Problem 4 - Biology Today and Tomorrow with Physiology

Question

Match the modes of natural selection with their best descriptions. a. eliminates extreme forms of a trait b. eliminates midrange forms of a trait c. shifts allele frequencies in one direction _____stabilizing _____directional _____disruptive

Step-by-Step Solution

Answer

Stabilizing: a; Directional: c; Disruptive: b.

Step 1: Match the Description for Stabilizing Selection

For stabilizing selection, think of it as selecting against the extremes, which means it eliminates traits that are too far from the average. In other words, it narrows down the distribution to keep the trait centered. Therefore, the best description that matches stabilizing selection is 'eliminates extreme forms of a trait'.

Step 2: Match the Description for Directional Selection

Directional selection occurs when an environmental change favors an allele that is not the average. This leads to a shift in allele frequencies toward one extreme. Thus, the correct match for directional selection is 'shifts allele frequencies in one direction'.

Step 3: Match the Description for Disruptive Selection

In disruptive selection, extreme forms of a trait are favored over the intermediate traits. This leads to the elimination of midrange forms and can result in a bimodal distribution. So, 'eliminates midrange forms of a trait' matches disruptive selection.

Key Concepts

Stabilizing Selection

Stabilizing selection is a fascinating mode of natural selection where the focus is on retaining the average or "normal" traits in a population, while weeding out the extreme variations. Think of it as a bell curve where the tallest point represents the most common trait. This type of selection aims to keep most individuals near the center of the bell curve.

• During stabilizing selection, individuals with extreme traits on either end of the spectrum are less likely to survive and reproduce.
• This process narrows the range of variation within a trait, focusing on traits that have already been optimized for the current environment.
• An example is human birth weight: babies that are too small or too large have lower survival rates than those in the middle range.

This process is crucial because it helps maintain a stable population by ensuring that the most advantageous traits are passed on to future generations.

Directional Selection

In directional selection, the environment changes in such a way that it favors a trait that is not currently the average. Imagine a graph where the curve shifts entirely to one side. This happens because individuals with one extreme of a trait are more successful in surviving and reproducing in the new environment.

• Directional selection leads to a shift in allele frequencies, pushing the population characteristics in one direction.
• It can result in a population evolving new traits over time, as the favored extreme becomes the average through successive generations.

A classic example of directional selection can be seen in the peppered moths of England during the Industrial Revolution. As soot blackened the trees, moths with darker coloring had a survival advantage and became more common, showing how the entire population shifted directionally toward a new average trait.

Disruptive Selection

Disruptive selection is an intriguing process where the extremes of a trait are favored, and the intermediate variations are selected against. This can result in a bimodal distribution, meaning two distinct traits become common within a population.

• Disruptive selection increases the genetic and phenotypic variance in a population.
• This process often occurs in environments where changes cultivate new niches, allowing different groups to thrive based on extreme traits.
• It can potentially lead to the formation of new species if the groups become too distinct over time.

An example of disruptive selection is seen in bird populations with different beak sizes. In environments where both large and small seeds are available, but not medium, birds with either very large or very small beaks have a feeding advantage, leading to two distinct groups.