The Evolution of Populations

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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

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

Frequency-Dependent Selection

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.