Chapter 20 Practice

Stabilizing selection

Intermediate phenotypes have the highest fitness, and the bell curve tends to narrow. For example, medium-green beetles might be the best camouflaged, and thus survive best, on a forest floor covered by medium-green plants. Stabilizing selection tends to narrow the curve.

Directional selection

One of the extreme phenotypes has the highest fitness. The bell curve shifts towards the more fit phenotype. For example, if the beetle population moves into a new environment with dark soil and vegetation, the dark green beetles might be better hidden and survive better than medium or light beetles. Directional selection shifts the curve towards the favorable phenotype.

Sexual selection in cardinals Female cardinals select male mates in part based on their bright red color. What effect would this have on a cardinal population that was in Hardy-Weinberg equilibrium?

The frequency of red alleles would be greater than those predicted by Hardy-Weinberg equilibrium

Natural selection can cause microevolution

change in allele frequencies), with fitness-increasing alleles becoming more common in the population.

In humans, babies that are born too small often lack the reserves to thrive, and babies that are too large are prone to difficult births. Since babies at the extremes are less likely to survive, evolutionary pressures favor moderately-sized babies.

stabilizing selection (favor the phenotypes in the middle)

Fitness

-Fitness = reproductive success -a measure of reproductive success (how many offspring an organism leaves in the next generation, relative to others in the group).

Genetic drift

-a mechanism of evolution in which allele frequencies of a population change over generations due to chance (sampling error). random changes -Genetic drift occurs in all populations of non-infinite size, but its effects are strongest in small populations.

Frequency of dominant allele The recessive phenotype of a trait occurs in 16% of a population. What is the frequency of the dominant allele?

0.60

Frequency of carriers A human autosomal recessive trait appears in 1 in 100 births. What percent of people homozygous dominant for this trait?

0.81

Natural selection on polygenic traits can take the form of:

1.Stabilizing selection 2.Directional selection 3.Disruptive selection

there are five basic Hardy-Weinberg assumptions

1.no mutation 2. random mating 3.no gene flow (no immigration/emigration)(also known as gene migration) 4. infinite population size 5. and no natural selection occurs

Frequency of an allele A population of flowers is in Hardy-Weinberg equilibrium with an allele frequency for white flowers (w) of 40%. What percentage of the flowers will have the colored or dominant phenotype?

84 % Explanation WW and Ww will be colored. If w is 40% then ww is 0.4 x 0.4 or 16% of the totals, so the remainder or 84% will be colored.

Interpret human birth weight graph In the graph above, how can the change in infant mortality be explained as birth weight increases from 2 to 7 pounds?

A larger baby will have more developed organs and thus have greater fitness

Disruptive selection

Both extreme phenotypes have a higher fitness than intermediate phenotypes. The bell curve develops two peaks. For example, if the beetles move into a new environment with patches of light-green moss and dark-green shrubs, both light and dark beetles might be better hidden (and survive better) than medium-green beetles.

A population of birds lived on an island where seeds are their primary food source. Birds with smaller beaks can eat small seeds more easily and birds with larger beaks can eat large seeds more easily. A fungus destroys the plants that make large seeds, leaving only the small seeds as food.

Directional selection (Directional selection favors one extreme or the other. In this case, the birds with small beaks will be selected for.)

A small group of squirrels gets separates them from the rest of the population due to a large flood. These squirrels, now confined to this new island, begin to start a new population.

Founder effect

Inheritance of sickle cell anemia Two parents who do not have sickle cell anemia have a child that has the disease. The parents are both:

Heterozygous for the sickle cell allele

Due to hunting, the population of Northern elephant seals was reduced to 20 individuals at the end of the 19th century. Since then, their population has since rebounded to over 30,000.

Individuals are genetically similar to each other. The population was drastically lessened which changed the allele frequencies and reduced genetic diversity. The surviving individuals reproduced and so the new population still suffers from low genetic diversity.

Blood types in native Americans The observation that most pureblood Native Americans have type O blood is best explained by _____

a founder effect

Mechanisms of evolution

correspond to violations of different Hardy-Weinberg assumptions. They are: 1.mutation, 2. non-random mating 3. gene flow 4. finite population size (genetic drift), 5. and natural selection.

Directional selection In a forest, trees that get more sunlight grow taller than other nearby trees. This is a form of ______

directional selection

In directional selection, over time During a drought on the Galapagos islands, finches with larger beaks were able to crack the large tough seeds produced by plants that survived the dry conditions. This is an example of ____.

directional selection

Genetic drift in small populations In a small population of cockroaches living in your kitchen, only a few roaches mate in one year. This can lead to random changes in allele frequency in the population through ______

genetic drift

What mechanism of evolution occurs when allele frequencies change over generations due to random chance?

genetic drift

When a population is in Hardy-Weinberg equilibrium for a gene

it is not evolving, and allele frequencies will stay the same across generations

Hardy-Weinberg equilibrium. If a population was in Hardy-Weinberg equilibrium then ____ would occur in that population.

no evolutionary changes

If the assumptions are not met for a gene, the population may evolve for that gene

the gene's allele frequencies may change

polygenic traits

traits determined by many genes. Polygenic traits in a population often form a bell curve distribution.

bottleneck effect

when a population is sharply reduced in size by a natural disaster

founder effect

when a small group splits off from the main population to found a colony