Ecological speciation

Across the years, biological thought on the causal associations between ecological factors and species formation has evolved from an initially implicit to an increasingly explicit level of recognition. Darwin (1859) recognized that adaptation to divergent environments via natural selection promoted the formation of new ‘kinds’. Shortly thereafter, Benjamin Walsh (1864) offered prescient insights on the relationship between host-plant-associated divergence, interbreeding and species status in herbivorous insects (see also Bates (1862) for related inferences). Dobzhansky (1937) and Mayr (1942) later more explicitly invoked the reproductive isolation between populations as the defining characteristic of ‘biological species’, the concept adopted for this chapter. Simpson (1944, 1953) noted the association of ecological shifts with increased species diversity in ‘adaptive radiations’. However, the questions of why and how why access to novel resources should promote the reproductive isolation required for increased speciation were long treated as a black box. Other workers of the synthesis provided verbal models that – with varying degrees of explicitness – illuminated this box. These models described how divergent adaptation might be expected to incidentally yield reproductive isolation between populations from different environments as a byproduct (Mayr 1942, 1947; Muller 1942; Dobzhansky 1951). This might occur via the pleiotropic effects of selected loci, or the direct effects of loci in linkage disequilibrium with them, on reproductive barriers.