In a recent, thought-provoking
paper published in Trends in Ecology and Evolution, and entitled 'What do we
need to know about speciation?', members of the Marie Curie Speciation Network
(2012) outline some important themes and questions that should, among others,
be central to future studies on speciation.
 |
| Darwin's famous scribble. (c) www.fossilmuseum.net |
What struck me most about this article was how fundamental some of
these questions are, and yet how far we seemingly are from even beginning to
answer many of them. As I came to discover, this is partly because answering
any general questions about speciation is a task fraught with challenges.
However, scientists also need to take some of the blame, for two reasons:
Firstly, many arguments and lines of thinking that have passed their sell-by
date are still followed by many; secondly, studies of speciation were
surprisingly rare up until surprisingly recently.
But first, let's have a look at some of the questions that
illustrate just how much we have yet to learn about speciation. Bear in mind
that the authors restricted themselves to questions concerning speciation in
sexually reproducing eukaryotes; the fundamental differences in how asexually
reproducing eukaryotes and prokaryotes diversify, and even in how we define the
term 'species' for them, is yet another illustration of the complexity of
speciation. Even excluding them, the scope of the issue is enormous.
Their first question asks which barriers are most important in
restricting gene flow between populations. In other words, which barriers
evolve first? Prezygotic isolation is often cited as generally being more
important, firstly because there are many examples where species can produce
viable hybrids but generally do not interbreed, and secondly because an
organism must anyway first mate outside its own population before any postzygotic
effects come into play. However, this doesn't account for the fact that
sometimes the first step in speciation is reduced fitness of hybrids in the
context of differential adaptation, i.e. postzygotic isolation.
Such uncertainty is caused by the tricky nature of the task of
figuring out which barriers are involved, and to what extent. For example,
measuring behavioral reproductive isolation between populations seems simple:
How often do members of different populations mate? However, the fact that mating events between members of separate populations
are generally rare and field observations cannot be made continuously means
that such events are easily overlooked. Furthermore, lab experiments are
hindered by the fact that experimental design can have a huge impact on their
outcome, and that for most species we don't know which experimental conditions
are most appropriate.
The theme that I find most interesting in this paper is that on
connecting speciation to patterns of biodiversity. For example, question 12
asks what factors contribute to variation in speciation rate (i.e. how quickly
clades diversify). One of the biggest problems in answering this question
relates to the fact that ecological and genetic factors can influence both
speciation rate and extinction rate, both of which affect our current
'snapshot' view of biodiversity. This has been a central theme in the recent
debate surrounding the origins of the latitudinal diversity gradient.
The authors also ask what we no longer need to know about
speciation. In other words, what questions can we safely leave behind? Among
the three arguments that they identify is that surrounding the geographic basis
for speciation. They argue that the traditional distinction between allopatric,
parapatric and sympatric speciation processes is now redundant. For example, rather
than focusing on whether speciation can take place in sympatry, we are advised
to rather focus on whether it can occur in the face of gene flow. As the
authors say: 'The criteria for ‘proving’ sympatric speciation can be made so exacting that an unambiguous case is almost impossible' (p. 35), and
that rather than viewing spatial context as central to the speciation process,
it should be viewed as merely a single factor influencing it. I agree entirely
with these sentiments. Perhaps in undergraduate courses it should be made
clearer that while this argument has in the past been useful in studying
speciation, it is no longer so.
The title of this blog post borrows from one of the most famous
titles in any field. Yet there is a contrast here. Where the title of Darwin's
seminal work alluded to its implications for our understanding of how new
species arise, its focus, and that of many subsequent decades of evolutionary
research, was on the nature of change within species rather than their origin.
Only in the past 25 years has the study of speciation really taken off (Santini
et al. 2012). In keeping with this philosophy, the title of this blog post is
also somewhat out of sync with its focus. Understanding the nature of the study
of speciation is vitally important for those who wish to engage in it. Yet its
origins have been crucial in shaping its current state. Put simply, there is
still much we need to know, and plenty of lost time to make up for.
Key Reference
The Marie Curie Speciation Network. 2012. What do we need to know about speciation? Trends in Ecology and Evolution. 27 (1): 27-39.
Further Reading
An informative online discussion forum on this paper can be found here.
Santini F, Miglietta MP and Faucci A. 2012. Speciation: Where Are We Now? An Introduction to a Special Issue on Speciation. Evolutionary Biology. 39: 141-147.
