Problem 10 - Biology: The Dynamic Science

Question

If a storm kills many small sparrows in a population, but only a few medium- sized and large ones, which type of selection is probably operating? a. directional selection b. stabilizing selection c. disruptive selection d. intersexual selection e. intrasexual selection

Step-by-Step Solution

Answer

The type of selection probably operating is a. directional selection.

Step 1: Identify the Types of Natural Selection

First, understand the various types of natural selection: a) Directional selection favors one extreme phenotype. b) Stabilizing selection favors intermediate phenotypes. c) Disruptive selection favors both extreme phenotypes over intermediates. d) Intersexual selection involves choice of mates by one sex. e) Intrasexual selection involves competition within one sex for mates.

Step 2: Analyze the Effects of the Storm on the Sparrow Population

The storm predominantly affects small sparrows, leaving medium and large ones relatively unharmed. This indicates a selective pressure against one extreme of size in the population.

Step 3: Determine the Type of Selection

Given that small sparrows suffered the highest mortality and both medium-sized and large sparrows had higher survival rates, the population is likely experiencing directional selection, which is favoring the larger body sizes over the smaller ones.

Key Concepts

Directional Selection

Imagine a gust of wind so strong it scatters leaves across a park, but only the lightest ones are carried far away, while the heavier ones remain closer to their original place. This is akin to how directional selection operates within a population of organisms.

Directional selection occurs when environmental pressures favor individuals at one extreme of a trait spectrum, causing a shift in the population's genetic variance toward that extreme. For instance, in our exercise scenario, the storm has disproportionately killed smaller sparrows. If smaller size were linked to certain genetic variations, the deaths of these smaller sparrows would mean those genes become less common in the population over time.

In the grand tapestry of evolution, directional selection plays a crucial role by driving significant changes in a population's traits. This can lead to a whole host of adaptations, allowing species to thrive in new environments, escape predators, capture prey, or find mates more efficiently.

Stabilizing Selection

Stabilizing selection can be visualized as a valley between two hills, where organisms in the valley are more likely to survive and reproduce, whereas those on either hillside are less favored. It's a natural balancing act.

This type of selection is the Goldilocks scenario of evolution: it favors intermediate phenotypes and selects against both extremes. Let's take human birth weight as an example. Babies that are too small or too large at birth have higher rates of complications. Therefore, there is a 'sweet spot' for birth weight that is most beneficial for survival, and over generations, the traits associated with this optimal weight become more common.

Through stabilizing selection, the population's genetic diversity may decrease, but the overall fitness increases, as the majority of individuals come to possess the favored trait. It's nature's way of saying, 'If it isn't broken, don't fix it,' encouraging consistency over time.

Disruptive Selection

Now, let's flip the script with disruptive selection. It's as if evolution has a split personality, favoring extremes and snubbing the middle ground. Imagine a habitat that has very distinct microenvironments, like a rocky shore interspersed with sandy patches.

Certain traits that are advantageous in the rocks may be detrimental in the sand and vice versa. This form of natural selection favors organisms at both ends of the trait spectrum and is less kind to those in between. For instance, in our habitat, crabs that are adept at camouflaging in the rocks, as well as those proficient at burrowing in the sand, are more likely to survive predator attacks than those who are mediocre at both.

Over time, disruptive selection can lead to a bimodal distribution of traits in a population, and it can be a precursor to speciation, where one species gives rise to two or more new species. It's the evolutionary equivalent of 'diversify or perish,' promoting variety and catering to ecological niches.