Explain how reproductive isolation and natural selection can lead to speciation, and describe how the distribution of traits in a population changes as allele frequencies shift over generations (MA STE HS-LS4-3, HS-LS4-4, HS-LS4-5).

What this topic is asking

The Massachusetts STE framework (HS-LS4-3, HS-LS4-4, HS-LS4-5) asks you to use evidence and reasoning to explain how the distribution of traits in a population changes and how new species arise. On the High School Biology MCAS, this is tested with scenarios: a population split by a barrier, or a changing environment that shifts which trait is favored. You explain how allele frequencies change and how isolation can lead to a new species. The crosscutting concept is stability and change, applied to populations over time.

How new species form

The most common route to a new species is:

1. Isolation. A population is split into groups that can no longer interbreed, often by a geographic barrier (a new river, mountain, or stretch of sea). This is reproductive isolation.
2. Divergence. The separated groups live in different environments with different selection pressures, so different traits are favored. Mutation adds new variation independently in each group.
3. Speciation. Over many generations the groups become so genetically different that, even if reunited, they can no longer interbreed to produce fertile offspring. They are now separate species.

The test of whether two groups are separate species is exactly this inability to interbreed successfully, which the MCAS likes to ask about. This process explains the biodiversity you meet in biodiversity and classification.

Evolution as changing allele frequencies

At the level of a population, evolution is a change in allele frequencies over generations. Recall from patterns of inheritance that an allele is a version of a gene. In a population, you can think about the proportion (frequency) of each allele:

• If a selection pressure favors a trait, individuals with the allele for that trait survive and reproduce more, so the allele becomes more common (its frequency rises).
• Alleles that are disadvantageous become less common (their frequency falls).

So when the environment changes, the distribution of traits in the population shifts as the allele frequencies change. This is the same natural selection from natural selection, now described in terms of allele proportions.

The peppered moth: a worked example of changing frequencies

The peppered moth is the classic case. Originally most moths were light, camouflaged against pale tree bark. When pollution darkened the bark, dark moths became better camouflaged, so they survived predators better, reproduced more, and passed on the dark allele, whose frequency rose. When pollution was later reduced and the bark lightened again, the light allele became favored once more and rose in frequency. The moth population evolved in step with its environment, showing that allele frequencies track the selection pressure.

Try this

Q1. State what must happen for two populations to become separate species. [2]

• Cue. They must become so genetically different (after reproductive isolation and divergence) that they can no longer interbreed to produce fertile offspring.

Q2. Explain what happens to the frequency of an allele when the trait it codes for becomes advantageous. [2]

• Cue. Individuals with the allele survive and reproduce more, so they pass it on, and the allele becomes more common (its frequency rises) over generations.