Evolution as Genetic Change 16-2 Evolution as Genetic Change

Natural Selection acts on Phenotypes: Natural Selection on Single-Gene Traits Can lead to changes in allele frequencies and thus evolution. A change in environment that would favor a particular phenotype will likely increase the frequency of its allele. Natural Selection and Polygenic Traits Natural selection of traits controlled by two or more genes will likely lead to a normal bell-shaped distribution.

Normal Distribution of Finches Beak Size Number of Birds in Population

Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle, or at the other end. Key Directional Selection Low mortality, high fitness High mortality, low fitness Food becomes scarce.

Stabilizing Selection When individuals near the center of the curve have higher fitness than individuals at either end of the curve. Stabilizing Selection Key Low mortality, High fitness High mortality, Low fitness Selection against both extremes keep curve narrow and in same place. Percentage of Population Birth Weight

Disruptive Selection When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Disruptive Selection Largest and smallest seeds become more common. Key Low mortality, high fitness Population splits into two subgroups specializing in different seeds. Number of Birds in Population Number of Birds in Population High mortality, low fitness Beak Size Beak Size

Genetic Drift In small populations an allele can become more or less common merely by chance Remember, the smaller the group the less likely it will reflect the laws of probability Genetic drift: a random change in allele frequency due to chance Usually natural disasters If many chance occurrences favor an individual with a particular allele to leave more offspring, then the allele may become more common in the population Founder effect: a situation in which allele frequencies change as a result of the migration of a small subgroup of a population.

Sample of Original Population Descendants Founding Population A Founding Population B

Sample of Original Population Descendants Founding Population A Founding Population B

Sample of Original Population Descendants Founding Population A Founding Population B

Evolution Versus Genetic Equilibrium Hardy-Weinberg principle: the allele frequency in a population will remain constant unless one or more factors cause those frequencies to change A way to measure if evolution may be occuring 1. random mating all members of a population must have equal opportunity to produce offspring 2. large population the population must be large enough to avoid genetic drift 3. no movement into or out of the population no new alleles may leave or come into the population

4. no mutations 5. no natural selection no new alleles can be introduced into the population through mutation 5. no natural selection all genotypes in the population must have equal probabilities for survival and reproduction genetic equilibrium: if all the above factors are maintained in the population then the existing allele frequencies will remain constant No evolution is taking place.

Isolating Mechanisms speciation: the formation of new species As new species evolve, populations become reproductively isolated from each other reproductive isolation: when members of two populations cannot interbreed and produce fertile offspring (3 types) behavioral isolation capable of interbreeding but have differences in courtship rituals and other behaviors geographic isolation two populations are separated by geographic barriers (rivers, mountains, or bodies of water) temporal isolation two or more species reproduce at different times