Population Notes A population is a collection of individuals of a species in a given area. All members of a population can interbreed (meaning they can breed together), they share a common group of genes called a gene pool. A gene pool contains two or more alleles (or forms) of a certain gene for each trait. Example: A mouse population may have two alleles for fur color (B for black fur; b for brown fur). The frequency of these alleles will occur a certain amount of time within the gene pool. The relative frequency of an allele is the number of times that allele occurs in a gene pool compared with the number of times other alleles occur. Genetic Variations- mutations and genetic shuffling 1. Mutations- any change in a sequence of DNA. Do not always affect an organism‘s phenotype. Example: Point mutation of a DNA codon from GGA to GGU will code for the same amino acid, glycine. This mutation has no effect on the organism’s phenotype. Many mutations produce changes in the organism’s phenotype which can affect its fitness, or its ability to survive and reproduce. 2. Gene shuffling- causes most inheritable differences and occurs during the production of gametes (sex cells). Crossing over can further increase the number of different genotypes that can appear in offspring. Sexual reproduction can produce many different phenotypes. Natural Selection Acting on Traits Natural selection does not act directly on genes. Instead, it acts on phenotypes. It determines which alleles are passed on from one generation to the next. Natural selection can affect the distributions of phenotypes in any of three ways: stabilizing selection, directional selection, and disruptive selection. 1. Stabilizing Selection- When individuals near the center of the curve have higher fitness than individuals at either end of the curve. What happens? This keeps the center of the curve at its current position, but narrows the overall graph. Example: A mother bird would overheat small eggs and wouldn’t keep large eggs warm enough. The medium sized babies would survive. 2. Directional Selection- When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. What happens? The entire curve moves as the character trait changes. Example: When food became scarce, the finches with larger beaks were better able to survive and reproduce. Thus, the average size of the beaks increased. 3. Disruptive Selection- When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. What happens? Selection acts most strongly against individuals of an intermediate type. Example: There are only large nuts for birds with large beaks and tiny seeds available for birds with small beaks. If disruptive selection is strong enough and long enough, the single curve may split into two; creating two distinct phenotypes and possibly a different species.