15.3 NOTES

Genetic DriftA random change in the frequency of an allele in a population NOT due to natural selection.

More likely to occur in small populations.

Genetic DriftFounder effect: a small group separates from the population and lives somewhere elsethey carry a subset of the population’s gene

Ex: Amish community

Genetic DriftBottleneck: population declines to a very low number and then reboundsthe gene pool of the rebound population is similar to the original population

Ex: Northern Elephant SealsLarge population overhunting 20 Total

◦Population eventually recovered in size

◦Loss of genetic variationCould hurt long-term survival of species

Remember mutations? A random change in a sequence of DNA.

A small population with a lot of mutations has the greatest potential for evolution

Natural Selection

How does it alter phenotypes?

Stabilizing selection: selection against extreme expressions of a trait, selection for the average

Ex: birth weight in human babies

Directional selection: selection toward one extreme of a trait

Ex: Galapagos finches - large beaks during drought

Peppered moths - color

Disruptive selection: selection against the average, selection for both extremes

Ex: Cichlid fish size - males

Sexual selection: selection of a trait in males to attract a mate and intimidate other malesEx: peacocks

Reproductive Isolation

Some members of a population change so much that they can no longer produce offspring with members of the original population.

Eastern meadowlark and

Western meadowlarkDifferent mating

patterns LigerSterile

Geographic IsolationA separation of population by a barrierEx: mountains, rivers, the Grand Canyon

For speciation to happen, a population has to move away and then be reproductively isolated.

Allopatric Speciation 

A physical barrier divides one population into 2 or more populations

Abert squirrelSouth Rim

Kaibab SquirrelNorth Rim

Sympatric Speciation A species evolves into a new species without a physical barrier

The old species and new species live side by side during speciation

Adaptive Radiation(Divergent Evolution)The evolution of a new species in a relatively short period of timeone species evolves into several different forms that live in different habitats

homologous structures

Coevolution

A close relationship between 2 species

The evolution of one species influences the evolution of the other

Datura Plant & Hawk Moth

Convergent EvolutionUnrelated species evolve similar traits even though they live in environments that are really far apartsimilar ecology and climateanalogous structures

3. Convergent Evolution Unrelated species evolve

similar adaptations, due to environmental pressures (natural selection)

These adaptations may look similar from the outside, but actually evolve independently from each other Ex: sharks, dolphins, seals,

penguins

Analogous structures Similar in appearance and

function, but are developed from anatomically different parts

Evidence for convergent evolution

Ex: octopus eye versus vertebrate eye (both complex eyes

South American North American Pliocene Pleistocene

Natural Selection Common Adaptation**But common adaptations do not necessarily

imply common ancestor!

Gradualism:

Evolutionary change occurs gradually, over long periods of time

Punctuated Equilibrium

Patterns of long periods of stability (no change) interrupted by episodes of rapid change

Evolution can follow both patterns, depending on the situation and the time in evolutionary history