1. SB6d. Genetic Change as Evolution
Natural Selection
Genetic variation in a population may increase the fitness of an organism or the chance that some individuals will survive.
Genetic variation can lead to phenotypic variation.
Phenotypic variation is necessary for natural selection.
Genetic variation is stored in a population’s gene pool.
made up of all alleles in a population
allele combinations form when organisms have offspring

2. SB6d. Genetic Change as Evolution
Allele Frequency
An allele frequency is the number of times an allele occurs in a gene pool, as a percentage of the total occurrence of all alleles 40 40
Explain to students that because members of a population interbreed with one another, they share a common group of genes called a gene pool. A gene pool consists of all genes, including all alleles for each gene, that are present in a population. Researchers study gene pools by examining the numbers of different alleles they contain.
Tell students: Allele frequency is the number of times an allele occurs in a gene pool, as a percentage of the total occurrence of all alleles for the same gene in that gene pool. For example, in the mouse population shown, the allele frequency of the dominant B allele (black fur) is 40 percent, and the allele frequency of the recessive b allele (brown fur) is 60 percent. The allele frequency of an allele has nothing to do with whether the allele is dominant or recessive.
Click to reveal the following problem.
Ask: This picture shows a total of 50 alleles; 20 alleles are B (black) and 30 are b (brown). How many of each allele would be present in a total of 100 alleles?
Answer: In a total of 100 alleles, 40 alleles would be B (black) and 60 alleles would be b (brown).
Click to reveal the answers.
Tell students that allele frequency has nothing to do with whether the allele is dominant or recessive. 60 60

3. There are six factors, or mechanisms, that can lead to evolution.
SB6d. Genetic Change as Evolution
There are six factors, or mechanisms, that can lead to evolution.
Genetic drift
Gene Flow
Mutations
Sexual Selection
Natural Selection
Reproductive Isolation
Populations, not individuals, evolve.

4. Mechanisms of Evolution
SB6d. Genetic Change as Evolution
Mechanisms of Evolution
Genetic drift changes allele frequencies due to chance alone.

5. SB6d. Genetic Change as Evolution Mechanisms of Evolution
Genetic drift can occur through:
Founder Effect: small subgroup of the population migrates
Bottleneck Effect: population size is dramatically reduced due to environmental event.
Explain to students that natural selection is not the only evolutionary mechanism that can change allele frequencies. In small populations, individuals that carry a particular allele may leave more descendants than other individuals leave, just by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population.
Tell students: This kind of random change in allele frequency is called genetic drift.
Explain that sometimes, a natural disaster, such as a storm or flood, can kill many individuals in a population. The gene pool of the surviving population may have allele frequencies that differ from those of the original gene pool just by chance. If the reduced population later grows, its allele frequencies will also differ from the original population’s.
Tell students: The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population. A severe bottleneck effect can sharply reduce a population’s genetic diversity, and thus decrease diversity within the species.
Explain that genetic drift may also occur when a few individuals colonize a new habitat.
Click to show founding individuals.
Tell students: These founding individuals may carry alleles that differ in relative frequencies from those of the main population, just by chance. The new gene pool therefore starts out with allele frequencies that differ from those of the parent gene pool.
Explain that this situation, in which allele frequencies change as a result of the migration of a small subgroup of a population, is known as the founder effect.
Tell students: These two small groups from a large, diverse population could produce new populations that differ from the original group.
Click to reveal the descendants.
Point out that one example of the founder effect is the evolution of several hundred species of fruit flies on different Hawaiian islands. All those species descended from the same mainland fruit fly population. However, species on different islands have allele frequencies that are different from those of the original species.
Ask: Why are the two populations of descendants shown so different from each other?
Answer: The two populations of descendants are different because they were founded by small subgroups of the original population that were, by chance, genetically very different from each other.
Explain that the founder effect may be especially likely to occur following a major change in the environment, such as a forest fire, landslide, volcanic eruption, or human destruction of a habitat.
Ask: Can you describe a scenario in which an environmental change such as this leads to the founder effect?
Answer: A fire might isolate a few mice in a small remnant of forest.

6. SB6d. Genetic Change as Evolution Mechanisms of Evolution
Gene flow moves alleles from one population to another.

7. SB6d. Genetic Change as Evolution Mechanisms of Evolution
Mutations produce the genetic variation needed for evolution.

8. SB6d. Genetic Change as Evolution Mechanisms of Evolution
Sexual selection selects for traits that improve mating success.

9. SB6d. Genetic Change as Evolution Mechanisms of Evolution
Natural selection selects for traits advantageous for survival.

10. Evolution through natural selection is not random.
SB6d. Genetic Change as Evolution
Natural Selection
Evolution through natural selection is not random.
Natural selection can have direction.
The effects of natural selection add up over time.

11. SB6d. Genetic Change as Evolution
Natural Selection can change the distribution of a trait.
Directional selection
Occurs when one extreme phenotype is favored
Disruptive selection
Occurs when both extreme phenotypes are favored
Stabilizing Selection
Occurs when the intermediate phenotype is favored.

12. Directional Selection
SB6d. Genetic Change as Evolution
Directional Selection
Individuals at one end of the curve have higher fitness than
individuals in the middle or at the other end.
Explain that when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection occurs. The range of phenotypes shifts because some individuals are more successful at surviving and reproducing than are others.
Tell students: Let’s examine how a finite supply of resources, such as food, can affect fitness and survival. Seed-eating birds with big, thick beaks can feed more easily on large, hard, thick-shelled seeds. Suppose the supply of small and medium-size seeds runs low, leaving only larger seeds. Under these conditions, larger beaks would serve as an adaptation, because big-beaked birds could feed more easily than small-beaked birds.
Point out that the big-beaked adaptation produces higher fitness and greater reproductive success. Over time, the average beak size of this population would probably increase.
Ask: If large seeds became scarce and there were more small seeds available, how would this affect the curve for directional selection?
Answer: The curve would probably shift to the left.

13. Stabalizing Selection
SB6d. Genetic Change as Evolution
Stabalizing Selection
Individuals near the center of the curve have higher fitness than
individuals at either end.
Explain that if individuals near the center of the curve have higher fitness than individuals at either end, stabilizing selection takes place. The center of the curve remains at its current position, but it narrows the curve overall.
Tell students: Birth weight among human infants is a trait related to fitness. Very small babies are likely to be less healthy, and are thus less likely to survive. Extremely large babies are more likely to experience difficulty being born. The fitness of smaller or larger babies is therefore lower than that of average-size babies. As a result, human birth weight is under the influence of stabilizing selection.
Ask: If medical advances led to large babies having a better chance of surviving, how would this affect the curve for stabilizing selection?
Answer: The curve would shift to the right.

14. SB6d. Genetic Change as Evolution
Disruptive Selection
Phenotypes at the upper and lower ends of the curve have higher
fitness than individuals near the middle.
Explain to students that when phenotypes at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection can occur.
Point out that disruptive selection lowers the fitness of intermediate phenotypes. If the pressure of natural selection is strong and lasts long enough, the single curve can split into two. In other words, disruptive selection can create two distinct phenotypes, increasing diversity within species.
Tell students: Suppose a bird population lives in an area where medium-size seeds become less common than large and small seeds. Birds with unusually small or large beaks would have higher fitness. As shown in the graph, the population might split into two groups: one with smaller beaks and one with larger beaks.
Ask: If more medium-sized seeds became available and small and large seeds became scarce, how would this affect the curve for disruptive selection?
Answer: The curve would shift back to having one central peak.

15. SB6d. Genetic Change as Evolution
Genetic Equilibrium
If a population is not evolving, the population is in genetic equilibrium
Allele frequencies
do not change
Hardy-Weinberg
principle
Tell students: The Hardy–Weinberg principle predicts that five conditions can disturb genetic equilibrium and cause evolution to occur.
Click to reveal nonrandom mating.
Explain that in genetic equilibrium, individuals must mate at random. But individuals often select mates based on size, strength, or coloration, a practice known as sexual selection. When sexual selection occurs, genes for traits selected for or against are not in equilibrium.
Tell students: Female peacocks, like the one shown, choose mates on the basis of physical characteristics such as brightly patterned tail feathers. This is a classic example of sexual selection.
Click to reveal small population size.
Tell students: Genetic drift does not usually have major effects in large populations, but it can affect small populations. Evolution due to genetic drift thus happens more easily in small populations.
Click to reveal gene flow from immigration or emigration.
Explain that changes in allele frequency can be produced by gene flow, the movement of genes into or out of a population. Individuals who join a population (through immigration) may introduce new alleles, and individuals who leave (through emigration) may remove alleles. If allele frequency in the population changes, gene flow has caused evolution to occur.
Click to reveal mutations.
Tell students: Mutations can introduce new alleles into a gene pool, changing allele frequencies and causing evolution to occur.
Click to reveal natural selection.
Tell students: If different genotypes have different fitness, natural selection will disrupt genetic equilibrium, and evolution will occur.
Explain that one or more of these conditions usually holds for real populations. So, most of the time, in most populations, evolution happens.

16. SB6d. Genetic Change as Evolution
Reproductive isolation can occur between isolated populations.
members of different populations cannot mate successfully
final step to becoming separate species which is speciation.
Speciation is the rise of two or more species from one existing species.

17. The isolation of populations can lead to speciation.
SB6d. Genetic Change as Evolution
The isolation of populations can lead to speciation.
Populations become isolated when there is no gene flow.
Isolated populations adapt to their own environments.
Genetic differences can add up over generations.
Types of Reproductive Isolation:
Behavioral isolation
includes differences reproductive behaviors, including courtship or mating
Geographic barriers
physical barriers divide populations, such as rivers
Temporal isolation
timing of reproductive periods or seasons prevents mating
Tell students: Reproductive isolation can develop in several ways, including behavioral isolation, geographic isolation, and temporal isolation.
Click to reveal behavioral isolation.
Tell students: If two populations that were once able to interbreed evolve differences in courtship rituals or other behaviors, behavioral isolation can occur. For example, eastern and western meadowlarks are similar birds whose habitats overlap. But members of the two species will not mate with each other, partly because they use different songs to attract mates. Eastern meadowlarks don’t respond to western meadowlark songs, and vice versa.
Click to reveal geographic isolation.
Tell students: When two populations are separated by geographic barriers such as rivers, mountains, or bodies of water, geographic isolation occurs.
Click to reveal temporal isolation.
Tell students: A third isolating mechanism, known as temporal isolation, happens when two or more species reproduce at different times. For example, suppose three similar species of orchids live in the same rain forest. Each species has flowers that last only one day and must be pollinated on that day to produce seeds. Because the species bloom on different days, they cannot pollinate one another.

18. SB6d. Genetic Change as Evolution