1. HOW DOES EVOLUTION WORK? By: A Good Student

2. 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

3. How Does Evolution Work?  Evolution depends on genetic variation and the resulting phenotypic variation in populations.  Sources of genetic variation  Mutations  Recombination  Sex

4. Mechanisms of Evolution  Natural Selection  Sexual Selection  Mutation  Genetic Drift  Gene Flow (Migration)

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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.

12. 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.

13. 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

14. Sources  http://evoultion.berkeley.edu/evosite/Evo101.html 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 http://evolution.berkeley.edu/evosite/search/searc h_lessons.phpsort_by=audience_rank&topic_id=&k eywords=teach+about+homologies&Submit=Searc h