Hybridisation is the mating and production of offspring between individuals from morphologically and/or genetically distinct populations. The production of fertile (or semi-fertile) offspring allows alleles to pass from one population to another and will both increase genetic diversity within populations and reduce genetic distinctiveness among populations (Slatkin 1987; Harrison 1993).
In New Zealand we have documented many examples of hybridisation (Morgan-Richards et al. 2009). Natural hybridisation can involve distinct species producing fertile hybrids (such as grasshoppers at Alexandra), and some examples of hybridisation involve morphologically distinct populations that are considered part of the same species (such as little and large geckos on Wellington’s south coast), still other examples of hybridisation involve individuals that are morphologically identical but genetically distinct, such as the chromosome races of the tree weta. New Zealand examples of natural hybridisation are reviewed in Morgan-Richards et al. 2009.
Members of the Phoenix lab study hybridisation because it provides a window into the understanding of evolutionary process such as speciation, selection, gene flow and the evolution of reproductive isolation.
Hybridisation can result in new species. The stick insect genus Acanthoxyla probably arose from a hybrid between two distinct species (Morgan-Richards & Trewick 2005). All species of Acanthoxyla are all-female, and individuals reproduce without males (parthenogenetic reproduction). All species of Acanthoxyla feed on a range of native and introduced plant species. They vary in colour (green, brown, grey, speckled) and in how spiny they are (some individuals are covered in black-tipped spines from head to abdomen, some are smooth), and some Acanthoxyla are diploid and some triploid (Myers et al. 2012).
Biogeography explores the way species are distributed around the earth. It is in part descriptive (what species or combinations of species where), but increasingly has a more robust hypothesis testing objective. A valuable development in recent years has been in the inclusion of molecular phylogenetic information which can tell us about the relatedness of species in different areas of the globe, and also provide estimates of how far back in the past species shared common ancestors. It cannot, although this is often overlooked, tell us where in the world those ancestors existed. A particular problem in this respect is the effect on interpretation of the geographic history of plants and animals, of missing representatives of a group at a particular location. This might be be due to lack of sampling by researchers, extinction of a lineage at one or other place (or, worse everywhere!) or a true historical absence of a lineage at a place.
(See Crisp et al. 2011. Hypothesis testing in biogeography. Trends in Ecology and Evolution 1317: 1-7.)
Phylogeography combines species’ geneologies with geographic distributions to study evolution.
New Zealand has long been a conundrum to biogeographers, possessing as it does geophysical and biotic features characteristic of both an island and a continent. This schism is reflected in provocative debate among dispersalist, vicariance biogeographic and panbiogeographic schools. A strong history in biogeography has spawned many hypotheses, which have begun to be addressed by a flood of molecular analyses. The time is now ripe to synthesize these findings on a background of geological and ecological knowledge. It has become increasingly apparent that most of the biota of New Zealand has links with other southern lands (particularly Australia) that are much more recent than the breakup of Gondwana. A compilation of molecular phylogenetic analyses of ca 100 plant and animal groups reveals that only 10% of these are even plausibly of archaic origin dating to the vicariant splitting of Zealandia from Gondwana. Effects of lineage extinction and lack of good calibrations in many cases strongly suggest that the actual proportion is even lower, in keeping with extensive Oligocene inundation of Zealandia. A wide compilation of papers covering phylogeographic structuring of terrestrial, freshwater and marine species shows some patterns emerging. These include: east–west splits across the Southern Alps, east– west splits across North Island, north–south splits across South Island, star phylogenies of southern mountain isolates, spread from northern, central and southern areas of high endemism, and recent recolonization (postvolcanic and anthropogenic). Excepting the last of these, most of these patterns seem to date to late Pliocene, coinciding with the rapid uplift of the Southern Alps. The diversity of New Zealand geological processes (sinking, uplift, tilting, sea level change, erosion, volcanism, glaciation) has produced numerous patterns, making generalizations difficult. Many species maintain pre-Pleistocene lineages, with phylogeographic structuring more similar to the Mediterranean region than northern Europe. This structure reflects the fact that glaciation was far from ubiquitous, despite the topography. Intriguingly, then, origins of the flora and fauna are island-like, whereas phylogeographic structure often reflects continental geological processes.
(Wallis & Trewick 2009. New Zealand phylogeography: evolution on a small continent. Molecular Ecology, 18, 3548–3580., Trewick et al. 2011 The invertebrate life of New Zealand: a phylogeographic approach. Insects. http://www.mdpi.com/journal/insects/ )
Assembly of the New Zealand fauna
See Goldberg et al. 2008 Evolution of New Zealand's terrestrial fauna: a review of molecular evidence. Philosophical Transactions of the Royal Society, London 363: 3319–3334
Trewick & Gibb 2010. Vicars, tramps and assembly of the New Zealand avifauna: a review of molecular phylogenetic evidence. IBIS. 152: 226-253. Trewick 2011. Vicars & Vagrants. Ausralian Science.
Long distance dispersal
See Shepherd et al. 2009. Multiple colonizations of a remote oceanic archipelago by one species: how common is long-distance dispersal? Journal of Biogeography 36: 1972-1977.
See Morgan-Richards et al. 2010. Geographic parthenogenesis and the common tea-tree stick insect of New Zealand. Molecular Ecology 19(6): 1227-1238.
New Zealand Ecology and Ecophysiology
The distribution and abundance of species can be investigated by focusing on biotic and abiotic interactions. We are interested in combining phylogeographic patterns that inform us about the historic distribution of species, with ecological data about what limits species ranges’. Species ranges’ can be limited by environmental factors such as climate, and competition and predation, and resources such as pollinators and food sources. These environmental factors interact with historical climate change and population adaptation.
What Limits a Weta?
Biodiversity can be measured at many levels, from ecosystem to genetics, but most often is examined at the level of species. Many distinct species means high biodiversity. On earth the majority of species level diversity is microscopic, each individual consisting of a single cell – bacteria, protozoa, amoeba, flagella, algae, protists, some fungi and archaea. However, when we think about conserving biodiversity we usually think about giant pandas, kakapo and whales. To ensure one large vertebrate species does not become extinct usually requires a whole ecosystem for that species to inhabit – and thus all the plants and invertebrates and unicellular organisms within the forest or ocean will also be preserved. But when we lose a rare habitat, or an unusual species we often have little knowledge of the dozens of species that use and require that space or resource. Thus one important aspect of conservation is to document biodiversity – knowing what we have before we loose it.
Within the Phoenix group we are carrying out many studies that inform about New Zealand’s biodiversity. Research projects such as investigating the systematics of New Zealand cave weta, and the NZ ground weta (Hemiandrus) lead directly to a better understanding of our biodiversity. These studies are naming new species and describing where they are found and their whakapapa. Our projects sometimes focus on endangered species to identify conservation units and highlight populations that need attention (Powellphanta; Trewick et al. 2008; Trewick 1999), and our genetic studies can lead to a better understanding of the dangers of population fragmentation (skinks), and where species boundaries are (gecko). On the East Coast on North Island we are documenting the numbers of species of marine snails living in rock pools and by using species ranges and genetic data can learn how the currents and coast line are connecting and separating populations. This information will link directly to iwi plans for a rahui (protected area). But studies of species interactions and ecosystems services are also important for understanding biodiversity and the way unrelated taxa rely on each other for food or pollination or seed dispersal. Parasitic wasps need aphids to eat, weta need trees to hide in and leaves to eat, orchids need gnats to transfer pollen, thus all these studies have biodiversity implications.
Conservation biology makes use of genetic tools to both delimit species boundaries and understand and manage the genetic consequences of small populations.
Many population genetic and phylogenetic studies improve our understanding of the specific status of populations and thus are important for management of endangered species, helping determine priorities and plan conservation programmes. In our lab we have been involved in the delimitation of species boundaries of critically endangered species such as the robust grasshopper (Brachaspis robustus) (Trewick 2001, Trewick & Morris 2008), giant land snails (Powelliphanta augusta) (Trewick et al. 2008, Walker et al. 2008) and geckos (Hoplodactylus duvaucilli), identifying cryptic species such as peripatus (Trewick 1998; 2000), and assessing conservation status of Philippine freshwater crocodile (Crocodylus mindorensis). Genetic studies sometimes reveal fewer species exist than the taxonomy suggests (e.g. stick insects (Trewick et al. 2005), cave weta (Cook et al. 2010).
Endangered species can suffer from inbreeding depression and loss of genetic diversity, resulting from small popualtions. In our lab, students have studied crocodile conservation genetics and skink conservation genetics with this in mind. Other current research includes threatened grasshoppers, the status of New Zealand falcon, and population genetics of weka and Auckland Island rails.
There are lots of opportunities for research in Conservation Genetics in our lab.
Rates, dates & extinction
Latitudinal biodiversity gradient
Calibrations of molecular clocks with living fossils
The distinctiveness of New Zealand’s large endemic orthopterans (weta) and lack of small mammals in our forest ecosystems led to the description of weta as ecologically equivalent to rodents in other countries. However, if these taxa are to be compared, the details of their ecology are important and the scale of their influence in an ecosystem must be taken into account. In particular the ‘invertebrate mouse’ cliché is misleading. For example, reproductive potential and scale of change in population size differ greatly between mice (Mus musculus) and tree weta (Hemideina sp.). Endothermic mice have a metabolic rate almost 20 times faster than ectothermic tree weta, an intrinsic rate of increase some 275 times higher, and consume a high quality diet dominated by seeds and invertebrates and devoid of leaves, in contrast to tree weta diets. (See Griffin et al. 2011 Exploring the concept of niche convergence in a land without rodents: the case of weta as small mammals. New Zealand Journal of Ecology. In press.)
Mutualisms or interspeciﬁc interactions involving net mutual beneﬁts, are an important component of ecological theory, although effectively demonstrating mutualism is notoriously difﬁcult. Among two New Zealand endemics, a slightly elevated germination rate of Fuchsia excorticata seeds after passage through tree weta compared with seeds manually extracted from fruit, led to the proposal that a mutualistic relationship exists between this plant and animal. An improved germination rate, or any other single trait, however, does not alone constitute evidence for mutualism; the relative costs and beneﬁts of numerous components of the interaction need to be accounted for. We considered the costs and beneﬁts to F. excorticata of the putative seed dispersal mutualism with tree weta (Wyman et al. 2010). Tree weta provided with F. excorticata fruits destroyed 78% of the seeds they consumed, did not move fruit; and faeces containing seeds were deposited near their roost holes (which are naturally in trees). The seeds remaining after fruit consumption and those that are ingested but survive gut passage are unlikely to be deposited in suitable habitat for seedling survival. Plant food preferences of captive tree weta assessed using pairwise leaf choice tests showed that the leaves of F. excorticata were the least preferred of six commonly encountered plants. In addition, we found that tree weta did not show a preference for F. excorticata fruit over a standard leafy diet, indicating they are unlikely to be actively seeking fruit in preference to other sources of food. These observations indicate that any interaction between tree weta and F. excorticata is likely to be opportunistic rather than mutualistic, and highlight the difﬁculty of characterizing such interactions.
Both tree and ground wētā have been proposed as potential seed dispersers of some New Zealand fruit. We have examine evidence for coevolution of ground wētā and fleshy fruits (Morgan-Richards, Trewick & Dunavan 2008). We found that although ground wētā consume fruits from Gaultheria depressa and G. antipoda, they do not do so in a way that would suggest they had coevolved as dispersers with these or other New Zealand plants. We observed a positive preference for eating fruits of plants with seeds that were too big for ground wētā to ingest. Thus there is little support to the proposal that ground wētā have coevolved with New Zealand plants resulting in the unusual characteristics displayed by many species (pale fruit presented within a divaricating canopy).