natural selection on polygenic traits. mechanisms of evolution –the main causes of evolutionary...
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Natural Selection on Polygenic Traits
Mechanisms of Evolution
– The main causes of evolutionary change are• genetic drift,• gene flow, and• natural selection.
– Natural selection is the most important, because it is the only process that promotes adaptation.
Mechanisms of Evolution: Genetic Drift
• Allele can become more or less common BY CHANCE
• Random 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
Genetic 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
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Generation 1p 0.7q 0.3
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Only 2 of 10plants leave
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Generation 1p 0.7q 0.3
Generation 2p 0.5q 0.5
Generation 3p 1.0q 0.0
Genetic Drift: Bottleneck Effect
– The bottleneck effect• is an example of genetic drift• results from a drastic reduction in population size• could be caused by earthquakes, floods, etc
– Passing 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, and
• results in a loss of individual variation and hence adaptability.
Genetic Drift: Bottleneck Effect
Originalpopulation
Bottleneckevent
Survivingpopulation
Genetic Drift: Bottleneck Effect
– Cheetahs appear to have experienced at least two genetic bottlenecks:
1. during the last ice age, about 10,000 years ago, and
2. during 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.
Genetic Drift: The Founder Effect• Founder Effect – allele frequencies change as a
result of the migration of a small subgroup of a population:
Sample of Original Population
Founding Population A
Founding Population B
Descendants
Genetic Drift: The Founder Effect• Founder Effect – allele frequencies change as a
result of the migration of a small subgroup of a population:
Sample of Original Population
Founding Population A
Founding Population B
Descendants
Genetic Drift: The Founder Effect• Founder Effect – allele frequencies change as a
result of the migration of a small subgroup of a population:
Sample of Original Population
Founding Population A
Founding Population B
Descendants
Gene 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, and• tends to reduce genetic differences between
populations.
Hardy-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
Hardy-Weinberg Equilibrium(page 259)
• 5 conditions are required to maintain genetic equilibrium from generation to generation:– Random mating – equal chance of passing genes– Large population – genetic drift has less effect– No movement into or out of the population – must
maintain gene pool– No mutation – can’t have new alleles– No natural selection – no “survival of the fittest”
Hardy-Weinberg Equilibrium(page 259)
• Meeting these five conditions is difficult to do and often cannot be met, so evolution will occur– Many organisms mate with chosen mates
based on things like strength, color, etc– Not all populations are large– Migration occurs all the time– Mutations are bound to happen– Natural selection/survival of the fittest often
cannot be avoided!
Natural Selection on Polygenic Traits
• Effects of natural selection on polygenic traits are very complex
• Can affect the distributions of phenotypes in any of three ways:– Directional selection– Stabilizing selection– Disruptive selection
Directional Selection
• Individuals at one end of the curve have a higher fitness than individuals in the middle or at the other end
• Entire curve moves as the trait changes– Ex. Finches and beak size – food became
scarce; finches compete for food; bigger beaks are able to find food, survive and reproduce = beak size increases
Directional Selection
Directional Selection
Food becomes scarce.
Key
Low mortality, high fitness
High mortality, low fitness
Stabilizing Selection
• Individuals near the center of the curve have higher fitness than individuals at either end of the curve
• Keeps the center of the curve at its current position, but narrows the graph– Ex. Weight of human infants – small babies
less likely to survive and large babies have difficulty being born = average-sized babies are favored
Stabilizing Selection
Key
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Birth Weight
Selection against both
extremes keep curve narrow and in same
place.
Low mortality, high fitness
High mortality, low fitness
Stabilizing Selection
Disruptive Selection
• Individuals at upper and lower ends of the curve have higher fitness than individuals near the middle
• Can cause graph to split into two, creating two distinct phenotypes– Ex. Bird – 2 different seed sizes (small and
large); birds with large beaks and birds with small beaks survive = two distinct beak sizes
Disruptive Selection
Disruptive Selection
Largest and smallest seeds become more common.
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Beak Size
Population splits into two subgroups specializing in different seeds.
Beak Size
Nu
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Low mortality, high fitness
High mortality, low fitness
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