Chaetodon fremblii x C. multicinctus (?) in the aquarium of Mr. Hirosaki. Identification of hybrids often involves speculation based on intermediate phenotypes displayed by both parents. In this case, Chaetodon fremblii is quite clearly one of two species involved. Photo credit: Kengo Zeze.
Hybridization is a common biological process best defined as the interbreeding of individuals from two distinct populations (or species), distinguishable on the basis of heritable characters. Throughout the dawn of time, since humans started developing a keen interest in the natural world, hybrids from most all organism classes have been recognized and subsequently studied. Marvel comes not surprisingly, for these examples often display spectacular intermediary characteristics of their parents, and in extreme cases, novel and wildly expressive phenotypes. While hybrids are often elusive and associated with the mythos of rarity, the phenomenon itself is not uncommon, occurring in a considerably large proportion of species. This challenges the fundamental biological definition of a species, and what defines it. Evolutionary biology has much to gain from the study of hybridization for many reasons, in addition serving to provide unique insights into reproductive isolation and the process of speciation. For example, a fundamental dogma of sympatric speciation is that there are behavioral or semi-permeable physical barriers to reproduction amongst distinct components of a population, leading through time to the eventual divergence of these populations into distinct species (Bush, 1969; Munday et al., 2004). In other words, two naturally overlapping species are able to maintain their separate path of evolutionary divergence through genetic purity by the lack of inbreeding. This then begs the question. What exactly limits hybridization, and what level of it can occur while species continue to diverge? We see numerous examples of sympatric species procreating in fairly regular occurrences. Hybridization can also increase genetic diversity and produce novel genotypes that enable hybrids to exploit new, unoccupied or altered habitats, setting the stage for hybrid originated speciation (Seehausen, 2004; van Oppen and Gates, 2006; Mallet, 2007). Cirrhilabrus katherinae, Centropyge shepardi and Chaetodon flavocoronatus in the Mariana Arc are thought to be hybrid-derived species through allopatric isolation from their parents. The biogeography and evolution of these species have been widely discussed in my previous article on Cirrhilabrus and Roaops. Lastly, hybridization can play a significant role in the generation and loss of species through extinction or reverse-speciation. Repeated backcrossing of fertile hybrids with pedigree parental species may result in elimination of a single species altogether, resulting in a population of intermediate hybrids spanning a wide spectrum of genetic and phenotypic plasticity.… More:
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