An example of “ecological opportunity” … that does not reflect adaptive radiation or “hybrid swarm”

Here is another example of gene-environment (G x E) interactions. How does a new species expand into any new ecological environment? “Ecological opportunity” is frequently proposed as the sole ingredient for adaptive radiation into novel niches. An additional trigger may be genome-wide hybridization –– resulting from “hybrid swarm.” However, these hypotheses have been difficult to test, due to the rarity of comparable control environments that lack adaptive radiations.

In the attached report, the author exploits such an ecological opportunity pattern in microendemic radiations of Caribbean pupfishes. In order to enjoy a nice, warm long winter vacation on a tropical island, he cleverly decided to perform a comparative evolutionary experiment in the Bahamas. He showed that a sympatric three-species radiation on San Salvador Island (Bahamas) diversified 1,445-times faster than that on neighboring islands in jaw length –– due to evolution of a novel scale-eating adaptive zone from a generalist-ancestral niche.

He then sampled 22 generalist populations on seven neighboring islands and measured: morphological diversity, stomach content diversity, dietary isotopic diversity, genetic diversity, lake/island areas, macro-algae richness, and Caribbean-wide patterns of gene flow. None of these standard metrics of ecological opportunity or gene flow was associated with adaptive radiation –– except perhaps for slight increases in macro-algae richness. Therefore, exceptionally rapid trophic diversification is highly localized, despite myriad generalist populations in comparable environmental and genetic backgrounds. This fascinating study provides a strong counterexample to the ecological and “hybrid swarm” theories of adaptive radiation, and suggests that diversification of novel specialists on a sparse-fitness landscape is constrained by more than ecological opportunity and gene flow.

Evolution   Nov 2o16; 70: 2504–2519

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