Explain how natural selection acts on heritable variation to produce adaptation in populations over time, and identify the conditions required for it to occur (TEKS Biology, Reporting Category 3; cause and effect; stability and change).

What this topic is asking

The Biology TEKS ask you to explain how natural selection acts on heritable variation to produce adaptation over time. For STAAR Reporting Category 3 you need the conditions natural selection requires and the cause-and-effect chain from variation to a changed population. This is a cause-and-effect and stability and change topic, and it is one of the most heavily tested ideas on the exam.

The conditions for natural selection

These conditions are the backbone of any natural-selection answer. If you can name variation, heritability, overproduction, and differential survival and reproduction, you can explain almost any example STAAR presents.

How selection produces adaptation

The logic runs as a chain of cause and effect:

1. Individuals in a population vary, and some variation is inherited.
2. More offspring are born than the environment can support, so they compete for limited resources (food, space, mates).
3. Individuals whose traits make them better suited to the environment are more likely to survive and reproduce.
4. They pass those traits to more offspring, so the helpful trait becomes more common in the next generation.
5. Over many generations, the population becomes better suited, or adapted, to its environment.

Where the variation comes from

Natural selection does not create variation; it acts on variation that already exists. The original source of new variation is mutation, which produces new alleles, and sexual reproduction (through meiosis and fertilization), which shuffles existing alleles into new combinations. Selection then favors whichever variants happen to be better suited. This is why mutation and meiosis (Reporting Category 2) underpin evolution.

A key idea STAAR tests: the environment does not cause the helpful variation to appear on demand. The variation is already present by chance; the environment simply determines which variants survive and reproduce. Antibiotic resistance is the classic case, the resistant bacteria existed before the antibiotic; the antibiotic just selected them.

Adaptation over time and stability

When the environment is stable, selection tends to keep a well-suited population much the same. When the environment changes, the traits that are favored can shift, and the population changes with it. This is the stability and change theme: selection can both maintain a population and drive it to change, depending on conditions. Over long spans, repeated selection in different environments contributes to the formation of new species.

Try this

Q1. State the four conditions required for natural selection. [2]

• Cue. Variation; the variation is heritable; overproduction of offspring (a struggle to survive); and differential survival and reproduction.

Q2. Explain why the environment does not cause a helpful mutation to appear. [2]

• Cue. Variation arises by chance (mutation and sexual reproduction) before selection; the environment only determines which existing variants survive and reproduce.