All this week we're celebrating Darwin and the advances in evolutionary theory that have built upon his work.
I asked Joseph Tobias of Oxford's Department of Zoology about birds, their traits, and how they evolved:
OxSciBlog: Darwin was fascinated by finches: What is it about birds that makes them ideal case studies for evolution?
Joseph Tobias: Viewpoints differ, as always. One regularly hears that birds are not ideal systems for evolutionary research. Listen to any entomologist or bacteriologist, for example, and they will tell you that birds are too much work to keep in the lab, or too slow to reproduce. A botanist will add that wild birds wake up impossibly early and move around too much. An ornithologist, however, will point out that birds make rewarding subjects because they can be studied in natural settings, and they offer several key measurable traits.
Some of these traits - including the number of eggs produced in a clutch - are associated with life-history, while others are linked to ecology. The beak, for example, is the main foraging apparatus of birds, and therefore strongly shaped by diet. Variation in beak design is a feature of adaptive radiations, as was evident in Darwin’s collection of finch specimens from the Galápagos, thus providing an inspiration for the theory of natural selection. Similarly, elaborate plumage ornaments and complex songs were central to the development of Darwin’s 'other theory', sexual selection.
Beaks, plumes and songs continue to be mainstays of evolutionary research. Meanwhile, birds have also made a disproportionate contribution to our understanding of behaviour as an agent of selection, partly because wild birds are easier to catch, mark, track and observe than many other animals.
To some extent, the role of ornithology as a driving force within evolutionary biology can be boiled down to the fact that birds, like us, are audio-visual creatures with acute colour vision and mid-range hearing. From a signalling perspective, they speak our language, and this explains a large part of their allure.
OSB: What can birds tell us about Darwin's 'mystery of mysteries', the origin of species?
JT: One upshot of the popularity of birds is that we know lots about them, and this simple fact accentuates their importance for studies of speciation. We have at our disposal more information about species limits, global ranges, ecological niches, and life-history strategies for birds than for any other diverse group of animals or plants. The same can be said of sequence data, as a flood of phylogenetic studies is rapidly filling out the avian tree of life. Given that the molecular clock is relatively consistent in birds, biologists can use these sequence data not only to determine the relationships between the branches of the tree but to estimate the timing of evolutionary events, including speciation. By putting these resources together, and applying increasingly powerful computational methods, it is becoming possible to test previously intractable hypotheses about the factors promoting reproductive isolation and shaping patterns of diversity.
Genetic data have already taught us more in the past decade about the way lineages diversify than we learned in the other 190 years since the birth of Darwin. The good news is that this exhilarating period of discovery is set to continue, with birds as a common theme.
OSB: What have your studies of Amazonian birds revealed about the evolution of signals?
JT: Most Amazonian birds have simple, genetically determined songs. By quantifying their structure, and conducting experiments to test perception, we are investigating how these signals are shaped by ecology and interactions.
For example, we have found that songs of different species are adapted to the transmission properties and noise regimes of their respective microhabitats. Whether by this kind of ecological adaptation or by random mutation, the process of signal divergence in isolated forest patches can be fairly rapid: a few thousand years can produce enough difference in song structure to reduce responses between populations. On the other hand, we have found compelling evidence that interspecific competition can drive convergence in territorial songs, suggesting that social selection can operate across species boundaries.
Further work on dueting species has shown that temporal coordination in joint signals is promoted by intersexual conflict rather than, as commonly assumed, cooperation. These are a few examples of how a comparative and experimental approach can clarify the mechanisms underlying speciation and phenotypic evolution.
OSB: How might research into evolution help in the conservation of rain-forest species?
JT: Some apparent flaws in current conservation practice can be attributed to a species-based mentality. This means that, in deciding what we want to conserve, we use as our currency a system of units that represent a mere snapshot of evolution, and which in any case we find impossible to define. Of course there will always be powerful flagship causes like pandas and whales, but evolutionary biology and conservation genetics can expand our consciousness beyond the ‘species’.
These disciplines encourage a consideration of the longer view, and of process rather than pattern. They highlight the importance, for example, of connectivity between habitats rather than isolated reserves. They reveal the remarkable genetic diversification within rainforest ‘species’, and draw attention to zones of future evolutionary potential. The pattern-based view directs resources towards the conservation of island faunas, where populations are naturally rare and often short-lived, whereas the evolutionary view argues for the re-direction of some of those resources to continental habitats, and to rainforests in particular. These are the powerhouses of terrestrial evolution, but they will only continue to function as such if we succeed in maintaining them at something like their present size.
Dr Joseph Tobias is a Departmental Lecturer in Evolutionary Ecology at Oxford’s Department of Zoology.