Mechanisms of Evolution

Gene Pool Combined genetic information of all the members of a particular population

Directional Selection Form of natural selection in which the entire curve moves. Favors an extreme trait one extreme of the trait experiences selection against it so trait shifts toward the other extreme Example: there was a selection pressure against short giraffe necks, since giraffes with short necks could not reach as many leaves to feed. As a result, the distribution of neck length shifted to favor individuals with long necks Put example with picture for all selection slides

Stabilizing Selection Form of natural selection by which the center of the curve remains in its current position. selects against the two extremes of a trait EX. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease.

Disruptive Selection Form of natural selection in which a single curve splits into two Select against individuals in the middle Example: imagine a plant that is pollinated by three different pollinators, one that was attracted to short plants, another that preferred plants of medium height and a third that visited only the tallest plants. If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against and the population would lean toward both short and tall, but not medium height plants

Genetic Drift A random event that results in part of a population being removed, regardless of their adaptations. Resulting population is successful because they are lucky not necessarily better adapted.

Bottleneck Effect Caused by genetic drift: populations size is reduced; can reduce genetic variation. Example of a bottleneck: Northern elephant seals have reduced genetic variation because of a population bottleneck humans inflicted on them by hunting them. Hunters reduced population to 20 individuals.Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

Founder’s Effect Caused by genetic drift: Occurs when a new colony is started by a few members of the original population. Can reduce genetic variation. Example, the Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency. Instead of picture paste example in its place

Gene Flow Transfer of genes from one population to another; causes genetic variation Ex. Immigration

Speciation Formation of a new species When 2 populations of species become so drastic they can no longer interbreed, they are considered 2 different species . Ex. Salamanders in California

Reproductive Isolation Separation of a species or population so that they cannot interbreed and produce fertile offspring Ex. Horse + Donkey=Mule What’s special about a mule? What sex is a mule? Can they reproduce? Why?

Behavioral Isolation Form of reproductive isolation in which two populations have differences in courtship rituals or other types of behavior that prevent them from interbreeding Ex. Birds dance/pose/sing to attract a mate

Geographical Isolation Form of reproductive isolation in which two populations are separated physically by geographic barriers such as rivers, mountains, or stretches of water

Temporal Isolation Form of reproductive isolation in which two populations reproduce at different times Closely related species fireflies mate at different times of night Frog species live and breed same pond reproduce different times of year Plants frequently have differing flowering seasons Put picture with words can make smaller to fit or leave one off

Mimicry An organism that evolves to look like another organism in order to survive Ex. King cobra and coral snake

Camouflage Organism that evolves to look like the environment in order to survive Ex. Iguana and walking stick

Resistance The capacity of a species or strain of microorganism to survive exposure to a toxic agent (as a drug) formerly effective against it Ex. Bacteria and penicillin A population is treated with a chemical. Individuals with a mutation are immune to chemical and don’t die. Next generation is not susceptible to chemical because most offspring had immune(resistant) parents who were able to survive and reproduce