2Mechanisms of Evolution The main causes of evolutionary change aregenetic drift,gene flow, andnatural selection.Natural selection is the most important, because it is the only process that promotes adaptation.
3Mechanisms of Evolution: Genetic Drift Allele can become more or less common BY CHANCERandom change in allele frequency is genetic drift (caused BY CHANCE!)In small populations, 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 = genetic drift
4Genetic Drift (continued) May occur when a small group of individuals colonies a new habitat. These individuals may carry alleles in different relative frequencies than the larger population that they came from. If so, the population that they create will be genetically different from the parent population
6Only 5 of 10 plants leave offspring p 0.7 q 0.3 p 0.5 q 0.5 RRRRrrRRRrRrrrRRRRrrRrRrRRRrrrRRRRRrRrRrGeneration 1p 0.7q 0.3p 0.5q 0.5
7Only 5 of 10 plants leave offspring Only 2 of 10 plants leave RRRRrrRRRRRrRrRRRRrrRRRRrrRRRRRrRrRRRRRRRrrrRRRRRRRrRrRrRRRRGeneration 1Generation 2Generation 3p 0.7q 0.3p 0.5q 0.5p 1.0q 0.0
8Genetic Drift: Bottleneck Effect The bottleneck effectis an example of genetic driftresults from a drastic reduction in population sizecould be caused by earthquakes, floods, etcPassing through a “bottleneck,” a severe reduction in population size,decreases the overall genetic variability in a population because at least some alleles are lost from the gene pool, andresults in a loss of individual variation and hence adaptability.
10Genetic Drift: Bottleneck Effect Cheetahs appear to have experienced at least two genetic bottlenecks:during the last ice age, about 10,000 years ago, andduring the 1800s, when farmers hunted the animals to near extinction.With so little variability, cheetahs today have a reduced capacity to adapt to environmental challenges.
11Genetic Drift: The Founder Effect Founder Effect – allele frequencies change as a result of the migration of a small subgroup of a population:Sample ofOriginal PopulationDescendantsFounding Population AFounding Population B
12Genetic Drift: The Founder Effect Founder Effect – allele frequencies change as a result of the migration of a small subgroup of a population:Sample ofOriginal PopulationDescendantsFounding Population AFounding Population B
13Genetic Drift: The Founder Effect Founder Effect – allele frequencies change as a result of the migration of a small subgroup of a population:Sample ofOriginal PopulationDescendantsFounding Population AFounding Population B
14Gene Flow Gene flow is another source of evolutionary change, is separate from genetic drift,is genetic exchange with another population,may result in the gain or loss of alleles, andtends to reduce genetic differences between populations.
15Hardy-Weinberg Equilibrium (page 259) Hardy and Weinberg – 2 scientists that asked: “Are there any conditions under which evolution will NOT occur?”If allele frequencies stay the same – the population does NOT evolve
16Hardy-Weinberg Equilibrium (page 259) 5 conditions are required to maintain genetic equilibrium from generation to generation:Random mating – equal chance of passing genesLarge population – genetic drift has less effectNo movement into or out of the population – must maintain gene poolNo mutation – can’t have new allelesNo natural selection – no “survival of the fittest”
17Hardy-Weinberg Equilibrium (page 259) Meeting these five conditions is difficult to do and often cannot be met, so evolution will occurMany organisms mate with chosen mates based on things like strength, color, etcNot all populations are largeMigration occurs all the timeMutations are bound to happenNatural selection/survival of the fittest often cannot be avoided!
18Natural Selection on Polygenic Traits Effects of natural selection on polygenic traits are very complexCan affect the distributions of phenotypes in any of three ways:Directional selectionStabilizing selectionDisruptive selection
19Directional Selection Individuals at one end of the curve have a higher fitness than individuals in the middle or at the other endEntire curve moves as the trait changesEx. Finches and beak size – food became scarce; finches compete for food; bigger beaks are able to find food, survive and reproduce = beak size increases
21Stabilizing Selection Individuals near the center of the curve have higher fitness than individuals at either end of the curveKeeps the center of the curve at its current position, but narrows the graphEx. Weight of human infants – small babies less likely to survive and large babies have difficulty being born = average-sized babies are favored
22Stabilizing Selection KeyLow mortality, high fitnessHigh mortality, low fitnessSelection against both extremes keep curve narrow and in same place.Percentage of PopulationBirth Weight
23Disruptive SelectionIndividuals at upper and lower ends of the curve have higher fitness than individuals near the middleCan cause graph to split into two, creating two distinct phenotypesEx. Bird – 2 different seed sizes (small and large); birds with large beaks and birds with small beaks survive = two distinct beak sizes
24Disruptive Selection Disruptive Selection Key Largest and smallest seeds become more common.KeyLow mortality, high fitnessPopulation splits into two subgroups specializing in different seeds.Number of Birds in PopulationNumber of Birds in PopulationHigh mortality, low fitnessBeak SizeBeak Size