20.2 Natural Selection
Key Takeaways
Natural selection requires heritable variation and differential reproductive success, and it edits existing variation rather than creating a mutation an organism needs.
Mutation is random with respect to usefulness.
Directional selection shifts the mean; dark peppered moths on sooty bark remain the classic directional example still taught that way.
Stabilizing selection favors the mean, as in human birth weight, and disruptive selection favors both extremes.
Sexual selection is differential mating success, and the fittest individual leaves more offspring in that environment rather than marching toward a predetermined goal.
20.2 Natural Selection
Natural selection is the mechanism Darwin and Wallace proposed, and CLEP Biology asks you to apply it. A stem will say who survives, who reproduces, and whether a trait was already present. Decide whether selection can explain the shift, which mode the pattern shows, and what fitness means in that environment. The College Board phrase differential reproduction is the sorting step: some individuals contribute more offspring than others.
What selection requires
Natural selection occurs when individuals vary, the variation is heritable, and some variants leave more offspring than others. Those requirements are one process. Variation that is not inherited cannot be sorted into the next generation. Inherited variation that does not affect reproductive success is not being selected. Differential reproductive success means some individuals contribute more offspring to the next generation than other individuals do.
Heritable variation means offspring tend to resemble their parents for the trait because of genes, not because the parent practiced the trait. A population of genetically identical individuals has nothing for selection to edit at that gene. A population in which the differences come only from diet or training also gives selection nothing to pass on. The raw material is difference that already exists and that gametes can carry.
Selection edits existing variation. It does not create a mutation because an organism needs one. A dark allele can spread on a dark tree only if moths that already carry it leave more offspring. If the allele is absent, selection has nothing to favor. Mutation is random with respect to usefulness. Mutations occur whether or not they help. The appearance of a mutation and its later spread are different events.
Three modes that move a distribution differently
Textbooks sort selection on a measurable trait into three modes. The trait has a distribution of values. Selection changes which part of that distribution leaves more offspring. Match the description to the mode and to the effect on the mean.
Directional selection favors one extreme, so the mean shifts toward that extreme. The classic example is the peppered moth on sooty bark. Dark moths were harder for birds to see, left more offspring, and the average color shifted toward dark. When cleaner air lightened the bark, the mean shifted again. The original peppered-moth story is still taught as directional selection. Soot did not create a dark mutation because moths needed one. The dark variant had to exist already.
Stabilizing selection favors values near the mean and removes both extremes. The mean stays, and the spread around it shrinks. Human birth weight is the standard example. Newborns much lighter or much heavier than the intermediate range have historically had higher mortality, so infants near the middle survived at higher rates. A stem that rewards the center and punishes both tails is stabilizing selection.
Disruptive selection favors both extremes and disfavors the middle. The mean can stay while variation increases. In black-bellied seedcracker finches, small bills and large bills each handle a different seed better than intermediate bills, so both tails leave more offspring. Directional selection rewards one tail. Disruptive selection rewards both.
Sexual selection is not a fourth row in that table. It is differential mating success. Individuals that gain more mates leave more offspring even when the winning trait is costly in survival. Antlers and bright courtship plumage are the familiar cases. It is still a count of offspring, not a drive toward a predetermined goal.
| Mode | Who leaves more offspring | What happens to the mean | Example to remember |
|---|---|---|---|
| Directional | One extreme | The mean shifts toward that extreme | Dark peppered moths on sooty bark |
| Stabilizing | Individuals near the mean | The mean stays; extremes decline | Human birth weight |
| Disruptive | Both extremes | The mean can stay while the middle thins | Seedcracker bills at both size extremes |
Fitness is a count of offspring in a place
The fittest individual is the one that leaves more offspring in that environment. Strength, speed, and size count only when they change that number. A moth that is not eaten can be fitter than a conspicuous moth. Change the environment, and the ranking can change. There is no fitness score that ignores the local setting.
Selection is not a drive toward a predetermined goal. No future target form is stored for the population to reach. The current environment filters variants that already exist, and the filter changes when the environment changes. Selection cannot build a trait whose mutation has not occurred. The peppered-moth reversal shows the point: the favored color was whichever color left more offspring on that bark.
Sorting a selection stem
Read three facts before you name a mode.
- Is the difference heritable, or is it a lifetime change such as a trained muscle?
- Which part of the distribution leaves more offspring: one tail, the center, or both tails?
- Does the question count survival only, or does it include mating success?
One tail, with a shifting mean, is directional selection. The center is stabilizing selection. Both tails are disruptive selection. Mating success is sexual selection and still counts offspring. A stem that says the organism produced the mutation it needed has reversed the order: mutation is random with respect to need, and selection only edits a heritable variant that already exists.
Warning
The fittest individual is the one that leaves more offspring in that environment, not necessarily the strongest. Selection is not a drive toward a predetermined goal. It edits heritable variation that already exists. It does not create a mutation because an organism needs one.
What must be true for natural selection to change a population, and what does selection not do?
Individuals must acquire traits by use during life, and selection writes a useful mutation whenever a trait is needed.
The population must have heritable variation and differential reproductive success, and selection edits variation that already exists.
Mating must be nonrandom and the population must be tiny, because those conditions are the definition of selection.
The population must be moving toward a predetermined goal, and mutation appears because that goal requires it.
Historically, human newborns near the middle of the birth-weight range survived at higher rates than newborns who were much lighter or much heavier. Which mode of selection is that?
Stabilizing selection, which favors the mean.
Sexual selection, defined as a change in allele frequency caused only by a population bottleneck.
Directional selection, which shifts the mean toward one extreme.
Disruptive selection, which favors both extremes over the mean.
On sooty bark, dark peppered moths were harder for birds to find and left more offspring, so the average color of the population shifted. The favored color later shifted again when the bark lightened. Which statement fits this example?
Soot created the dark mutation because the moths needed it, and selection was pursuing a fixed dark goal.
The case is disruptive selection, and fitness means the largest body regardless of how many offspring are left.
The case is stabilizing selection, and the fittest moth is whichever moth has the strongest flight muscles.
The case is taught as directional selection, and fitness means leaving more offspring in that environment rather than progress toward a predetermined goal.
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