Genetic drift and gene flow are random and so do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment.
Slide 31
Sexual selection is natural selection for mating success.
It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics.
Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival.
Slide 32
Sexual Selection
Slide 33
How do female preferences evolve?
The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should be selected for.
Slide 34
The Preservation of Genetic Variation
Various mechanisms help to preserve genetic variation in a population:
Diploidy maintains genetic variation in the form of hidden recessive alleles.
Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes. Natural selection will tend to maintain two or more alleles at that locus.
The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance.
Slide 35
Heterozygote Advantage
0–2.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
Frequencies of the
sickle-cell allele
2.5–5.0%
7.5–10.0%
5.0–7.5%
>12.5%
10.0–12.5%
Slide 36
In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population.
Selection favors whichever phenotype is less common in a population.
Slide 37
Frequency Dependent Selection
“Right-mouthed”
1981
“Left-mouthed”
Frequency of
“left-mouthed” individuals
Sample year
1.0
0.5
0
’82
’83
’84
’85
’86
’87
’88
’89
’90
Slide 38
Neutral Variation
Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage.
For example,
Variation in noncoding regions of DNA
Variation in proteins that have little effect on protein function or reproductive fitness.