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Evolution as Genetic Change in Populations
Mechanisms that affect change in populations Make Up #11
Lesson Objectives
Explain how natural selection affects single-gene and polygenic traits.
Describe genetic drift.
Explain how different factors affect genetic equilibrium.
Lesson Summary
How Natural Selection Works Natural selection on a single-gene trait can lead to changes in allele frequencies and changes in phenotype frequencies. For polygenic traits, populations often exhibit a range of phenotypes for a trait. When graphed, this range usually forms a bell curve, with fewer individuals exhibiting the extreme phenotypes than those with the average ones (in the case of beak size, the extremes may be either small or large beaks). Natural selection on polygenic traits can cause shifts to the bell curve depending upon which phenotype is selected for.
Directional selection takes place when individuals at one end of the bell curve have higher fitness than those near the middle or at the other end of the curve. For example, when large seeds are plentiful, large-beaked birds in a population may be selected for.
Stabilizing selection takes place when individuals near the middle of the curve have higher fitness than individuals at either end.
Disruptive selection takes place when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.
Genetic Drift In small populations, alleles can become more or less common simply by chance. This kind of change in allele frequency is called genetic drift.
The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population.
The founder effect is a change in allele frequency that may occur when a few individuals from a population migrate to and colonize a new habitat.
Evolution Versus Genetic Equilibrium If allele frequencies in a population do not change, the population is in genetic equilibrium. Evolution is not taking place.
The Hardy-Weinberg Principle states that allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change. These factors include: non-random mating, small population size, immigration or emigration, mutations, and natural selection.
Populations are rarely in genetic equilibrium. Most of the time, evolution is occurring. For example, many species exhibit non-random mating patterns. Sexual selection, or the process in which an individual chooses its mate based on heritable traits (such as size or strength), is a common practice for many organisms.
How Natural Selection Works
1.If a trait made an organism less likely to survive and reproduce, what would happen to the allele for that trait?
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2.If a trait had no effect on an organism’s fitness, what would likely happen to the allele for that trait?
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Use the table showing the evolution of a population of mice to answer Questions 3–5.
3.Is the trait for fur color a single-gene trait or a polygenic trait? Explain your answer.
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4.Describe how the relative frequency of fur color alleles is changing in this population and propose one explanation for this change.
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5.Suppose a mutation causes a white fur phenotype to emerge in the population. What might happen to the mouse population after 40 generations?
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Each graph below shows a change that has occurred in a population over time. Answer the questions next to each graph.
6.What type of selection is shown in graph A?
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7.Describe one scenario in which changes in the environment could have caused the shift in graph A.
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8.What type of selection is shown in graph B?
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9.Give an example of one other trait that might exhibit the type of selection shown in graph B.
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10.Describe one scenario in which changes in the environment could have caused the shift shown in graph C.
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11.What might happen if a barrier (for example, a new dam is built that divides the lake into two parts) separates the two groups shown in graph C?
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Genetic Drift
For Questions 12–14, complete each statement by writing the correct word or words.
12.In small populations, random changes in ______is called genetic drift.
13.A situation in which allele frequencies change as a result of the migration of a small subgroup of a population is known as the ______.
14.The ______is a change in allele frequency following a dramatic reduction in the size of a population.
15.Complete the concept map.
Evolution Versus Genetic Equilibrium
16.What is genetic equilibrium?
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17.List the five conditions that can disturb genetic equilibrium and cause evolution to occur.
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18.Suppose a population of insects live in a sandy habitat. Some of the insects have tan bodies and some have green bodies. Over time, the habitat changes to a grass-filled meadow.
Use the ideas of natural selection to explain how and why the insect population might change.
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