Speciation: Introduction

Abstract

Speciation is the formation of two or more new species from one ancestral species. How new species form is a complex process and a fundamental biological question. Answering this question provides insights into patterns of biodiversity and informs conservation strategies. Research on the causes and patterns of speciation is heavily influenced by species concepts – how we define species. The biological species concept is perhaps the most influential, and it groups individuals into species on the basis of the ability to interbreed. Consequently, most research on the process of speciation is driven by the search for mechanisms that affect reproductive compatibility among closely related lineages. Once formed, a new species can maintain its identity by sharing advantageous genes among dispersed and reproductively compatible populations. Conversely, distinguishing traits can be maintained between species via isolating mechanisms. These contrasting mechanisms produce discrete clusters of phenotypic variation rather than a continuum of forms. Key Concepts: Speciation tends to form clusters of biodiversity rather than continuous gradients. Species concepts heavily influence conclusions about how speciation occurs. Choice of species concept depends on the goal of the research and the organisms being studied. Species remain distinct from each other because of various isolating mechanisms, which prevent gene flow between species. Isolating mechanisms have a genetic basis. Hybridisation between species pairs often occurs in nature and can yield insights into the causes of speciation. Hybridisation can lead to the fusion of two species into one, to reinforcement of reproductive barriers, or even to the formation of new ‘hybrid species’ that are ecologically distinct and genetically isolated from their parent species.

Gene flow as a simple cause for an excess of high‐frequency‐derived alleles

“…In our models, we used an unsampled or “ghost” population as the source of gene flow (Excoffier et al., 2013), which simply models a reservoir for some divergent lineages now found in the sampled population (Beerli, 2004; Slatkin, 2005). It can represent a population that separated a long time ago from the sampled population, as in the case of a secondary contact after a period of isolation, like in hybrid zones at the population or species level (Alcala, Jensen, Telenti, & Vuilleumier, 2016; Alcala & Vuilleumier, 2014; Hvala & Wood, 2012; Tine et al., 2014). Such hybridization events have occurred repeatedly in human evolution (e.g.…”

“…In our models, we used an unsampled or "ghost" population as the source of gene flow (Excoffier et al, 2013), which simply models a reservoir for some divergent lineages now found in the sampled population (Beerli, 2004;Slatkin, 2005). It can represent a population that separated a long time ago from the sampled population, as in the case of a secondary contact after a period of isolation, like in hybrid zones at the population or species level (Alcala, Jensen, Telenti, & Vuilleumier, 2016;Alcala & Vuilleumier, 2014;Hvala & Wood, 2012;Tine et al, 2014) or after long-distance dispersals between population of distinct ancestries (Fortes-Lima et al, 2018;Sedghifar, Brandvain, Ralph, & Coop, 2015;Verdu et al, 2014). Interestingly, if the source population is actually sampled, the joint SFS for the source and the target populations will reveal in the target population an excess of rare or event quite frequent derived alleles, for small and large immigration rates, respectively (Supporting Information 12), as previously reported in population or species having recently reconnected (Alcala et al, 2016;Alcala & Vuilleumier, 2014;Fraïsse et al, 2018;Tellier et al, 2011;Tine et al, 2014).…”

Spatial distribution pattern of mustelids in the eastern edge of the Qinghai–Tibet Plateau

“…the accumulation of differences between closely related populations) and speciation [9]. When the newly evolved species encountered [10][11][12], severe competition would occur owing to their high degree of similarities and thus the overlap of their niches, which could be detrimental to their survival (Gause's law [12,13]). Therefore, natural selection would favour a taxon whose species avoid spatial association, by which the severe competition would be avoided.…”