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Assignment # Natural Selection As A Mechanism For Evolution Mrs. McCarthy Biology Wednesday, May 24, 2017 I. Natural Selection- The process by which populations change in response to their environment. This causes the frequency of certain alleles in a population to increase or decrease over time. II. Three types of natural selection A. Directional Selection- natural selection that favors extreme variations of a trait 1. Example: In giraffes necks those with longer necks could reach the leaves in the high trees and thus passed down their genes 2. Allele frequencies in the gene pool tend to move in one direction B. Stabilizing selection- natural selection that favors average individuals in a population 1. Example: Small Mice- low frequency Large Mice- Low Frequency Medium Mice- High Frequency 2. Allele frequencies in the gene pool tend to decline on the extremes C. Disruptive Selection- natural selection that favors individuals with traits at either extreme 1. Example: – – – – Normal crab species are tan to dark brown Crabs living in sandy areas are tan Crabs living in muddy areas are dark brown These two colors of crabs become different species 2. Example- Squirrels are normally brown – An earthquake changes the climate of an area – Some squirrels turn white – Some squirrels turn black 3. Allele frequencies in the gene pool tend to favor the extremes. II. Speciation- the process of making new species A. Geographic isolation- a species is separated by a physical barrier 1. A species is divided by some sort of physical barrier and can no longer interbreed 2. Over time, the species adapt to each environment and become two distinct species B. Reproductive Isolation- occurs when formerly interbreeding organisms can no longer mate C. Adaptive radiation- the evolution of species from one common ancestor to fill a variety of niches 1. Darwin’s finches diverged from a single seed eating finch into several different species D. Hybridization- mating two genetically similar species to create a new sterile species 1. Mule- horse and donkey 2. Liger- Lion and Tiger