Comparing diversification rates in lakes, rivers, and the sea

Abstract

The diversity of species inhabiting freshwater relative to marine habitats is striking, given that freshwater habitats encompass <1% of Earth's water. The most commonly proposed explanation for this pattern is that freshwater habitats are more fragmented than marine habitats, allowing more opportunities for allopatric speciation and thus increased diversification rates in freshwater. However, speciation may be generally faster in sympatry than in allopatry, as illustrated by lacustrine radiations such as African cichlids. Such differences between rivers and lakes may be important to consider when comparing diversification broadly among freshwater and marine groups. Here I compared diversification rates of teleost fishes in marine, riverine and lacustrine habitats. I found that lakes had faster speciation and net diversification rates than other aquatic habitats. However, most freshwater diversity arose in rivers. Surprisingly, riverine and marine habitats had similar rates of net diversification on average. Biogeographic models suggest that lacustrine habitats are evolutionarily unstable, explaining the dearth of lacustrine species in spite of their rapid diversification. Collectively, these results suggest that strong diversification rate differences are unlikely to explain the freshwater paradox. Instead, this pattern may be attributable to the comparable amount of time spent in riverine and marine habitats over the 200‐million‐year history of teleosts.

Key roles for the freezing line and disturbance in driving the low plant species richness of temperate regions

“…This method is most useful in detecting rate shifts deeper in the phylogeny as BAMM assigns tip diversification rates to whole clades, making statistical assessment difficult. Various evaluations have found that the simpler DR statistic correlates well with BAMM estimates (e.g., Igea & Tanentzap, 2020;Miller, 2021;Title & Rabosky, 2019), and we found it very difficult to run BAMM on such a large phylogeny, as did Igea and Tanentzap (2020). We calculated mean phylogenetic distance (MPD) as the average distance between the 100 top species that contribute to each motif; results are similar to those using DR and are reported in Supporting Information Figure S3.…”

Riverscape genomics of cichlid fishes in the lower Congo: Uncovering mechanisms of diversification in an extreme hydrological regime

“…Higher rates of diversification in freshwater fishes compared to marine fishes have been hypothesized to be shaped by barriers that reduce gene flow (e.g., allopatric processes), but meta-analyses of diversification mechanisms have produced mixed results (Miller, 2021). Our study suggests that major rapids correspond to species boundaries in LCR lamprologines, and therefore appear to be important drivers of speciation processes.…”

“…However, the mechanisms by which some rivers generate unusually high ichthyofaunal diversity and endemicity remain poorly known. Previous studies have inferred allopatric processes as a primary mechanism driving the separation of lineages within and between rivers (Boschman et al, 2021;Dias et al, 2013;Lujan et al, 2020;Miller, 2021); however, diversification in sympatry or parapatry has also been hypothesized for some groups (Burress et al, 2018;Sullivan et al, 2002) including another cichlid genus in the lower Congo (Alter et al, 2017). In this study, we investigated a previously suggested rapids-driven allopatry hypothesis in the LCR (Alter et al, 2015(Alter et al, , 2017Markert et al, 2010;Schwarzer et al, 2011) using genome-wide SNP data across and within four endemic lamprologine cichlids.…”

“…Future studies include surveying environmental variation across species' habitats to investigate ecological/ sympatric speciation, which has been suggested in some cichlid clades (Barluenga et al, 2006;Elmer et al, 2014;Kautt et al, 2016;Malinsky et al, 2015;Poelstra et al, 2018). Furthermore, we suggest that it is important to compare diversification rates among riverine habitats varying different habitat complexity, for example, between the LCR and other subdrainages in the Congo Basin, in addition to comparison with other freshwater systems and marine habitats (Miller, 2021). Further examination of regional-based diversification rates may provide more nuanced insight into the complex diversification processes in the hydrologically extreme LCR revealed in the present study as well as the "freshwater fish paradox".…”

Reeling in answers to the “freshwater fish paradox”

“…She repeated her analysis in a 2021 article, this time excluding cichlids. Sure enough, diversification rates for the remaining freshwater and saltwater groups roughly matched (15). Probing deeper, Miller classified the remaining fishes as lake, river, or marine species and compared their diversification rates across habitats.…”