“How humans drive speciation as well as extinction”

CITATION:
Bull, J.W. and Maron, M. 2016. Proceedings of the Royal Society B 283: 20160600.
ON-LINE AVAILABILITY:
ABSTRACT:
A central topic for conservation science is evaluating how human activities influence global species diversity. Humanity exacerbates extinction rates. But by what mechanisms does humanity drive the emergence of new species? We review human-mediated speciation, compare speciation and known extinctions, and discuss the challenges of using net species diversity as a conservation objective. Humans drive rapid evolution through relocation, domestication, hunting and novel ecosystem creation—and emerging technologies could eventually provide additional mechanisms. The number of species relocated, domesticated and hunted during the Holocene is of comparable magnitude to the number of observed extinctions. While instances of human-mediated speciation are known, the overall effect these mechanisms have upon speciation rates has not yet been quantified. We also explore the importance of anthropogenic influence upon divergence in microorganisms. Even if human activities resulted in no net loss of species diversity by balancing speciation and extinction rates, this would probably be deemed unacceptable. We discuss why, based upon ‘no net loss’ conservation literature—considering phylogenetic diversity and other metrics, risk aversion, taboo trade-offs and spatial heterogeneity. We conclude that evaluating speciation alongside extinction could result in more nuanced understanding of biosphere trends, clarifying what it is we actually value about biodiversity.

We humans are clearly sending an unprecedented number of species into evolutionary oblivion. A few of these extinctions are well documented, such as the famous demise of the Passenger Pigeon. But most species are actually going very quietly, and the majority of the extinct species may never have been described by a scientist. Although the number of well documented cases of extinction is small, it is clear that most cases are not documented, and that many species that are barely hanging on are on the way to extinction (see Beckman, 2016 for an example). In general, species that are not in the category of “charismatic megafauna” will go quietly.

From an anthropocentric, strictly utilitarian point of view this is a tragedy: we may be losing important plants and animals that could feed us or cure human diseases. It’s not hard to imagine that, as you read this, a plant species that contains the chemical to cure cancer is being bulldozed in the Amazonian rainforest. (Donald Rumsfeld might call this a known unknown.) Obviously the possible bad effects on human beings is not all that is wrong with extinction. The loss of a species of life is bad in and of itself—and it typically has cascading bad effects on other members of the ecosystem the species was part of. The current extinction crisis is one of the great wrongs associated with the Anthropocene, and we had a pair of posts from Tom Lekan earlier this year that talked about ways the loss of species is being mourned.

But the paper I’m presenting here makes the very interesting point that humans do not only cause species to go extinct—at least to some extent, we also cause new species to arise.

Now according to the standard biological definition, a species is a group of organisms that can breed amongst each other, but not with members of other species; in other words, they are reproductively isolated. This definition has some problems, but is widely accepted. Typically species are formed when two distinct lineages of a common ancestor split apart by becoming increasingly different, usually due to differing environmental influences. But there can be strong change over time even within a single lineage—i.e. one species can evolve into another.

Bull and Maron discuss several mechanisms by which human beings play a role in the evolution of new species. I’ll discuss them briefly.

One mechanism they propose is domestication. This is a really interesting point—everyone is familiar with the cases where humans have interacted with wild species, domesticating them. This has been considered as a mechanism for speciation. But despite their major importance for human beings as sources of food, clothing, and power, the number of species created this way is actually very small.

An extremely interesting wrinkle to domestication is added with direct DNA manipulation. Zev’s recent post made clear that the door to a new world using CRIPR is being opened right now, as always with unknown consequences.

Also important in this context are the effects of different forms of selection. Artificial selection is directed by human beings, who choose to cultivate certain traits in the organisms they domesticate. Darwin modeled the idea of natural selection on artificial selection, arguing that the “choice” is “made” by natural circumstances—the environmental conditions to which species must adapt.

But is there something in between, where those two processes intersect? Bull and Maron take up the case of hunting and fishing. Killing the animals that have the traits we prefer means that we in effect determine what traits will persist in the populations we prey on. Because this is intentional, one could argue that we should look at it as artificial selection.

Bull and Maron also discuss the issue of relocation, whereby species that human beings transport actively or passively can evolve into new species in their new habitat. In several cases rapid evolution of species was noticed, but this has so far not led to new species, something which is nonetheless possible. But notice something about the language here: relocated species is what most ecologists would call invasive species. Relocated is clearly the less loaded term, right?

Finally, according to Bull and Maron, humans create novel ecosystems that can facilitate speciation. I find this argument fascinating for various reasons—primarily because the best known example makes a very interesting connection to human habitation and urban ecology. Bull and Maron mention what is arguably the best example for speciation in a human provided ecosystem: the famous London Underground mosquito, which split from the regular mosquito (Culex pipiens) and is now populating the London tube system. Other massive effects on the behavior of the many species of animals that live with us in cities are well described. One really interesting example is that some birds adjust the pitch of their songs so that they can be heard above the noise of traffic.

Bull and Maron end their article with a core argument we need to consider here: is there some kind of numerical balance in which species lost to extinction can be replaced by species resulting from human induced speciation in a virtual spread sheet? If there is no net loss of species, is all good? Can this be true? Are all species of equal value and importance? Who decides that? Is a green plant really just a green plant?

These questions are similar to those explored by Emma Marris in her book Rambunctious Garden. But let me emphasize that ecologists actually distinguish between the roles individual species play in an ecosystem. Ecologists speak of some species as “keystone species:” species that play a pivotal role in their ecosystem and cannot be replaced easily. Indeed, in a recent paper Worm and Paine identify homo sapiens as a “hyperkeystone” species, which affects other keystone species disproportionally.

In sum, it is crucial to bear in mind that humans do not only cause extinctions, we also create new species in various ways. I think this is a very valuable observation, but I hesitate to believe that this might in any way counter the catastrophic vortex of extinctions we are currently in.


FURTHER READING:
Benkman, C.W. 2016. The Natural History of the South Hills Crossbill in Relation to Its Impending Extinction. The American Naturalist, vol. 188, no. 6. DOI: 10.1086/688904. Describes the complex mechanisms that contribute to this bird’s vulnerability to extinction.
Worm, B. and Paine, R.T. 2016. Humans as a Hyperkeystone Species. Trends in Ecology & Evolution, vol. 31, no. 8. DOI: http://dx.doi.org/10.1016/j.tree.2016.05.008. Discusses peculiar role of humans as the “species that drives complex interaction chains by affecting other keystone actors across different habitats.”
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