Speciation across the Tree of Life

Abstract

Much of what we know about speciation comes from detailed studies of well‐known model systems. Although there have been several important syntheses on speciation, few (if any) have explicitly compared speciation among major groups across the Tree of Life. Here, we synthesize and compare what is known about key aspects of speciation across taxa, including bacteria, protists, fungi, plants, and major animal groups. We focus on three main questions. Is allopatric speciation predominant across groups? How common is ecological divergence of sister species (a requirement for ecological speciation), and on what niche axes do species diverge in each group? What are the reproductive isolating barriers in each group? Our review suggests the following patterns. (i) Based on our survey and projected species numbers, the most frequent speciation process across the Tree of Life may be co‐speciation between endosymbiotic bacteria and their insect hosts. (ii) Allopatric speciation appears to be present in all major groups, and may be the most common mode in both animals and plants, based on non‐overlapping ranges of sister species. (iii) Full sympatry of sister species is also widespread, and may be more common in fungi than allopatry. (iv) Full sympatry of sister species is more common in some marine animals than in terrestrial and freshwater ones. (v) Ecological divergence of sister species is widespread in all groups, including ~70% of surveyed species pairs of plants and insects. (vi) Major axes of ecological divergence involve species interactions (e.g. host‐switching) and habitat divergence. (vii) Prezygotic isolation appears to be generally more widespread and important than postzygotic isolation. (viii) Rates of diversification (and presumably speciation) are strikingly different across groups, with the fastest rates in plants, and successively slower rates in animals, fungi, and protists, with the slowest rates in prokaryotes. Overall, our study represents an initial step towards understanding general patterns in speciation across all organisms.

Repeated divergence of amphibians and reptiles across an elevational gradient in northern Madagascar

“…de León et al, 2010; Kautt et al, 2020; Losos, 2009; McKinnon et al, 2004). A recent review found that ecological speciation is prevalent across the tree of life, often associated with abiotic habitat factors such as temperature (Hernández‐Hernández et al, 2021).…”

Population genomics indicate three different modes of divergence and speciation with gene flow in the green-winged teal duck complex

“…In some cases, geographic isolation alone has been shown to drive divergence when isolated populations reside in allopatry. For example, allopatric speciation is common in molluscs (78% of species pairs), driven in part by their low dispersal rates (Hernández-Hernández et al, 2021). Similarly, among terrestrial snails that live on island archipelagos, allopatric species pairs predominate (96%), with most species being endemic to a single island (Holland & Cowie, 2009;Rundell, 2008;Jordaens et al, 2009;Hernández-Hernández et al, 2021).…”

“…While these factors can operate within each category of the geographically oriented modes of speciation (allopatric, parapatric, and sympatric;Gavrilets, 2003), their presence is particularly important for enabling parapatric or sympatric speciation to progress (speciation with gene flow; Rundle & Nosil, 2005;Nosil, 2008;Feder et al, 2012). For example, ecological and sexual selection are considered major evolutionary forces that often drive insect speciation (Arnqvist et al, 2000;Forbes et al, 2017;Hernández-Hernández et al, 2021), and spatial relationships can become less important. A classic example of this occurs in apple maggot flies (Rhagoletis pomonella), in which ecological divergence enables speciation through divergent adaptation to the preferred host plants of these phytophagous flies (Filchak et al, 2000).…”

“…Another example of ecological divergence occurs in Timema walking stick insects where ecotypes live on different host plants (Nosil et al, 2008), in which greater reproductive isolation evolves between populations adapting to contrasting environments than between populations adapting to similar environments (Rice & Hostert, 1993;Schluter & Nagel, 1995;Schluter, 2009). Environmental factors are also often an important driver of divergence (Hernández-Hernández et al, 2021), and this occurs in many vertebrate groups, including salamanders (Kozak & Wiens, 2010), frogs (Moen & Wiens, 2017), birds (Cooney et al, 2016), and mammals (Castro-Insua et al, 2018).…”

“…Finally, behavior is commonly involved in prezygotic isolating barriers, including ecological and behavioral differences between species (Hernández-Hernández et al, 2021). For example, species-specific vocalization and communication are often important reproductive isolating mechanisms in birds and frogs (e.g., Edwards et al, 2005;Hoskin et al, 2005;Boul et al, 2007;Uy et al, 2018).…”