Solving the conundrum of cryptic species through integrative taxonomy
A new taxonomy
“Species are not immutable”, somebody once said, after looking into it* - and they were absolutely right. Since soon after that, generations of dedicated taxonomists have been classifying and naming organisms according to the position they occupy in the web of life that this mutability has woven.
For the majority of this time, the principle characteristics used to delineate groups were morphological - the colour patterns of fish, the beaks of finches. However, not all diversity is
manifested morphologically, and so some species appeared (and may still appear) indistinguishable from others. These are called cryptic species, with multiple such species forming cryptic species complexes.
Over time, though, scientific and technological advances have provided new lenses through which to compare individuals. Sophisticated cameras can image light from the non-visible spectrum, computer programs can analyse complex bioacoustic data and molecular genetic techniques can quantify the divergence between lineages.
Although this led to a degree of subsequent 'frank discussion' in the scientific literature around how these new kinds of data should fit into the established taxonomic model, it is now widely recognised that the future of taxonomy is integrative - combining all available data, especially morphological and genetic. It’s now generally agreed that neither morphology nor genetic divergence is more important - what really matters is what they can tell us, in combination, about the underlying evolutionary processes in a given situation.
Now with this varied arsenal we are better equipped to tackle the conundrum of cryptic species. By comparing and contrasting data within a pluralistic framework we can hone in on changes in traits that are particularly relevant to speciation in the species being studied, to tease apart those cryptic species differences.
Why are species cryptic?
There is much we don’t yet know about cryptic speciation, but from discovered examples it appears that two drivers predominate: either cryptic species differ in non-visual mating signals, or they are under selection to remain morphologically unchanged (or in ‘stasis’).
Cryptic species may be relatively common in groups that use signals such as mating calls, or pheromones to communicate. Generating differences in these traits does not necessarily leave detectable morphological change, so taxonomists would need to look at bioacoustic or biochemical data to help tease species apart. Such studies have revealed cryptic species in birds, bats, frogs and insects.
Under certain harsh environmental conditions species may be under selection to retain a certain morphology because it is vital to their survival. In this instance, even when lineages are geographically divided and speciate by subsequent drift (or an alternative mechanism), we would expect them to retain the same morphology. Evidence for this hypothesis is provided by the large numbers of cryptic species discovered in arctic and deep sea ecosystems.
Why does identifying cryptic species matter?
The importance of identifying cryptic species pervades many of the ways that we as humans interact with nature. Failing to recognise biologically important species means that we cannot utilise, study or conserve them effectively.
Much of modern medicine hinges on the discovery of natural compounds with pharmaceutical applications, a better understanding of species boundaries in biologically important taxa could well give more opportunities to discover new medicines.
The primary mosquito vector of malaria in Africa has been found to be a complex of seven cryptic species, some of which do not bite humans. Large investments are currently being made in biological and ecological means of controlling these mosquitoes, this money can now be better directed towards only those species which impact human health.
These are just two examples, but the relevance extends to all aspects of how we aim to contain invasive species, utilise others as biological controls.
There are also significant implications of the study of cryptic species for conservation. A broadly distributed species could be discovered to actually be a cryptic complex of several, smaller, range-restricted populations, each at greater risk of extinction and in greater need of protection.
The severity of the current extinction crisis has led to conservation effort being concentrated in hotspots identified by their species richness and rates of endemism. The prevalence of cryptic species may be masking the true values of these measures and causing a misallocation of conservation resources that could have irreversible consequences.
Summary
For much of the history of taxonomy the morphological traits of individuals were the best clues available to what evolutionary processes were at work on them. Integrating modern techniques into classical taxonomy enables identification of characters that are functionally relevant to speciation in a particular group of organisms. This in turn facilitates the discovery of cryptic species complexes, getting us closer to the true picture of earth’s biological diversity - to the benefit of future appreciation, study and preservation of all true species.
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For more detail on cryptic species see the excellent review by Bickford et al. (2006) and their references. A great overview of integrative taxonomy is given in Padial et al. (2010).
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*It was Darwin, in ‘On the Origin of species..’ (1859).