1. The evolution of Populations

2. What is a Population?
A group of organisms of the same species that live in a specific geographical area and interbreed

3. Population Size The number of individuals in a population
Can affect the population’s ability to survive

4. Very Small Populations
Most likely to become extinct
Random events or natural disturbances, such as a fire or flood, endanger small populations
Tend to experience interbreeding
Only relatives may be available as mates
Reduces the population’s fitness

5. Genetic variation
Gene pool: collection of alleles found in all of the individuals of a population
Allele frequency: proportion of one allele, compared with all the alleles for that trait, in the gene pool

6. Genetic variation comes from several sources
Mutation: a random change in the DNA of a gene
Mutation rates in nature are very slow
Genes mutate only about 1-10 times per 100,000 cell divisions
Mutation is the source of variation and makes evolution possible
Recombination: new alleles that form in offspring during meiosis

7. Natural selection acts on distributions of traits
Normal distribution: distribution in a population in which allele frequency is highest near the mean range value and decreases progressively toward each extreme end

8. Natural selection can change the distribution of a trait in one of three ways

9. Directional selection
Pathway of natural selection in which one uncommon phenotype is selected over a more common phenotype

10. Stabilizing selection
Pathway of natural selection in which intermediate phenotypes are selected over phenotypes at both extremes

11. Disruptive selection
Pathway of natural selection in which two opposite, but equally uncommon, phenotypes are selected over the most common phenotype

12. Gene flow is the movement of alleles between populations
Increases the genetic variation of the receiving population
Caused by the migration of individuals to or from a population
New individuals (immigrants) add alleles
Departing individuals (emigrants) take away alleles

13. Genetic drift is change in allele frequencies due to chance
Small populations, like small sample sizes, are more likely to be affected be chance
Some alleles will likely decrease in frequency and become eliminated
Genetic drift causes a loss of genetic diversity in a population

14. Genetic Drift Example: the cheetah
Undergone drastic population declines in the last 5000 years
Those alive today are the descendants of only a few individuals, and each cheetah is almost genetically uniform

15. Bottleneck effect
Genetic drift that results from an event that drastically reduces the size of a population
Example: northern elephant seals

16. Founder effect
Genetic drift that occurs after a small number of individuals colonize a new area
Example: Older Order Amish

17. Effects of genetic drift
Can cause several problems for populations
Population loses genetic variation
Individuals less likely to be able to adapt to a changing environment
Lethal genes could become more common due to chance

18. Sexual selection occurs when certain traits increase mating success
Males
Make sperm continuously
Value of each sperm is relatively small
Females
Limited to the number of offspring they can reproduce in each reproductive cycle
Each investment is more valuable
“The choosy sex”

19. Sexual selection
Selection in which certain traits enhance mating success
Traits are passed on to offspring
Intrasexual selection: competition among males for the female (i.e. male deer)
Intersexual selection: when males display certain traits to attract the female (i.e. male bird of paradise)
Some traits that were originally selected for quality and good health are now exaggerated through sexual selection (i.e. the male peacock feathers)

20. Hardy-weinberg equilibrium describes populations that are not evolving
Genotype frequencies stay the same in a population if certain conditions are met:
Very large population
No emigration or immigration
No mutations
Random mating
No natural selection
Real populations rarely meet all 5 conditions

21. The hardy-weinberg equation is used to predict genotype frequencies in a population
p2 + 2pq + q2
p2 = Frequency of individuals that are homozygous for allele A
2pq = Frequency of heterozygous individuals with alleles A and a
q2 = Frequency of individuals that are homozygous for allele a
Know that: p + q = 1

22. Hardy-weinberg example

23. Hardy-Weinberg Principle
The principle that states that the frequency of alleles in a population does not change unless evolutionary forces act on the population
IMPOSSIBLE IN NATURE!

24. There are five factors that can lead to evolution
Genetic drift
Gene flow
Mutation
Sexual selection
Natural selection

25. Speciation through isolation
The isolation of populations can lead to speciation
Reproductive Isolation: Final stage in speciation, in which members of isolated populations are either no longer able to mate or no longer able to produce viable offspring

26. Populations can become isolated in several ways
Behavioral isolation: due to differences in courtship or mating behavior
Geographic isolation: due to physical barriers
Temporal isolation: due to barriers related to time, such as differences in mating periods or differences in the time of day that individuals are most active

27. Example: Reproductive Isolation
Kaibab squirrel
North Rim of the Grand Canyon, Arizona
Black belly
Abert squirrel
South Rim of the Grand Canyon, Arizona
White belly
Have been isolated for about 10,000 years and cannot interbreed
Some biologist consider them different species

28. A New Species
Divergence: the accumulation of differences between groups
Leads to the formation of new species
Speciation: The formation of new species as a result of evolution by natural selection

29. Subspecies
Populations of the same species that differ genetically because of adaptations to different living conditions
The members of newly formed subspecies have take the first step toward speciation
Eventually, the subspecies may become so different that they can no longer interbreed successfully…they would then be separate species

30. Subspecies

31. Evolution through natural selection is not random

32. Convergent evolution
Evolution toward similar characteristics in unrelated species, resulting from adaptations to similar environmental conditions

33. Divergent evolution
Evolution of one or more closely related species into different species; resulting from adaptations to different environmental conditions

34. Species can shape each other over time
Beneficial relationships through coevolution
Evolutionary arms race

35. Species can become extinct
Background extinctions
Mass extinctions

36. Speciation often occurs in patterns
Punctuated equilibrium: theory that states that speciation occurs suddenly and rapidly followed by long periods of little evolutionary change
Adaptive radiation: process by which one species evolves and gives rise to many descendent species that occupy different ecological niches