Understanding Evolution: An Introduction to Populations and Speciation
One of the challenges for discussing evolution within evangelical Christian circles is that there is widespread confusion about how evolution actually works. In this (intermittent) series, I discuss aspects of evolution that are commonly misunderstood in the Christian community. In this first post, we examine how speciation is something that happens to populations over many generations, and discuss how this informs our understanding of human speciation.
One abiding misconception of evolution among Christians (and even for students of biology) is that the production of new species is a sudden event, or one that begins through a single breeding pair (for sexually-reproducing organisms). In reality, new species arise through incremental changes to populations, not individuals.
Speciation is a population-level phenomenon
Imagine a child’s flip-book, where the image on each page is slightly modified from the one preceding it. Let each page in the book represent one generation for a population of organisms (NOT individuals). Each page in the book is very similar to pages on either side of it: indeed, if you placed adjacent pages side-by-side you would need to study them carefully to find the tiny differences. Pages widely separated in the book would have more obvious differences. So too for populations over time. Each generation, as a population, is slightly different at the genetic level than the one preceding it. These changes need not be new mutations, though each generation does introduce a small number of new mutations into a population. It may be that certain variants of genes (called alleles) have increased or decreased in frequency in the population, or that genetic recombination during sexual reproduction has generated new combinations of previously existing alleles. These subtle differences may slightly change the average characteristics of the population, just like turning a page in the flip-book.
The fact that the average characteristics of a population can slowly change over time creates an interesting possibility: if two separated populations of the same organism accumulate enough differences over a large number of generations, then the resulting populations may become different enough to prevent interbreeding even if the opportunity later arises. All that is needed to start this process is some mechanism to reduce or eliminate the exchange of genetic material between the populations (so-called “gene flow”). This blockage is needed, or the average genetic makeup of the two populations will remain the same. Only when a genetic barrier is in place can genetic differences begin to build up between the two groups. There are different mechanisms by which this occurs in nature, some of which we will examine below.
Separation by geography (allopatric speciation)
A simple way to block gene flow is to have what was once a single population become physically separated into two populations. There are many ways this can take place: a fire or a landslide could break a habitat into two areas, a few members of a population could be dispersed to a new location far away from their founding population, and so on. Note that separation does not guarantee that speciation will occur – the two populations may reconnect soon enough that nascent genetic differences are again averaged out, or the two populations may track each other in parallel closely enough (either through chance, low genetic variability to start with, or due to similar selection pressures) such that no changes capable of blocking interbreeding arise before the populations come together again. What separation does achieve is the possibility of speciation. Now that the two populations are genetically isolated from each other, differences can begin to slowly build up. To return to our flip-book analogy, these two populations start from the same page, but then go their separate ways, with their average characteristics shifting slightly, but now independently from each other. Given enough time, the differences that build up may affect characteristics relevant to reproduction (i.e. genetic exchange): traits used for courtship, competition for mates, or behaviors suited for rearing young in a specific environment. These changes now begin to “genetically enforce” the separation of the two groups, even if they come into geographic contact again and some interbreeding does happen. Once these barriers are in place, the two groups no longer interbreed as successfully between groups as they do within their own group, and the groups are recognized as closely-related species. This process is called allopatric speciation (in ancient greek, allo = “different”, patrida = “fatherland”) indicating that physical separation was the initial barrier between members of the same population.
Separation by niche (sympatric speciation)
While allopatric speciation is relatively easy to envision, sometimes species arise without ever being geographically separated. If speciation occurs without separation, it is said to be a case of sympatric speciation (in ancient greek, sym = “same”). The availability of a new ecological niche can lead to sympatric speciation, as different subsets of a starting population exploit different environments within their geographic range. This process is not as abrupt as geographical separation, and it may take longer for substantial differences to arise that impede cross-breeding between the two sub-populations. Allopatric and sympatric speciation are in fact extremes on a continuum from no geographical overlap (allopatric) and complete geographical overlap (sympatric). In real life, a speciation event may lie somewhere in between.
Beyond the textbook
One real-world example of speciation that we have touched on before that exemplifies these complexities is Dolph Schluter’s work on the evolution of British Columbia’s freshwater sticklebacks: small lake fish descended from sea-dwelling ancestors. What we observe is lakes with stickleback species pairs, both descended from the same sea-dwelling ancestor. These same-lake species pairs were once thought to be classical examples of sympatric speciation, but further research has confirmed a more complicated picture, where two independent colonization events by the marine form occurred. The first colonization is a textbook example of allopatric speciation, where the marine form adapts to a new environment. The later, second colonization events introduce the marine form into lakes that already have the first species in a specific niche. The new invaders manage to persist in a different niche, one more similar to the one used by the marine form. Additionally, there is evidence of interbreeding between some species pairs, and in one lake the two forms have collapsed back into one species as a result of new environmental pressures. The lake forms are distinct from the ancestral marine form, and these species have been shaped by allopatric and sympatric factors. Some of the genes (and their various alleles) involved in the major differences between these species pairs have been identified. These genetic variations are present in the marine species, but in the lake species certain alleles have been nearly lost from the one species, and kept in the other, enforcing the physical and lifestyle differences between the two. These differences, in turn, raise a barrier to reproduction that largely keeps the species separate, though interbreeding remains possible, if unlikely.
The relevance of this discussion for human speciation may not be immediately obvious, but hopefully a few points are now more accessible. The first point is that speciation happens at the level of a population, as its average characteristics change over time relative to other populations from which it is reproductively isolated. There are other tools and techniques that geneticists can use to estimate how large a population was as it went through a speciation event, and we now know that our species has never been below around 10,000 individuals at any point in our evolutionary history. Some of those tools use genomes of our close relatives (e.g. chimpanzees) as a basis for comparison, while others look solely at genetic variation within our species, and thus do not require any “evolutionary” assumptions. These methods, though based on distinct methodologies with independent assumptions, nevertheless give the same answer for our population size over the last several million years. Darrel Falk and I have written about this evidence here on BioLogos in the past.
A second point is that this process proceeds through incremental steps. Like those pages in a flip book, each generation is nearly indistinguishable from the one preceding it, and characteristics change as averages, not as sudden shifts. Over time, these changes can build up and raise reproductive barriers between populations, producing new species. This suggests that many of the features we recognize as distinctly human arose incrementally. For some evangelicals, this understanding can be troubling if a more discrete event of “becoming human” was expected, or if they hold the view that natural explanations remove the need for God’s activity. One of the challenges for believers who accept the evidence for human evolution is mapping characteristics of theological importance, such as the declaration that humans are created in the image of God, onto this understanding of our biological history. This is an ongoing discussion, and one that has, and should, generate much discussion.
A third point is that as we became human, the population that eventually became Homo sapiens did not suddenly cease to interbreed with other groups. Like sticklebacks, our evolutionary path appears to be a mix of allopatric and sympatric speciation, with complex patterns of geographical migration and interbreeding with other hominid groups. We will have more to say about this in upcoming posts.