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How does natural selection cause populations to change over time?

Explain how variation, overproduction, competition and differential survival lead to natural selection, and how this changes the proportion of traits in a population over time (NYSSLS LS4, cause and effect; patterns).

A NYSSLS-level answer on natural selection for the New York Life Science: Biology Regents: how variation, overproduction, competition and differential survival drive evolution, with the Beaks of Finches investigation and worked examples.

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  1. What this topic is asking
  2. The logic of natural selection
  3. Adaptation
  4. The Beaks of Finches investigation
  5. Natural selection in action
  6. Try this

What this topic is asking

NYSSLS LS4 (Biological Evolution) centers on natural selection: the mechanism that changes populations over time. The Life Science: Biology Regents wants you to explain the logic (variation, overproduction, competition, differential survival) and apply it to a scenario or to the Beaks of Finches investigation. The crosscutting concepts are cause and effect and patterns.

The logic of natural selection

The reasoning has four linked steps the exam expects you to use:

  1. Variation. Individuals in a population differ in their traits. The original source of new variation is mutation, reshuffled by sexual reproduction (see mutations and biotechnology).
  2. Overproduction and competition. Populations produce more offspring than the environment can support, so individuals compete for limited resources (food, space, mates).
  3. Differential survival and reproduction. Individuals whose traits suit the environment are more likely to survive and reproduce; this is sometimes called "survival of the fittest", where fitness means reproductive success.
  4. Inheritance. Because the favorable traits are inherited, the survivors pass them on, so the trait becomes more common in the next generation.

Adaptation

A camouflaged color, a beak shape suited to the available food, or resistance to a drug are all adaptations. The exam often asks why a trait spread: the answer is that individuals with it survived and reproduced more, passing it on.

The Beaks of Finches investigation

The New York Beaks of Finches lab models natural selection using tools to represent different beak shapes picking up "seeds". Some tools gather food more easily than others; the ones that gather more represent finches more likely to survive and reproduce. If the available seeds change (for example to larger, harder seeds), a different beak shape becomes favorable. The lab makes the abstract logic concrete: the trait best suited to the current food is selected for, and clusters draw on this style.

Natural selection in action

Natural selection explains many observed changes: the spread of antibiotic resistance in bacteria (resistant variants survive treatment and reproduce), the peppered moth becoming darker where bark was darkened, and changes in finch beak size after droughts. In each case, the environment selects among existing variation, and the favorable trait becomes more common. This links to the larger evidence for evolution (see evidence for evolution).

Try this

Q1. State the original source of the variation that natural selection acts on. [1]

  • Cue. Mutation (new alleles), reshuffled by sexual reproduction.

Q2. Explain why a favorable inherited trait becomes more common in a population over time. [2]

  • Cue. Individuals with the trait are more likely to survive and reproduce, so they pass the trait to more offspring, increasing its proportion each generation.

Exam-style practice questions

Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Regents (Life Science sample, 2024)3 marksA population of beetles varies in color from light to dark. A new predator hunts mainly on dark tree bark, where dark beetles are camouflaged. Over several generations the population becomes mostly dark. (a) State the source of the original color variation. (b) Explain why dark beetles became more common. (c) Explain why this is an example of natural selection.
Show worked answer →

A 3-point constructed-response item assessing cause and effect and patterns.

(a) 1 point: mutation (which produces new variation), shuffled by sexual reproduction.
(b) 1 point: dark beetles were better camouflaged on the dark bark, so they were more likely to survive the predator and reproduce, passing on the alleles for dark color.
(c) 1 point: there is variation, more offspring are produced than survive, and those with the favorable trait survive and reproduce more (differential survival), so the favorable allele becomes more common over generations.

Markers reward mutation as the source, the survival advantage of camouflage, and the variation-overproduction-differential-survival logic.

Regents (Life Science CR, 2025)2 marksIn the Beaks of Finches investigation, different tools represent different beak shapes used to pick up seeds. (a) Explain how this models natural selection. (b) Explain why a finch with a beak poorly suited to the available seeds is less likely to survive.
Show worked answer →

A 2-point item built on the Beaks of Finches lab.

(a) 1 point: the tools (beaks) vary in how well they pick up the seeds (food); those that gather more food represent finches more likely to survive and reproduce, modelling selection for the better-suited beak.
(b) 1 point: a poorly suited beak gathers less food, so the finch is more likely to starve and less likely to survive and reproduce, so its trait becomes less common.

Markers reward linking food gathered to survival and reproduction, and the disadvantage of a poorly suited beak.

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