1. The Evolution of Populations Ch. 11

2. Genetic Variation in Populations 11.1

3. Gene pool —the combined alleles of all of the individuals in a population
There are typically 2 or more alleles for a certain trait. (dominant or recessive)
Allele frequency —the measure of how common a certain allele is in a population.

4. Allele Frequency
If brown skin color became more advantageous, what would likely happen to the frequencies of alleles G and g in this gene pool?

5. Sources of Genetic Variation
The 2 main sources of genetic variation are mutations & the genetic shuffling that results from sexual reproduction (recombination)

6. Natural Selection in Populations 11.2

7. Microevolution
Microevolution: change in allele frequencies of a population over time
Natural selection is one process that leads to microevolution

8. Natural selection acts on the distribution of traits.
Remember: the range of most phenotypes seen in a population is the result of polygenic traits rather than single-gene traits

9. NS never acts directly on genes
NS can only affect which individuals survive & reproduce, & which do not
Also, remember that it is populations, not individual organisms, that can evolve over time

10. Natural selection acts on the distribution of traits.
Normal distribution: phenotypes near the middle of the range tend to be most common, while extremes are less common
Ex: height tall/short height
less common
 medium height
more common

11. Natural selection acts on the distribution of traits.
Natural selection can change the distribution of a trait in three ways:
1.) directional selection
2.) stabilizing selection
3.) disruptive selection

12. Directional Selection - occurs when individuals at 1 end of the curve have higher fitness than individuals in the middle or at the other end (large beaks or small beaks have higher fitness)

13. Stabilizing selection - occurs when individuals near the
Stabilizing selection - occurs when individuals near the center of a curve have higher fitness than individuals at either end (medium
beaks have
higher fitness)

14. Disruptive selection - when individuals at the upper & lower ends of the curve have higher fitness than individuals near the middle (large and small = high fitness
medium = low fitness)

15. Other Mechanisms of Evolution 11.3

16. Other Mechanisms of Evolution
Other factors besides natural selection may lead to evolution
Other mechanisms of evolution include: 1.) gene flow
2.) genetic drift
3.) sexual selection

17. Gene Flow
Gene flow: the movement of alleles from one population to another
Occurs when individuals move between populations
Gene flow b/t neighboring species keeps their gene pools similar
A lack of gene flow may cause neighboring populations to evolve into different species

18. Genetic Drift
Genetic drift: changes in allele frequencies due to chance alone
Causes a loss of genetic diversity
Occurs in small populations
Caused by 2 things:
1.) population bottleneck
2.) the founding of a small population

19. Bottleneck effect: Genetic Drift that occurs after a destructive event reduces the size of a population

20. Founder effect: Genetic Drift that occurs after a small number of individuals colonize a new area

21. 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

22. Sexual Selection
Sexual selection occurs when certain traits increase mating success

23. Evolution vs. Genetic Equilibrium 11.4
Genetic equilibrium - when allele frequencies remain constant, the population will not evolve

24. 2.) No emigration or immigration 3.) No mutations 4.) Random mating
5 conditions are required to maintain genetic equilibrium and prevent evolution:
1.) Very large population
2.) No emigration or immigration
3.) No mutations
4.) Random mating
5.) No natural selection

25. Speciation Through Isolation 11.5

26. The Process of Speciation
Speciation - formation of a new species
Gene flow between 2 populations must stop in order for them to become new species

27. Isolating Mechanisms
As new species evolve, populations become reproductively isolated from each other
Reproductive isolation - when members of 2 populations can’t interbreed & produce fertile offspring

28. Isolating Mechanisms
1.) Behavioral isolation - when 2 populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior
Ex.) Birds with diff. mating songs

29. Isolating Mechanisms
2.) Geographic isolation - 2 populations are separated by geographic barriers like rivers, mountains, or bodies of water
Ex.) Squirrels split by Colorado River

30. Isolating Mechanisms
3.) Temporal isolation - when 2 or more species reproduce at different times
Ex.) Orchids releasing pollen on different days

31. Patterns in Evolution 11.6

32. Convergent Evolution
Convergent evolution: evolution toward similar characteristics in unrelated species
Ex: Dolphins (mammals) and sharks (fish) have evolved similar tail fins as each has adapted to similar environmental conditions

33. Divergent Evolution
Divergent Evolution: closely related species evolve in different directions
Ex: red fox & kit fox

34. Coevolution
Coevolution: two or more species evolve in response to changes in each other

36. Extinction Extinction: the elimination of a species from Earth
2 types:
1.) background extinctions
2.) mass extinctions

37. Speciation often occurs in patterns.
Punctuated equilibrium: episodes of speciation occur suddenly and are followed by long periods of little evolutionary activity
Adaptive radiation: many species evolve from one species; descendent species usually adapt to a wide range of environments

38. Origin of Eukaryotic Cells
Endosymbiotic theory - proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms