The Island Lizard and the Shrinking Gene Pool
A population of lizards lives on a small island. A storm introduces a predatory snake species. Within a few generations, the average body size of the lizards decreases, and lizards with longer legs become more common. A biologist samples the population before and after the snake introduction and finds that the allele frequencies at several loci have changed. She also notes that the island population is small (about 200 individuals) and that migration from the mainland is rare. A colleague argues that the change in body size is 'just natural selection acting on individuals,' while another argues that the population is too small to evolve.
1. Explain how natural selection could produce the observed changes in body size and leg length, and why this is a change in the population rather than in individuals.
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Natural selection acts on heritable variation among individuals. If smaller body size and longer legs improve escape from the snake predator, individuals with those traits survive and reproduce more, passing their alleles to the next generation. Over generations, the frequency of alleles associated with smaller size and longer legs increases, so the population average shifts. Individual lizards do not change their genes during their lifetime; the change is in the distribution of alleles in the population. The observed shift in allele frequencies at several loci supports a genetic response to selection rather than phenotypic plasticity alone, though plasticity could contribute to the initial variation.
2. Evaluate the claim that the population is 'too small to evolve.'
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The claim is incorrect. A population of about 200 individuals can evolve if there is heritable variation and differential reproduction. Small size does not prevent evolution; it changes the dynamics. Small populations are more affected by genetic drift, which can cause random allele frequency changes and loss of variation, and they are more vulnerable to inbreeding depression. But selection can still act, and the observed allele frequency changes show evolution is occurring. The real concern is that small size and low migration reduce genetic diversity, which may limit future adaptive potential. So the population can evolve now, but its long-term evolutionary resilience is reduced.
3. What role does the rare migration from the mainland play in this population's evolution?
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Rare migration introduces new alleles into the island population, increasing genetic variation and potentially counteracting drift. Even low migration rates can reduce the loss of diversity in a small population. However, if migration is very rare, its effect is limited, and drift and selection dominate. Migration can also bring in alleles that are maladaptive on the island, but at low rates this is unlikely to overwhelm selection. Overall, rare migration acts as a minor source of variation and gene flow, slowing the erosion of diversity but not preventing local adaptation to the snake predator.