Lesson 1 of 6 / Evolution, species and phylogeny
Selection changes populations
Why does an individual's survival matter only if it affects inheritance?
In this lesson: Explain variation, selection pressures and population-level evolution.
About 7 min
The key ideaHeritable variants associated with greater reproductive success become more frequent across generations under the relevant conditions.
Explore the idea
Change the environment, then the frequency
Resistant lineages have twice the relative reproductive contribution of susceptible lineages in this hypothetical environment. Across generations their share rises because they leave more descendants. Generation 0: 10.0% resistant.
Green is resistant; purple is susceptible. Both variants exist before the environment is applied. The control changes their relative reproductive contribution, not their DNA in response to need.
What is held constant in this model?
This is a deterministic clonal-inheritance model. Each frequency is weighted by its group's stated reproductive contribution and the two values are normalised to sum to one. Mutation, migration, random sampling and changing conditions are omitted. The bars show frequency, not the size of the whole population; the numbers are original examples, not clinical antibiotic effects.
Explanation
Mutation creates new alleles, while meiosis and sexual reproduction reshuffle alleles into new combinations. Individuals in a population can therefore differ in heritable traits before a new environmental pressure appears. The environment does not direct a population to produce whichever mutation it needs.
A selection pressure affects survival or reproductive success. For example, an antibiotic can reduce susceptible bacteria while resistant variants leave more descendants; a predator can favour camouflage that lowers detection. The advantage depends on the particular environment and may carry costs elsewhere.
If the difference is heritable, alleles associated with greater reproductive contribution can become more frequent in later generations. Survival without reproduction is not enough to increase an allele's transmission. Fitness in this context concerns reproductive success, not physical strength or moral value.
A population is the smallest unit that evolves because its inherited composition changes across generations. An individual can acclimatise or develop, but does not change the population's allele frequencies by deciding to adapt. Selection acts through differences among individuals, while evolution is recorded at the population level.
Step by step
- 1
Identify pre-existing variation
Name a heritable trait rather than a need.
- 2
Identify differential reproduction
Explain why one variant leaves more descendants.
- 3
State the population change
Describe a frequency shift over generations.
Worked example
Work through the evidence
After antibiotic exposure, resistant bacteria rise from 2% to 70%. Does this show the drug deliberately created the resistance mutation?
One way to explain it
No. The change is consistent with selection of resistant variants already present or arising independently; the frequency data alone do not show directed mutation.
Why this answer works
- A frequency increase and a mutation's origin are separate questions.
- Susceptible and resistant lineages contribute differently after exposure.
Is this true? "An organism mutates because it needs to survive."
Mutations are not directed by need; selection changes the representation of variants.