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/search_lessons.phpsort_by=audience_rank&topic_id=&keywords=teach+about+homologies&Submit=Search