Problem 3 - Biology Concepts and Applications

Question

Match the modes of natural selection with their best descriptions. $$ \text { stabilizing} \quad \quad\quad\quad\quad \text {a. eliminates extreme forms of a trait} $$ $$ \text { directional} \quad \quad\quad\quad\quad \text { b. eliminates midrange forms of a trait \(r\) } $$ $$ \text { disruptive } \quad \quad\quad\quad\quad \text { c. shifts phenotypes in one direction } $$

Step-by-Step Solution

Answer

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

Step 1: Analyze the Modes of Natural Selection

Understand that modes of natural selection refer to the effects that selection pressures have on the frequency distribution of traits in a population. There are three main types: stabilizing, directional, and disruptive selection.

Step 2: Understand Stabilizing Selection

Stabilizing selection eliminates extreme forms of a trait, favouring the intermediate variants. This means it acts to reduce genetic diversity and maintain the status quo for a particular trait.

Step 3: Understand Directional Selection

Directional selection involves shifts in the population's trait distribution in one particular direction, whether towards the dominant or recessive trait. This means it increases the frequency of one phenotype at the expense of others.

Step 4: Understand Disruptive Selection

Disruptive selection increases genetic diversity by favouring extreme variants over intermediate ones. Essentially, it eliminates or reduces the frequency of the intermediate phenotype, favouring both ends of the distribution.

Step 5: Match Descriptions to Definitions

Match each type of natural selection with the appropriate description: - Stabilizing selection corresponds to description a: it eliminates extreme forms of a trait. - Directional selection corresponds to description c: it shifts phenotypes in one direction. - Disruptive selection corresponds to description b: it eliminates midrange forms of a trait.

Key Concepts

Stabilizing Selection

In the fascinating world of natural selection, "Stabilizing Selection" plays a crucial role. It works by favoring the average or intermediate forms of a trait while eliminating the extremes. Imagine a population of birds where the size of their beaks is critical to their survival. If the environment consistently supports those with an average-sized beak, birds with too large or too small beaks would be less likely to survive and reproduce.

What happens here is a reduction in genetic variance. It narrows the range of traits, keeping the population closer to the existing norm.

• This is common when environmental conditions are stable and the existing trait is already optimized for survival.
• Stabilizing selection results in the population becoming more uniform and cohesive over time.

By favoring stability over change, this mode of selection helps maintain the balance and continuation of a species over generations.

Directional Selection

"Directional Selection" showcases how natural forces can steer a population's trait in a particular direction. It occurs when one phenotype is favored over others, leading to a shift in the population's genetic makeup. Think of a scenario where a particular environment suddenly changes, like an area becoming colder, favoring animals with thicker fur.

This type of selection results in increased frequency of a beneficial trait, effectively moving the average trait value toward one of the extremes.

• For instance, if thicker fur becomes advantageous, over time, more individuals will possess it.
• This process leads to evolutionary changes as the population adapts to new conditions.

Directional selection is a driving force behind the adaptation of species to their environments, allowing them to better survive in changing conditions.

Disruptive Selection

"Disruptive Selection" offers a twist by promoting variations. Instead of pushing a population towards a single trait, it favors the extremes and eliminates the intermediate forms. Imagine a habitat where two distinct food sources are available: one suited for small, nimble beaks and another for large, powerful beaks.

With disruptive selection, birds with these extreme beak sizes would thrive, while those with average-sized beaks might struggle. This leads to an increase in genetic variability as both extremes become more common.

• This is particularly important in diverse environments where multiple traits can ensure species survival.
• In some cases, it can even lead to the emergence of two distinct species over time.

Disruptive selection demonstrates nature's ability to diversify and innovate, allowing for a wide array of adaptations and forms within a species.