1. Genes Within Populations
Chapter 20

2. 5 agents of evolutionary change
Mutation
Rates generally low
Other evolutionary processes usually more important in changing allele frequency
Ultimate source of genetic variation
Makes evolution possible

3. Gene flow Movement of alleles from one population to another
Animal physically moves into new population
Drifting of gametes or immature stages into an area
Mating of individuals from adjacent populations

4. Nonrandom mating Assortative mating Disassortative mating
Phenotypically similar individuals mate
Increases proportion of homozygous individuals
Disassortative mating
Phenotypically different individuals mate
Produces excess of heterozygotes

5. Genetic drift
In small populations, allele frequency may change by chance alone
Magnitude of genetic drift is negatively related to population size
Founder effect
Bottleneck effect

6. Genetic drift can lead to the loss of alleles in isolated populations
Alleles that initially are uncommon are particularly vulnerable

7. Northern Elephant Seal
Bottleneck case study
Nearly hunted to extinction in 19th century
As a result, species has lost almost all of its genetic variation
Population now numbers in tens of thousands

8. Selection
Some individuals leave behind more progeny than others, and the rate at which they do so is affected by phenotype and behavior
Artificial selection
Natural selection

9. 3 conditions for natural selection to occur and to result in evolutionary change
Variation must exist among individuals in a population
Variation among individuals must result in differences in the number of offspring surviving in the next generation
Variation must be genetically inherited

10. Natural selection and evolution are not the same
Natural selection is a process
Only one of several processes that can result in evolution
Evolution is the historical record, or outcome, of change through time
Result of evolution driven by natural selection is that populations become better adapted to their environment

11. Common sulphur butterfly
Caterpillar usually pale green
Excellent camouflage
Bright blue color morph rare and kept at low frequency by predation

12. Pocket mice come in different colors
Population living on rocks favor dark color
Populations living on sand favor light color

13. Genetic Variation and Evolution
Differences in alleles of genes found within individuals in a population
Raw material for natural selection
Evolution
How an entity changes through time
Development of modern concept traced to Darwin
“Descent with modification”

14. “Through time, species accumulate differences; as a result, descendants differ from their ancestors. In this way, new species arise from existing ones.”
Charles Darwin

15. Darwin was not the first to propose a theory of evolution
Unlike his predecessors, however, Darwin proposed natural selection as the mechanism of evolution.

16. Alfred Wallace
Wallace made many contributions to the development of evolutionary theory besides being co-discoverer of natural selection.
These included:
warning coloration in animals
and the Wallace effect, a hypothesis on how natural selection could contribute to speciation by encouraging the development of barriers against hybridization.

17. Jean-Baptiste Lamarck
Rival theory of Jean-Baptiste Lamarck was evolution by inheritance of acquired characteristics
If organisms adapt throughout their lives, then their offspring will be born with those improved characteristics.

20. Selection Many traits affected by more than one gene
Selection operates on all the genes for the trait
Changes the population depending on which genotypes are favored
Types of selection
Disruptive
Directional
Stabilizing

21. Acts to eliminate intermediate types
Disruptive selection
Acts to eliminate intermediate types
Different beak sizes of African black-bellied seedcracker finch
Available seeds fall into 2 categories
Favors bill sizes for one or the other

22. Birds with intermediate-sized beaks are at a disadvantage with both seed types – they are unable to open large seeds and too clumsy to efficiently process small seeds

23. Directional selection Acts to eliminate one extreme
Often occurs in nature when the environment changes
In Drosophila, artificially selected flies that moved toward the light
Now fewer have that behavior

24. Directional selection for negative phototropism in Drosophila

25. Stabilizing selection
Acts to eliminate both extremes
Makes intermediate more common by eliminating extremes
In humans, infants with intermediate weight at birth have the highest survival rate

26. Stabilizing selection for birth weight in humans

27. Limits of selection Multiple phenotypic effects of alleles
Larger clutch size leads to thinner shelled eggs
Lack of genetic variation
Gene pool of thoroughbreds limited and performance times have not improved for more than 50 years
Phenotypic variation may not have genetic basis

28. Selection for increased speed in racehorses is no longer effective

29. Fitness and its measurement
Individuals with one phenotype leave more surviving offspring in the next generation than individuals with an alternative phenotype
Relative concept; the most fit phenotype is simply the one that produces, on average, the greatest number of offspring

30. Fitness has many components
Survival
Sexual selection – some individuals more successful at attracting mates
Number of offspring per mating
Traits favored for one component may be a disadvantage for others
Selection favors phenotypes with the greatest fitness
Phenotype with greater fitness usually increases in frequency

31. Larger female water striders lay more eggs per day
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8 50
200 40 6
150
Number of Eggs
Laid per Day 30 4
Life Span of Adult
Female (days)
Number of Eggs Laid
During Lifetime
100 20 2 50 10 12 13 14 15 16 12 13 14 15 16 12 13 14 15 16
Length of Adult Female Water Strider (mm)
Length of Adult Female Water Strider (mm)
Length of Adult Female Water Strider (mm)
Larger female water striders lay more eggs per day
Large females survive for a shorter period of time
As a result, intermediate-sized females produce the most offspring over the course of their entire lives and thus have the highest fitness

32. Interactions Mutations and genetic drift may counter selection
In nature, mutation rates are rarely high enough to counter selection
Selection is nonrandom but genetic drift is random
Drift may decrease an allele favored by selection
Selection usually overwhelms drift except in small populations

33. Hardy–Weinberg principle
Hardy–Weinberg equilibrium
Proportions of genotypes do not change in a population if
No mutation takes place
No genes are transferred to or from other sources (no immigration or emigration takes place)
Random mating is occurring
The population size is very large
No selection occurs

34. Principle can be written as an equation
Used to calculate allele frequencies
For 2 alleles, p and q
p = B for black coat color
Black cat is BB or Bb
q = b for white coat color
White cats are bb
p2 + 2pq + q2 = 1
BB + Bb + bb = 1

37. If all 5 assumptions for equilibrium are true, allele and genotype frequencies do not change from one generation to the next
In reality, most populations will not meet all 5 assumptions
Look for changes in frequency
Suggest hypotheses about what process or processes at work