Problem 3 - Biology: The Unity and Diversity of Life

Question

Match the modes of natural selection with their best descriptions. ________stabilizing \(\quad\)a. eliminates extreme forms of a trait. ________disruptive \(\quad\)b. eliminates midrange form of a trait

Step-by-Step Solution

Answer

Stabilizing selection - a; Disruptive selection - b.

Step 1: Understanding Modes of Natural Selection

Natural selection can lead to different outcomes based on how it acts on the range of phenotypes in a population. The three main modes are stabilizing selection, directional selection, and disruptive selection. This exercise focuses on stabilizing and disruptive selection.

Step 2: Stabilizing Selection

Stabilizing selection favors an average phenotype in a population and acts against extreme variations. This means it eliminates the extreme forms of a trait, leading to reduced variation and maintaining the status quo of the phenotypic feature.

Step 3: Disruptive Selection

Disruptive selection favors extreme phenotypes at both ends of the spectrum and acts against the average phenotype. This means it eliminates the midrange form of a trait and can lead to increased variation and possibly speciation.

Step 4: Matching Descriptions

Match the correct mode of natural selection to its description. Stabilizing selection corresponds to 'eliminates extreme forms of a trait,' and disruptive selection corresponds to 'eliminates midrange form of a trait.'

Key Concepts

Stabilizing Selection

Stabilizing selection is a fascinating aspect of natural selection that helps maintain equilibrium within populations. This mode acts like a balancing force, favoring individuals that exhibit average phenotypes while reducing those with extreme traits on either end of the spectrum.

Imagine a population of birds where individuals with wings that are too long or too short are less efficient at flying. If stabilizing selection is at play, those birds with average wing lengths will have a better chance of survival and reproduction. Over time, this leads to a reduction in variation regarding that trait within the population, consolidating the wing length around the average size.

In practical terms, stabilizing selection can be advantageous in stable environments where being average translates to better adaptation and survival odds. This is because extreme traits may hinder survival, especially in environments that do not undergo significant changes.

Disruptive Selection

Disruptive selection, in contrast to stabilizing selection, operates by favoring individuals at the extremes of a trait distribution over those with intermediate characteristics. This can lead to an increase in diversity among phenotypes, essentially pulling the population into divergent paths.

Consider a scenario where a certain fish species exists in an environment with both very large and very small pebbles. Fish with smaller or larger mouths might be able to exploit these different niches more effectively than their counterparts with medium-sized mouths. As a result, both small and large-mouthed fish thrive while the medium-mouthed fish dwindle in number, potentially paving the way for the evolution of two distinct species.

Disruptive selection can significantly contribute to biodiversity by pushing populations into new adaptive zones. It's one of the mechanisms that can lead to speciation, where new species arise from a single ancestral lineage.

Phenotypes

Phenotypes refer to the observable traits or characteristics of an organism, such as morphology, development, biochemical or physiological properties, and behavior. These are determined by the organism's genotype in conjunction with the environment.

When we talk about selection, it's these phenotypic traits that are subject to the pressures of natural selection. Some phenotypes offer advantages in survival and reproduction, leading to the propagation of the beneficial traits over generations.

Natural selection acts on the level of the phenotype, not the genotype, because the interaction of an organism with its environment is typically influenced more directly by its physical traits. Thus, understanding phenotypes is crucial for studying how natural selection shapes the diversity and adaptation of species.