sexual selection in speciation.
Explanation:
Discussion of most topics within Evolutionary Biology begins with Darwin. Indeed, On The Origin of Species (1859) continues to influence much of modern Evolutionary Biology. Darwin viewed evolution by natural selection as a very gradual mechanism of change within populations, and postulated that new species could be the product of this very same process, but over even longer periods of time. This eventual process of speciation by natural selection is illustrated by a sketch drawn by Darwin in his personal notebook nearly 20 years before the Origin of Species was published. Here, he proposed a model whereby lineages form from their ancestors by evolving different characters over relatively long periods of time. Darwin indicated that species could form by the evolution of one species splitting into two, or via a population diverging from its extant ancestor to the point it was a new species. Darwin's insights into evolution were brilliant, especially in light of their being made in the absence of genetics. Indeed, ideas about heredity and the introduction of new genetic material via mutation were to come long after Darwin's founding theories of evolution.
Darwin’s famous sketch indicating that evolution within species may eventually give rise to entirely new ones.
The role of sexual selection in speciation.
A view that is becoming increasingly popular is that sexual selection, or selection related to variation in reproductive success, plays a role in speciation (Panhuis et al. 2001, Ritchie 2007). This model suggests that differential patterns of trait variation related to reproductive success within populations contribute to the reproductive isolation among populations. A compelling example is related to the explosive radiation of cichlid fishes in the African Rift Lakes, where populations with overlapping distributions are diverging as a function of the differential preference of male color in mate selection (Seehausen et al. 2008).
A current debate is whether sexual selection can lead to speciation in the absence of ecological divergence (van Doorn et al. 2009). Indeed, compelling examples that implicate an important role of sexual selection leading to new species sometimes also involve the evolution of different signals used in mate-selection among populations in different ecological contexts, such as light environment (Seehausen et al. 2008, Maan & Seehausen 2010). Here, signals used in mate-selection become adapted to new ecological environments where the transmission of these traits is more perceptible or audible in a new habitat.
Genetics/Genomics: New Directions with Genetics
Genetic studies have long-been at the forefront of speciation research (Coyne & Orr 2004). For example, studies examining the genetic basis of hybrid sterility and inviability have supported the existence of ‘Dobzhansky-Muller Incompatibities' and patterns predicted by ‘Haldane's Rule'. Recent advances in genomics now allow such studies to be taken to the genome-wide level, where biologists can examine hundreds of thousands of gene regions, rather than just a handful. To help understand this genome-wide variation, biologists have developed the metaphor of ‘genomic islands of divergence' (Turner et al. 2005). A genomic island is any gene region, be it a single nucleotide or an entire chromosome, which exhibits significantly greater differentiation than expected under neutrality (i.e., divergence by genetic drift alone). The metaphor thus draws parallels between genetic differentiation observed along a chromosome and the topography of oceanic islands and the contiguous sea floor through which they are connected. Following this metaphor, sea level represents the threshold above which observed differentiation is significantly greater than expected by neutral evolution alone. Thus, an island is composed of both directly selected and tightly linked loci. Major remaining questions concern the size, number and distribution (i.e., chromosomal location) of these genomic islands, and how variation in these factors affects the process of speciation. Clear answers to these questions will likely require experimental studies that measure selection at the genomic level to directly quantify how selection acts on the genome. Nevertheless, the integration of geographic, ecological, and new genomic approaches is likely to yield new insight into speciation over the coming decades.
