Evaluate evidence that changes in environmental conditions may result in changes to populations, the rise of new species, or extinction (Louisiana Student Standards for Science, High School Biology, HS-LS4-5).

What this topic is asking

Louisiana's LS4 standards (HS-LS4-5) ask you to evaluate evidence that changes in environmental conditions can change populations, produce new species, or cause extinction. For LEAP 2025 Biology you should understand how isolation and separate selection lead to speciation, and why a population either adapts or dies out when conditions change. The test often gives a scenario (a barrier forming, a rapid climate shift) and asks you to predict and justify the outcome.

Environmental change drives population change

A population is not fixed: when the environment changes (climate shifts, a new predator or disease arrives, a food source disappears), the selection pressures change, so different traits become advantageous. Natural selection then changes which alleles are common. The standard asks you to evaluate what happens next, and the outcome depends on the variation present in the population.

Speciation: forming a new species

So the recipe for speciation is isolation plus separate selection over time. The river-splitting-a-lizard-population example is the standard illustration.

Extinction: when a species dies out

Extinction is the permanent loss of a species. It happens when the environment changes faster than, or more severely than, a population can adapt, and no individuals have the heritable traits needed to survive and reproduce in the new conditions. Causes include rapid climate change, loss of habitat, new predators or diseases, and competition. Mass extinctions in the fossil record show that large environmental changes have repeatedly wiped out many species at once, while also opening opportunities for survivors to diversify.

Adapt or go extinct: it depends on variation

The key idea to evaluate a scenario is this: when conditions change, a population adapts only if it already contains heritable variation suited to the new conditions. If such variation exists, the suited individuals survive, reproduce, and the population persists (and may eventually become a new species). If it does not, too few survive and the species goes extinct. This is why genetic variation (from mutation and meiosis) is so important for a population's long-term survival.

Try this

Q1. Explain how isolation can lead to the formation of a new species. [2]

• Cue. A barrier separates a population so the groups cannot interbreed; each group undergoes separate natural selection and changes differently until they can no longer produce fertile offspring together, forming a new species.

Q2. State the conditions under which a species is likely to go extinct when the environment changes. [2]

• Cue. When the change is rapid or severe and no individuals have heritable traits suited to the new conditions, so too few survive and reproduce.