Allopatric speciation & sympatric speciation examine mating barriers and geographic location; alloneossic taxonation adds in adaptation and niche shift.
I am pre-publishing this sequence of essays here and in social media to elicit comments and other feedback. They will form the framework for my next book examining evolution, innovation, and progress in biology, human creativity, and machine learning: Darwin, Dada, Dalí, Duke, & Devadevàya. Please feel free to comment below, or contact me.
New Species and Reproductive Potential
In previous posts, we discussed the New Synthesis, the Extended Evolutionary Synthesis, and the critical contributions both have made to our understanding of biology. We also pointed out that in between those two important advances, the discovery/recognition of punctuated equilibria opened up yet more puzzles and possibilities for exploring the evolution of life.
One highly productive area of research has focused on the formation of new species. In modern biology, a species is defined by reproductive potential: if two living things can produce viable, fertile offspring, they belong to the same species.This is known as Mayr’s Biological Species Concept, after the noted biologist Ernst Mayr. If they cannot, they belong to different species.
Researchers have used that definition to explore how species are generated. The inability of two closely-related populations to mate is often considered the first step in creating a new species.
Allopatric Speciation, Sympatric Speciation
The thinking about speciation breaks this breeding barrier into two paths. The first assumes that mating barriers appear when a population is divided by a geographic barrier. This is known as allopatric speciation, because a population is isolated by splitting part of it into some ‘other country’: allo + patria. In this case, the two populations cannot mate because they don’t physically interact. With time, non-geologic barriers to mating will appear to reinforce the separation, including different recognition and attraction factors; different mating times; genetic incompatibilities, and others. If the geographic barrier is subsequently removed, the two species remain distinct.
The second approach assumes that the reproduction barriers emerge within a single geographic area. This is named sympatric speciation, as it takes place in the ‘same country’ as the parent stock. In this case, it is postulated that the barriers that appeared secondarily with allopatric speciation, appear first, without the two populations needing to be physically separated.
Speciation and Taxonation
I previously noted that definitions are essential for clarifying our thinking, but like all tools, definitions have strengths and weaknesses. One downside of definitions is that the limits they describe can become constrictions. If we don’t step back from time to time and consider how a concept fits the larger picture, we can miss opportunities for further exploration.
I suspect that this is the case with our definitions of speciation. Because as I commented previously, while a mating barrier clearly separates the horse from the amoeba, it also leaves out many other very interesting things. In addition, the disruptive nature of punctuated equilibria gives us an opportunity to go beyond the origin of species, to begin investigating the origins of higher taxa, i.e., the problem of taxonation.
Adaptation by Natural Selection
To look at the shortfall of our current definition here, recall that our fundamental mechanism for understanding evolution is natural selection. The complete phrase, however, is adaptation by natural selection. Adaptation gives a fuller picture of the difference between the horse and the amoeba: they have adapted differently to the problems of life. So a helpful way of considering the differences among taxa is that they have evolved to adapt to different niches: they use different tools and strategies to ply different trades. This allow them to solve the challenges of life in very different ways.
Consider the many known examples of allopatric speciation. In virtually all of them, no dramatic niche differentiation of the two species has occurred. They are reproductively isolated, but other than minor adjustments to differences to local ecosystems, small changes in food sources and nesting, for the most part they occupy very similar niches. In fact, for reasons we will cover in a moment, if allopatric speciation had moved part of a population into a different geographic area, and into a different niche as well, we should expect to end up with three species, or just one species, rather than two.
The New Synthesis transmuted evolution from theory into an experimental field built on hard data. The work of the early biological pioneers elevated biology to peerage with the other great sciences. The research that allowed these advances, including our current concepts of speciation, focused on Darwinian gradualism and small evolution— i.e., a shifting emphasis among previously existing alleles.
Punctuated equilibria expands those advances, and opens up considerations of higher taxonation. Evolutionary punctuations are very rare in the lineage of a species, and their extreme rarity means they are highly improbable. This, in turn, suggests they are not readily amenable to the traditional statistical approaches of small evolution, which focuses on the likely and the probable. Because of these, punctuations are not typically selections among existing alleles, but signal the arrival of something completely new. To generate truly new alleles—or neolleles—requires the great improbability and large evolution made possible by biological numbers.
The preceding show that large evolution, taxonation, and punctuated equilibria will often begin with an attempted niche shift. For an organism to attempt to move from an old niche for which it is adapted, into a new niche for which it is not, requires the addition of alleles that it currently does not have, and may not have had at any point in its lineage. We can assume this because, if the needed allele(s) were already contained in the gene pool of the ancestral species, the new species would long ago have had the necessary genes for invading the new niche, and it would have split off much earlier in the history of the lineage. The addition of new alleles which comprise more than a few point mutations, is a highly unlikely occurrence.
Another aspect that suggests niche shift requires highly improbable evolution, is the very long stasis that punctuated equilibria comprises. The long stasis again suggests that the alleles necessary for invading a new niche were a long time in coming: since living things are constantly reshuffling their genes, and mutating new genes, there should be a correlation between the length of time necessary to solve the problems of niche shift, and the improbability an evolutionary solution will appear.
The distinction between speciation and taxonation, and the role played by mechanisms of breeding barriers and niche shift, can be illustrated with our breeds of dog. All dogs, from the Chihuahua to the Great Dane, can potentially interbreed. At the same time, many of them have been artificially bred to occupy somewhat different niches. If some of our breeds were released into the wild, and there were sufficient prey to support them—rats for the rat terriers, badgers for the dachshunds, bears and boar for the Mastiffs, wolves for the wolfhounds—then we can envision a situation in which different packs, hunting different game, might still be able to interbreed; but the hybrids would typically be poor members of either pack. At some point, those packs that cease interbreeding, through any number of mechanisms, would produce more useful pack members. This example, of course, is small evolution, not large; but it nevertheless provides an example of how niche shift might precede other mechanisms for both speciation and taxonation.
As we noted above, however, pure allopatry does not move species into new niches; and we also noted that if it did, we should end up with either three species, or one species, rather than two. Consider two populations of a species that become physically isolated. If one of them begins moving into a new niche, we would not expect all members to abandon the original niche; they should not leave behind a perfectly good trade where there are resources available, and which the species is adapted to maximally exploit. So simply splitting a population geographically changes nothing in terms of niche shift. If, however, one of them does begin moving into a new niche, some of their companions will still remain in the old one, which brings us back to the situation of sympatric speciation. This is why allopatry with niche shift produces three species: one species on one side of a geographic barrier, and two species in the other.
On the other hand, if part of a population becomes geographically separated, and ends up in a situation where the old niche no longer exists, that will almost always lead to extinction. This was the importance of the previous essay, in explaining why evolutionary pressure can’t produce large evolution: pressure selects among existing alleles, but it doesn’t force the generation of neolleles. Innovation is produced through massive randomization; selective pressure may have a small impact on the rate of randomization, but it has no effect on the extraordinary improbability of success. If the old niche is no longer available, the overwhelming likelihood is that the isolated population will go extinct. In this case we remain with only one species.
The point is, taxonation should almost always begin with niche shift and adaptation.
Niche Shift & Alloneossic Taxonation
Finally, we need a term for these niche shifts. I will start with the same humorous criticism of allopatry that I used with microevolution and macroevolution, I began with a quote about television: “The word is half-Latin and half-Greek. No good came come of it.” In the same logic, allo is Greek, while patria is Latin. A better choice might have been ‘alterpatric’ speciation, or even ‘aliopatric’ speciation.
There is, however, the other odd word in the title of this post: alloneossic. Allo, as noted, is Greek for ‘other.’ We also need, however, a word corresponding to ‘niche.’ But that concept didn’t exist for the ancient Greeks. One possible etymology for ‘niche’ is from the Latin nidus, ‘nest.’ Working from there, an ancient Greek word for ‘nest’ is neossía, νεοσσία. Adaptation is often initiated when an organism attempts ‘another nest/niche,’ giving us alloneossic speciation and taxonation.
These insights provide new approaches to explain evolution and the diversity of life. In the next few essays we will cover a few more insights from punctuated equilibria and large evolution, and then use those to begin discussing human progress and other forms of innovation and progress.
For an ecological explanation of sexual dimorphism, click here.
Graphic edited from an old bookplate of fish species courtesy of Bibliodyssey and Paul K via Creative Common License 2.0.
|↑1||This is known as Mayr’s Biological Species Concept, after the noted biologist Ernst Mayr.|