Speciation: How One Species Becomes Two

Speciation is the process by which one species evolves to form two or more new species. Often members of different species are unable to interbreed due to the evolution of reproductive isolating barriers. Exactly how to define a species is a matter of active debate among biologists, but most agree that a species is an interconnected population of organisms that have an evolutionary trajectory independent from other species. This independence is critical to the evolution and maintenance of biological diversity. Populations evolve new traits by way of natural selection, where traits that are adaptive (beneficial to the survival or reproductive output of individuals bearing them) are favored and become more common in a population. However, whether a trait is beneficial or not depends on the species it arises in. For example, while conspicuous coloration in a poisonous frog is beneficial, the same trait would be disastrous for harmless species. If all frogs were a single species this trait would be quickly purged from the population. Reproductive boundaries, formed by the process of speciation, provide a mechanism for species to evolve independently from one and another, and accumulate distinctively adaptive traits.

The study of speciation boils down to asking the question “where do species come from?” While speciation is a fundamental part of evolution, our knowledge of exactly how the process of speciation works is surprisingly incomplete. For many years the prevailing theory of speciation focused on isolation. In this model, populations of a single species are physically separated from one and other. This could be due to the formation of some impenetrable barrier, like a mountain or glacier, or due to dispersal, like arriving on a remote island. Once isolated, these populations evolve independent from one another, and given enough time, they will randomly accumulate enough differences to be considered distinct species. The spectacularly blue-dewlapped Anolis conspersus from Grand Cayman island is one likely example of speciation by isolation. This species’ closest relative is Anolis grahami from Jamaica. The ancestors of present-day A. conspersus arrived on Grand Cayman from Jamaica thousands or millions of generations ago and formed a new population. Over, time the Jamaican and Caymanian populations accumulated differences in isolation such that they now considered distinct species. 

Scientists often observe that a species and its closest relative differ in their ecology. These sister species may occupy different habitats or different structural parts of their environment. This pattern is not predicted by the isolation theory and this observation gave rise to an alternative theory of speciation where adaptation plays a central role. The theory of ecological speciation posits that speciation occurs as a side-effort of populations of a single species adapting to different habitats or environments. Adaptive evolution can proceed very rapidly, especially compared to the random accumulation of differences in the isolation theory. While there is evidence for both speciation by isolation and ecological speciation we don’t know which (or if either) are responsible for most of the species diversity on earth.  

Anoles are a great group to study speciation for a number of reasons. First, there are over 400 species of anole, so they are clearly adept at speciating. Second, many anole species appear to be in the process of speciation, for example Anolis distichus populations in Hispaniola are partially reproductively isolated from one and other. Finally third, anoles are particularly well suited to investigate which theory of speciation best fits what we see in nature. Thanks to many decades of evolutionary and functional ecology research, we have a strong grasp on what anole traits are ecologically adaptive. With this knowledge, we can use anoles to test the relative importance of isolation time and adaptive evolution in driving the process of speciation. 

Ecological differences may accelerate the speciation process. Anolis cooki and Anolis cristatellus are two Puerto Rican species that are closely related but occupy very different habitats.[/caption]