Case Study Caribbean Anole Lizards

Often, when scientists compare groups of species, the scientists find patterns that are best explained as evolution in progress. An example is the comparison of anole lizard species (genus Anolis) on the Caribbean islands of Cuba, Hispaniola, Jamaica, and Puerto Rico. Among these lizards, each species' body type correlates with the habitat in which the species lives, as shown in Figure 15-11. For example, anole species that live mainly on tree trunks have stocky bodies and long legs. In contrast, those that reside on slender twigs have thin bodies, short legs and tails, and large toe pads. Grass-dwelling anoles tend to be slender and have very long tails. In all, there are at least six anole body types that are each adapted to their environment in a unique way. Also, distinct species of anoles with the same body types occur on different islands. For example, a distinct species of twig-dwelling anole is found on each island.

Many different hypotheses could explain these observations. Two possibilities are that (1) an ancestral anole species specialized for living on twigs originally lived on one island and later migrated to other islands or that (2) each twig-dwelling species evolved independently on each island from distinct ancestor anole species.

Anolis Pulchellus
(a) Anolis cybotes (b) Anolis insolitus (c) Anolis pulchellus

A small number of animals of a species of lizards happens to drift to an island, carried, for example, by a hurricane.

A small number of animals of a species of lizards happens to drift to an island, carried, for example, by a hurricane.

The island's lizard population increases. The lizards exhibit hereditary variation in toe-pad size and leg length.

Animals that have unfavorable combinations of leg length and toe-pad size die at a faster rate in specific habitats.

The population splits into several groups, each of which is adapted to a specific habitat. Eventually, each group may become a separate species.

The island's lizard population increases. The lizards exhibit hereditary variation in toe-pad size and leg length.

Animals that have unfavorable combinations of leg length and toe-pad size die at a faster rate in specific habitats.

The population splits into several groups, each of which is adapted to a specific habitat. Eventually, each group may become a separate species.

Biologists tested these hypotheses by comparing DNA from various species to look for closely related species. The DNA evidence supported hypothesis 2—twig-dwelling species evolved independently on each island. In other words, each twig-dwelling species came from different ancestors but evolved similar adaptations to similar habitats. The process by which different species evolve similar traits is called convergent evolution. Many other examples of convergent evolution can be found in nature.

Divergence and Radiation

A model of Caribbean anole evolution must also explain how the lizards became adapted to their particular habitats. Studies showed that long-legged trunk-dwelling species could run faster on flat surfaces than short-legged twig-dwelling species, but the twig-dwelling species could cling to twigs better and did not fall as often. However, both kinds of lizards on each island were closely related.

The best explanation for this pattern of phylogeny is that divergent evolution occurred on each island. Divergent evolution is a process in which the descendants of a single ancestor diversify into species that each fit different parts of the environment. Lizards with genes for large toe pads and short legs ran so slowly on the trunk and ground that predators often caught them, and lizards with long legs and small toe pads often slipped if they climbed thin branches.

Sometimes, a new population in a new environment, such as an island, will undergo divergent evolution until the population fills many parts of the environment. This pattern of divergence is called adaptive radiation. Figure 15-12 illustrates a possible scenario for the evolution of Caribbean anole lizards. Fossil evidence suggests many cases of adaptive radiation on the geologic time scale.

figure 15-12

This diagram shows a possible scenario to explain, through natural selection, the evolution of a variety of anole lizard species in the Caribbean islands by descent from common ancestors.

Observing Adaptations Around You

Materials paper and pencil Procedure Observe organisms around your school grounds or around your home. Describe any traits that seem to be adaptations to a particular environment or way of life. Also, look for and describe variations within groups of organisms that you see. Explain your reasoning for each inference you make about adaptations. Analysis Which variations in the traits that you observed might increase or decrease the fitness of the organisms? Explain your reasoning.

figure 15-13

Recent DNA evidence shows that despite the enormous variation among domestic dogs, all varieties descended from Asian wolves. By artificially selecting the dogs that will be the parents of the next generation, people have increased the rate of divergent evolution among domestic dogs.

figure 15-13

Recent DNA evidence shows that despite the enormous variation among domestic dogs, all varieties descended from Asian wolves. By artificially selecting the dogs that will be the parents of the next generation, people have increased the rate of divergent evolution among domestic dogs.

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Responses

  • yolanda took-took
    How are lizards toepads hereditary?
    4 months ago

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