KEY CONCEPT Natural selection is not the only mechanism through which populations evolve.

NGSSS SC.912.L.15.14* Discuss mechanisms of evolutionary change other than natural selection such as genetic drift and gene flow. (MODERATE)

Four causes of evolutionary change: Mutation: fundamental origin of all genetic (DNA) change. Crossing-over …others at grosser chromosome level

Gene flow is the movement of alleles between populations. bald eagle migration Gene flow occurs when individuals join new populations and reproduce. Gene flow keeps neighboring populations similar. Low gene flow increases the chance that two populations will evolve into different species. This map shows the locations where young banded eagles were found during the first summer after hatching

Genetic drift is a change in allele frequencies due to chance. Genetic drift causes a loss of genetic diversity. It is most common in small populations. A population bottleneck can lead to genetic drift. It occurs when an event drastically reduces population size. The bottleneck effect is genetic drift that occurs after a bottleneck event.

Four causes of evolutionary change: Mutation: fundamental genetic shifts. Genetic Drift: isolated populations accumulate different mutations over time. In a continuous population, genetic novelty can spread locally.

Four causes of evolutionary change: Mutation: fundamental genetic shifts. Genetic Drift: isolated populations accumulate different mutations over time. Local spreading of alleles

Four causes of evolutionary change: Mutation: fundamental genetic shifts. Genetic Drift: isolated populations accumulate different mutations over time. Local spreading of alleles

Four causes of evolutionary change: Mutation: fundamental genetic shifts. Genetic Drift: isolated populations accumulate different mutations over time. Spreading process known as ‘gene flow’.

Four causes of evolutionary change: But in discontinuous populations, gene flow is blocked.

Four causes of evolutionary change: Variations accumulate without inter-demic exchange

Four causes of evolutionary change: Of course, this works at many loci simultaneously

The founding of a small population can lead to genetic drift. It occurs when a few individuals start a new population. The founder effect is genetic drift that occurs after start of new population.

Founder Effect.-- Human example: your tribe had to live near the Bering land bridge…

Founder Effect.-- …to invade & settle the ‘New World’!

Genetic drift has negative effects on a population. less likely to have some individuals that can adapt harmful alleles can become more common due to chance

Sexual selection occurs when certain traits increase mating success. Sexual selection occurs due to higher cost of reproduction for females. males produce many sperm continuously females are more limited in potential offspring each cycle

There are two types of sexual selection. intrasexual selection: competition among males intersexual selection: males display certain traits to females

Four causes of evolutionary change: Mutation: fundamental genetic shifts. Genetic Drift: isolation  accumulation of mutations Founder Effect: immigrant sampling bias. Natural Selection: reproductive race These 4 interact synergistically