More species, more genes?

Submitted by editor on 11 April 2017.Get the paper!

More species, more genes? Why positive correlations between species and genetic diversity might not be the rule

Diversity is an essential facet of natural ecosystems that cuts through many organizational scales. The diversity of species observed in a given area (species diversity) often reflects the diversity of traits among organisms and is linked to ecosystem functioning. Within species, the diversity of genes/alleles constitutes the raw material of evolution. These two levels of diversity were originally the focus of distinct fields of research until they progressively became more integrated only a few decades ago. Due to similarities in the processes controlling both levels, it was hypothesized that species diversity and genetic diversity should be positively correlated in natural systems, a pattern referred to as species-genetic diversity correlations (SGDCs). Based on 15 years of empirical investigations, our paper first shows that positive SGDCs are not the rule in nature – most studies in fact did not find a correlation, and we even detected a publication bias in favor of positive values. This is not extremely surprising since, as we show, SGDCs are influenced by multiple factors that can have either positive or negative effects.

Drawing from ecological and evolutionary theory, we provide a framework for interpreting SGDCs when genetic diversity is derived from neutral genetic markers. We summarized the factors affecting SGDCs, teasing apart factors that are due to spatial variation in some habitat characteristics from those that reflect species interactions within communities. All these factors can influence SGDCs in very different ways. Our analysis of the links between factors and effects on SGDCs focuses on how ecological similarity among species, i.e. the similarity of their realized niches and dispersal abilities, is key to interpret the contribution of any factor to SGDCs. In a second part, we invite researchers to not only assess SGDCs, but also to decompose them into the relative contributions of the factors we identified. Taking a freshwater snail metacommunity inhabiting a network of ponds as an example, we illustrate how structural equation modelling (SEM) can be used to quantify the predominant positive contribution of habitat connectivity to the observed SGDC, due to the fact that most species are ecologically similar with respect to their dispersal abilities (i.e. passive dispersal). However, the marginal niche occupied by the focal species along the disturbance gradient explained that habitat instability reduces the overall SGDC.

We finally show how such a link between SGDC and ecological similarity among species can be of general use to test assembly rules in ecology. SGDC represents a new promising pattern between ecology and evolution pending that particular attention is paid to the processes underlying it. Such pattern can also be extended to the analysis of the correlations between β-diversities at species and genetic levels to further contribute to our understanding of community assembly. The SGDC approach is complementary to other approaches across biodiversity levels, such as community phylogenetics, with the merit of being rooted in a well understood theoretical framework.

The authors through Thomas Lamy

Figure 1. D. Depressissium the focal species of the study example.

Figure 2. A pond located on the Guadeloupe Archipelago.

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