11. Natural Selection & Genetic Drift

Jean Baptiste Lamarck

Lamarck believed that organisms acquired traits through use or disuse.

Alfred Wallace and Charles Darwin

AW: Saw evidence of evolution by natural selection which he wrote about to Darwin

CD: Published the theory of evolution by natural selection

Darwin's Postulates on Evolution by Natural Selection

1. There is variation between individuals.

2. In each generation some individuals will die.

3. Some individuals with specific features are more likely to survive.

4. Organisms who survive pass on their traits

Natural Selection

A process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates BECAUSE of those traits

Natural Selection pt. 2

- NOT random -> selecting for traits

- Traits that increase fitness get passed down

- Does not lead to perfection; Not progressive

- Phenotypic constraints to adaptation

Peppered Moth

- Initially white with black spots

- Industrial revolution sends soot everywhere

- White-black moths are too visible to predators

- Selection towards black pigmentation to blend in

Directional Selection

Favors one extreme of a spectrum of phenotypes

Shift phenotypes from one end to the other

Artificial Selection

Human-driven selection for specific phenotypes

Ex: Overfishing large fish leads to selection towards small, early-reproducing fish

Stabilizing Selection

Favoring average phenotype

Acting against extreme phenotypes

Ex: Female pelvis size

Disruptive Selection

Bimodal distribution

Favoring the extreme phenotypes, not average

Frequency-dependent Selection

Fitness of a specific trait is tied to its frequency in the population

Rare phenotypes can be good or bad

Heterozygote Advantage

Heterozygote has increased fitness or advantage compared to either homozygous genotype

- Keeps deleterious allele in the population

Genetic Drift

Random changes in relative allele frequency due to random chance

- Probability of becoming fixed = allele frequency

- Probability of becoming removed = 1 - alelle frequency

- "Drunk man's walk"

- Strongly affects small populations

Genetic Bottleneck

Genetic Bottleneck: Massive decrease in population size die to random events

- Decreases genetic diversity

Founder Effect

Founder Effect: Process where a small group of individuals colonize a new habitat

- Do not represent the diversity of parent group

- Ex: Pitcairn island