While there is only one species of Galápagos Island tortoise, there are several subspecies. Larger islands with more wet highlands have lush vegetation near the ground. Tortoises there tend to have high-domed shells and shorter necks, which restrict upward head movement. They also have shorter limbs. They are the heaviest and largest of the subspecies.

Smaller, drier islands are inhabited by tortoises with longer necks and limbs and with shells that are elevated above the neck, which allow them to browse taller vegetation.

Based on the information given, which of the following is a plausible explanation for the ancestry of the tortoise subspecies?

Question

While there is only one species of Galápagos Island tortoise, there are several subspecies. Larger islands with more wet highlands have lush vegetation near the ground. Tortoises there tend to have high-domed shells and shorter necks, which restrict upward head movement. They also have shorter limbs. They are the heaviest and largest of the subspecies.

Smaller, drier islands are inhabited by tortoises with longer necks and limbs and with shells that are elevated above the neck, which allow them to browse taller vegetation.

Based on the information given, which of the following is a plausible explanation for the ancestry of the tortoise subspecies?

BlackTom AI · Accepted Answer

This question asks us to evaluate explanations for why tortoise subspecies on different Galápagos islands evolved distinct shell shapes, neck lengths, and limb proportions based on the information given about their environments.
Option 1: 'The subspecies share a recent common ancestor whose neck length, shell shape, and leg length were intermediate between the two subspecies.' This suggests a recent, uniform ancestor with intermediate traits. However, the described differences align with adaptation to distinct environments on large wet islands versus small dry islands, implying divergence due to selective pressures rather than simply retaining intermediate ancestral traits. The scenario described in the prompt fits adaptive divergence more than a static intermediate ancestor.
Option 2: 'Random mutations coupled with the inheritance of acquired characteristics resulted in distinct subspecies.' The idea of acquired characteristics being inherited is a hallmark of Lamarckian thinking and contradicts modern genetics, which shows that acquired traits (like learned behavior or temporary changes) are not passed to offspring in the way suggested. Therefore, this option misstates how heritable variation arises and would not reliably explain the structured, environment-consistent differences seen between island subspecies.
Option 3: 'Individuals with different adaptations in shell shape and leg length best exploited the food resources and left more surviving offspring on each island.' This is a classic example of natural selection in action: different environments favor different morphologies, leading to differential survival and reproduction. The larger islands with lush, ground-level vegetation select for shorter necks and more heavy build, while smaller, drier islands favor longer necks and limbs for reaching taller vegetation. This explanation coherently accounts for the observed divergence as an adaptive response to island-specific resources.
Option 4: 'The tortoises with shorter legs and necks were most easily preyed on as young animals by the rats that were introduced, so they survived only on a few islands.' While predation by introduced rats could influence survival on some islands, this option frames the discrepancy as a selective filter for preexisting short-limbed morphologies without tying it to how the subspecies arose in the first place. It also doesn’t address the consistent pattern of shell and limb adaptations aligned with resource use across islands, making it a less comprehensive explanation for ancestry.
In summary, Option 3 provides the logically coherent mechanism—natural selection driven by island-specific resource availability—that best explains how distinct subspecies could arise from a shared lineage by exploiting different ecological niches, whereas the other options rely on outdated genetics, incomplete causal links, or misstate the role of predation alone in shaping ancestry.

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