I think we should have an Evolution sticky post to refer to and add to whenever we get an inquiring newbie or even an obstinate creationist. We can cover the basics at first, and then delve into the more nuanced details of the theory.
Anyway, here goes.
What is Evolution?In biology, the process of evolution is the change in a population's genetic structure over successive generations. Specifically, it is the change in allele frequency over time. The many sub-processes of evolution account for the diversity of life, such as genetic inheritance, which accounts for the continuity of traits, mutation, which accounts for novel traits, and natural selection, which accounts for the environmental filtering of traits.
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Initially variation is introduced by mutation, a process which can create new alleles. Mutations are commonly seen as something bad, often associated with cancer, but can also be good, or have no effect at all. 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.
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LoginWhat is 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.
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LoginIn addition to simple survival, organisms must reproduce for their genes to be passed on to later generations. 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 be, 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 male Birds of Paradise have large bright feathers, which provide no advantage to flying, and might actually impede flight, thus attracting predators. These spectacular feathers have evolved because they attract potential mates.
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LoginWhat is Speciation?Speciation is the evolutionary process by which new biological species arise.
There are four geographic modes of speciation in nature, based on the extent to which speciating populations are isolated from one another: allopatric, peripatric, parapatric, and sympatric. Speciation may also be induced artificially, through animal husbandry, agriculture, or laboratory experiments.
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