Rates of molecular evolution and gene flow : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology, Massey University, New Zealand.

Abstract

Species diversity is driven by speciation, extinction and immigration. In this thesis I explore species diversity among regions and the process of speciation via four related studies. The latitudinal biodiversity gradient (LBG) describes the pattern of higher diversity levels towards the tropics. One of the popular hypotheses to explain the LBG is the evolutionary speed hypothesis, which suggests that species in the tropical regions are evolving faster than those in temperate regions. I demonstrate that current work on the LBG often focuses on describing the pattern as distinct from understanding the process that might drive the LBG (chapter 1). Second, I show that the most popular method used to measure differences in rate of molecular evolution between taxa from different regions, the sister-species method, does not give consistent results and estimated rates of molecular evolution can vary widely depending on outgroup selection and gene analysed. The inherent problems revealed within the current approaches raise questions as to the validity of inferences made from putative variation in rate of molecular evolution between high and low latitudinal taxa. In particular, I show that studies of the LBG that use very close sister-species pairs, rely on the most problematic datasets and should therefore be treated with considerable scepticism (chapter 2). A phenomenon such as the LBG must derive from small-scale processes in population dynamics. Species are sometimes defined as reproductively isolated units, with hybridisation viewed as evidence of failure to speciate. But it is now increasingly acknowledged that speciation is a dynamic process during which species and populations can exchange alleles. I found relatively high levels of admixture and gene flow between morphologically distinct biological entities in two very different study systems: insects and snails. Analyses of a pair of grasshopper species provided no evidence of deviation from random mating in the region where they are sympatric. However, analysis of morphological traits provided no evidence for hybrid phenotypes between these Sigaus grasshoppers. I infer that some loci associated with these morphological characters are subject to strong natural selection but neutral genetic material is being extensively shared between the two species (chapter 3). A similar situation was found in terrestrial snails that I studied, but in this case there was a clear association of neutral genetic makers with phenotype. The mtDNA haplotypes, 3,764 SNP loci and morphometric data revealed two clear genotypic clusters among New Caledonian Placostylus, despite strong evidence for gene flow (chapter 4). Although it is convenient and popular to define species on the basis of their reproductive isolation, a more dynamic model that allows for the possibility of gene flow is closer to reality for these taxa. Speciation can be complex and our current understanding of the process of speciation does not suggest it is limited by the genomes' rate of molecular evolution. Results from my research shows, in support of other recent research, that the process of speciation is often the product of adaptation.