Evolution of Populations

Stored in

Gene pool: the combined alleles of the population

Allele frequency: how common an allele is in a population

Sources of genetic variation:

Sources

Mutation:

• random change in DNA
• can be passed to offspring

Recombination:

• occurs through sexual reproduction
• DNA is not the same as its parents
• meiosis

The observable change in the allele frequency over time

Microevolution

Microevolution:

• Occurs on a small scale - within a single population
• natural selection can cause microevolution
• directional selection
• stabilizing selection
• disruptive selection
• can change how a population looks and behaves

Gene flow

The movement of alleles from one population to another

• when an organism joins a new population and reproduces
• the genes leave one gene pool and are added to another
• may occur with fungi and plant populations when their spores or seeds are spread to new areas

The less gene flow that occurs, the MORE GENETICALLY DIFFERENT THE TWO POPULATIONS CAN BECOME

Gene Flow

Bottleneck Effect

Genetic drift: changes in allele frequency DUE TO CHANCE

genetic drift that occurs after an event greatly reduces the size of a population

• overhunting of elephant seals
• cheetahs

Genetic Drift

Founder Effect

occurs after a small number of individuals colonize a new area

• the new gene pool is often very different from those of larger populations
• Amish of Pennsylvania (Ellis-van Creveld syndrome = dwarfism)

Mating can have an important effect on the evolution of the population

Sexual Selection

sexual selection occurs when certain traits increase reproductive success

intrasexual selection: competition among males (headbutting of bighorn sheep), the winner gets to mate with the female

intersexual selection: males display certain traits that attract the female (peacocks fanning their tails)

5 factors that can lead to evolution

5 factors

1. genetic drift

2. gene flow

3. mutation

4. sexual selection

5. natural selection

Why do real populations rarely reach HW equilibrium?

Reproductive isolation occurs when members of different populations can no longer mate successfully

The final step to becoming two different species

SPECIATION

Reproductive Isolation

Behavioral barriers

Chemical scents

Courtship dances

Courtship dances of frogs

Changes in these sexual signals can prevent mating between populations

2000 species of firefly

• produce flashes of light that attract mates of their own species

Physical barriers that divide a population into two or more groups

Geographic barriers

Lakes

Rivers

Mountains

Dried lakebeds

• the formation of the isthmus of Panama made it so that many marine organisms could no longer easily cross between the Atlantic and Pacific oceans

Temporal barriers

Species that breed during different times of day, different seasons, or different years cannot mix gametes

The Eastern spotted skunk (top) and Western spotted skunk (bottom) overlap in range but Eastern mates in later winter and Western mates in late summer

Convergent Evolution

Evolution TOWARDS similar characteristics

analagous structures: wings on birds and wings on insects

• tail fins on fish and marine mammals

Convergent Evolution

Divergent Evolution

Closely related species evolve in different directions - and become increasingly different

Divergent Evolution

Beneficial relationships through evolving together

two or more species evolve in response to changes in the other

• an also occur in competitive relationships (pressure to adapt or else they'll die)

Coevolution

Punctuated Equilibrium

bursts of evolutionary activity are followed by long periods of stability

Gradualism

change happens slowly over long periods of time

Patterns

Adaptive Radiation

the rapid evolution of many diverse species from an ancestral species

• descendent species are usually adapted to a wide range of environments