Diversity
Latitudinal gradients in Diversity
Moving from the poles towards the equator, the diversity of organisms increases dramatically. This latitudinal gradient in diversity is one of the most striking patterns in nature, yet we still do not fully understand why it occurs. A variety of evidence supports the role of speciation as a mechanism creating higher diversity in the tropics, and we have been expanding upon this evidence in our recent work in this area, focusing on fossil evidence (Martin, Bonier & Tewksbury, in review), patterns of genetic isolation of present day populations (Martin & McKay 2004), and patterns of taxanomic diversity at the subspecies, and congeneric level (Martin and Tewksbury, in review).
One of the reasons we are currently unable to pinpoint a reason for higher speciation rates at lower latitudes is because we know little about the process of speciation in nature. We contend that in most groups, speciation occurs by the gradual divergence of populations in allopatry, whereby populations become phenotypically, genetically, and adaptively divergent, and eventually reproductively isolated. While there is general agreement that these four aspects of divergence occur during speciation, there is little consensus on their order or importance in either the process of speciation, or the definition of a species.
Our lab is interested in understanding the relative contribution of each of these mechanisms to the process of species formation across latitude. Understanding how these four components of speciation vary geographically would provide important insight into how and why speciation varies with latitude.
Our current work on this topic is the first to examine all four potential mechanisms that could underlie the increase in diversity towards the tropics: speciation, extinction, emigration and immigration (Martin et al. in review). In addition, we have now shown that the latitudinal gradient in diversity is comprised of two distinct patterns (not one pattern as previously thought) that represent increased rates of speciation in the tropics, and a greater propensity of taxa to coexist in the tropics (Martin and Tewksbury in review). These two patterns covary over latitude, but otherwise act independently, and are likely caused by different abiotic causes (Figures 1, 2). In addition, we are also examining the importance of latitudinal variation in the synchrony of seasons, an overlooked hypothesis that has great potential to explain patterns of populations divergence and speciation.
