Mammalian speciation is not usually an instantaneous event. It is a protracted process, the course and outcome of which is determined by the interaction between demographic and genetic processes. These demographic processes include the formation of population isolates and the persistence of these isolates. The role and regulators of the demographic determinants of speciation have received some empirical attention, but mostly across birds and squamates. The point of this thesis is to ask what factors regulate the demographic determinants of speciation in mammals, and how these relationships shape macroevolutionary patterns of speciation and species richness. The thesis is divided into two sections that approach the question from different angles. The first is focused on extant mammals and uses subspecies as a proxy for population isolates to ask how ecological factors (1) regulate population isolate formation and (2) mediate the evolutionary trajectory of these isolates. In taking this approach, this section also sheds new light on the debated evolutionary ‘role’ of subspecies in mammals. The second section focuses on a particular subgroup of mammals as a case study: hominins. Hominin speciation rate is calculated across three phylogenies based on different taxonomic approaches. I then test whether climate, time, and interspecific competition explains variation in these rates, and so ask what abiotic or biotic factors determined hominin speciation. Differences between results obtained across more and less inclusive (“lumping” and “splitting”, respectively) phylogenies are then used to link these results to the main question in the thesis. It is suggested that taxa in “split” taxonomies are populations of taxa in “lumped” taxonomies, so that determinants of speciation on “split” taxonomies represent determinants of population isolate formation, and those of speciation on “lumped” taxonomies represent determinants of the rate at which those isolates become detached from the rest of the species.

Mammalian population isolate formation provides the raw material for speciation, and the rate at which it occurs is regulated by barriers in the landscape, the level of habitat fragmentation, and climate variability. Speciation is the road less travelled for most mammalian subspecies: and general determinants of the evolutionary trajectory of a mammalian population isolate include ecological substrate and metapopulation niche availability. Across all mammals, terrestrial population isolates become species less often than their non-terrestrial equivalents; and this pattern holds up in hominins. Patterns found across hominins suggest that the probability of a population isolate becoming a novel species can also be determined by whether or not species-level niche space is saturated. An important bridge between population isolate formation and speciation in mammals is the length of population isolate persistence: extended persistence, rather than extending the time it takes to become a full species, increases speciation rate. Again, abiotic factors—particularly climate and possibly habitat fragmentation—are important determinants of this process. Of interest here is the contradicting effects of these abiotic factors: habitat fragmentation promotes population isolate formation across mammals, but it can also curtail how long these isolates persist for. In hominins, a previously unknown role for climate is in mediating the link between intraspecific processes and speciation: the results suggest that longer-term persistence of populations necessary for these to split from the rest of the species tended to occur in more stable climates. Macroevolutionary patterns of mammalian species richness are the cumulative outcome of the balance between population isolate formation and persistence over time: and including demographic determinants of speciation rate in evolutionary models can provide novel insights into why, and how, mammalian species form.