The Science of Evolution

Part 4b: More on Reproductive Isolation, Speciation, and the Emergence of Evolutionary Novelties

By Ardea Skybreak

Revolutionary Worker #1164, August 25, 2002, posted at http://rwor.org

As we discussed in the last installment of this series ("The Science of Evolution, Part 4a: How Evolution Produces Whole New Species," RW #1163), divergence between two reproductively isolated populations which had once belonged to the same species is one of the main ways new species can come into being. There is evidence to suggest that new species are especially likely to emerge out of especially small pockets of individuals which in one or another way have become reproductively cut off from the original population and which then begin to accumulate evolutionary changes at a disproportionately rapid rate (and with disproportionately significant effects),as compared to the kind of more routine ongoing small-scale evolutionary changes which continue to take place in the larger original population. In such cases, the original ancestral species may simply continue to exist, in the usual environment and in a relatively unchanged state, while the new species emerging out of the small "founder population" may fairly quickly spread out, often occupying and interacting in new ways with a somewhat different local environment than the original species.

How can two populations become reproductively isolated from each other in the first place? There are actually many different ways this can happen. Often the initial reproductive separation can be caused by a physical geographic barrier,as can happen when two populations remain separated for some time by sandbars, canyons (as in the case of the diverging species of Kaibab and Abert squirrels discussed in an earlier installment), bodies of water, mountain ranges, and the like. But sometimes, especially when two related populations have already remained geographically separate for some time--and have accumulated enough changes to at least begin the process of diverging into two different species--the two populations will continue to remain effectively "reproductively isolated" even if they once again come to occupy the same local area. This might be because they have already accumulated sufficient genetic differences that they no longer reproduce at the same time (for instance, when two populations of trees, even in the same habitat, tend to flower at slightly different times, or when two populations of closely related animals no longer mate in quite the same season), or because they have adapted to occupying somewhat different "niches" (sub-sets of environmental variables) and to using somewhat different means of exploiting the resources of the habitats they occupy even in the very same local area (as when closely related species live and feed at ground level or in the canopy level [tree-tops] of a rain forest). In the case of many mammal and bird species, sufficient reproductive isolation for full speciation to take place often seems to occur when different populations have accumulated significant behavioral differences in genetically encoded mating rituals or other forms of communication : in such cases, individuals from two populations no longer recognize each other as potential mates even if they come across each other in the same general habitat, and therefore they don't even attempt to mate.¹

I want to stress again the importance to the speciation process of the kinds of changes that can rapidly accumulate in especially small reproductively isolated populations. Many of the routine genetic reshufflings which occur in the bigger gene pool of a large ancestor population may have dampened (or limited) effects (slight back and forth quantitative increases or decreases in one or another developmental variable, for instance) and lead to little in the way of significant evolutionary modifications. Such a large original population may remain for long periods in a relatively unchanged state (in what is sometimes referred to as "relative stasis"), especially if the external environment is not changing to any significant degree, and if the process of evolution has had a relatively long period of time through which to "fine-tune" many of this population's interactions with its relatively stable environment. But in a really small and reproductively isolated off-shoot population--which by definition starts off with only a small portion of the total genetic variability available in the ancestral stock--even relatively minor genetic mutations and recombinations (as well as changes in total genetic variation caused by migration or simple genetic drift) may have disproportionately large effects on the overall genetic make-up of the small population.²

So the particularities of the gene pool of a particularly small population may facilitate the appearance and consolidation of genuine evolutionary novelties which might be much less likely to appear, or to become consolidated, in a larger ancestor population. This has been suggested in particular by the veteran evolutionary biologist Ernst Mayr, who has perhaps contributed more than anyone else to our understanding of speciation (including through the "budding off" from a much larger population of "founder populations," consisting of very small numbers of individuals). Mayr points out that people often mistakenly assume that new species arise only when one species splits into two completely new species, which are then assumed to both proceed to accumulate evolutionary changes at about the same rates. But this misses the fact that an ancestor species often continues to maintain itself in a given environment even after a "daughter" species has "budded off," and that the rate of accumulation of subsequent evolutionary modifications (as well as the qualitative impacts of any such changes) may be very different in the smaller reproductively isolated off-shoot population than in the larger populations of the original parent species.³

It is also important to realize that smaller off-shoot populations (which have become reproductively isolated from a parent population) often encounter different sets of environmental conditions than their parent populations. This is especially likely to happen if the off-shoot population has ended up in a very different locality, where it may encounter such things as different food sources or a different mix of competitors or predators. But even if the off-shoot population finds itself interacting with what appears to be a very similar environment as the parent population, it may do so in completely new ways,simply because changes in the underlying genetic variation of the population have brought forth some significantly different individual features. So, for these reasons as well, natural selection may more rapidly drive especially small reproductively isolated populations to diverge rapidly away from the parent population.

For some of the very same reasons discussed above, very small populations are also more likely to produce changes which actually increase their chances of extinction.The appearance of radically new features of anatomy, development, or behavior in a tiny population whose environment presents no dramatically altered "challenges" can easily make it impossible for individuals of that small population to continue to survive and produce descendants. In fact, many people think that small, reproductively isolated off-shoot populations are, on average, more likely than not to go extinct relatively quickly in most circumstances, rather than continuing to evolve in new directions-- especially if they are not encountering environmental changes in relation to which some quirky new features might actually constitute a brand new selective advantage.

It is also common in nature for reproductive isolation (a necessary condition for speciation) to get reversed.This can happen when a population which has been temporarily reproductively isolated from a parent population (perhaps by some temporary physical barrier) ends up rejoining the parent population before there has been a sufficient period of separation for full speciation to take place. In fact, this happens a lot in real life, which is one reason it is rare to get a chance to observe full speciation taking place from start to finish in the framework of a human lifetime.

Biologists are constantly expanding their understanding of the overall process of speciation and of the kinds of factors which contribute to making speciation more or less likely (and more or less frequent) in different periods, conditions and environments. So evolutionary biologists are naturally always debating such things as whether certain factors are, or are not, likely to affect the pacing and rhythm of evolutionary change in different plant and animal lines; or whether major evolutionary turning points in the history of life always come about only as a result of a very gradual accumulation of microevolutionary modifications, or whether large-scale evolutionary trends are more typically characterized (as has been suggested by Niles Eldredge and Stephen Jay Gould in their theory of "punctuated equilibria") by long periods of relative stasis (during which many species and evolutionary lines change mainly in minor ways), separated (or "punctuated") by shorter periods in which relatively more rapid modifications take place through concentrated "bursts" of speciation and overall diversification.

Evolutionary biologists today are working to bring together such things as elements of our understanding of speciation processes (including in especially small isolated populations) with a growing realization that random external factors (such as the sudden impact of a meteorite colliding with the earth 65 million years ago) may have had profound effects on the overall pace and rhythm of speciation and related processes throughout the history of life. Hypotheses are being proposed and experiments are being designed to try to get a better handle on just what mix of factors may encourage unusually high rates of speciation (as well as, relatedly, unusually high rates of extinctions) at different times and under different environmental conditions.

There is still much to learn about all this. But this doesn't mean, as the Creationists often try to claim, that "evolutionists can't even agree among themselves, so this must mean evolution is wrong." First of all, as a point of basic method, even if evolutionists were wrong (which is clearly not the case) this still wouldn't mean Creationists are necessarily right! More importantly, healthy debate among evolutionists about "cutting-edge" questions in the field does not mean the field itself is in disarray! The opposite is in fact the case. Evolutionary biology is currently an extremely dynamic field in science in large part because advances are being made on the basis of a very solid, comprehensive and well-proven foundational theory.Once again, there is total agreement among working evolutionary biologists the world over that new species have emerged (and continue to emerge) only as modifications of their immediately preceding ancestor species.This is a matter of established fact and general scientific consensus. It has been supported not only by the fossil record and many other types of indirect evidence but also by close direct observations of populations of living plants and animals, which have been observed and monitored while they were in the process of diverging and developing reproductive isolation (again, readers of this series may recall, as just one example, the case of the divergence of the Kaibab and Abert squirrels on opposite sides of the Grand Canyon).

Creationists like to argue that the fossil record does not provide evidence for evolution because it is "incomplete" and "full of gaps," and they claim it contains no "intermediate fossils" which show any step-wise evolutionary transitions between different life-forms. We will discuss these false accusations (and the flawed methodology of the Creationists) more fully in a later installment. For now, let me just say that some of their "accusations" are just outright lies and distortions. For instance, there are in fact quite a few clear sequences of fossils which include intermediate forms! Some examples of this are: the well-known fossils of Archaeopteryx (which is anatomically intermediate between reptiles and birds and which has characteristics of both, such as reptilian teeth and bird-like feathers); the well-preserved fossil sequences which show that the ancestors of the first whales were four-legged terrestrial mammals which evolved into marine mammals through a well-preserved series of intermediate steps; or, ironically (since the Creationists are particularly concerned to present human beings as the result of god's special Creation and not descended from any other species), the fossil record of our own evolution, which is particularly rich in "transitional" fossils, linking our more ape-like and small-brained ancestors of a few million years ago to our modern human species through a variety of obviously intermediate steps. (We will discuss human evolution more in a future installment of this series.)

As for the claim that the fossil record is incomplete: of course it is "incomplete"! For one thing, only a small percentage of plants and animals ever get preserved as fossils in the first place. But that doesn't mean that there aren't plenty enough fossils (and more are found every day) for the vast majority of scientists to feel extremely confident about the basic time progressions and ancestor-descendant sequences linking all the various evolutionary lineages. And, as for those "gaps" between species: yes, there are some gaps between species in the fossil record, but this doesn't represent a "problem" for the theory of evolution as the Creationists would like to make people believe. The Creationists have a reactionary social and political agenda, based in religious fundamentalism, that they are seeking to impose on people, so they often misrepresent what things like gaps in the fossil record actually mean (and the facts be damned!). Many critics of Creationist methods have pointed out that every time someone finds a new intermediate fossil which actually fills a gap between two fossil species, the Creationists act as if all this means is that there are now two additional new gaps in the fossil record, one on either side of the new fossil species! (At this rate they'll keep complaining about "gaps" forever, no matter how many gap-filling fossils are found!)

On the other hand, leaving aside the Creationist lunacy, it is a fact that the theory of evolution actually predicts that there would likely continue to be significant "gaps" in series of fossils, because of the conditions under which new species are thought to emerge in the first place. Paleontologists and other fossil collectors have often noticed that fossils of new species never before encountered often seem to appear rather "suddenly" in a geological layer. But this is what you would expect to find if new species do in fact evolve out of relatively small numbers of individuals which have somehow become reproductively isolated from a larger population (as seems often to be the case in living species). If this is how new species typically evolve, we should expect the fossil record to appear to exhibit many "gaps" and discontinuities, not just because of the obvious fact that most individual organisms decay completely and never get preserved as fossils in the first place, but also because many (and perhaps most) initial speciation events of past periods likely occurred in small pockets of only a handful of isolated individuals.This in itself would make it highly unlikely that any of the "original" members of a new species would ever be found by fossil collectors.

And again, if emerging species manage to avoid going extinct almost as soon as they emerge, they are likely to interact with their environments in some brand new ways that are unique to them, and this in turn may allow them to expand rapidly, radiate out in new directions, and even to rapidly produce additional substantial evolutionary modifications (and further diversify into more species) in the process of adapting to new environmental challenges. When we look at the fossil record, we can't see the exact moment in time at which a new lineage first diverged from an ancestor lineage; what we more typically find is evidence of what looks like a relatively sudden appearance of large numbers of new organisms never previously encountered, but whose preserved forms nevertheless show clear signs of being related to the members of an earlier line. This is what you would expect to find if speciation events typically start off in small isolated populations containing only relatively small numbers of individuals, and if it then takes time for populations of a new species to build up their numbers and perhaps "settle down," so to speak, as a distinct species (maintaining themselves for an extended period of time) in relation to their new environments. It is reasonable to expect that we would start finding their distinct fossils only after such a process has had a chance to unfold for some time.

Given this, it is all the more amazing that the fossil record is actually as good as it is!

There do seem to have been times and environmental conditions in the past which were particularly favorable for lots of speciations and a great deal of overall species diversification to take place, even in relatively brief and concentrated periods of time. Other periods and environmental conditions seem to have been more characterized by a sort of general maintenance of the status quo in many species and lineages: but even then, populations would have been evolving in more minor ways (through ongoing natural selection and related phenomena) in a constant dynamic interaction with their local environments. How much or how little speciation takes place (within a particular lineage of plant or animal species, and in general among all lineages characterizing the plant and animal life of a particular period in earth's history) seems to vary in relation to factors and conditions that are not yet very well understood. As yet, no one quite understands why some entire lineages remain largely unchanged for millions of years while other lineages change frequently and rapidly. Or exactly why (as can be deduced from the relative diversity of fossils found in different rock layers), some geological eras were characterized by tremendous "explosions" of species diversification, while others were not.

Fossil ancestors of modern sea turtles can be found in rock layers dating back 100 million years (before the time of the dinosaurs) but, anatomically at least, they seem almost identical to the sea-turtles which swim in today's oceans. How can they have changed so little in 100 million years? The same question can be posed about other lineages of so-called "living fossils," including the horseshoe crabs and ginkgo trees, which also seem to have hardly changed in hundreds of millions of years. On the other hand, there is also the opposite situation: there are currently about 400 distinct species of cichlids (a kind of fish) in just one Lake in Africa (Lake Victoria). They are all related to each other and are obviously descended from a single ancestor species. The truly amazing thing is that we know from the geological record that the lake was dry 12,000 years ago, so all this incredible amount of evolutionary diversification from a common ancestor-- all these repeated speciations--had to have taken place in no more than 12,000 years, which is like a blink of the eye on the geological time scale! So, on the one hand, you have some sea turtles that barely changed at all in 100 million years; and, on the other hand, you have some fish which splintered off into hundreds of different species in only 12,000 years!

Exactly why some lineages of plants or animals have tended to evolve and diversify more frequently and dramatically than others over the course of history is one of the more interesting questions being explored by modern evolutionary biologists. As we discussed earlier, it may be that some species and whole lineages are more rigidly "constrained" (restricted) genetically and developmentally by past evolutionary modifications than others; if so, they may be more likely to maintain themselves pretty much "as is" (or simply go extinct) than to accumulate really significant modifications over time. This could explain some of the features of so-called "living fossils." But it is also important to remember that populations of living organisms don't evolve in a vacuum: they exist in a constant state of dynamic interaction with features of their particular local habitats and environments, which include not only physical features like climate and terrain but also "biotic" (living) features, such as the other living plants and animals they may be encountering as competitors or as predators. Many evolutionary biologists suspect that the rhythm and pacing of overall larger-scale evolutionary trends (including the rate at which new speciation events take place) may depend to a great extent on how often and to what extent evolving populations may be encountering significantly new environmental "challenges" and habitat "disruptions."

We know from direct experience that, in living species, changing environmental conditions can exert strong new selection pressures on populations of organisms, both on a large scale (for example, if there is a change in global climate, or an entire ecosystem is being disrupted) and on a small scale (as when an ecosystem remains relatively stable but a change in just one environmental variable--such as the introduction into the area of a new predator species--exerts a powerful new selective pressure on populations of even just one species in a local area). It seems pretty clear that some lines are driven to rapidly evolve repeated modifications in relation to repeated changes in external environmental pressures. This is often seen in the back-and-forth co-evolution of a predator and a prey species, sometimes referred to as a biological "arms race"--over time, the predator species keeps evolving new ways of going after its prey, and the prey species keeps evolving new ways of escaping its predator. It also seems clear from living species that evolutionary modifications can definitely affect the way a species interacts with its environment, including by enabling species to tap into previously unexploited resources and previously unoccupied habitats.

It is a fact, however, that not all new environmental challenges or opportunities can necessarily be "met" (so to speak) by an evolving species. It really depends on what kind of genetic variation in inheritable characteristics happens to be available within a species at a given time. There are never any guarantees that reproductively "advantageous" features will emerge among individuals in a population in the first place, regardless of how strongly natural selection might favor such features if they happened to emerge.

As discussed in the last installment, natural selection itself involves an interplay of both random and non-random factors: first of all, random ("chance") factors such as mutations, recombinations and genetic drift, lead to a continual re-shuffling of the deck of cards which makes up the total genetic variation in a population and which serves as the raw material of evolutionary change; but then, on the basis of that randomly produced variation, natural selection proceeds to very much non- randomly sort out some of the resulting features of organisms in relation to the demands and challenges of a given external environment.

But, again, there is never any guarantee that a species will evolve a particular feature which might allow it to meet a new environmental pressure or challenge. Some degree of evolutionary change is always going on in all living populations, but the options for which specific modifications can take place at any given time are severely limited, both by chance events and by the constraining effects of past evolutionary modifications. Sometimes a population is able to evolve and keep pace with transformations of the outside world, and sometimes it is not able to do so. Small-scale extinctions of individual species (also known as "background extinctions"), as well the phenomenon of entire plant or animal evolutionary lines running into evolutionary "dead-ends," are as much part and parcel of the history of life as the birth of new species and lineages.

As discussed previously in this series, the fossil record reveals that, so far in the history of the planet, there have been at least five waves of "mass extinctions" of a large proportion of all the living plant and animal species. The most famous of these mass extinctions occurred about 65 million years ago and seems to have been caused by the impact of a large meteorite which smashed into the planet at a speed of hundreds of thousands of miles per hour. The large impact crater which resulted from this collision can still be seen today on the Yucatan Peninsula of Mexico, and bands of the mineral iridium (which is commonly found in meteorites) can be found in 65-million-year-old rock layers all around the planet.

It is estimated that this powerful meteorite impact would have sent up such huge amounts of dirt and debris into the atmosphere that it likely would have blocked sunlight and dropped temperatures all around the globe to such an extent that much of the sunshine-dependent plant life would have been rapidly killed off. This in turn would have led, in a chain-reaction, to the deaths of most of the animals dependent on plants for food and many of their predators. In fact, it can be seen in the fossil record that all the plant- eating dinosaurs and their meat-eating predators became extinct 65 million years ago, most likely in relation to this catastrophic event.

It is very unlikely that any dinosaur species could have evolved new features rapidly enough to allow them to avoid extinction in the face of such a devastating and sudden event. But for those animal species that did happen to survive (including the ancestors of modern-day mammals which, 65 million years ago, were just small shrew or mouse-like creatures, who may have escaped sudden extinction because they were nocturnal and lived in burrows), the elimination of many of the ecologically dominant species of the era (including the dinosaurs) may well have created an environmental "opening" and opportunity into which any surviving species might be able to rapidly expand and further diversify. In fact, there are clear indications that the geological period following the mass extinction event of 65 million years ago saw a tremendous burst in diversification of mammal species.

In most cases, speciations take too long (too many generations) for human beings to witness the full process from beginning to end. But there are some cases (especially in plants and in fast- reproducing species) where it has been possible to directly observe speciation as it was happening.

For instance, many years ago the great geneticist Theodosius Dobzhansky experimentally demonstrated how speciation can occur in fruit flies: in one of many lab experiments, he took one species of fruit fly and divided it into two separate populations and then proceeded to artificially "select" for different characteristics (like the number of bristles on their legs) in the two populations over successive generations. After 20 or so generations, he put the two populations of flies back together in the same place but found that the individuals of the two populations could no longer interbreed--they had accumulated enough underlying genetic differences (through Dobzhansky's manipulations "favoring" just one or two apparently superficial features) that the flies in the two different populations could no longer mate with each other--they had effectively become different species!

Another experiment involved taking a wild, natural population of fruit flies collected in a tropical forest and simply dividing it up into two equal groups, which were then kept and allowed to reproduce in separate tanks for a period of five years without interfering with them in any way: at the end of that time (plenty of time for fruit flies in each group to separately produce a great many generations!), the two populations were reunited--but here again it was found that they had lost their ability to interbreed. The genetic differences which had been accumulating separately for five years had rendered the two populations reproductively incompatible;observations of their anatomical differences, changed behaviors, and differences in their underlying DNA revealed how much they had diverged - they really had become two separate species.

Why then aren't we always finding brand new species of plants and animals emerging in the natural world everywhere we look? There are a number of reasons for this. One of the most obvious reasons is that most species just don't produce series of new generations as fast as things like fruit flies or bacteria, and it takes many generations for enough distinct genetic variation to accumulate in a population to even have a chance of turning it into a distinct species. People can observe many fruit fly generations over a few months or years, but think about how much time it would take (relative to an average human lifespan) to observe changes taking place over 20 or 30 generations in most mammal species, for instance! Most major evolutionary changes take place over at least thousands of generations, so people living today are much more likely to witness the total extinction of a species (which we unfortunately witness all too frequently these days) than the emergence of a brand new species which has never before existed.

Also, in the real world it is not so easy for conditions of absolute reproductive isolation to be maintained between populations of a species for a long enough period of time for full speciation to take place. Populations in nature which have been kept apart for a while by some physical barrier (a dry channel, a sandbar, a logging swath cut through a forest, and so forth) often recombine when the physical barrier breaks down-- a blocked channel between two lakes may reopen, for instance, allowing previously separated fish populations of the same species to once again interbreed. This has the effect of interrupting the speciation process which had only just begun. Keep in mind also that it is not enough for new features to "emerge" in a relatively reproductively isolated population -- evolutionary "novelties" may well emerge in such a population, and may even confer some reproductive advantages on the individuals which have these novel features, but even that is not a guarantee that the change will get preserved and consolidated in the population as a whole --lots of "accidental" processes can intervene to prevent this "consolidation" of novelty, such as the sudden introduction into an area of a new predator or competitor species; the impact of a factor leading to dramatic climate changes; the effects on a local physical environment of such things as wildfires or floods, etc. Any such factors can easily lead to particular individuals or even entire populations getting wiped out long before any advantageous novelty has had a chance to spread to enough descendants to make it likely that it will be preserved. Especially in the case of a really small population, it can be a real crapshoot whether it hangs in there long enough for any potentially unique evolutionary innovations to get an evolutionary toe-hold.

Despite factors which can prevent the speciation process from getting completed in any given instance, or cause a newly emerged species to peter out, full speciation has nevertheless occurred innumerable times over the past 3 and a half billion years and it is an ongoing process. It is what explains the truly amazing diversity of past and present life-forms on this planet--a diversity which is all the more wonderful in that it has flowered and unfolded on its own, without the involvement of any imaginary outside "engineer" or super-natural "designer."