1. CHAPTER 20
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2. 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”

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

4. Darwin was not the first to propose a theory of evolution
Unlike his predecessors, however, Darwin proposed natural selection as the mechanism of evolution
Rival theory of Jean-Baptiste Lamarck was evolution by inheritance of acquired characteristics

7. Genetic variation is the raw material for selection
Population genetics
Study of properties of genes in a population
Evolution results in a change in the genetic composition of a population
Genetic variation is the raw material for selection
In nature, genetic variation is the rule

8. 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

9. 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

12. 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

13. 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

14. 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

15. 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

16. 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

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

18. 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

19. 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

20. 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

21. 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

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

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

24. Housefly has pesticide resistance alleles at
pen gene decreases insecticide uptake
kdr and dld-r genes decrease target sites for insecticide

25. 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

26. 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

27. Larger female water striders lay more eggs per day
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200
150
100 50 8 6 4 2 40 30 20 10 12 13 14 15 16
Number of Eggs
Laid per Day
Length of Adult Female Water Strider (mm)
Life Span of Adult
Female (days)
Number of Eggs Laid
During Lifetime
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

28. 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

29. Gene flow can be Constructive Constraining
Spread beneficial mutation to other populations
Constraining
Can impede adaptation by continual flow of inferior alleles from other populations

30. Slender bent grass at copper mines
Resistance allele occurs at intermediate levels in many areas
Individuals with resistance gene grow slower on uncontaminated sites
Gene flow between sites high enough to counteract selection

31. Maintenance of variation
Frequency-dependent selection
Fitness of a phenotype depends on its frequency within the population
Negative frequency-dependent selection
Rare phenotypes favored by selection
Rare forms may not be in “search image”
Positive frequency-dependent selection
Favors common form
Tends to eliminate variation
“Oddballs” stand out

32. Negative frequency-dependent selection
In water boatman, fish eat the most common color type more than they would by chance alone

33. Positive frequency-dependent selection

34. Oscillating selection
Selection favors one phenotype at one time and another phenotype at another time
Effect will be to maintain genetic variation in the population
Medium ground finch of Galapagos Islands
Birds with big bills favored during drought
Birds with smaller bills favored in wet conditions

35. Heterozygote advantage
Heterozygotes are favored over homozygotes
Works to maintain both alleles in the population
Sickle cell anemia
Hereditary disease affecting hemoglobin
Causes severe anemia
Homozygotes for sickle cell allele usually die before reproducing (without medical treatment)

36. Why is the sickle cell allele not eliminated?
Leading cause of death in central Africa is malaria
Heterozygotes for sickle cell allele do not suffer anemia and are much less susceptible to malaria

37. 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

38. 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

39. 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

40. 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

41. Directional selection for negative phototropism in Drosophila

42. 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

43. Stabilizing selection for birth weight in humans

44. Experimental studies
To study evolution, biologists have traditionally investigated what has happened in the past
Fossils or DNA evidence
Laboratory studies on fruit flies common for more than 50 years
Only recently started with lab and field experiments

45. Guppy coloration
Found in small streams in northeastern South America and Trinidad
Some are capable of colonizing portions of streams above waterfalls
Different dispersal methods
Other species not able to make it upstream
Dispersal barriers create 2 different environments
Predators rare above waterfall

46. Pike cichlid (predator) rare above waterfall
Killifish rarely eats guppies
Guppy males larger and gaudier
Predator common below waterfall
Individuals more drab and reproduce earlier

47. Guppy lab study Other explanations are possible for field results
10 large pools
Added pike cichlids to 4, killifish to 4, and 2 left as controls
14 months and 10 guppy generations later
Guppies in killifish and control pool – large and colorful
Guppies in pike cichlid pools – smaller and drab

49. 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
Interactions between genes – epistasis
Selective advantage of an allele at one gene may vary from one genotype to another

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

51. Differences in the number of ommatidia in fly eyes does not have a genetic basis