1. Mechanisms of Evolution Microevolution
Population Genetics

2. Key Concepts
23.1: Population genetics provides a foundation for studying evolution
23.2: Mutation and sexual recombination produce the variation that makes evolution possible
23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition
23.4: Natural selection is the primary mechanism of adaptive evolution

3. Concept 23.2: Mutation and sexual recombination produce the variation that makes evolution possible
Two processes, mutation and sexual recombination produce the variation in gene pools that contributes to differences among individuals

4. Mutation Mutations Cause new genes and alleles to arise
Are changes in the nucleotide sequence of DNA
Cause new genes and alleles to arise
Figure 23.6

5. Point Mutations A point mutation Is a change in one base in a gene
Can have a significant impact on phenotype
Is usually harmless, but may have an adaptive impact

6. Mutations That Alter Gene Number or Sequence
Chromosomal mutations that affect many loci
Are almost certain to be harmful
May be neutral and even beneficial
Gene duplication
Duplicates chromosome segments

7. Mutation Rates Mutation rates
Tend to be low in animals and plants
Average about one mutation in every 100,000 genes per generation
Mutations are spread more rapidly in microorganisms because of short generation times

8. Mutations
Can only be passed on to offspring only if they occur in the germ line
Are the ultimate source of genetic variation (new genes and alleles)
But are NOT considered a significant source of genetic change, especially in slowly reproducing plants and animals

9. Sexual Recombination
In sexually reproducing populations, sexual recombination is far more important than mutation in producing the genetic differences that make adaptation possible
Most variation is produced by genetic differences that result from recombination of existing alleles
Recombination may affect genotype frequencies but usually has no effect on allele frequencies

10. Concept 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition
Three major factors alter allele frequencies and bring about most evolutionary change
Natural selection
Genetic drift
Gene flow

11. Natural Selection Differential success in reproduction
Results in certain alleles being passed to the next generation in greater proportions

12. Genetic Drift Genetic drift is chance changes in the gene pool
Chance changes have more of an effect on a small gene pool
Statistically, the smaller a sample the greater the chance of deviation from a predicted result

13. Genetic drift
Describes how allele frequencies can fluctuate unpredictably from one generation to the next
Tends to reduce genetic variation
Figure 23.7
CRCR
CRCW
CWCW
Only 5 of
10 plants
leave
offspring
Only 2 of
Generation 2
p = 0.5
q = 0.5
Generation 3
p = 1.0
q = 0.0
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3

14. The Bottleneck Effect In the bottleneck effect
A sudden change in the environment may drastically reduce the size of a population
The gene pool may no longer be reflective of the original population’s gene pool
Figure 23.8 A
(a)
Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent.
Original
population
Bottlenecking
event
Surviving
population

15. Understanding the bottleneck effect
Can increase understanding of how human activity affects other species
Figure 23.8 B
(b)
Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.

16. The Founder Effect The founder effect
Occurs when a few individuals become isolated from a larger population
Can affect allele frequencies in a population

17. Gene Flow Gene flow Causes a population to gain or lose alleles
Results from the movement of fertile individuals or gametes
Tends to reduce differences between populations over time

18. Concept 23.4: Natural selection is the primary mechanism of adaptive evolution
Accumulates and maintains favorable genotypes in a population
Is the only deviation from the Hardy-Weinberg that leads to adaptation (of the population to the environment)
Requires genetic variation

19. Variation Phenotypic variation
Occurs between individuals in populations of all species
Is not always heritable
Only the genetic component can have evolutionary consequences
Figure 23.9 A, B
(a) Map butterflies that emerge in spring: orange and brown
(b) Map butterflies that emerge in late summer: black and white

20. Variation Within a Population
Both discrete and quantitative characters contribute to variation within a population

21. Quantitative characters
Discrete characters
Can be classified on an either-or basis
Quantitative characters
Vary along a continuum within a population

22. Polymorphisms Phenotypic polymorphism Genetic polymorphisms
Describes a population in which two or more distinct morphs for a character are each represented in high enough frequencies to be readily noticeable
Genetic polymorphisms
Are the heritable components of characters that occur along a continuum in a population

23. Measuring Genetic Variation
Population geneticists
Measure the number of polymorphisms in a population by determining the amount of heterozygosity at the gene level and the molecular level
Average heterozygosity
Measures the average percent of loci that are heterozygous in a population

24. Variation Between Populations
Most species exhibit geographic variation in the gene pools of separate populations or population subgroups 1
2.4
3.14
5.18 6
7.15 XX 19
13.17
10.16
9.12
8.11
2.19
3.8
4.16
5.14
6.7
15.18
11.12
9.10
Figure 23.10

25. Heights of yarrow plants grown in common garden
Cline
Some geographic variations occur as a cline, which is a graded change in a trait along a geographic axis
Figure 23.11
EXPERIMENT Researchers observed that the average size of yarrow plants (Achillea) growing on the slopes of the Sierra
Nevada mountains gradually decreases with increasing
elevation. To eliminate the effect of environmental differences
at different elevations, researchers collected seeds
from various altitudes and planted them in a common
garden. They then measured the heights of the resulting plants.
RESULTS The average plant sizes in the common garden were inversely correlated with the altitudes at
which the seeds were collected, although the height
differences were less than in the plants’ natural
environments.
CONCLUSION The lesser but still measurable clinal variation in yarrow plants grown at a common elevation demonstrates the role of genetic as well as environmental differences.
Mean height (cm)
Atitude (m)
Heights of yarrow plants grown in common garden
Seed collection sites
Sierra Nevada Range
Great Basin Plateau

26. Five Causes of Microevolution
Two processes, mutation and sexual recombination produce the variation in gene pools that contributes to differences among individuals and that makes evolution possible
Three major factors alter allele frequencies and bring about most evolutionary change
Natural selection
Genetic drift
Gene flow