Natural Selection and Artificial Selection
In the 1990s, hospitals noticed something alarming: MRSA, a staph bacterium, kept surviving antibiotics that used to wipe it out. Nobody "taught" the bacteria anything. So how does a population change in response to its environment? Natural selection is the answer, and the same logic explains how people turned a roadside weed into broccoli.
The four conditions for natural selection
Natural selection is not a force or a goal. It is an outcome that happens automatically whenever four conditions are true:
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Heritable variation. Individuals differ in a trait, and the differences are passed from parents to offspring (through DNA).
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Overproduction. More offspring are born than the environment can support.
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Competition. Offspring compete for limited resources (food, mates, space) or face dangers such as predators and disease.
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Differential survival and reproduction. Individuals with certain traits survive and reproduce more than others in that environment.
If all four hold, the next generation has more copies of the favorable alleles. The population evolves. Notice that individuals do not change. The population changes, because the proportions of traits shift from one generation to the next.
Where does the variation come from? Random mutation, plus the shuffling of alleles in meiosis and sexual reproduction. Mutations are not caused by the need for them. They happen at random, and the environment only "tests" them afterward.
List the four conditions that must be met for natural selection to occur in a population. Briefly explain why heritable variation is essential.
List the four conditions that must be met for natural selection to occur in a population. Briefly explain why heritable variation is essential.
Fitness and adaptation
Fitness is an individual's reproductive success: how many surviving, reproducing offspring it leaves compared with others in the population. Survival matters only because it lets an organism reproduce.
Fitness depends on the environment. A white coat is a high-fitness trait in snow but a low-fitness trait on dark soil. The environment does the "selecting," which is why it is called the selective pressure.
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…
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.
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%…
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.
Selection in action and its patterns
Selection acts on the phenotype, and over time it changes the distribution of a trait in three typical ways.
| Type | Which individuals do best? | Effect on the distribution |
|---|---|---|
| Directional | One extreme | Mean shifts toward that extreme |
| Stabilizing | Intermediate | Narrower; mean stays the same |
| Disruptive | Both extremes | Two peaks; middle shrinks |
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)…
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…
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.
Measuring selection and heritability
Biologists can measure how strongly selection acts.
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Selection differential = mean of the parents that reproduced − mean of the whole original population.
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Response to selection = mean of the offspring − mean of the original population (or of the previous generation).
If the response is close to the selection differential, the trait is highly heritable, meaning differences are mostly due to genes. If offspring look like the original population, the differences were mostly environmental.
Selection also reverses. If heavy rain makes small, soft seeds abundant, large beaks lose their advantage. Smaller, quicker-feeding beaks may be favored, so the mean could shift back. Selection always depends on the current environment.
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…
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.
Artificial selection
Artificial selection is selective breeding: people choose which individuals reproduce, based on traits they want. The mechanism is the same as natural selection (heritable variation, and only some individuals reproduce), but the selector is different.
| Natural selection | Artificial selection | |
|---|---|---|
| Who or what selects? | The environment (predators, climate, disease, mates) | Humans |
| Trait favored | Whatever raises fitness in that habitat | Whatever humans want (size, taste, oil content) |
| Genetic diversity | Maintained or shifted by environment | Often reduced, since few individuals breed |
The classic example is wild mustard. Breeders picked plants with large leaf buds (kale), flower clusters (broccoli and cauliflower), a big terminal bud (cabbage), and so on. All of these are one species that diverged by human choice.
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…
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.
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…
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.
Back to the start: MRSA did not learn anything. Resistant cells already existed by random mutation, antibiotics removed their competitors, and the resistant cells reproduced. That is natural selection.
Summary
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Natural selection needs heritable variation, overproduction, competition, and differential survival and reproduction.
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Fitness is the number of surviving, reproducing offspring. Adaptations are heritable traits that raise fitness in a given environment.
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Selection can be directional, stabilizing, or disruptive depending on which phenotypes do best.
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Populations evolve, not individuals. Mutations are random, and the environment selects among them.
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A response to selection close to the selection differential means the trait is highly heritable.
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Artificial selection uses the same mechanism, but humans choose the parents, often reducing diversity.