Problem 3 - Biology Concepts and Applications

Question

Match the modes of natural selection with their best descriptions. ______ stabilizing ______ directional ______ disruptive a. eliminates extreme forms of a trait b. eliminates midrange forms of a trait c. shifts allele frequency in one direction

Step-by-Step Solution

Answer

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

Step 1: Understanding Stabilizing Selection

Stabilizing selection favors intermediate variants and acts against extreme phenotypes. Therefore, it eliminates extreme forms of a trait to preserve existing population averages.

Step 2: Understanding Directional Selection

Directional selection occurs when an environmental change makes one extreme of a trait more advantageous. This shifts allele frequencies in one direction as one phenotype is favored over all others.

Step 3: Understanding Disruptive Selection

Disruptive selection favors phenotypes at extreme ends and acts against intermediate phenotypes, causing the population to split into two distinct groups by eliminating midrange forms.

Step 4: Matching Stabilizing Selection

Match 'stabilizing' with description 'a': Stabilizing selection eliminates extreme forms of a trait.

Step 5: Matching Directional Selection

Match 'directional' with description 'c': Directional selection shifts allele frequency in one direction.

Step 6: Matching Disruptive Selection

Match 'disruptive' with description 'b': Disruptive selection eliminates midrange forms of a trait.

Key Concepts

Stabilizing Selection

Stabilizing selection is a type of natural selection where genetic diversity decreases as the population stabilizes on a particular trait value. This selection process favors the average traits and acts to eliminate extreme variants.
This process can be visualized with a bell curve where the organisms with intermediate phenotypes are most favored. For example, in a human population, stabilizing selection may favor average birth weights because very low or very high weights can pose survival challenges.
Key characteristics of stabilizing selection include:

• Intermediate traits are advantageous.
• Elimination of extreme trait forms.
• Maintains the status quo in trait distribution over time.

By maintaining these average traits, populations are better suited to stable environments where conditions do not rapidly change.

Directional Selection

Directional selection occurs when environmental changes make a particular trait more beneficial, causing a shift in the population's trait distribution. This means that one extreme form of a trait becomes favored over others, leading to a change in allele frequency over time.
Imagine a population of birds where longer beaks suddenly become advantageous due to a change in available food sources. The population would then shift gradually towards having longer beaks as this trait becomes selected for.
Important points about directional selection include:

• Shifts in allele frequencies toward a selected phenotype.
• One extreme phenotype is consistently favored.
• Can lead to significant evolutionary change over several generations.

Directional selection is often seen when rapid changes in environmental conditions make survival depend heavily on a specific trait.

Disruptive Selection

Disruptive selection is a scenario where extreme trait values at both ends are favored over intermediate values. This mode of selection can lead to a population dividing into two distinct groups, each specialized in a different direction.
Think of a population of fish where smallest and largest sizes offer different survival advantages - small ones might hide easily from predators, while large ones may compete more effectively. The average-sized fish might then be at a disadvantage.
Key elements of disruptive selection include:

• Favors both extreme phenotypes over the intermediate.
• Elimination of intermediate trait values.
• Potential result in two or more diverse populations.

Disruptive selection can play a critical role in speciation as populations split and adapt to different ecological niches.