Use a model to explain how changes in environmental conditions and reproductive isolation can cause populations to change and new species to form (Tennessee Academic Standards for Science, Biology I, BIO1.LS4).

What this topic is asking

The Tennessee LS4 standards ask you to model how populations change over time and how new species form. For the Biology I EOC that means understanding that environmental change shifts which traits are favored, that reproductive isolation (often from a geographic barrier) can split one species into two, and that when variation cannot meet a challenge the result may be extinction. Items often describe a population being split or facing a sudden change and ask you to predict the outcome.

How environmental change drives population change

When the environment changes, the traits that give an advantage can change too, so natural selection favors different variants and the population's allele frequencies shift. A classic example is a change in climate, food supply, or predators that suddenly makes a previously rare trait advantageous. Because selection acts on existing variation, a population with more genetic variation is more likely to contain individuals that can cope with the new conditions, and so more likely to adapt rather than decline.

Reproductive isolation and speciation

Once two groups are isolated, they no longer share genes. Each group experiences its own mutations and its own selection pressures, so over many generations they accumulate different changes. If the differences become large enough that the two groups could no longer interbreed and produce fertile offspring, even if the barrier were removed, they have become separate species. The EOC scenario is usually a population split by a barrier and asked whether and how a new species could form.

Speciation as part of common descent

When variation is not enough: extinction

Not every population adapts. If the environment changes faster than a population can respond, or if the population lacks the favorable variation needed, the population can die out, which is extinction. Mass extinctions in the fossil record (such as the one that ended the dinosaurs) show that even successful groups can be lost when conditions change dramatically. The lesson the EOC draws is that survival depends on having heritable variation that suits the new conditions; without it, selection has nothing favorable to act on.

Try this

Q1. Explain how a geographic barrier can lead to the formation of two species from one. [3]

• Cue. The barrier reproductively isolates two groups so they cannot interbreed; each group accumulates different mutations and faces different selection over generations; if they become unable to interbreed even if reunited, they are separate species.

Q2. State one reason a population might go extinct when its environment changes. [1]

• Cue. It lacks the favorable heritable variation needed to survive the new conditions (so selection has nothing advantageous to act on), or the change is too fast to adapt to.