Question 1

What do biologists mean by fitness in the context of natural selection?

Accepted Answer

In biology, fitness refers to an organism's reproductive success relative to others in its population, not physical strength or speed. Organisms with greater relative fitness possess adaptations that enable them to produce more offspring that survive and reproduce. This reproductive advantage allows their traits to become more common in subsequent generations.

Question 2

How do scientists calculate relative fitness and selection coefficient?

Accepted Answer

Scientists calculate relative fitness (w) by multiplying an organism's survival rate by its reproductive rate, then dividing by the highest product value among all phenotypes. The selection coefficient (s) is then calculated by subtracting relative fitness from 1. Higher selection coefficients indicate stronger selection against a particular phenotype.

Question 3

What is directional selection and how does it change populations?

Accepted Answer

Directional selection favors extreme values of a trait in one direction, causing populations to shift toward that extreme over time. This occurs when environmental changes make one phenotype more advantageous. For example, after industrial pollution increased, dark-colored peppered moths had higher survival and reproductive success, becoming more common in populations.

Question 4

How does stabilizing selection maintain trait values in populations?

Accepted Answer

Stabilizing selection holds a trait at an optimum value by selecting against extreme phenotypes. Deviations from this optimum are disadvantageous. For instance, robins laying four eggs represents an optimum: fewer eggs risk insufficient offspring, while more eggs increase malnutrition risk. This maintains most individuals near the average trait value.

Question 5

What is disruptive selection and what distribution does it create?

Accepted Answer

Disruptive selection favors two different extremes of a phenotype while selecting against intermediate forms, creating a bimodal distribution. West African seedcracker finches exemplify this: large-billed finches crack hard seeds efficiently, small-billed finches handle soft seeds well, but medium-billed finches cannot open either seed type effectively and are rarely observed.

Question 6

Why are phenotypes important for understanding natural selection?

Accepted Answer

Phenotypes are the visual expressions of traits that natural selection acts upon. Organisms with phenotypes providing survival or reproductive advantages pass these traits to more offspring. Variation in phenotypes across populations provides the raw material for evolution, allowing selection to favor beneficial traits and increase their frequency across generations.

Question 7

How do adaptations like the snowshoe hare's color change demonstrate natural selection?

Accepted Answer

Snowshoe hares display seasonal coat color changes—brown in summer, white in winter—that provide camouflage against predators. Hares with color-changing adaptations have higher survival rates and produce more offspring that survive to reproduce. This heritable trait increases in frequency because it correlates with greater reproductive success relative to non-changing phenotypes.

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