Natural Selection on Polygenic Traits. Mechanisms of Evolution –The main causes of evolutionary change are genetic drift, gene flow, and natural selection

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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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.


Originalpopulation

Bottleneckevent

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– 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

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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


• 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”


• 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



Food becomes scarce.

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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


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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



Largest and smallest seeds become more common.

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Population splits into two subgroups specializing in different seeds.

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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