As part of my Thesis Development Seminar, I ask students to do a critical read of a paper they enjoy and place it in a broader context. This week, MA student Vijay Ramesh takes on a challenging but interesting paper by Dan Rabosky
Blog Piece 2: Diversity-dependence, Ecological speciation and the Role of competition in Macroevolution – Dan Rabosky (2013)
This interesting paper talks about the ways in which speciation, extinction (& its rates) and ecological divergence are related. Rabosky does a fine job of talking about diversity dependent models of diversification and impinges on why it is important to consider this perspective today. This manuscript starts by talking about the need to have a conceptual basis for unifying global patterns of biodiversity and a diversity dependent model provides that basis. It all boils down to variation in rates of speciation and extinction at whichever scale one wants to consider. He defines viewing diversity as a mechanism as well as a pattern. Interspecific competition is considered to explain a mechanistic framework, but as a pattern, a diversity curve at the macroevolutionary scale is used. Rabosky defines macroevolutionary rates as events over geological timescales, but the most of the studies regarding rates of speciation and extinction are however considered at ecological time spans (smaller scales).
Today, the literature of speciation and extinction is confounded with phenomenological models of pattern vs mechanistic models of process. The former quantifies statistical behaviour of entities without considering behaviour as a proxy at lower levels, while the latter looks at genetic and demographic processes. The author moves on to highlight the importance of carrying capacity and the fact that it makes sense only for a diversity dependent process. However, to look at macroevolutionary patterns of diversification, it is crucial to understand speciation rates and extinction rates and the author highlights the missing fossil record not giving us a clear picture since failed speciation is never recognized.
Rabosky then highlights how macroevolutionary rates are regulated by factors, intrinsic and extrinsic. He then suggests two interpretations of diversity dependence: a) Darwinian diversity dependence and asymptotic diversity dependence. As the name suggests, the first method of diversity dependence is concerned with how Darwin interpreted the origin of new species. He linked it mainly to extinction and the fact that the ecological contribution to speciation rates was mainly concerned with persistence of diverging populations in time, rather than reproductive isolation. On the other hand, asymptotic diversity dependence implies a limit on species richness and that there will be regulation of diversity even if the long term trend is positive.
The author talks about decoupling time and species richness and highlights three types of studies which have been carried out in this regard. Some of them include integrating biome areas and its ages, using phylogenetic methods and studying relationships between named clades and their species richness. Since this review is about diversity dependent expansion, Rabosky highlights an important point on what ecological mechanisms are involved. He says that reproductive isolation is not the same as speciation and a graph shown suggests that the rate at which species evolve reproductive isolation and their macroevolutionary rates of speciation are different from the same at local scales. This could imply that there are other factors limiting diversification. He concludes by highlighting the importance of both methods of diversity dependence he alludes to (Darwinian and asymptotic). The former is a population level mechanism and the latter does not propose a causal mechanism. He suggests that it is imperative to understand if reproductive isolation is involved at all in affecting macroevolutionary dynamics.
Dan Rabosky makes a very compelling case for a diversity-dependent mode of diversification by highlighting numerous examples and situations where it would make more sense to think of diversity dependence rather than non-independent processes. He suggests the fact that it is crucial to consider processes occurring at a local scale, since that would indirectly affect processes at a global level. However, there are diversity-independent processes such as the effect of tectonics and climatic factors, which could influence species persistence at a global as well as a local scale.
He suggests that rates of origination and rates of extinction behave as though they are unregulated by other species interactions in a diversity-independent process. He goes on to say that diversity independent processes if left unchecked could generate more number of species than is possible. But, I would disagree in this case since there are mechanisms by which populations are regulated in diversity independent processes and it need not be biotic interactions alone which have to regulate populations. Also, his models are similar in many ways to Jack Sepkoski’s models of rates of speciation and extinction, the former decreasing with increases in diversity and vice versa.
Some of the big assumptions which Rabosky makes while talking about diversity-dependence is that biotic resources are limited and there is a carrying capacity which is reached by most populations. In some sense, he also assumes that the biosphere is a closed ecological system, when in reality it is not. It is a non-equilibrial system and all living organisms exchange matter and energy with the universe. Furthermore, it is important to consider rates of mutation and fixation for density dependent processes, which Rabosky has not highlighted. He says that predation and other non-competitive factors are not discussed here, but these factors could be potentially affecting diversity of populations at the small scale.
If we do consider a diversity dependent process of diversification, it is very hard to explain the Phanerozoic curve. This curve suggested for marine family level fauna by Sepkoski (1990) shows that diversity has been only increasing and never shown stasis after the Permian. Since Rabosky argues for every species having its carrying capacity, one should observe a logistic growth curve, but one does not notice that. One could also argue that the geological record has not really been complete to show that diversity has been continuously increasing. Although the fossil record highlights many cases of increasing diversity, there could be cases of extinctions as well and we do not notice that due to our incomplete record.
Interestingly, climatic factors and other extrinsic controls such as the environment are important to be considered while explaining diversification, but these are not considered. Although a local population could reach its carrying capacity due to limiting factors, it is important to consider other factors which could in turn affect the carrying capacity of the population. For example, “Simpsonian innovations” (Simpson 1953) states how species enter into new adaptive zones and this could be either due to extinction of a competitor, dispersal or due to evolution of a key innovation and in majority of the cases do not depend on carrying capacities. I believe that in some sense, one could think of the Red Queen hypothesis where species evolve continually in response to their predators and parasites continually evolving, which in turn would ensure that species diversify.
He finally mentions how diversity dependence is a challenge to study speciation. Evolutionary biology has unfortunately, not developed a concrete explanation to talk about extinction, but we are trying to grapple with what little we know about speciation. Rabosky mentions that most empirical studies of speciation consider a diversity-independent approach where biological diversity would depend on rate at which reproductive isolation evolves (Coyne & Orr 2004). However, if reproductive isolation is the only criterion to be considered in diversification, one could go wrong in estimating the true diversity of species. It is important to consider the right species concept in mind while understanding concepts of species diversification, since this will influence your overall estimate of the biological diversity. Today, most systematists favour the Phylogenetic species concept.
Rabosky concludes by suggesting that a diversity dependent perspective is important to be considered at different scales. I feel that it is critical to have holistic approach to the concept of species diversification and one must consider both approaches of diversity-dependence as well as diversity independence at different scales accordingly. Only time and better datasets could help us understand species diversification better.
- Coyne JA, Orr HA. 2004. Speciation. Sunderland, MA: Sinauer
- Simpson GG. 1953. The Major Features of Evolution. New York: Columbia Univ. Press
- Sepkoski, J. John. “The taxonomic structure of periodic extinction.” Geological Society of America Special Papers247 (1990): 33-44.