From Plant Press Vol. 8, No. 1, January 2005.
Why do some plants diversify through the process of speciation? Jonathan Price and Warren Wagner attempt to answer this question in a study published in the October issue of Evolution, “Speciation in Hawaiian angiosperm lineages: cause, consequence, and mode” (58: 2185-2200. 2004). The Hawaiian flora provides an ideal opportunity to pose questions about speciation thanks to its extreme isolation, self-contained geologic history and highly unique flora. All Hawaiian plant species are descended from a limited and identifiable set of original colonists. Some of these, such as the famous Silversword alliance, diversified into many distinct species, while others remained a single species. By comparing such features as how species are pollinated and dispersed, and what range of islands and habitats they occupy, Price and Wagner were able test whether certain ecological characters drive the diversification process.
Dispersal appeared to be the most important feature in speciation. Price and Wagner believe on the one hand the lineages with poor dispersal ability (those with large seeds, or those that depend on floatation) had few species because they tended not to colonize very many islands and thus could not readily establish isolated populations that begin the speciation process. On the other hand, lineages with highly dispersible fruits (especially those sought out and ingested by birds) also have on average few species because they colonize many different islands but disperse too much permitting gene flow between populations. Those that are dispersed by external adhesion to birds (with barbs or sticky surfaces, or with tiny seeds that become embedded on bird’s feet) had more species than those with other modes of dispersal. This may be because they disperse enough to colonize all major islands, but not so much that populations continue to interbreed.
Despite all of this, in many cases and especially species-rich groups, the type of dispersal or pollination shifted, often through the evolution of unique, specialized features. For example, in eleven lineages there was a shift toward bird pollination, evidenced by red tubular flowers that differ from those of their presumed ancestor. More dramatically, in four lineages a capsular fruit has evolved into a fleshy fruit that is used as a food source for dispersing birds. These shifts underscore the reciprocal relationship between evolution and ecology and caution against drawing too many conclusions about cause and effect.
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