Mechanisms of Evolution Biology Mr. Solis

Populations, Not Individuals Evolve An organism cannot evolve a new phenotype, but rather natural selection acts on the range of phenotypes in a population. Evolution occurs as a populations genes and their frequencies change over time. Gene Pool- All the alleles of a populations genes. Allelic Frequency- the percentage of any specific allele in a gene pool. Genetic Equilibrium- frequency of alleles remains the same over generations.

Changes in genetic equilibrium A population in genetic equilibrium is not evolving. Anything that affects the genes in gene pool and affects the frequency will cause the population to evolve. Genetic Drift- the alteration of allelic frequencies by chance events.

Natural Selection acts on variations Some variations increase or decrease an organisms chance of survival in an environment. Three types of natural selection that act on variations. –Stabilizing –Directional –Disruptive

Stabilizing Natural selection that favors average individuals in a population. Example: organisms that range from small, medium and large. The large ones are easily seen and eaten, the small have difficulty finding food. Therefore the average size has a selective advantage.

Directional Selection Natural selection favors one of the extreme variations. Example an insect invades an area populated with long beak woodpeckers and short beak woodpeckers. The insect lives deep within the bark only allowing birds with long beaks to reach them. Therefore having the selective advantage.

Disruptive Selection Favors both extreme variations of a trait. Example: When you have a light colored, medium colored and dark colored organism and the environment contains surroundings of light and dark color areas. This allows the dark and light colored organisms to survive but the medium color gets eliminated.

The Evolution of Species Speciation: occurs when members of similar populations no longer interbreed to produce fertile offspring with in their natural environment.

Physical Barriers can prevent interbreeding Geographic Isolation: occurs when a physical barrier divides a population. Example: A single species of a frog may live in the forest, within time a river forms creating a new trait. Eventually this new trait will procreate and create a sub population of its own. Eventually natural selection will occur and these frogs will never inter breed with each other.

Reproductive Isolation Occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring. There are two types of Reproductive Isolation. –When the genetic material is so different that fertilization will not occur. –When reproductive behaviors are different.

Speciation Rates Gradualism- species originate through a gradual change in adaptation. Punctuated Equilibrium: speciation happens quickly, in a rapid burst, with long periods of genetic equilibrium in between.

Patterns of Evolution Adaptive radiation: when an ancestral species evolves into an array of species to fit a number of diverse habitats. Divergent Evolution: When one species evolves to two subpopulations and eventually become two different species. Convergent Evolution: A pattern of evolution in which distantly related organisms evolve similar traits.