Problem 5 - Campbell Biology

Question

The upper forelimbs of humans and bats have fairly similar skeletal structures, whereas the corresponding bones in whales have very different shapes and proportions. However, genetic data suggest that all three kinds of organisms diverged from a common ancestor at about the same time. Which of the following is the most likely explanation for these data? (A) Forelimb evolution was adaptive in people and bats, but not in whales. (B) Natural selection in an aquatic environment resulted in significant changes to whale forelimb anatomy. (C) Genes mutate faster in bats than in humans or whales. (D) Whales are not properly classified as mammals.

Step-by-Step Solution

Answer

Option (B) is the most likely explanation.

Step 1: Understand the Question

The question asks why the upper forelimbs of humans and bats are similar in structure, while the bones in whales are different, despite genetic data suggesting a common ancestor.

Step 2: Analyze Each Option

(A) Forelimb evolution was adaptive in people and bats, but not in whales: This suggests that evolutionary pressures affected humans and bats similarly but not whales.

Step 3: Evaluate Aquatic Environment Impact

(B) Natural selection in an aquatic environment resulted in significant changes to whale forelimb anatomy: Natural selection could explain the significant difference in whale forelimb anatomy if whales adapted to living in water.

Step 4: Consider Mutation Rates

(C) Genes mutate faster in bats than in humans or whales: This would imply more variation in bat genes, but does not specifically explain why whale forelimbs are more different.

Step 5: Taxonomic Classification

(D) Whales are not properly classified as mammals: This suggests a classification error, but does not address the anatomical differences in forelimbs.

Step 6: Select the Best Explanation

Option (B) offers a logical explanation for the anatomical differences based on natural selection adapting whales to an aquatic environment.

Key Concepts

natural selection

Natural selection is a fundamental mechanism of evolution as proposed by Charles Darwin. It explains how species evolve over time to adapt to their environment. Traits that enhance an organism's ability to survive and reproduce are more likely to be passed on to future generations. This process leads to the gradual accumulation of advantageous traits in a population.
Natural selection operates on the variability within a population—differences that arise from mutations, genetic recombination, and other sources. Individuals with traits that provide a better fit to their environment will generally have higher survival rates and produce more offspring. These advantageous traits become more common in the gene pool over generations.

homologous structures

Homologous structures are anatomical features shared by different species due to a common ancestor. These structures might have different functions now due to evolutionary changes, but their similarity points to an evolutionary connection.
For example:

• The upper forelimbs of humans, bats, and whales originally evolved from the same forelimb in a common ancestor.
• Despite different functionalities—flying in bats, manipulating objects in humans, and swimming in whales—the underlying bone structure is similar.

Studying homologous structures provides insights into how species adapt through divergent evolution, where different environments lead to the modification of a common structural template.

adaptive evolution

Adaptive evolution refers to changes in a population that make the organisms better suited to their environment. This process is driven primarily by natural selection and can lead to the development of new traits and behaviors.
Adaptive evolution occurs when there is a consistent selective pressure. These pressures can include environmental changes, predators, diseases, and competition for resources. Traits that enhance survival and reproduction tend to increase in frequency within the population.
In the case of whales, their aquatic environment exerted selective pressures that led to significant modifications in their forelimbs for swimming. Unlike humans and bats, their limbs adapted to enhance aquatic mobility, demonstrating an example of adaptive evolution.

convergent evolution

Convergent evolution is when unrelated species evolve similar traits independently, usually as a result of adapting to similar environments or ecological niches. This leads to analogous structures which perform similar functions but are not derived from a common ancestor.
Examples include:

• The wings of bats and butterflies—both used for flying but structurally different and evolved separately.
• The streamlined bodies of sharks and dolphins, which are adapted to moving efficiently through water, despite one being a fish and the other a mammal.

Convergent evolution shows how similar environmental pressures can shape the evolution of different species in analogous ways.