When most people think of a species they assume it is a unique, inviolate category of organism. Most of us learned in middle and high school the Biological Species Concept, that organisms of different species are unable to interbreed. For the most part this works, think of any two species and more than likely you can’t think of a case when they interbreed. That is, unless you are a Napolean Dynamite fan and then Ligers immediately jump to mind.
If you are into horses you know that a mule is a sterile horse/donkey hybrid. If you’ve read this blog in the past you might have read about how Red Wolves are not considered a separate sub-species of wolf, but instead a hybrid of wolves and coyotes, as are Eastern coyotes. So, the ability to interbreed between species is actually not all that rare…which makes determining what a species is tricky, especially for animal populations that have only recently diverged and who’s progeny are fertile. We also know that determining whether a population is a separate species can have a big impact on whether they are protected under laws like the endangered species act.
One of the problems with determining a species taxonomically is that there are various metrics used to delineate species, and sometimes they conflict. Some species look very different but are genetically very close, while others look almost identical but their genetics show extremely large differences that should keep them from interbreeding. Some species are still able to interbreed, but appear to be evolving away from each other. So, the question is, how do you explain that grey area where species are diverging, and perhaps are geographically separated, but might still be considered the same species? Where do you draw the line between species that are that closely related?
A new paper out this week in PLOS Biology aims to shed light on this grey area. Most of the research on this topic is species specific, case studies looking at the degree of separation within a single species or a group of species. Because each study is so unique it’s hard to get a handle on whether there are broad trends in speciation across animal populations that are very different from each other. Do species from earthworms to gorilla’s share anything in common when it comes to how new species are made, and when they diverge enough to be considered different species?
Camille Roux and his team examined 61 pairs of animal populations that showed differing levels of divergence. Some were considered different species, others simply sub-populations. looking for clues about how their genomes diverge. Essentially, Roux et al. looked at the level of difference between the genomes of two populations to determine, across all the groups, how much divergence made for populations that were completely separate, and what were the edges of the “grey zone” where determining whether populations are indeed different species is controversial.
What they found is interesting. Regardless of the species or population, and across taxonomic groups from very simple animals to much more complex animals, the “grey zone” stayed about the same…between 0.5% and 2% of difference in genomes between populations. Further, they controlled for habitat, geography, life history and other traits and found that regardless of these differences this grey zone held true.
Does this paper tell us what is and isn’t a species? No, it doesn’t. But, it gives us an idea of how to understand the process of speciation across species. It also indicates that speciation happens (genetically at least) in pretty similar ways in all species regardless of the specifics of the population…which is good news for anyone interested in developing better ecological and evolutionary theories to explain how species come about through natural selection.
This study also gives us some knowledge about the shortcomings of the ways we currently determine species. This study looked at changes within the whole genome, or at least as much of the genome as is available. But most genetic studies look at only a few, small, known, snippets of mitochondrial genetic material (called microsatellites) to determine how many mutations have occurred between species. This paper indicates that genome-wide techniques are a better tool to really understand speciation, but the authors admit these techniques are still in the beginning stages and aren’t really available yet.
What’s the upshot of all of this? The skeptics among you are saying, “we already knew speciation was complex. So what?” True. But species definitions matter, especially for conservation. This paper shows that sometimes we just aren’t going to be able to determine what is and isn’t a species easiy, that darn grey zone is real!
They conclude that scientists and policy makers must be willing to make conservation decisions even when the evidence of speciation is unclear. This presents problems when many environmental laws require species designations. Heck, the Endangered Species Act has species right there in the name…and as we know from our look at the Red Wolf, whether a population is considered a species makes all the difference when it comes to whether it gets protection under the endangered species act. Scientists, policy makers, and the public need to better understand this fuzziness. We need to decide how and when species designations really matter, or whether simply conserving important genetic variation is the end goal.