Evolution of Populations

Sources of Genetic Variation Mutations Any change in the DNA sequence. Genetic Shuffling Results from combinations created during sexual reproduction.

Natural Selection Can lead to changes in the allele frequencies and cause evolution to take place in a population.

Natural Selection Affects Phenotypes Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle or opposite end. Stabilizing Selection When individuals near the center of the curve have higher fitness than individuals at either end of the curve. Disruptive Selection When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.

Types of Selection

Genetic Drift In a small population, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population.

Founder Effect Results from the situation in which allele frequencies change as a result of the migration of a small subgroup of a population.

Genetic Drift

Hardy-Weinberg Principle States that the allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. If they remain constant, it is called genetic equilibrium, and evolution will not occur.

Hardy-Weinberg Principle (cont.) The five conditions required for genetic equilibrium are: There must be random mating Population size must be very large No movement into or out of the population No mutations No natural selection

Speciation As new species evolve, populations become reproductively isolated from each other. Behavioral isolation occurs when 2 populations can interbreed but courtship rituals or other behaviors prevent it. Geographic isolation is when 2 populations are separated by a geographic barrier, like a river or mountain. Temporal isolation is when 2 or more species reproduce at different times.