Mechanisms and Types of Natural Selection

Natural Selection Individuals with favorable traits are more likely to leave more offspring better suited for their environment. Example: English peppered moth (Biston betularia) - light and dark phases

Artificial Selection The selective breeding of domesticated plants and animals by humans. Question: What’s the ancestor of the domesticated dog? Answer: WOLF

Population A localized group of individuals belonging to the same species.

Species Individuals have the potential to interbreed and produce viable or fertile offspring.

Gene Pool The total collection of genes in a population at any one time.

Microevolution Changes in a population’s gene pool over brief periods of geological time.

Four Mechanisms of Natural Selection (things that cause evolution) 1. Genetic drift: Change in the gene pool of a small population due to chance. Two examples: a. Bottleneck effect- drastic reduction in population size b. Founder effect- resulting from the colonization of a new location by a small number of individuals.

Four Mechanisms of Natural Selection 2. Gene Flow: The gain or loss of alleles from a population by the movement of individuals or gametes. Immigration or emigration.

Four Mechanisms of Natural Selection 3. Mutation: Change in an organism’s DNA that creates a new allele. 4. Non-random mating: The selection of mates other than by chance.

Types of Natural Selection Natural selection has three modes of action: 1. Stabilizing selection 2. Directional selection 3. Disruptive selection Number of Individuals Size of individuals Small Large

1. Stabilizing Selection Selects against the two extremes and favors the intermediate or average . Number of Individuals Size of individuals Small Large

Example The smallest beetles can’t compete and the largest beetles are easy prey

2. Directional Selection Favors one extreme. Number of Individuals Size of individuals Small Large

Example Only the beetles with the darkest bodies thrive in the new environment

3. Disruptive Selection Favors the both extremes but selects against the average. Number of Individuals Size of individuals Small Large

Example The beetles with the smallest wings thrive in the rotten tree trunks and the beetles with the largest wings thrive along the coastline

Stabilizing? Directional? Or Disruptive? During a drought, food becomes scarce. The finches with the larger beaks are better suited to survive.

Stabilizing? Directional? Or Disruptive? Only small and large seeds are available to a species of birds. Birds with large and small beaks have the advantage.

Stabilizing? Directional? Or Disruptive? If an insect is too large it can easily be seen but predators, and if it is too small it may not be able to capture and eat its prey.

Stabilizing? Directional? Or Disruptive? In humans, infants with low or high birth weights are less likely to survive.

Speciation The evolution or formation of new species.