MECHANISMS OF EVOLUTION

POPULATIONS, NOT INDIVIDUALS, EVOLVE

An organism cannot change its phenotype. A phenotype can become more predominant in a population, though.

Gene PoolAll the alleles of the population’s genes.

ALLELIC FREQUENCY

The percentage of any specific allele in the gene pool.

GENETIC EQUILIBRIUM

A population in which the allele frequencies remain the same over many generations.

CHANGES IN GENETIC EQUILIBRIUM

A population in genetic equilibrium is not evolving.

When equilibrium of the gene pool is disrupted, evolution occurs.

Changes in equilibrium may be the result of mutations.

Gene FlowOrganisms moving in (immigration) or out (emigration) of a population creates changes in the allelic frequencies of the gene pool.

Hardy-Weinberg Principle

• In order for genetic equilibrium to remain, these conditions must be met:–No mutations

–No immigration or emigration

–Random mating

GENETIC DRIFT

The alteration of allelic frequencies by chance events.

Can greatly affect small populations.

NATURAL SELECTION ACTS ON VARIATIONS

THE EVOLUTION OF SPECIES

SPECIATION

• Occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment.

PHYSICAL BARRIERS CAN PREVENT INTERBREEDING

-Geographic isolation

-Reproductive isolation

CHANGE IN CHROMOSOME NUMBER

SPECIATION CAN OCCUR QUICKLY OR SLOWLY

GRADUALISM

• The idea that species originate through a gradual change of adaptations. Example: horses in fossil record.

PUNCTUATED EQUILIBRIUM

• The idea that speciation occurs in rapid bursts with long periods of genetic equilibrium between.

• Caused by abrupt temperature changes or introduction of a competitive species.

PATTERNS OF EVOLUTION

ADAPTIVE RADIATION

• Many species evolve from a single species.

• Example – Darwin’s finches and Hawaiian Island honeycreepers.

DIVERGENT EVOLUTION

• Two or more species with the same common ancestor become different species due to isolation.

CONVERGENT EVOLUTION

• Two species with different ancestors evolve similar traits due to selective pressures in the environment.