How Populations Evolve

Charles Darwin : Theory of Evolution by Natural Selection

Evolution – Change over time of heritable characteristics

• Selection is a major driving force for evolution• Members of a population vary in their inherited traits• All species are capable of producing more offspring

than the environment can support• Individuals whose inherited traits give them a higher

probability of reproducing in a given environment leave more offspring

• Unequal reproduction leads to accumulation of favorable traits in a population over generations (adaptation to the environment)

Artificial Selection

Natural Selection

Fossil Record – Evidence for Evolution

Tiktaalik – “fishapod”

Archaeopteryx dino to bird transition

Whale evolution

Multiple Lines of Scientific evidence support evolution

• Biogeography – closely related species live in close proximity. Especially apparent on islands.

• Comparative Anatomy – closely related species share common anatomical features (homology).

• Molecular Biology – genetic code is shared across all life. Closely related species have more similar genetic sequences.

Homology across mammal forearms

Vestigial Structures

Phylogeny – evolutionary tree

Tetrapodlimbs

Amnion

Lungfishes

Amphibians

Mammals

Lizardsand snakes

Crocodiles

Ostriches

Hawks andother birds

Feathers

TetrapodsA

mniotes

Birds

1

2

3

4

5

6

Evolution of populations

• A population is a group of interbreeding individuals of the same species that live in the same region.– Have variation of traits within the population due

to mutation and sexual reproduction.– Populations evolve by changes in allele

frequencies.

Mutations create new alleles, very rarely are these new mutations beneficial

Sexual reproduction results in reshuffling of allele combinations

Hardy-Weinberg Equilibrium• A population whose allele frequencies do not

change (evolve) is in HWE

Hardy-Weinberg Equations

p + q = 1

p2 + 2pq + q2 = 1

Assumptions of HWE

• Very large population size• No gene flow between populations (no

migration)• No mutations• Random mating between individuals• No selection

Genetic Drift• Random change in allele frequency due to

sampling effect– Stronger in smaller populations

Natural Selection• Selection results in individuals that are

adapted to the environment to reproduce at a higher rate than those that are not.

• Over generations the frequency of alleles that are adaptive increase, while those that are maladaptive decrease.

Sexual Selection leads to differences between sexes

Balancing Selection maintains variation

Natural Selection does not produce perfect organisms

• Selection can only act on existing variation• Evolution is limited by historical constraints

(what was adapted in the past)• Adaptations are often compromises• Drift, selection, and environment interact to

produce allele frequencies