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Theory of evolution
This article is an overview of the Theory of Evolution. It is designed to be brief but precise. Follow the links to each process to find in depth articles about each mechanism of evolution.
Evolution is usually defined as "a change in allele frequency over time", or "heritable change" <ref>Talk origins:Defining evolution</ref>, or "lasting change in the mean phenotype of a population that transcends the life of an individual" . (See: Evolution for what the ToE is not). In layman's terms these translate as a change in the characteristics of living things over multiple generations. The idea of evolution had been thought of by several people such as Anaximander<ref>UoC:Ancient Evolution</ref> and Charles Darwin's grandfather Erasmus Darwin<ref>UoC:Erasmus Darwin</ref> before Charles Darwin's time but Darwin came up with the theory of evolution by means of Natural Selection, an explanation for how these changes occur, and gathered extensive material evidence to back up both the theory, and the fact that evolution has, and is occurring. Alfred Russel Wallace also came up with ToE by Natural Selection<ref>Western Kentucky University</ref>, but Darwin had much more extensive support for the theory; the field has since developed immensely.
The neo-Darwinian modern synthesis united the processes of variation and selection to give us the cornerstones of the modern theory of evolution. In Darwin's time there was little understanding of heredity<ref>Serendip</ref>, but since then genetics has developed, and we have traced heredity to units of DNA, known as genes. There are a number of well understood mechanisms by which variation is introduced into the gene pool.
Variation (increase in genetic diversity)
Main article: Variation
Main article: Mutation
Initially variation is introduced by mutation<ref>Mechanisms: the processes of evolution</ref>, a process which can create new alleles.<ref>Mutations</ref> Mutations are commonly seen as something bad, often associated with cancer, but can also be good, or have no effect at all.<ref>The peanut files</ref> Tissue cells divide by mitosis, and cells are dividing constantly in large multi-cellular organisms, like humans. Mutations, therefore, occur in humans every second. These may make the daughter cell more, or less, efficient. They may damage the mechanisms which control mitosis, making the cell divide uncontrollably, forming a tumor, or they may affect a gene which is never expressed in the cell, having no effect. Mutations in mitosis are never passed on, but mutations in meiosis are. Meiosis is the type of cell division which occurs in gametogenesis, the process by which gametes (sex cells) are produced. The same processes of mutation take place in both types of cell division.
Mutations are mistakes which are made when copying DNA, which result in a change to the sequence of base pairs in a gene. Mutations occur at a rate of 10-10 to 10-12 mutations per base pair.<ref>Mutation rates</ref> They can happen in a number of different ways. These in turn are classified by the length of the DNA involved in the mutation. In Gene mutations a single base pair, or very short length of DNA is involved, limiting the change to a single gene (gene defined here as a length of DNA which codes for one protein. In Chromosome mutations the length of DNA is much longer. In the prophase of meiosis, stretches of DNA break at areas called chiasmata, where they are then liable to mutation.
A cell's DNA could be compared to a set of encyclopedias, with one chromosome being represented by one volume. When copying out an encyclopedia entry you could make a typo and the word "not" could become the word "noy", so that the entry, or gene, no longer makes any sense. The protein that the gene makes is useless. This is a substitution mutation. Alternatively, "not" could become "note", and the entry means something completely different. This is an addition, or insertion mutation, similarly there are deletion mutations. Addition and deletion mutations can produce bigger changes than substitution mutations, as DNA codons are read as three consecutive base pairs. If a pair is inserted into the gene it will not just be the one amino acid in the resulting protein that has changed, but also each of the following amino acids. The final type of mutation is a reversal mutation, where a length of the DNA detaches itself and reattaches itself reversed. "Not", could become "ton", again changing the meaning of the entry.
Sorting and Shuffling
During the prophase of meiosis, when the chromatids break at chiasmata they may not rejoin to the correct chromatids. Since the chromatid pairs are in contact with each other during the prophase, it is easy for sections of the chromatids to swap, move to a different location on the chromosome, or very rarely break off entirely forming a new chromosome, or merge with another chromosome. This has the effect of shuffling our genes, so that the sets carried in the gametes are not identical to any of our own. This is important, because otherwise a good gene could be lost because it shares a chromosome with a bad gene, or a bad gene could spread because it shares a chromosome with good genes. This is called recombination or crossing over.
Further variation is added by sexual reproduction, where gametes carrying different sets of genes fuse. Sexual selection, though normally considered a mechanism which decreases variation, can actually increase variation, and many mammals have evolved to choose mates with greatly varying genomes to their own.
Main article: Gene flow
Gene flow occurs when genes from different populations mix, increasing the variation in each population. The term particularly applies when closely related species produce fertile hybrids which vector genes between species. Genes can be transferred between more distantly related species, called horizontal transfer, but this occurs less often.
Selection (decrease in genetic diversity)
Main article: Selection
Because of variation there will always be individual organisms in a population that are "fitter" than others. The variation in the population is reduced because of differential reproduction and non-random death rates - the selectors. Genes must not just make their carriers good at survival, they must also make them good at reproduction, and good at producing offspring that are good at reproduction.
Main article: Natural selection
Natural selection is described as "survival of the fittest", or a "struggle for survival". There is, of course, no conscious struggle, it is simply that those good at surviving survive, while those who aren't, don't. Genes, therefore, that make an individual good at surviving, also survive. Those which aren't so good don't survive.
Genes are selected for, or against, when there is a selection pressure, or collection of them. A commonly used example of a selection pressure is that of predator and prey running speed. This is also an example of different species "co-evolving" (changes to one species phenotype exert a selection pressure on another species). A population of big cats living in the African grasslands run at approximately the same speed as their preferred prey. By the mechanisms of variation the running speed of the prey increases slightly. The slowest big cats can no longer catch enough food, and they all die out. The average running speed of the population of big cats increases. By the mechanisms of variation the running speed of the big cats increases slightly - the new phenotype might be slightly longer legs or more streamlined body - the individuals who carry the new genes are better at catching prey, live longer and reproduce more. The new gene spreads through the population. By the mechanisms of variation the running speed of the big cats may decrease, the individuals carrying the new gene are less well adapted to hunting and starve, removing the deleterious gene from the gene pool.
There are a number of ways genes which seemingly prevent their own perpetuation remain in the gene pool. That is, they don't prevent their own perpetuation at all, it just looks at first like they do. The hereditary disease sickle-cell anaemia is a good example of this. The disease causes those who are homozygous (both of the chromosome pair have the same allele for the gene) for sickle-cell anaemia to produce defective haemoglobin (oxygen-carrying protein) which causes crippling and potentially fatal blood clots. Those who are heterozygous (only one of the chromosome pair carries the defective allele) for sickle-cell anaemia, however, are immune to malaria, and in areas where malaria is common the gene has spread. This is an important illustration of natural selection being environment-specific.
Main article: Sexual selection
As well as surviving organisms must reproduce for their genes to survive. There are many genes which control the processes involved in sexual reproduction, and this includes the choosing of partners. In sexual selection genes are selected for because genes for a particular phenotype, and genes for finding that phenotype desirable in a mate, are passed on to offspring such that those genes spread throughout the population.
Sexual selection can, but does not have to be, exclusive of natural selection. In English slang the word "fit" is often used to describe an attractive body, because attractive bodies are fit and healthy. When humans were hunters and gatherers, people who were fit and healthy were better at providing food for their families, so that their offspring, who carried their genes, did not starve but grew up to pass on their genes.
Sexual selection can occur against the pressures of natural selection, however. Or, at least, against many of the obvious pressures of natural selection. Peacocks and widow birds have large bright tails, which provide no advantage to flying, and attract predators. They have evolved because they attract potential mates. We can imagine a population of these birds' ancestors, with an optimum tail length dictated by natural selection, somewhere between the optimum length for flying, and the optimum for avoiding predators. There will be some males with longer than average tails, and some with shorter than average, but most close to the optimum. A gene may be introduced, by the processes of variation, that causes female carriers to prefer males with long tails. The carriers will therefore mate more often with males with long tails, and the offspring will carry the genes for both long tails and prefering long-tailed mates. While tail length is not an issue to non-carriers when choosing mates, the carriers choose only other carriers, so that the average tail length slowly increases. Any new gene that causes a further increase in tail length will spread through the population for the same reason, until tail length reaches an optimum between the pressures of sexual selection and the pressures of natural selection (predation, flight).
Main article: Artificial selection
Humans have been exploiting genetics and heredity for centuries to increase production and profitability from agriculture. Crops and livestock are selected for their yield, still by differential reproduction, but this time because farmers choose which individual animals are allowed to reproduce, and which seeds are sown for the next crop. By choosing carefully the phenotypes we desire we have made tall cerial crops which produce lots of grain, fat cows which produce lots of milk and meat, sheep which produce lots of wool, strong horses and chickens which produce lots of eggs. Artificial selection has been taking place for 10,000 years, since the first farmers of the fertile crescent. Wheat and cows were first domesticated over 8,000 years ago. Nowadays farmers keep records of milk production and only the cows with the highest milk yields are allowed to mate. Genes for high milk yield spread through the gene pool not because they increase that individual's chances of survival, but because they increase that individual's chances of being chosen for reproduction.
Natural selection only happens when there is a selection pressure, and a change - either a change in the environment, or the introduction of new alleles which change the reproduction rate of carriers. When there is no such change, stabilising selection takes place. If a new allele is introduced to the population of big cats which has no effect on the individual's reproductive rate, that allele is likely to be removed by stabilising selection. The offspring of the carrier of the new allele carry only ï¿½ of its genes, the grandchildren just ï¿½. The allele, therefore, disappears.
Since the ToE is all about change, stabilising selection is rarely mentioned.
Main article: Kin selection
Genetic drift (decrease in genetic diversity)
Main article: Genetic drift
Allele frequencies can change by chance alone, a process called genetic drift. The alleles which make up the next generation's gene pool are a sample of those that make up the current generation's gene pool. The frequency of alleles differs slightly between the generations by chance.
Some alleles will decrease in frequency, while others will increase in frequency. The average expected change is 0, since by genetic drift increase and decrease is equally probable.
Drops in population size often cause considerable changes in allele frequency, and the remaining alleles are not representative of the diversity before the population change. This is called the founder effect.
There has been much argument over how important genetic drift is to evolution.
Main article: Speciation
A species is a group of organisms with similar morphological, physiological, biological and behavioural features. That is, they look similar, behave in similar ways, and have similar DNA sequences. The organisms of a species can interbreed and produce fertile offspring. They can not breed with other species to produce fertile offspring. Individuals may take divergent evolutionary paths, by the processes above, to form separate species, a process called speciation.
Any population in a given geographical area will either be stable, or undergo directional evolution. When a geographical feature separates two populations, however, divergent evolution may take place. For example, a few individuals from a rodent population in northern Europe may be transported to England. The original population is large, in a stable habitat to which they are well adapted. The new population is small, in a habitat which varies in many ways from that which they are adapted to. Selection pressures on the new population may be wetter weather, different predators and different types of food. The average rodent generation may be 2 years, so after 100 years the changes in the population of migrant rodents may be enough to make them reproductively isolated from the original population. A speciation event has taken place.
Charles Darwin studied allopatric speciation on the Galapagos islands, where populations of finches had become isolated by the water between the islands.
Allopatric speciation has traditionally been thought to be the most common method of speciation, but current research on sympatric speciation suggests that things might not be that simple.
Also another type of speciation known as Artificial speciation occurs when humans select existing genetic variants within species or hybridizing different subspecies or breeds to create new species that are more suited to human use.
Not all speciation events are the result of geographical isolation. Divergent evolution could take place in populations that are not isolated, for example if a population of insects switch host plants and do not reproduce with those on the original plant. If divergent sexual selection takes place, such that a certain set of genes in a population is undergoing directional evolution in more than one direction, and the population is equally divided between the directions a speciation event may take place. A rare complication of meiosis is polyploidy, where a gamete ends up as a diploid (both sets of chromosomes) rather than haploid (single set of chromosomes). The offspring created by such an event will have more than the normal number of chromosomes, and will be reproductively isolated from the rest of the population.
An example of an observed sympatric speciation event is when the plant Oenothera lamarckiana gave rise to a new species of plant O. gigas in 1905 ce which could not successfully breed with its parents and also had a different chromosome number than its parents. (O. gigas had a chromosome number of 2N = 28 while its parent Oenothera lamarckiana had a chromosome number of 2N = 14.) <ref>http://links.jstor.org/sici?sici=0006-8071(190909)48%3A3%3C179%3ATBOTCI%3E2.0.CO%3B2-Y</ref> <ref>http://www.holysmoke.org/cretins/speci.htm</ref>
See also: Observed speciation.
Main article: Extinction.
Extinction, when there are no individuals of a species left alive, is the ultimate fate of all species. Extinction can be caused by competition with other species which occupy a similar niche, and are better adapted to it. A species' habitat may change such that the species can no longer survive in it. The species may be over-predated or their prey may become inedible or uncatchable. Some species remain relatively constant for millions of years, while others may become extinct relatively quickly, depending on the changes that take place to the habitats they occupy.
Mass extinctions are events of pruning of the evolutionary tree. Following a mass extinction there are many unoccupied niches, and new species evolve to fill these.
Main article: Punctuated equilibrium.
In the fossil record there are often many examples of each species, or genus, but transitionals between these are quite rare. This led Stephen Jay Gould and Niles Eldredge to develop their theory of punctuated equilibrium (punk eek). Gould and Eldredge proposed that species remain relatively constant for long periods of time when there is little pressure for the species to adapt, while they change quickly whenever their environment changes, exerting a new selection pressure. It was proposed that change usually occurs to a small peripheral population at the time of speciation. Punctuated equilibrium is presented such that speciation is analagous to mutation, and the replacement of one species by another as analogous to natural selection. The term species selection is sometimes used to describe the replacement of one species by another.
The proponents of punk eek have been criticised for overstating the importance of the theory. Critics point out that it has always been accepted that while rates of variation and selection are constant, the net rate of change varies. Most biologists also argue that species selection is neither analogous to nor as important as natural selection.
Evolution as the uniting theory of biology
The theory of evolution unites modern biology in the way that many things, such as homology would not make sense without a theory of evolution and common descent. Biological evolution explains these similar characteristics as being inherited from a common ancestor. The pattern of limb bones (pentadactyl limb) is an example of such homologous structures. The fossil record shows that the pentadactyl limb originated in primitive tetrapods in the Devonian Period. Predecessors, with more than five fingers, can be traced back even earlier to the fins of certain fossil fishes from which the first amphibians are thought to have evolved. It is still found in all classes of tetrapods (i.e. from amphibians to mammals) today.
Evolution explains Vestigial structures as once having had a function. As an example, with mole rats living underground, the eyes on a mole rat got smaller over time and eventually a layer of skin began to cover them, which eventually led to the form they are in today. Humans and other animals also bear some vestigial behaviors and reflexes. For example, the formation of goose bumps in humans under stress is a vestigial reflex; its purpose in human evolutionary ancestors was to raise the body's hair, making the ancestor appear larger and scaring off predators. Raising the hair is also used to trap an extra layer of air, keeping an animal warm. This reflex formation of goosebumps when cold is not vestigial in humans, but the reflex to form them under stress is. However these vestigial structures and behaviors are explained by evolution in the way that they once served a purpose but no longer serve a purpose in the new environment so they have become smaller and will eventually disappear.
The Importance of Evolution
- '"Nothing in biology makes sense except in the light of evolution." -- Theodosius Dobzhansky
In Darwin's time Evolution was a controversial theory. Since Origin of Species, however, the Theory of Evolution, and related areas such as genetics have come a long way. Though scientists still debate aspects of Evolution, such as the importance of punctuated equilibrium, the Theories of Evolution and Common descent are considered to be fact. Critics of Evolution, many of whom claim to be scientific, attack the theory claiming there is no evidence to support it, while presenting their own pseudoscientific hypotheses, claiming them to be theories, when they are little more than names. It is obvious, even to the majority of lay people, that creationism provides no scientific alternatives to evolution, and simply attacks the theory with lies in the hope of convincing those too dim to understand what's going on.
Evolution is the cornerstone of Biology. While a knowledge of evolution is not essential for understanding other biological concepts, when one does think of those concepts in terms of genes and selection, everything becomes obvious quickly. Complex organisms and metabolic processes can be looked at in terms of evolution, and make sense instantly.
The processes of the theory of evolution are capable of explaining the development of life on earth, by the theory of common descent, and with paleontology we can even find out how life developed, and what the world used to look like. Shared traits and distributions of organisms can be explained by common descent and macroevolution. Features and processes which otherwise make no sense, become clear when looked at in terms of our evolutionary past, as indeed do complex animal behavours, including many of our own behaviours.
- Colby, C., 1996. Talk.Origins introduction to evolutionary biology.
- Clegg, C.J., Advanced Biology. London: John Murray.
- Original article by Steinsky 23:24, 26 Aug 2003 (BST)