Fitness = offspring that live to breed, in this environment. Selection sorts existing variation; it never invents what's needed.
Natural selection doesn't give organisms what they need. It sorts what's already there: individuals differ in heritable ways, those that suit this environment leave more offspring, and their alleles become more common.
Key terms
- Natural selection — heritable traits that raise survival and reproduction in an environment become more common, generation by generation.
- Fitness — reproductive success: the number of offspring an individual leaves that survive to breed. Not strength, size, or lifespan.
- Heritable variation — differences among individuals that are coded in genes, so parents can pass them on.
- Adaptation — a heritable trait that raises fitness in a particular environment. Change the environment and it may stop helping.
- Selective pressure — anything in the environment, like a predator, drought, or drug, that makes some variants reproduce more than others.
- Population — one species' interbreeding group in one place. It's the unit that evolves; individuals don't.
How it works
- Individuals vary. New variants come from random mutations, which arise whether or not they will ever be useful.
- Some of that variation is heritable, so offspring tend to resemble their parents.
- More offspring are produced than the environment can support. They compete for food, mates, and space, and many die before breeding.
- Individuals whose traits suit the current environment tend to leave more offspring that survive to breed.
- Their alleles become more common in the next generation. Over many generations the population shifts, though no individual changed its genes.
- If the environment changes, the favored trait can change too. Selection has no goal; it tracks current conditions.
Why it matters
This loop sits under most of Unit 7. Data questions often give survival or offspring numbers in two settings and ask what they show. Measure fitness in offspring that reach breeding age, and compare within one environment. Artificial selection runs the same loop, with a breeder choosing who reproduces. The difference is the goal: a breeder has one, and nature doesn't.
Exam traps
| Trap | Actually |
|---|
| The biggest, strongest, or longest-lived individual is the fittest. | Fitness counts offspring that survive to breed. A short life with many such offspring beats a long life with few. |
| Organisms develop new traits because they need them. | Variants arise at random, often before they're useful. Selection changes how common they are; it doesn't create them. |
| Traits an animal gains during its life pass to its young. | Muscles built by use don't change the DNA in eggs or sperm. Only inherited differences let selection change the next generation. |
| Some traits are better than others, full stop. | Fitness depends on the environment. Dark fur hides a mouse on black lava and exposes it on pale sand. |
Worked example
Two meadows hold the same wildflower species. In meadow P, plants differ in height because they carry different alleles. Meadow Q was planted with clones, so every plant is genetically identical, and height differences come only from patchy soil. Deer move into both meadows and eat most plants taller than 40 cm before they set seed. Predict how the average height of the next generation changes in each meadow.
- Check the three requirements: variation, inheritance, and differential reproduction.
- Both meadows vary in height, and in both the deer let short plants reproduce more. Differential reproduction happens in both.
- Only in P is height inherited. P's short survivors pass on alleles for short stems, so the next generation is shorter on average.
- Q's survivors carry the same genes as the plants that were eaten. Their seeds, landing on the same patchy soil, grow the same range of heights as before.
Meadow P's next generation is shorter on average. Meadow Q's next generation averages the same height as before, because its height differences were never inherited.
Selection sorts genes only through the traits they produce. If the variation comes from the environment, the survivors have nothing different to pass on. The near miss is 'both meadows get shorter, since the deer removed the tall plants.' That's true of this generation's survivors, but evolution is measured in their offspring.