Lecture Topic: Natural Selection as the mechanism of adaptive evolution:

Importance:

• Natural Selection = differential survival and reproduction.

• Adaptation = traits that increase an individual's ability to survive and reproduce.

Are all traits adaptive?

• Natural selection (non-random changes) versus Genetic drift (random changes)

Evolution = non-random changes in allele frequencies within a population over time (across generations). Any gradual directional change.

  Artificial selection

Evolution of drug resistance in HIV

  Natural selection in action: beak evolution in one of Darwin’s finches

Principles of Natural Selection

1) More offspring are produced then can reproduce themselves.

2) Variation exists among individuals in survival and reproduction.

3) Variation is heritable, offspring resemble their parents! (genetic variation)

4) Differences in survival and reproduction is based on genetic variation among individuals.

Darwinian Fitness = the relative contribution an individual makes to the gene pool of the next generation.

• Fitness is relative!!!

• A more quantitative definition used by population geneticists:

• Fitness is the contribution of one genotype to the next generation compared to the contributions of alternative genotypes for the same locus/loci.

• survival alone does not guarantee fitness.

  Polymorphism in Snakes: same species but occur in different portions of the range.

Unit of Selection?

1) Individual Selection* most important!

* occurs within populations

• acts on the phenotype

Individuals are selected, but populations evolve

Types/Modes of Selection

• Directional Selection = an extreme phenotype is favored in the population often in the presence of novel environmental pressures.

• Stabilizing Selection = acts against extreme phenotypes and favors the more common intermediate variants.

  Modes of selection

Types/Modes of Selection

• Diversifying Selection = occurs when environmental conditions are varied in a way to favor both extremes in a population over an intermediate phenotype.

• Frequency Dependent Selection = the relative fitness of one morph is dependent on the frequency of another morph in the population.

  Modes of selection

Limits to Selection

1) Dependent on the presence of genetic variation.

• Ex., if only one allele present in a population, then a change in gene frequency is not possible unless a new mutation occurs.

2) Selection acts on phenotypes

Ex., does not operate on rare recessive alleles, and why we have genetic load within populations.

Normal and sickled cells heterozygote advantage: genotypes are HH, Hh, hh

HH hh

Mapping malaria and the sickle-cell allele

Frequency-dependent selection in a host-parasite relationship: infection rate of parasitic worm greater in common clones than the rare clone

Agents of Selection:

• The environment is the major selective force:

• Abiotic component =

• Biotic components =

  Clinal variation in a plant to abiotic factors (elevation and temperature variation).

Examples of Adaptation:

• Cryptic coloration = camouflage, often used to escape predation.

• Ex's.,

• Aposematic coloration = warning coloration, often used to advertise poisonous if eaten.

• Ex's.,

• And many more……………………….

Camouflage. Poor-will (left), lizard (right)

Deceptive coloration

Aposematic (warning) coloration