AP Biology · Lesson 1

Natural selection and the evidence for it

Know Know it

Natural selection needs four things. Individuals in a population vary in a trait. Part of that variation is heritable, passed on in alleles. More offspring are born than can survive, so they compete for food, mates and space. And individuals with some variants survive and reproduce more than others in that environment. Over generations, the alleles behind the favored variants become more common.

Fitness is reproductive success: the number of offspring an individual leaves that survive and go on to reproduce, compared with others in its population. A long life or great strength counts only if it leads to more surviving offspring.

Selection acts on the phenotypes of individuals, but it is the population that evolves, as its allele frequencies change across generations. The environment decides which variants are favored, so a trait that helps in one place can hurt in another.

Selection has three common patterns. Directional selection favors one extreme, so the average shifts. Stabilizing selection favors the middle, so the average stays and the spread narrows. Disruptive selection favors both extremes over the middle and can split a population into two forms. In artificial selection, people choose which individuals breed.

Several kinds of evidence show that species descend from common ancestors. Fossils show a sequence of forms in rock layers of known age. Homologous structures share one underlying plan though they do different jobs, while vestigial structures are reduced parts left from ancestors. Molecular evidence compares DNA and protein sequences: groups that split more recently differ in fewer positions, and nearly all life uses the same genetic code. Evolution can also be watched directly, as when bacteria become resistant to a drug over months.

Trap The need does not make the trait

A changed environment does not create the useful variant. It sorts the variation already in the population, or that arises by random mutation, by which individuals leave more offspring. And a change an animal gains during its life, such as stronger muscles from use, does not alter the alleles it passes on.

Similar shape is not always shared ancestry. Bird wings and insect wings do the same job but are built in different ways: they are analogous, from convergent evolution. Evidence of common ancestry comes from a shared underlying plan, not a shared use.

Worked example Easy

A fictional population of beetles varies in shell thickness. Which condition must hold for natural selection to change average shell thickness over many generations?

  1. Differences in shell thickness must be at least partly heritableAnswer
  2. Each beetle must try to grow a thicker shell during its life
  3. Every beetle must leave the same number of offspring
  4. New mutations must arise because the beetles need thicker shells

How to solve it

  1. Natural selection works on differences between individuals.
  2. Only differences passed on to offspring can build up over generations.
  3. So shell thickness must vary and be at least partly heritable.

Why each choice is right or wrong

  • A. Correct. Selection can shift a population only through variation that parents pass to their offspring.
  • B. Incorrect. Effort during a lifetime does not change the alleles a beetle passes on, so trying plays no role.
  • C. Incorrect. Equal reproduction would mean no selection at all; selection needs some variants to leave more offspring.
  • D. Incorrect. Mutations arise at random, not in answer to a need, so need is not a condition for selection.

Explain Explain it

Task verbs: explain, predict

To explain a change by natural selection, give every link in the chain, in order: the trait varies and is heritable; in this environment one variant helps individuals survive or reproduce; those individuals leave more offspring; so the alleles for that variant become more common over generations. Name the specific feature of the environment that does the favoring.

To name a mode of selection, compare the trait before and after. If the average moved, it is directional. If the average stayed but the extremes became rarer, it is stabilizing. If the middle became rarer and both extremes more common, it is disruptive.

Worked example Medium

In a fictional population of rock lizards, body mass was measured in spring. A harsh winter followed. Very light lizards ran out of stored fat, and very heavy lizards could not find enough food, so most survivors were of middle mass. The average mass of the survivors was about the same as the spring average.

Which kind of selection did the winter cause?

  1. Directional selection, because the winter pushed mass toward one end
  2. Stabilizing selection, because lizards of middle mass survived bestAnswer
  3. Disruptive selection, because both light and heavy lizards were affected
  4. Artificial selection, because the winter chose which lizards bred

How to solve it

  1. Compare before and after: the average mass stayed about the same.
  2. Both extremes, very light and very heavy, survived poorly.
  3. Favoring the middle while trimming both ends is stabilizing selection.

Why each choice is right or wrong

  • A. Incorrect. Directional selection would shift the average, but the survivors’ average mass stayed about the same.
  • B. Correct. Both extremes died more often and the middle survived, so the average held while the spread narrowed.
  • C. Incorrect. Disruptive selection favors both extremes, but here the light and heavy lizards were the ones that died.
  • D. Incorrect. Artificial selection means people choose the breeders; a winter is part of the natural environment.

Predict Your turn

A fictional nectar-feeding bat lives on an island. Tongue length varies among the bats and is heritable. Over many years, the island’s main flower is replaced by a new flower with a much deeper tube. Predict how the average tongue length of the bat population will change over many generations, and explain your prediction using natural selection.

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Compare with a model answer

A full answer does this

  • Predicts that the average tongue length of the population increases over generations
  • Explains that bats with longer tongues reach more nectar in the deep flowers, so they survive or reproduce more than short-tongued bats
  • Explains that these bats pass on the heritable trait, so alleles for longer tongues become more common in later generations

Model answer

Over many generations the average tongue length of the bats will increase. Bats with longer tongues can reach the nectar at the bottom of the deep new flowers, so they get more food and survive and reproduce more than bats with short tongues. Tongue length is heritable, so they pass the alleles for longer tongues to their offspring, and those alleles become more common in each later generation.

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