Explain how new species form when populations become reproductively isolated and diverge, and how environmental change can lead to extinction (NYSSLS LS4, cause and effect; stability and change).

What this topic is asking

NYSSLS LS4 wants you to explain how new species form (speciation) and why species become extinct. On the Life Science: Biology Regents these are usually tested with a scenario: a population split by a barrier, or a species facing rapid environmental change. The crosscutting concepts are cause and effect and stability and change.

What a species is

This definition is the key to speciation: a new species exists once a population can no longer interbreed with the original. The exam expects you to use "can no longer interbreed" as the marker of a new species.

Speciation

Speciation usually follows three steps:

1. Isolation. A population is split into groups that can no longer meet and interbreed. This is often geographic (a river, mountain range or new island), producing reproductive isolation.
2. Divergence. The separated groups experience different environments and undergo natural selection independently, and accumulate different mutations, so different traits become common in each group.
3. New species. Over many generations the groups become so different that, even if reunited, they could no longer interbreed to produce fertile offspring. A new species has formed.

Extinction

A species becomes extinct when the environment changes (climate shift, loss of habitat, a new predator or disease, loss of a food source) faster than the species can adapt or move, so too few individuals survive and reproduce. Extinction has happened throughout the history of life, including mass extinctions, and the fossil record shows that most species that ever lived are now extinct. Human activity is now a major cause of extinction (see human impact on ecosystems).

Why variation matters

A species with more genetic variation is more likely to survive environmental change, because it is more likely that some individuals already have traits suited to the new conditions. Those individuals survive and reproduce, so the species can adapt rather than die out. A species with little variation is more vulnerable, because if no individual is suited to the change, none survive. This links extinction risk to the importance of biodiversity.

Try this

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

• Cue. They must become reproductively isolated and diverge until they can no longer interbreed to produce fertile offspring.

Q2. Explain why a species with little genetic variation is more likely to become extinct when the environment changes. [2]

• Cue. With little variation it is unlikely that any individuals have traits suited to the new conditions, so few or none survive and reproduce, and the species can die out.