EVOLUTION 15.3 NOTES

Mechanisms of Evolution Genetic Drift A random change in the frequency of an allele in a population NOT due to natural selection. More likely to occur in small populations.

Mechanisms of Evolution Genetic Drift Founder effect: a small group separates from the population and lives somewhere else they carry a subset of the population’s gene Ex: Amish community

Mechanisms of Evolution Genetic Drift Bottleneck: population declines to a very low number and then rebounds the gene pool of the rebound population is similar to the original population

Northern Elephant Seals

Ex: Northern Elephant Seals Bottleneck Effect Ex: Northern Elephant Seals Large population  overhunting  20 Total Population eventually recovered in size Loss of genetic variation Could hurt long-term survival of species

Bottleneck Effect - Cheetahs

Mechanisms of Evolution Remember mutations? A random change in a sequence of DNA. A small population with a lot of mutations has the greatest potential for evolution

Mechanisms of Evolution Natural Selection How does it alter phenotypes?

Mechanisms of Evolution Stabilizing selection: selection against extreme expressions of a trait, selection for the average Ex: birth weight in human babies

Mechanisms of Evolution Directional selection: selection toward one extreme of a trait Ex: Galapagos finches - large beaks during drought Peppered moths - color

Mechanisms of Evolution Disruptive selection: selection against the average, selection for both extremes Ex: Cichlid fish size - males

Mechanisms of Evolution Sexual selection: selection of a trait in males to attract a mate and intimidate other males Ex: peacocks

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

Different mating patterns Eastern meadowlark and Western meadowlark Different mating patterns Liger Sterile

Isolation Geographic Isolation A separation of population by a barrier Ex: mountains, rivers, the Grand Canyon

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

Speciation Allopatric Speciation   A physical barrier divides one population into 2 or more populations Abert squirrel South Rim Kaibab Squirrel North Rim

Speciation 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

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

Adaptive Radiation

Patterns of Evolution Coevolution A close relationship between 2 species The evolution of one species influences the evolution of the other

Coevolution Datura Plant & Hawk Moth

Patterns of Evolution Convergent Evolution Unrelated species evolve similar traits even though they live in environments that are really far apart similar ecology and climate analogous 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!

How fast does speciation happen? Gradualism: Evolutionary change occurs gradually, over long periods of time

How fast does speciation happen? Punctuated Equilibrium Patterns of long periods of stability (no change) interrupted by episodes of rapid change

How fast does speciation happen?

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