Geological events such as glaciation and crustal uplift can impact on species' distributions and genetic diversity through the formation of dispersal barriers and fragmentation of habitat. This thesis investigates the effects of the Pliocene and Pleistocene glaciations on intra- and interspecific population diversity in a New Zealand aquatic invertebrate and an Antarctic terrestrial invertebrate. Caddisflies (Insecta: Trichoptera) are an aquatic invertebrate that are widespread and common throughout both islands of New Zealand. Species are used in stream health studies and as a measure of diversity, but knowledge of their genetic diversity is limited. Mitochondrial DNA (COI) sequence variability was used to examine levels of divergence among closely-related species of caddisflies collected from throughout New Zealand. Based on genetic analysis, seven closely related species pairs were identified, consisting of morphologically distinct species, each restricted to either the North or South Island. Another five species showed similar or greater levels of "intraspecific" divergence, with genetically distinct populations on each island. Sequence divergence between these twelve "species pairs" ranged from 0.41% in Confluens olingoides / Confluens hamiltoni, to 9.92% between the North and South Island populations of Pycnocentria evecta. Based on molecular clock estimates, divergences for these twelve species pairs were estimated to within the last 5 million years, with most dating to the beginning of the Pleistocene (2 Mya). I conclude that population fragmentation during the Pleistocene glaciations and subsequent closing of the Cook Strait land bridge have both played important roles in the isolation and speciation of the New Zealand caddisflies. Mitochondrial DNA (COI) analysis was also used to examine the levels of genetic variability within and among populations of three endemic springtail species (Arthropoda: Collembola), collected from the Mackay Glacier region of southern Victoria Land, in the Ross Dependency, Antarctica. I tested the hypothesis that genetic divergences would occur among glacially-isolated habitats within a fragmented landscape. Mitochondrial analysis of 97 individuals showed high levels of genetic divergence at small spatial scales (<15km). High levels of genetic divergence were found among populations for two of the three species. Gomphiocephalus hodgsoni, a widespread and common species showed 7.6% sequence divergence on opposite sides of the Mackay Glacier. Similar divergences were also found for Neocryptopygus nivicolus, a more range-restricted species showing 4.0% sequence divergence among populations. Based on molecular clock estimates, divergence of these populations occurred in the last 5 Mya. It was suggested that glaciations during the Pliocene (5-2 Mya) and throughout the Pleistocene (2 Mya - 10 Kya) have fragmented springtail populations in this region and isolated them in small, refugial nunataks. I conclude that glaciation has promoted and maintained the levels of diversity observed among populations of springtails and that isolation has occurred on extremely small spatial scales. Collectively, the two studies presented here suggest that the Pleistocene glaciations are responsible for habitat fragmentation, the genetic signatures for which can be observed in invertebrate taxa in both New Zealand and southern Victoria Land, Antarctica.