Recognize the factors that influence the genetic makeup of populations and lead to speciation, including mutation, gene flow, genetic drift, and reproductive isolation (TEKS Biology, Reporting Category 3; cause and effect; patterns).

What this topic is asking

The Biology TEKS ask you to recognize the factors that change the genetic makeup of populations and lead to speciation. For STAAR Reporting Category 3 you need to know mutation, gene flow, and genetic drift as sources of genetic change, and how reproductive isolation turns one species into two. This is a cause-and-effect and patterns topic that builds on natural selection.

Factors that change a population

Natural selection is not the only thing that changes a population's genes. The Biology TEKS list several factors:

• Mutation. The original source of new alleles; without it, there would be no new variation to act on.
• Gene flow. The movement of alleles into or out of a population as individuals migrate and breed, or as pollen and seeds move. Gene flow tends to make populations more similar.
• Genetic drift. Random changes in which alleles happen to be passed on, especially in small populations, where chance events can quickly change allele frequencies (and even remove an allele entirely).
• Natural selection. The non-random survival and reproduction of better-suited variants (covered in natural selection and adaptation).

A useful distinction STAAR tests: natural selection is non-random (it favors specific traits), while genetic drift is random (chance alone changes the frequencies).

Reproductive isolation

Once two groups are reproductively isolated, they no longer share genes. Each group then accumulates its own mutations and experiences its own selection pressures and drift, so they drift apart genetically.

Speciation

The classic pattern is: one population, a barrier appears, the two groups are isolated, they change in different ways over a long time, and eventually they are distinct species. The squirrels on either side of a canyon and the birds blown to an isolated island are textbook examples. Speciation is how the branching tree of life (see cladograms and phylogeny) gains new branches.

Why population size matters

Genetic drift highlights the role of chance and population size. In a large population, random events tend to average out and have little effect on allele frequencies. In a small population, a chance event (a few individuals failing to reproduce, or a storm killing a random subset) can dramatically change which alleles are passed on. This is why small, isolated populations can change quickly and are more vulnerable to losing genetic variation.

Try this

Q1. State the difference between gene flow and genetic drift. [2]

• Cue. Gene flow is the movement of alleles between populations (which makes them more similar); genetic drift is random change in allele frequencies, strongest in small populations.

Q2. Explain how a geographic barrier can lead to speciation. [2]

• Cue. It reproductively isolates two groups so they no longer interbreed; they change independently over time until they can no longer interbreed, forming separate species.