Question

As the climate got colder during the Ice Age, a particular species of mammal evolved a thicker layer of fur. This is an example of what kind of selection? A. Stabilizing selection B. Directional selection C. Disruptive selection D. Speciation

Short answer

B. Directional selection

Step-by-step solution

Identify Key Information

Recognize that the exercise describes a species of mammal developing a thicker layer of fur in response to the climate becoming colder.

Understand Types of Selection

Recall the definitions of the types of selection: Stabilizing selection favors the average trait, Directional selection favors one extreme trait, Disruptive selection favors multiple extreme traits, and Speciation leads to the formation of a new species.

Analyze the Change in Trait

Consider that the change described (thicker fur) is a specific trait that has evolved in one particular direction as a response to colder climate.

Match Trait Change to Selection Type

Determine which type of selection fits the scenario: Since the change is directional (favoring thicker fur), this aligns with Directional selection.

Select the Correct Answer

Choose 'Directional selection' as the type of selection described in the exercise.

Key concepts

Directional selection

Directional selection happens when natural selection favors one extreme trait over others. This leads to a shift in the population's trait distribution over time towards one direction. In the context of the Ice Age example, as the climate got colder, mammals with thicker fur had a survival advantage. Here, natural selection continuously favored those with thicker fur, gradually making it the common trait. Key points about directional selection:

• It results in a shift towards one extreme trait.
• Often occurs in response to a changing environment.
• Can lead to significant changes in a species over time.

The mammal species evolving thicker fur is a textbook example. The population's fur thickness constantly increased, showing a clear directional shift.

Stabilizing selection

Stabilizing selection occurs when natural selection favors the average traits and selects against the extremes. This type of selection reduces variability and maintains the status quo of a trait in the population. For example, in a stable environment, animals with average sizes might survive better than very large or very small ones. Key points about stabilizing selection:

• It favors intermediate traits.
• Reduces extremes in trait distribution.
• Maintains genetic stability in a population.

In the Ice Age scenario, if stabilizing selection were at play, the fur thickness of the mammals would remain around an average value, rather than shifting towards thicker fur.

Disruptive selection

Disruptive selection happens when natural selection favors extreme traits at both ends of the spectrum, rather than intermediate traits. This can lead to a bimodal distribution of traits within a population. For instance, in a population of birds, if both very small and very large beaks are advantageous (but not medium ones), disruptive selection would increase both extremes. Key points about disruptive selection:

• It favors multiple extreme traits.
• Can result in a split in the population.
• Might lead to the evolution of new traits or even new species.

In the given Ice Age example, if disruptive selection had occurred, we might see some mammals with extremely thick fur and others with extremely thin fur, but fewer with intermediate fur thickness.

Speciation

Speciation is the process through which new species arise from existing ones. It occurs when populations of a species become isolated (genetically or geographically) and evolve independently. Over time, these populations accumulate enough differences that they can no longer interbreed, forming distinct species. Key points about speciation:

• Involves the formation of new species.
• Can arise from various types of selection.
• Needs genetic isolation and time for divergence.

In the Ice Age scenario, if the mammal populations with thicker fur were isolated from others, they could undergo divergent evolution leading to a new species. However, simply evolving thicker fur due to cold climates represents directional selection, not speciation.