The genetics of speciation
Given the strong influence of genetic identity on the process and outcomes of the speciation process, it seems a natural connection to use genetic information to study speciation and species identities. There is a plethora of genetics-based tools we can use to investigate how speciation occurs (both the evolutionary processes and the external influences that drive it). One clear way to test whether two populations of a particular species are actually two different species is to investigate genes related to reproductive isolation: if the genetic differences demonstrate reproductive incompatibilities across the two populations, then there is strong evidence that they are separate species (at least under the Biological Species Concept; see Part One for why!). But this type of analysis requires several tools: 1) knowledge of the specific genes related to reproduction (e.g. formation of sperm and eggs, genital morphology, etc.), 2) the complete and annotated genome of the species (to be able to find and analyse the right genes properly) and 3) a good amount of data for the populations in question. As you can imagine, for people working on non-model species (i.e. ones that haven’t had the same history and detail of research as, say, humans and mice), this can be problematic. So, instead, we can use other genetic information to investigate and suggest patterns and processes related to the formation of new species.
Is reproductive isolation naturally selected for or just a consequence?
A fundamental aspect of studies of speciation is a “chicken or the egg”-type paradigm: does natural selection directly select for rapid reproductive isolation, preventing interbreeding; or as a secondary consequence of general adaptive differences, over a long history of evolution? This might be a confusing distinction, so we’ll dive into it a little more.
Of the two proposed models of speciation, the by-product of natural selection (the second model) has been the more favoured. Simply put, this expands on Darwin’s theory of evolution that describes two populations of a single species evolving independently of one another. As these become more and more different, both in physical (‘phenotype’) and genetic (‘genotype’) characteristics, there comes a turning point where they are so different that an individual from one population could not reasonably breed with an individual from the other to form a fertile offspring. This could be due to genetic incompatibilities (such as different chromosome numbers), physiological differences (such as changes in genital morphology), or behavioural conflicts (such as solitary vs. group living).
Certainly, this process makes sense, although it is debatable how fast reproductive isolation would occur in a given species (or whether it is predictable just based on the level of differentiation between two populations). Another model suggests that reproductive isolation actually might arise very quickly if natural selection favours maintaining particular combinations of traits together. This can happen if hybrids between two populations are not particularly well adapted (fit), causing natural selection to favour populations to breed within each group rather than across groups (leading to reproductive isolation). Typically, this is referred to as ‘reinforcement’ and predominantly involves isolating mechanisms that prevent individuals across populations from breeding in the first place (since this would be wasted energy and resources producing unfit offspring). The main difference between these two models is the sequence of events: do populations ecologically diverge, and because of that then become reproductively isolated, or do populations selectively breed (enforcing reproductive isolation) and thus then evolve independently?
Reproductive isolation through DMIs
The reproductive incompatibility of two populations (thus making them species) is often intrinsically linked to the genetic make-up of those two species. Some conflicts in the genetics of Population 1 and Population 2 may mean that a hybrid having half Population 1 genes and half Population 2 genes will have serious fitness problems (such as sterility or developmental problems). Dramatic genetic differences, particularly a difference in the number of chromosomes between the two sources, is a significant component of reproductive isolation and is usually to blame for sterile hybrids such as ligers, zorse and mules.
However, subtler genetic differences can also have a strong effect: for example, the unique combination of Population 1 and Population 2 genes within a hybrid might interact with one another negatively and cause serious detrimental effects. These are referred to as “Dobzhansky-Müller Incompatibilities” (DMIs) and are expected to accumulate as the two populations become more genetically differentiated from one another. This can be a little complicated to imagine (and is based upon mathematical models), but the basis of the concept is that some combinations of gene variants have never, over evolutionary history, been tested together as the two populations diverge. Hybridisation of these two populations suddenly makes brand new combinations of genes, some of which may be have profound physiological impacts (including on reproduction).
How can we look at speciation in action?
We can study the process of speciation in the natural world without focussing on the ‘reproductive isolation’ element of species identity as well. For many species, we are unlikely to have the detail (such as an annotated genome and known functions of genes related to reproduction) required to study speciation at this level in any case. Instead, we might choose to focus on the different factors that are currently influencing the process of speciation, such as how the environmental, demographic or adaptive contexts of populations plays a role in the formation of new species. Many of these questions fall within the domain of phylogeography; particularly, how the historical environment has shaped the diversity of populations and species today.
A variety of different analytical techniques can be used to build a picture of the speciation process for closely related or incipient species. A good starting point for any speciation study is to look at how the different study populations are adapting; is there evidence that natural selection is pushing these populations towards different genotypes or ecological niches? If so, then this might be a precursor for speciation, and we can build on this inference with other complementary analyses.
For example, estimating divergence times between populations can help us suggest whether there has been sufficient time for speciation to occur (although this isn’t always clear cut). Additionally, we could estimate the levels of genetic hybridisation (‘introgression’) between two populations to suggest whether they are reasonably isolated and divergent enough to be considered functional species.
The future of speciation genomics
Although these can help answer some questions related to speciation, new tools are constantly needed to provide a clearer picture of the process. Understanding how and why new species are formed is a critical aspect of understanding the world’s biodiversity. How can we predict if a population will speciate at some point? What environmental factors are most important for driving the formation of new species? How stable are species identities, really? These questions (and many more) remain elusive for a wide variety of life on Earth.