Darwin Day follow-up: Speciation, Darwin’s “mystery of mysteries”

Darwin’s and Wallace’s theory of evolution had to answer two fundamental questions. Why are species’ traits so well adapted to their environment? And why might a species diverge into two or more new species? Both men answered the first question by showing how heritable variation plus natural selection necessarily caused a population to adapt to its environment.

The Galapagos Islands. NASA image.

The Galapagos Islands. NASA image.

Darwin and Wallace both soon realized that this also provided a partial solution to the second problem. If a population became split by some barrier, and environmental conditions were different on each side of the barrier, then natural selection would eventually sculpt the two subpopulations into different forms, each adapted to its own environment. Darwin thought this is why the Galapagos mockingbirds differed from mainland forms. He also thought that because each island differed slightly from its neighbors in environmental conditions, natural selection would cause the forms on each island to be distinctive.

The San Cristobal (Chatham) Mockingbird (top), Espanola (Hood) Mockingbird (middle), and Galapagos Mockingbird (bottom). Mockingbirds are not strong flyers, and have speciated on different island groups. Photos courtesy Roger Ahlman.

The San Cristobal (Chatham) Mockingbird (top), Espanola (Hood) Mockingbird (middle), and Galapagos Mockingbird (bottom). Mockingbirds are not strong flyers, and have speciated on different island groups. Photos courtesy Roger Ahlman.

Darwin and Wallace realized that for speciation to occur, the barrier between the subpopulations (like open water or a mountain range) had to be effective enough to keep the members of one subpopulation from interbreeding freely with the other. A small island very close to a continent would generally not have bird species distinct from the nearby continental forms, because birds regularly flew between the continent and island. Islands farther from a continent were more likely to evolve forms distinct from their continental relatives. For the same reason, island organisms with poor dispersal ability would be more likely to diverge from their mainland counterparts than organisms with good dispersal abilities.

We can see this pattern on the Galapagos. Mockingbird do not often fly long distances, and they did speciate on different groups of islands. However, the Galapagos plant and animal species that are good dispersers have not speciated on the different islands. The swallow-tailed gull (Creagrus furcatus), for example, is a strong flyer and has not speciated on different islands.

The strong-flying Swallow-tailed Gull is endemic to the Galapagos but is the same on all the islands. Photo courtesy of Roger Ahlman.

The strong-flying Swallow-tailed Gull is endemic to the Galapagos but is the same on all the islands. Photo courtesy of Roger Ahlman.

Plants in the Galapagos show the same pattern. Plants with poorly dispersing seeds are different from island to island, and also different from their relatives on the mainland. Plants that are really good dispersers not only fail to differentiate on different islands, they don’t even differentiate from their peers on the mainland. Orchids are examples of this. Of all the plant families, orchids have the smallest seeds, spore-like naked embryos without endosperm, which are wind-dispersed over long distances. It should come as no surprise, then, that twelve of the fifteen Galapagos orchid species are also mainland species. It appears enough seeds are blowing across 500 miles of open ocean to maintain gene flow (enough to prevent differentiation, anyway) between the Galapagos and the mainland.

Even though the Galapagos Islands are 500 miles from the continent, they share most of their orchid species, because orchids are good dispersers.

Even though the Galapagos Islands are 500 miles from the continent, they share most of their orchid species, because orchids are good dispersers.

But this raises something of a mystery, as I discuss in this short clip from a talk I gave at the Carnegie Institution for Science, at Stanford University, in December 2014:

In later posts I’ll try to explain this mystery, based on what I’ve learned from studying the orchid distributions of EcoMinga’s main area of focus, the upper Rio Pastaza watershed near Banos, Ecuador.

Lou Jost

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