Presentation on theme: "15.3 NOTES. 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."— Presentation transcript:
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.
Genetic Drift Founder effect: a small group separates from the population and lives somewhere else they carry a subset of the populations gene Ex: Amish community
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
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
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 males Ex: 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 meadowlark Different mating patterns Liger Sterile
Geographic Isolation A separation of population by a barrier Ex: 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 squirrel South Rim Kaibab Squirrel North 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 time one 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 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 AmericanNorth 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