Explain how populations change over time and how reproductive isolation can lead to the formation of new species (North Carolina Standard Course of Study, Biology, LS.Bio.10).

What this topic is asking

North Carolina LS.Bio.10 asks how populations change over time and how reproductive isolation can lead to new species. For the Biology EOC you need to know what a species is, how geographic isolation can start the process, how populations diverge through different mutations and selection, and that reproductive isolation completes speciation. You also need to connect environmental change to population change. Items often give an isolation scenario.

What a species is

This definition is the key to speciation: as long as two populations can still interbreed and produce fertile offspring, they are the same species. They become different species only when they can no longer do so, a state called reproductive isolation.

How new species form

The classic illustration is a river or mountain range splitting a population. With gene flow cut off, the two groups evolve independently. Over a long time the genetic differences can become so large that interbreeding is no longer possible, producing two species from one. Other forms of isolation (such as differences in mating time or behavior) can also prevent interbreeding, but geographic isolation is the example the EOC most often uses.

Environmental change and population change

Beyond making new species, evolution constantly changes populations in response to the environment. When conditions change, natural selection favors different traits, so the proportions of alleles in a population shift over time. A trait that was rare can become common if it suddenly helps survival (as with antibiotic resistance), and a once-common trait can fade if it stops being useful. If the environment changes faster than a population can adapt, or too drastically, the species may decline and even go extinct, which links this topic to biodiversity and extinction.

Try this

Q1. State the usual test for whether two organisms belong to the same species. [1]

• Cue. They can interbreed to produce fertile offspring.

Q2. Explain how geographic isolation can lead to the formation of a new species. [3]

• Cue. A barrier separates a population; the groups face different environments and undergo different mutations and selection, so they diverge; eventually they can no longer interbreed, becoming separate species.