IV. Evolution as Genetic Change *Populations can evolve over time in different situations. A. Natural Selection on Single-Gene Traits -Natural Selection on single-gene traits can lead to changes in allele frequencies & thus to evolution. Example : Brown vs. Black vs. Red lizards. If brown is less visible, red & black may be eaten by predators.

B. Natural Selection & Polygenic Traits -When dealing with a range of phenotypes, a bell curve is used for comparison. -Natural selection can affect the distribution of phenotypes in any of three ways : 1. Directional selection 2. Stabilizing selection 3. Disruptive selection

Directional Selection Directional selection – when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. -The range of phenotypes shifts as some individuals die & others succeed, ex: Darwin’s finches & increase in large seed availability.

Stabilizing Selection Stabilizing selection – when individuals near the center of the curve have higher fitness than individuals at either end of the curve. -The center of the curve stays at its current position, but narrows the overall graph, ex: human birth weight.

Disruptive Selection Disruptive selection – when individuals at the upper & lower ends of the curve have higher fitness than individuals near the middle. -Selection acts most strongly against individuals of an intermediate type. -In disruptive selection, the pressure of natural selection can become strong enough to split a single curve in two, creating two distinct phenotypes, ex : Darwin’s finches & availability of large & small seeds only.

C. Genetic Drift Genetic drift – random change in allele frequencies that occurs in small populations. -In small populations, some individuals with particular traits may leave more descendants than others by chance. -Over time, a series of chance occurrences of this type can cause an allele to become common in a population. -Can occur when a small group colonizes a new habitat. Founder effect – change in allele frequencies as a result of the migration of a small subgroup of a population.

D. Evolution vs. Genetic Equilibrium *To understand how evolutionary change works, we must consider if there are any situations in which evolution doesn’t happen. Hardy-Weinberg principle – principle that states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. Genetic Equilibrium – situation in which allele frequencies remain constant. *No allele change = no evolution. -In order to maintain genetic equilibrium 5 conditions must be kept : 1. Random mating- no choosing mates. 2. Large population – no genetic drift. 3. No moving into/out of population. 4. No mutations. 5. No natural selection.