Defining the speciation continuum

Abstract

A primary roadblock to our understanding of speciation is that it usually occurs over a timeframe that is too long to study from start to finish. The idea of a speciation continuum provides something of a solution to this problem; rather than observing the entire process, we can simply reconstruct it from the multitude of speciation events that surround us. But what do we really mean when we talk about the speciation continuum, and can it really help us understand speciation? We explored these questions using a literature review and online survey of speciation researchers. Although most researchers were familiar with the concept and thought it was useful, our survey revealed extensive disagreement about what the speciation continuum actually tells us. This is due partly to the lack of a clear definition. Here, we provide an explicit definition that is compatible with the Biological Species Concept. That is, the speciation continuum is a continuum of reproductive isolation. After outlining the logic of the definition in light of alternatives, we explain why attempts to reconstruct the speciation process from present‐day populations will ultimately fail. We then outline how we think the speciation continuum concept can continue to act as a foundation for understanding the continuum of reproductive isolation that surrounds us.

Regions of genetic divergence in depth‐separated Sebastes rockfish species pairs: Depth as a potential driver of speciation

“…Many cases of heterogeneous gene flow among different genomic sites in species pairs with intermediate levels of divergence have been documented (Roux et al, 2016). A newer model of speciation focuses on the degree to which species are reproductively isolated along a continuum (Stankowski & Ravinet, 2021). While the present study was not focused on quantifying the strength of barriers to reproduction, for both the Smi ‐ Scr pair and Sca ‐ Sch pair (Buonaccorsi et al, 2011) hybridization still occurs, and F1s were both viable and capable of backcrossing to some extent.…”

Strong yet incomplete reproductive isolation in Vermivora is not contradicted by other lines of evidence: A reply to Toews et al.

“…The hybrid index score is estimated for each individual from multiple unlinked, highly divergent loci (Jiggins and Mallet, 2000). If reproductive isolation is weak, the association of alleles among regions will decay, producing a unimodal distribution of hybrid index scores (Jiggins andMallet, 2000, Stankowski &Ravinet, 2021). At the other extreme, if reproductive isolation is complete, a bimodal distribution of hybrid index scores is expected (Jiggins andMallet, 2000, Stankowski &Ravinet, 2021).…”

“…If reproductive isolation is weak, the association of alleles among regions will decay, producing a unimodal distribution of hybrid index scores (Jiggins andMallet, 2000, Stankowski &Ravinet, 2021). At the other extreme, if reproductive isolation is complete, a bimodal distribution of hybrid index scores is expected (Jiggins andMallet, 2000, Stankowski &Ravinet, 2021). Trimodal distributions of hybrid index scores are expected in scenarios of strong yet incomplete reproductive isolation (Jiggins andMallet, 2000, Stankowski &Ravinet, 2021).…”

“…These results hold if either of the two divergent regions on the Z chromosome are excluded from the analyses (thus accounting for potential linkage and non-independence of these regions). This suggests there is strong yet incomplete reproductive isolation between V. cyanoptera and V. chrysoptera (Jiggins andMallet, 2000, Stankowski &Ravinet, 2021). Importantly, the proportion of individuals with different phenotypes in extensive field surveys by Bennett et al (2017) at this site in 2015 (cyanoptera: 0.53, chrysoptera: 0.32, hybrids: 0.15) is similar to the proportions in the genotypic samples of Toews et al (2016) (cyanoptera: 0.47, chrysoptera: 0.37, hybrids: 0.16), suggesting no sampling bias that would increase deviation from a unimodal distribution of hybrid index scores (i.e., a scenario of no reproductive isolation).…”

Diversification dynamics and (non-)parallel evolution along an ecological gradient in African cichlid fishes

“…Speciation is commonly defined as the evolution of reproductive isolation through the building up of barriers to gene flow (2). This has typically been considered a continuous process, during which genetic and phenotypic differences accumulate more or less gradually between diverging populations, and reproductive barriers become stronger, until complete reproductive isolation is reached -a progression often referred to as speciation continuum (3,4). In the genic view of speciation, genomic regions under divergent natural (or sexual) selection become resistant to introgression at the initial stages of diversification, creating 'genomic islands' of strong differentiation; as the populations diverge, genetic differentiation expands across the genome, leading to stronger and more genome-wide patterns of differentiation (3,5,6).…”