organism's lifetime could be passed on to its offspring. He looked for examples, such as large ground feeding birds getting stronger legs and weaker wings until, like the ostrich, they could no longer fly.^[8] This misconception, called the inheritance of acquired characters, had been part of the theory of transmutation of species put forward in 1809 by Jean-Baptiste Lamarck. In the late 19th century the theory became known as Lamarckism. Darwin produced an unsuccessful theory he called pangenesis to try to explain how acquired characteristics could be inherited. In the 1880s August Weismann's experiments indicated that changes from use and disuse were not heritable, and Lamarckism gradually fell from favour.^[9]

The missing information necessary to help explain the emergence of new traits in offspring was provided by the pioneering genetics work of Gregor Mendel. Mendel’s experiments with several generations of pea plants demonstrated that heredity works by reshuffling the hereditary information during the formation of sex cells and recombining that information during fertilization. Mendel referred to the information as factors; however, they later became known as genes. Genes are the basic units of heredity in living organisms. They contain the biological information that directs the physical development and behavior of organisms.^[10] Later research by Thomas Hunt Morgan showed that genes are linked in a series on chromosomes and it is the reshuffling of these chromosomes that results in unique combinations in offspring.

Modern synthesis

Modern evolutionary synthesis was the outcome of merging several different scientific fields into a more uniform and cohesive understanding of evolutionary theory. In the 1930s and 1940s, efforts were made to merge Darwin's theory of natural selection, research in heredity, and understandings of the fossil records into a unified theme.^[11] The application of the principles of genetics to naturally occurring populations by scientist such as Theodosius Dobzhansky and Ernst Mayr led to advancements in the understanding of the processes of evolution. Dobzhansky 1937 work Genetics and the Origin of Species was an important step in bridging the gap between geneticists and field biologist. Mayr, based on an understanding of genes and direct observations of evolutionary processes from field research, introduced the biological species concept, that defined a species as a group of interbreeding or potentially interbreeding populations that are reproductively isolated from all other populations. The paleontologist George Gaylord Simpson contributed to the incorporation of research on the fossil records into this broader understanding of evolution known as modern synthesis. The fossil record illustrates a pattern that was consistent with the branching, and non-directional pathway of evolving organism predicted by the modern synthesis.

The discovery of the genetic material known as DNA filled many of the gaps in information concerning the processes of evolution. DNA is the chemical that contains the genetic instructions used in the development and functioning of all known living organisms. It is now known that the genetic variations in a population arise by chance mutations in DNA. Natural selection then acts upon the variations in these genes. The fusion of such diverse fields clarified many of the principles of evolution. For example, it is now understood that evolution is not a chance phenomena. Although the mutations are random, natural selection is not a process of chance: the environment determines the probability of reproductive success. The end products of natural selection are organisms that are adapted to their present environments. Natural selection does not involve progress towards an ultimate goal. Evolution does not necessarily strive for more advanced, more intelligent, or more sophisticated life forms.^[12] For example, fleas (wingless parasites) are descended from a winged, ancestral scorpionfly,^[13] and snakes are lizards that no longer require limbs. Organisms are merely the outcome of variations that succeed or fail, dependent upon the environmental conditions at the time. Environmental changes that occur over short periods typically lead to extinction.^[14] Of all species that have existed on Earth, 99.9 percent are now extinct.^[15] Since life began on Earth, five major mass extinctions have lead to significant reduction in species diversity. The most recent, the Cretaceous–Tertiary extinction event, occurred 65 million years ago, and has attracted more attention than all others because it killed the dinosaurs.^[16]

Evidence for evolution

Scientific evidence for evolution comes from many aspects of biology, and includes fossils, homologous structures, and molecular similarities between species' DNA.

The fossil record

Research in the field of paleontology, the study of fossils, supports the idea that all living creatures are related. Fossils provide evidence that accumulated changes in organisms over long periods of time have led to the diverse forms of life we see today. A fossil itself reveals the organism's structure and the relationships between present and extinct species, allowing paleontologists to construct a family tree for all of the life forms on earth.^[18]

Modern paleontology began with the work of Georges Cuvier (1769–1832). Cuvier noted that, in sedimentary rock, each layer contained a specific group of fossils. The deeper layers, which he proposed to be older, contained simpler life forms. He also noted that many forms of life from the past are no longer present today. One of Cuvier’s successful contributions to the understanding of the fossil record was establishing extinction as a fact. In an attempt to explain extinction, Cuvier proposed the idea of “revolutions” or catastrophism in which he speculated that geological catastrophes had occurred throughout the earth’s history, wiping out large numbers of species. ^[19] Cuvier's theory of revolutions was later replaced by uniformitarian theories, notably those of James Hutton and Charles Lyell who proposed that the earth’s geological changes were gradual and consistent. ^[20] However, current evidence in the fossil record supports the concept of mass extinctions. As a result, the general idea of catastrophism has re-emerged as a valid hypotheses for at least some of the rapid changes in life forms that appear in the fossil records.

A very large number of fossils have now been discovered and identified. These fossils serve as a chronological record of evolution. The fossil record also provides examples of transitional species that demonstrate ancestral links between past and present life forms.^[18] One such transitional fossil is Archaeopteryx, an ancient creature that had the distinct characteristics of a reptile, yet also had the feathers of a bird. The implication from such a find is that modern reptiles and birds arose from a common ancestor.^{[21] [7]}

Comparative anatomy

Taxonomy

Taxonomy is the branch of biology that names and classifies all living things. Scientists use morphological and genetic similarities to assist them in categorizing life forms based on ancestral relationships. For example, orangutans, gorillas, chimpanzees, and humans all belong to the same taxonomic grouping referred to as a family – in this case the family called Hominidae. These animals are grouped together because of similarities in morphology that come from common ancestry (called homology).^[22]

Strong evidence for evolution comes from the analysis of homologous structures in different species that no longer perform the same task.^[23] Such is the case of the forelimbs of mammals. The forelimbs of a human, cat, whale, and bat all have strikingly similar bone structures. However, each of these four species' forelimbs performs a different task. The same bones that construct a bird's wings, which are used for flight, also construct a whale's flippers, which are used for swimming. Such a "design" makes little sense if they are unrelated and uniquely constructed for their particular tasks. The theory of evolution explains these homologous structures: all four animals shared a common ancestor, and each has undergone change over many generations. These changes in structure have produced forelimbs adapted for different tasks. Darwin described such changes in morphology as descent with modification.^[24]

Embryology
In some cases, anatomical comparison of structures in the embryos of two or more species provides evidence for a shared ancestor that may not be obvious in the adult forms. Such homologies might be lost or take on different functions as the embryo develops. Part of the basis of classifying the vertebrate group (which includes humans), is the presence of a tail (extending beyond the anus) and pharyngeal gill slits. Both structures appear during some stage of embryonic development but are not always obvious in the adult form.^[25]

Because of the morphological similarities present in embryos of different species during development, it was once assumed that organisms re-enact their evolutionary history as an embryo. It was thought that human embryos passed through an amphibian then a reptilian stage before completing their development as mammals. Such a re-enactment, called ontogeny recapitulates phylogeny, is not supported by scientific evidence. What does occur, however, is that the first stages of development are similar in broad groups of organisms.^[26] At the pharyngula stage, for instance, all vertebrates are extremely similar, but do not exactly resemble any ancestral species. As development continues, the features specific to the species emerge from the basic pattern.

Vestigial structures

Homology also includes a unique group of shared structures referred to as vestigial structures. Vestigial refers to anatomical parts that are of minimal, if any, value to the organism that possesses them. These apparently illogical structures are remnants of organs that played an important role in ancestral forms. Such is the case in whales which have small vestigial leg bones that appear to be remnants of the legs that their ancestors used to walk on land.^[27] Humans also have many vestigial structures, including the ear muscles, the wisdom teeth, the appendix, the tail bone, body hair (including goose bumps), and the semilunar fold in the corner of the eye.

Convergent evolution

Anatomical comparisons can also be misleading, however, as not all anatomical similarities indicate a close relationship. Organisms that share similar environments will often develop similar physical features in a process known as convergent evolution. Both sharks and dolphins have similar body forms, yet are only distantly related – sharks are fish and dolphins mammals. Such similarities are a result of both populations being exposed to the same selective pressures. Within both groups, changes that aid swimming would be favored. Thus, over time, they develop similar morphology, even though they are not closely related.^[28]

Artificial selection

Artificial selection is the controlled breeding of domestic plants and animals. In controlled breeding, humans determine which animals will reproduce, and to some degree, which genes will be passed on to future generations. The process of artificial selection has had a significant impact on the evolution of domestic animals. For example, people have produced different types of dogs by controlled breeding. The differences in size between the Chihuahua and the Great Dane are the result of artificial selection. Despite their dramatically different physical appearance, they and all other dogs evolved from a few wolves domesticated by humans in what is now China fewer than 15,000 years ago.^[29]

Artificial selection has also produced a wide variety of plants. In the case of maize (corn), recent genetic evidence suggests that domestication occurred 10,000 years ago in central Mexico.^[30] Prior to domestication, the edible portion of the wild form was small and difficult to collect. Today The Maize Genetics Cooperation • Stock Center maintains a collection of more than 10,000 genetic variations of maize that have arisen by random mutations and chromosomal variations from the original wild type.^[31]

Darwin drew much of his support for natural selection from observing the outcomes of artificial selection.^[32] Much of his book On the Origin of Species was based on his observations of the diversity in domestic pigeons arising from artificial selection. Darwin proposed that if dramatic changes in domestic plants and animals could be achieved by humans in short periods, then natural selection, given millions of years, could produce the differences between living things today. There is no real difference in the genetic processes underlying artificial and natural selection. As in natural selection, the variations are a result of random mutations; the only difference is that in artificial selection, humans select which organisms will be allowed to breed.^[23]

Molecular biology

Every living organism contains molecules of DNA, RNA, and protein. If two organisms are closely related, these molecules will be very similar.^[33] On the other hand, the more distantly related two organisms are the more molecular differences they will have. For example, two brothers will be very closely related and will have very similar DNA, while distant cousins will have more differences in their DNA. Comparing these molecules is extremely useful when studying species that are very closely related. The extent of their relationship is shown by how similar these molecules are. For example, comparing the DNA of chimpanzees with that of gorillas and humans showed that there is as much as a 96% similarity between the DNA of humans and chimps, suggesting humans and chimpanzees are more closely related to each other than to gorillas.^{[34] [35]}

Scientists have made great strides in analyzing these molecules, particularly the DNA that makes up organisms' genes. Genes are the pieces of DNA that carry information and influence the properties of an organism. Genes determine a person's general appearance and to some extent behavior. Since close relatives have similar genes they tend to look alike. The exact form of the genes in an organism is called the organism's genotype and this set of genes influences the properties (or phenotype) of an organism.^[36] The field of molecular systematics focuses on measuring the similarities in these molecules and using this information to work out how different types of organisms are related through evolution. These comparisons have allowed biologists to build a relationship tree of the evolution of life on earth.^[37] They have even allowed scientists to unravel the relationships of organisms whose common ancestors lived such a long time ago that no real similarities remain in the appearance of the organisms.

Co-evolution

Co-evolution is a process in which two or more species influence the evolution of each other. All organisms are influenced by life around them; however, in co-evolution, there is evidence that genetically determined traits in each species directly resulted from the interaction between the two organisms.^[33]

An extensively documented case of co-evolution is the relationship between Pseudomyrmex, a type of ant, and the acacia, a plant that the ant uses for food and shelter. The relationship between the two is so intimate that it has led to the evolution of special structures and behaviors in both organisms. The ant defends the acacia against herbivores and clears the forest floor of the seeds from competing plants. In response, the plant has evolved swollen thorns that the ants use as shelter and special flower parts that the ants eat.^[38] Such co-evolution does not imply that the ants and the tree choose to behave in an altruistic manner. Rather, across a population small genetic changes in both ant and tree benefited each and the benefit gave a slightly higher chance of the characteristic being passed on to the next generation. Over time, successive mutations created the relationship we observe today.

Population genetics

Some definitions

Hardy-Weinberg equilibrium

The Hardy-Weinberg principle states that the frequencies of alleles (variations in a gene) in a sufficiently large population will remain constant if the only forces acting on that population are the random reshuffling of alleles during the formation of the sperm or egg and the random combination of the alleles in these sex cells during fertilization.^[40] A population in which the frequencies of alleles are constant is not experiencing evolution.

  Suppose a group of mice inhabit a barn. In this population, there are only two alleles for the gene that controls fur color. One allele for the fur color in the mouse population produces black mice and accounts for 75% of the genes. The other allele for the fur color produces white mice and makes up the remaining 25% of the genes in the population. If the only factors determining an allele’s chance of being represented in the next generation are random shuffling of alleles during the formation of sex cells (meiosis) and the random recombination that takes place during fertilization, the allele frequencies will stay the same from one generation to the next. In this example, the composition of the gene pool in the mouse population remains 75% black-coding alleles and 25% white-coding alleles. Because there is no change in the allelic frequencies, there is no evolutionary change in fur color. This population is in Hardy-Weinberg equilibrium or is non-evolving.^[41] It is very rare for natural populations to experience no change in the frequency of alleles from generation to generation.

Changing gene pool and genetic drift

Frequencies of alleles in a gene pool typically change, resulting in evolution of populations over successive generations. Several forces may alter the composition of the gene pool. One such process is the exchange of alleles between members of different populations. Exchanges occur when new members join the population or when others leave. This migration between populations is called gene flow.^[41] Mutations can also alter the gene pool by creating new alleles and thus changing the frequency of the pre-existing alleles.^[41] The introduction of a single allele by the process of mutation would have very little impact on the frequencies of alleles in a large population. However, in a population of only a few individuals, the introduction of single allele would be statistically significant.^[41] In addition, preference for any particular allele during mate selection will affect the frequency of alleles. Another factor influencing the frequencies of an allele in the gene pool is natural selection. Differential survival and reproductive success as a consequence of inheriting a particular allele will alter the allele frequencies and possibly result in detectable changes in the gene pool.^[41] The chance that each allele has for survival and reproduction must be the same if the frequency of alleles is to remain constant.

Small populations are very susceptible to chance fluctuations in the number of individuals, a condition known as genetic drift.^[23] Two common situations may arise, affecting the genetic makeup of a small population and resulting in genetic drift. The first is the Bottleneck Effect, which occurs because random chance events, such as fires or floods, significantly reduce the number of individuals in a population.^[41] The remaining members may not accurately represent the original gene pool. The second situation that can lead to genetic drift is The Founder Effect. This occurs when only a few individuals colonize a new habitat. The smaller the founding party, the less likely its gene pool will represent the gene pool of the original population.^[23]

Species

Given the right circumstances, and enough time, evolution leads to the emergence of new species. Scientists have struggled to find a precise and all-inclusive definition of species. Ernst Mayr (1904–2005) defined a species as a population or group of populations whose members have the potential to interbreed naturally with one another to produce viable, fertile offspring. (The members of a species cannot produce viable, fertile offspring with members of other species.)^[42] Mayr's definition has gained wide acceptance among biologists, but does not apply to organisms such as bacteria, which reproduce asexually.

Speciation is the lineage-splitting event that results in two separate species forming from a single common ancestral population.^[6] A widely accepted method of speciation is called allopatric speciation. Allopatric speciation begins when a population becomes geographically separated.^[23] Geological processes, such as the emergence of mountain ranges, the formation of canyons, or the flooding of land bridges by changes in sea level may result in separate populations. For speciation to occur, separation must be substantial, so that genetic exchange between the two populations is completely disrupted. In their separate environments, the genetically isolated groups follow their own unique evolutionary pathways. Each group will accumulate different mutations as well as be subjected to different selective pressures. The accumulated genetic changes may result in separated populations that can no longer interbreed if they are reunited.^[6] Barriers that prevent interbreeding are either prezygotic (prevent mating or fertilization) or postzygotic (barriers that occur after fertilization). If interbreeding is no longer possible, then they will be considered different species.^[43]

Those who reject evolution as a viable theory often claim that speciation has never been observed. However, speciation has been observed in several groups of organisms, including bacteria, round worms, insects, and fish, as well as in several groups of plants. In addition, past speciation events are recorded in fossils.^[44] Scientists have documented the formation of five new species of cichlid fishes from a single common ancestor that was isolated fewer than 4000 years ago from the parent stock in Lake Nagubago. The evidence for speciation in this case was morphology (physical appearance) and lack of natural interbreeding. These fish have complex mating rituals and a variety of colorations; the slight modifications introduced in the new species have changed the mate selection process and the five forms that arose could not be convinced to interbreed.^[45]

Barriers to breeding between species

Reproductive barriers that prevent interbreeding can be classified as either prezygotic barriers or postzygotic barriers.^[46] The distinction between the two lies in whether the barrier prevents the generation of offspring before fertilization of the egg or following such fertilization.

Barriers that prevent fertilization

Prezygotic barriers prevent mating between species or prevent the fertilization of the egg if the species attempt to mate.^[23] Some examples are:

• Temporal isolation – Occurs when species mate at different times. Populations of the western spotted skunk (Spilogale gracilis) overlap with the eastern spotted skunk (Spilogale putorius) yet remain separate species because the former mates in summer and the latter in late winter.^[46]
• Behavioral isolation – Signals that elicit a mating response may be sufficiently different to prevent a desire to interbreed. The rhythmic flashing in male fireflies is species-specific and thus serves as a prezygotic barrier.^[47]
• Mechanical isolation – Anatomical differences in reproductive structures may prevent interbreeding. This is especially true in flowering plants that have evolved specific structures adapted to certain pollinators. Nectar-feeding bats searching for flowers are guided by their echolocation system. Therefore, plants which depend on these bats as pollinators have evolved acoustically conspicuous flowers that assist in detection.^[48]
• Gametic isolation – The gametes of the two species are chemically incompatible, thus preventing fertilization. Gamete recognition may be based on specific molecules on the surface of the egg that attach only to complementary molecules on the sperm.^[46]
• Geographic/habitat isolation – Geographic: The two species are separated by large-scale physical barriers, such as a mountain, large body of water, or physical barriers constructed by humans. Such barriers disrupt gene flow between the isolated groups. This is illustrated in the divergence of plant species on opposite sides of the Great Wall of China.^[49] Habitat: The two species prefer different habitats, even if they live in the same general area, and therefore do not encounter each other. For example, two different species of garter snakes in the genus Thamnophis occur in the same area but one prefers the water while the other prefers dry land.^[28]

Barriers acting after fertilization

  Postzygotic barriers occur after fertilization, usually resulting in the formation of a hybrid zygote^[50] that is either not viable^[51] or not fertile. This is typically a result of incompatible chromosomes in the zygote.^[23] Examples include:

• Reduced hybrid viability – A barrier between species occurs after the formation of the zygote, resulting in incomplete development and death of the offspring.^[52]

• Reduced hybrid fertility – Even if two different species successfully mate, the offspring produced may be infertile. Crosses of horse species within the genus Equus tend to produce viable but sterile offspring. For example, crosses of zebra x horse and zebra x donkey produce sterile zorses and zedonks. Horse-donkey crosses produce sterile mules. Very rarely, a female mule may be fertile.^[53]

• Hybrid breakdown – Some hybrids are fertile for a single generation but then become weak or inviable.^[54]

Perspectives on the mechanism of evolution

The theory of evolution is widely accepted among the scientific community, serving to link the diverse specialty areas of biology. Evolution provides the field of biology with a solid scientific base. The significance of evolutionary theory is best described by the title of a paper by Theodosius Dobzhansky (1900–1975), published in American Biology Teacher; "Nothing in Biology Makes Sense Except in the Light of Evolution".^[55]

Nevertheless, the theory of evolution is not static. There is much discussion within the scientific community concerning the mechanisms behind the evolutionary process. For example, the rate at which evolution occurs is still under discussion. In addition, the primary unit of evolutionary change, the organism or the genes, is not agreed on.

Rate of change

Two views exist concerning the rate of evolutionary change. Darwin and his contemporaries viewed evolution as a slow and gradual process. Evolutionary trees are based on the idea that profound differences in species are the result of many small changes that accumulate over long periods.

The view that evolution is gradual had its basis in the works of the geologist James Hutton (1726–1797) and his theory called "gradualism". Hutton's theory suggests that profound geological change was the cumulative product of a relatively slow continuing operation of processes which can still be seen in operation today, as opposed to catastrophism which promoted the idea that sudden changes had causes which can no longer be seen at work. A uniformitarian perspective was adopted for biological changes. Such a view can seem to contradict the fossil record, which shows evidence of new species appearing suddenly, then persisting in that form for long periods. The paleontologist Stephen Jay Gould (1940–2002) developed a model that suggests that evolution, although a slow process in human terms, undergoes periods of relatively rapid change over only a few thousand or million years, alternating with long periods of relative stability, a model called "punctuated equilibrium" which explains the fossil record without contradicting Darwin's ideas.^[56]

Unit of change

It is generally accepted amongst biologists that the unit of selection in evolution is the organism, and that natural selection serves to either enhance or reduce the reproductive potential of an individual. Reproductive success, therefore, can be measured by the volume of an organism's surviving offspring. The organism view has been challenged by a variety of biologists as well as philosophers. Richard Dawkins (1941–) proposes that much insight can be gained if we look at evolution from the gene's point of view; that is, that natural selection operates as an evolutionary mechanism on genes as well as organisms.^[57] In his book The Selfish Gene, he explains:

{{quote|Individuals are not stable things, they are fleeting. Chromosomes too are shuffled to oblivion, like hands of cards soon after they are dealt. But the cards themselves survive the shuffling. The cards are the genes. The genes are not destroyed by crossing-over; they merely change partners and march on. Of course they march on. That is their business. They are the replicators and we are their survival machines. When we have served our purpose we are cast aside. But genes are denizens of geological time: genes are forever.^[58]

Others view selection working on many levels, not just at a single level of organism or gene; for example, Stephen Jay Gould called for a hierarchical perspective on selection.^[59]

Summary

The theory of evolution, which explains the variations in biological species, is founded on several basic observations. The first is that there are genetic variations within a population. Some offspring, by chance, have features that allow them to survive and thrive better than others. The offspring that survive will be more likely to have offspring of their own. Some of these useful features are then passed along to new generations. Evolution is therefore not a random process for creating new life forms. Mutations are (partly) random, but natural selection is far from random. Evolution is an inevitable result of imperfectly copying, self-replicating organisms reproducing over billions of years under the selection pressure of the environment.

There are misconceptions about evolution, and for various reasons there have been objections to the theory of evolution.^[61] However, the theory of evolution is supported by evidence, which includes direct observation in the laboratory and field studies. Domesticated animals have evolved as a result of selectively breeding for certain traits. The record of past evolution is found in fossils as well as in the genetic code, which demonstrates common ancestry of all organisms, both surviving and extinct. Evolution is a successful scientific theory and is universally accepted in every field of biology. An understanding of evolution is an essential component of biological sciences and much of medicine.

Notes

Further reading

• Berra, Tim (1990). Evolution and the myth of creationism: a basic guide to the facts in the evolution debate. Stanford, Calif: Stanford University Press. ISBN 0-8047-1548-3. 
• Darwin, Charles (1872). The Origin of Species. London: Murray.  6th Edition
• Darwin, Charles (1979). The illustrated Origin of species (abridged & introduced by Richard E. Leakey; consultants: Bynum WF, Barrett JA). London: Faber and Faber. ISBN 0-571-11477-6. 
• Dawkins, Richard (1995). River out of eden: a Darwinian view of life. New York: Basic Books. ISBN 0-465-01606-5. 
• Dawkins, Richard (1996). Climbing mount improbable. New York: W.W. Norton. ISBN 0-393-03930-7. 
• Dawkins, Richard (1976). The selfish gene. Oxford University Press, USA. ISBN 0-19-929114-4. 
• Gould, Stephen Jay (1980). The panda's thumb: more reflections in natural history. New York: Norton. ISBN 0-393-01380-4. 
• Gould, Stephen Jay (1995). Dinosaur in a haystack: reflections in natural history. New York: Harmony Books. ISBN 0-517-70393-9. 
• Gould, Stephen Jay (1989). Wonderful life: the Burgess Shale and the nature of history. New York: W.W. Norton. ISBN 0-393-02705-8. 
• Mayr, Ernst (2001). What evolution is. New York: Basic Books. ISBN 0-465-04425-5. 
• Ridley, Matt (2003). The red queen: sex and the evolution of human nature. New York, NY: Perennial. ISBN 0-06-055657-9. 
• Sagan, Carl & Druyan, Ann (1992). Shadows of forgotten ancestors: a search for who we are. New York: Random House. ISBN 0-394-53481-6. 
• Sis, Peter (2003). The tree of life: a book depicting the life of Charles Darwin, naturalist, geologist & thinker. New York: Farrar Straus Giroux. ISBN 0-374-45628-3. 

Videos about evolution

• Evolution (provided by PBS)
• Evolution of Life Explained by Carl Sagan
• Natural Selection Explained by Carl Sagan

Printable Introduction to evolution

• The Big Picture on Evolution (pdf), with description, from the Wellcome Trust

Genetics

• University of Utah Genetics Learning Center animated tour of the basics of genetics