Selection on phenotype/behavior changes the underlying genetics (allele frequencies)

Selection on phenotype/behavior changes the underlying genetics (allele frequencies)

Directional selection on allele frequency

Selections alters the allele frequency of the pop.

FF has highest fitness

SS has lowest fitness

The rate of change in allele frequency is determined by the strength of selection (i.e., the fitness differential of the phenotypes)

Intensity determines rate of increase in allele frequency.Intensity depends on fitness differentials

Selection on Allele Frequency

• Directional selection—graph it– One allele will be lost, the other will be “fixed”

• Balancing selection

Balancing Selection

• Allele frequency is maintained by natural selection at some frequency between 0 and 1.0—Graph it

• Can be due to:– heterozygote advantage, – frequency dependent selection, or – multiple niches

Malaria and the S allele

Balancing selection on allele frequency: Due to heterozygote advantage (overdominance)

AA ASSS

Fitness w/malaria: 0.9 1.00.1

AS has the highest fitness when malaria is present, so natural selection will preserve both alleles

A and S never have equal frequency in a population, though

Balancing selection on A and S alleles

• Frequency A stays at about 90%• Frequency S stays at about 10%• No fixation/loss of A or S because AS is best genotype• Why is the frequency of A > S?

Because fitness of AA = 0.9 (pretty high) SS = 0.1 (very low).Therefore, AA will contribute more alleles than SS.

Balancing Selection on Allele Frequency due to frequency dependent selection

1 gene, two alleles, dominant allele right mouthed

Balancing Selection on Allele frequency due to multiple niches

Black-belliedSeed crackers

Environment offers large seeds and small seeds, but not intermediate seeds

Adaptation

• As a Process: the population as a whole becomes better suited for its environment over time as a result of natural selection– The avg. fitness of the pop. increases

• As a Product: a feature of an organism that has been molded by natural selection to suit its current use– Examples?

• “The sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition [birth], and no doubt they facilitate, or may be indispensable for this act; …

• …but as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals.”

Harvey and Pagel, 1991

• To be considered an adaptation, a characteristic must be derived and have evolved in response to a specific selective agent

Why does the giraffe have a long neck?

Why are polar bears white?

Why are pronghorn so fast?

Pronghorn running

Why do birds have feathers?

Why is blood red?

“The Adaptationist Programme”

Be careful about adaptationist explanations…

1a) The “use” of a feature is not always obvious (video)

Be careful about adaptationist explanations…

1b) The “use” of a feature is not always obvious(video)

Be careful about adaptationist explanations…

2) The feature may have arisen in a different environment

Cheetah in North America until 10,000 ya

Be careful about adaptationist explanations…

3) Some features were originally selected for another purpose (exaptation)

Feathers arose before flight;

Heat retention?Display?

Be careful about adaptationist explanations…

4) Neutral theory:some variation in nature [especially genetic] can be

selectively neutral

Be careful about adaptationist explanations…

5) A feature may be a by-product of selection for another trait

Why is blood red?

Color is a by-product of high iron content probably selectively neutral