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HOW DOES EVOLUTION WORK? By: A Good Student What is Evolution? Evolution is descent with modification. On a small scale, this can be changes in the gene frequency of a population over time. On a large scale, this can be the descent of different species from a common ancestor How Does Evolution Work? Evolution depends on genetic variation and the resulting phenotypic variation in populations. Sources of genetic variation Mutations Recombination Sex Mechanisms of Evolution Natural Selection Sexual Selection Mutation Genetic Drift Gene Flow (Migration) Natural Selection Variation exists in populations Variation is heritable Variation influence reproductive success Helpful variations are passed on to the next generation in greater abundance Natural Selection: an example Variation in beak size in Galapagos finches Beak size is a heritable trait After a drought, individuals with bigger beaks survived better and reproduced more often The next generation had a larger average beak size Sexual Selection A “special case” of natural selection Occurs because of nonrandom mating Acts on an individual’s ability to attract a mate Example: Male guppies have bright spots to attract females Can work in the opposite direction of natural selection Example: Predators can see bright males more easily Mutation A mutation could cause parents with genes for one trait to produce children for genes with another trait Example: Marfan Syndrome Spontaneous mutation causes a connective tissue disorder This is passed on to offspring Genetic Drift Due to random factors or chance (not selective pressures), individuals with one genotype produce more offspring Occurs in small population sizes Founder effect: a small group breaks off from the large, limiting the gene pool Bottleneck effect: population size reduced, genetic variation decreases Genetic Drift: an example Northern elephant seals were hunted by humans Decreased population size and genetic variation Population size has rebounded Genetic variation has not Display less variation than southern elephant seals that were hunted less Gene Flow When organisms from one population migrate to another population, they change the composition of the gene pool of both populations Example: If all the red headed people left Scotland, the next population would have less people with the trait. The place where they migrated to, would have more. Speciation Mutation, Natural Selection, and Genetic Drift would all act to increase the likelihood of speciation because they increase genetic diversity between the two populations. Gene flow would reduce the likelihood of speciation because it makes populations more genetically similar. Speciation Speciation occurs when two or more species (group of interbreeding individuals) come from a single species. The general causes of speciation are genetic drift and gene flow. Allopatric speciation: occurs when a specific barrier exists to isolate two populations geographically, preventing them from interbreeding Peripatric speciation: a type of allopatric speciation that occurs when one of the isolated populations is very small Parapatric speciation: occurs when there is no specific geographic barrier present; organisms are more likely to reproduce with individuals closer to them than further away in the geographic range of the species Sympatric speciation: occurs when there is no geographic distance or barrier present; instead some individuals might start to exploit a new niche, creating a divide between the population Sources http://evoultion.berkeley.edu/evosite/Evo101.html http://evolution.berkeley.edu/evosite/search/searc h_lessons.phpsort_by=audience_rank&topic_id=&k eywords=teach+about+homologies&Submit=Searc h