1. Populations Mechanisms
Evolution
Populations
Mechanisms

2. KEY CONCEPT A population shares a common gene pool.

3. Genetic variation in a population increases the chance that some individuals will survive.
Genetic variation leads to phenotypic variation.
the physical appearance of a trait based on alleles.
Alternative forms of genes  Ex. Green/Brown skin
Phenotypic variation is necessary for natural selection.
Genetic variation is stored in a population’s gene pool.
made up of all alleles in a population
allele combinations form when organisms have offspring

4. Allele frequencies measure genetic variation.
measures how common the allele is in the population
can be calculated for each allele in gene pool

5. Populations
Organisms of the same species living at the same place at the same time.
Populations evolve
NOT individuals

6. Natural selection acts on distributions of traits.
A normal distribution graphs as a bell- shaped curve.
highest frequency near mean value
frequencies decrease toward each extreme value
Traits not undergoing natural selection have a normal distribution.

7. Natural selection can change the distribution of a trait in one of 3 ways.
Microevolution is evolution within a population.
Can result from natural selection
Natural selection can take one of three paths.
Directional selection
Stabilizing selection
Disruptive selection

8. Directional selection favors phenotypes at one extreme.

9. Stabilizing selection favors the intermediate phenotype.

10. Disruptive selection favors both extreme phenotypes.

11. Natural selection is not the only mechanism through which populations evolve.
Gene flow – the movement of alleles between populations
When organisms join a new population and reproduce, their alleles become part of that population’s gene pool.
Increases the genetic variation of the receiving population

12. Gene flow keeps neighboring populations similar.
Low gene flow increases the chance that two populations will evolve into different species.

13. 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 event 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.

14. 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 the start of a new population.

15. 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

16. 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

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

18. Application
Male Irish elks, now extinct, had a 12-foot antler span. Describe how sexual selection could have caused such an exaggerated trait to evolve.
It may have been viewedby females as a “quality” trait in selecting mates

19. KEY CONCEPT New species can arise when populations are isolated.
The isolation of populations can lead to speciation.
Populations become isolated when there is no gene flow.
Isolated populations adapt to their own environments.
Genetic differences can add up over generations.
Speciation – the rise of two or more species from one species

20. Reproductive Isolation
When members of different populations can no longer mate successfully with one another.
Usually final step to becoming separate species

21. Behavioral Isolation
Isolation caused by differences in mating or courtship behaviors
Example: Fire Flies
Some species flash every 2 seconds
Some species flash every 5 seconds
These will not interbreed because they are not attracted to different flashing sequences

22. Geographic Isolation
Isolation due to physical barriers that divide a population
Barriers
Oceans
Canyons
Rivers
Mountains

23. Temporal Isolation
Exists when timing prevents reproduction between populations
Eastern spotted skunk
Western spotted skunk
Are considered different species because of mating times

24. Application
The Monterey Pine sheds its pollen in February. The Bishop Pine, which lives in the same region, sheds its pollen in April.
Write a paragraph explaining what type of isolation this represents.
Explain why you picked the type of isolation that you did.
Explain why it isn’t any two of the remaining types.