Exercises: Natural Selection and Artificial Selection

List the four conditions that must be met for natural selection to occur in a population. Briefly explain why heritable variation is essential.

Define fitness in the context of evolution. Then decide which of two rabbits has higher fitness: rabbit 1 lives 9 years and leaves 2 surviving offspring, or rabbit 2 lives 3 years and leaves 6 surviving offspring. Explain your reasoning.

Wild mustard has been bred by people into broccoli, kale, cabbage, and cauliflower. Define artificial selection, and state one way it differs from natural selection in terms of what determines which individuals reproduce.

A population of 1,000 beetles contains 500 green beetles and 500 brown beetles, and color is heritable. Birds find green beetles on brown tree bark more easily. Only 30% of green beetles survive to reproduce, compared with 60% of brown beetles. (a) How many green and brown beetles survive to reproduce? (b) Assuming each survivor leaves the same number of offspring, what percent of the next generation will be green? (c) Predict what happens to the frequency of the green phenotype over many generations if this pressure continues.

A population of $10^9$ bacteria contains, on average, 1 resistant cell per $10^6$ cells, because of random mutations that occurred before any antibiotic was used. (a) How many resistant cells are expected before treatment? (b) After an antibiotic kills nearly all susceptible cells, explain why the population becomes mostly resistant. (c) Explain why it is incorrect to say that the bacteria "decided" or "learned" to become resistant.

Classify each scenario as directional, stabilizing, or disruptive selection, and describe how the distribution of the trait changes:

(a) During a drought, only finches with large, deep beaks can crack the remaining hard seeds;

(b) human babies with very low or very high birth weight have lower survival than babies of intermediate weight;

(c) in a population of seed-eating birds, birds with very small or very large beaks feed efficiently on the available seeds, but birds with medium beaks do poorly.

In a hypothetical study of a finch population on an island, the mean beak depth before a severe drought was 9.0 mm. The mean beak depth of the birds that survived and reproduced was 9.8 mm. The mean beak depth of their offspring was 9.7 mm. (a) Calculate the selection differential (survivors minus original population) and the change in the mean from the parents' generation to the offspring generation. (b) Explain what these values suggest about the heritability of beak depth. (c) After several years of heavy rain, small soft seeds become abundant. Predict how selection might change, and explain whether evolution always moves in one direction.

In a hypothetical long-term breeding experiment, farmers selected only the corn plants with the highest oil content in each generation. The mean oil content rose from 4.5% in generation 0 to 12% in generation 50 and 17% in generation 100, but the increase per generation slowed. (a) Explain how this is an example of artificial selection acting on heritable variation. (b) Propose two reasons the rate of increase slowed. (c) Compare this process with natural selection acting on a wild plant population, addressing who or what selects, what trait is favored, and the effect on genetic diversity. (d) Argue one risk of a farming system that relies on a single genetically uniform crop variety.