Evolution of speciation ideas, from gradualism to punctuated equilibrium. Source: Wikimedia Commons; author: wooptoo.
Evolution of ideas on speciation
The beginning of Darwin’s title for his epochal book is On the Origin of Species….1 The Origin, of course, was the work that convinced the thinking world that life has evolved; and its title tended forever after to equate the term evolution with the “origin of species.” To Darwin:
- Species evolve through the development and further modifications of adaptations under the guidance of natural selection.
- For the most part, evolutionary change was a slow, steady and gradual affair.
- Species are temporary stages in the continuous evolution of life.
1930s and 1940s
New thinking on species developed in the 1930s and 1940s. Geneticist Theodosius Dobzhansky2 and systematist Ernst Mayr3 developed the idea that:
- Species are reproductive communities, with their members capable of interbreeding among themselves, and not, as the general rule, with members of other species.
- Evolution of new species centers on how changes occur in adaptations so that an ancestral species is split into two (occasionally more) descendant species, with interbreeding no longer possible between the members of what have evolved into descendant, or “daughter,” species.
In general, both biologists argued that physical, geographic isolation must be a precursor to speciation. In this, the notion of “allopatric speciation,” environmental change might be imagined to separate previously continuous species distributions. A seaway, for example, might develop between two formerly connected areas of land; conversely, land might emerge separating once connected oceans — as it happened 2.5 million years ago when the Isthmus of Panama was completed, and the connection between the Caribbean Sea and the Pacific Ocean was finally broken. Though biologists disagree on the extent of evolutionary change — and true speciation — among marine species on either side of the Isthmus, as we shall see below, the evolutionary effects of this environmental change were actually global in extent.
Thus we have two major connections drawn between environmental change and evolution by the time the centennial of Darwin’s book rolled around in 1959:
- Darwin’s image of natural selection tracking environmental change, thus modifying adaptations
- Dobzhansky- Mayr’s picture of speciation in geographically isolated regions which may reflect a result of environmental change as well
1960s and 1970s
Although Darwin’s perspective was being redefined by new discoveries in genetics in the 1960s and 1970s, geologically-trained paleontologists were discovering repeated patterns in the history of life, supporting the validity of Dobzhansky’s and Mayr’s insights of the previous decade. For example, Eldredge4 and Eldredge with Stephen J. Gould5 rediscovered the pattern of remarkable species stability (“stasis”) that was first discussed by paleontologists in Darwin’s time.
Towards a modern view Paleontologists now generally agree that stasis — where species may persist in recognizably the same form, with little or no accumulated change, for millions of years (5-10 million in marine species; somewhat shorter durations in the more volatile terrestrial environments) — is a common phenomenon. Nineteenth century evolutionists essentially ignored stasis, so contrary to the Darwinian perspective it did seem. But Eldredge and Gould, in their notion of “punctuated equilibria,” saw that stasis fits in well with the Dobzhansky-Mayr notion of speciation:
- species arise by a process of splitting
- this may happen relatively quickly (5-50,000 years, say) compared with the vastly longer periods of time in a species history
- it all occurs between a species’ origin via speciation and its eventual extinction.
But why such stability? What, in other words, causes stasis? Ecologists and evolutionary biologists have recently joined in the search for explanations of stasis. Currently, two general categories of explanation of the evolutionary phenomenon seem to be favored:
Instead of prompting adaptive change through natural selection, environmental change instead causes organisms to seek familiar habitats to which they are already adapted. In other words, “habitat tracking,” rather than “adaptation tracking” is the most expected biological reaction to environmental change — which is now understood to be inevitable. For example:
During the past 1.65 million years, there have been four major, and many minor, episodes of global cooling resulting in the southward surge of huge fields of glacial ice in both North America and Eurasia.
Yet, despite this rhythmically cyclical pattern of profound climate change, extinction and evolution throughout the Pleistocene was surprisingly negligible.
Instead, ecosystems (e.g., tundra, boreal forest, mixed hardwood forest, etc.) migrated south in front of the advancing glaciers.
Though there was much disruption, most plant species (through their seed propagules) and animal species were able to migrate, find “recognizable” habitat, and survive pretty much unchanged throughout the Pleistocene Epoch.
Botanist Margaret Davis6 and colleagues, and entomologist G. R. Coope7 have provided especially well-documented and graphic examples of habitat tracking as a source of survival of species throughout the Pleistocene.
Species also remain stable because of the very nature of their internal structural organization; all species are broken up into local populations that are integrated into local ecosystems. This means that:
A population of the American robin, Turdus migratorius, faces a very different existence in, say, the wet woodlands of the Adirondack Mountains in the Northeastern United States, compared to what the local populations of the same species experience in Santa Fe, New Mexico.
Such disjunct populations encounter very different food, water availability, ambient temperatures, potential predators, and possibly even disease vectors.
This of course implies that natural selection (as initially seen by Sewall Wright8,9) will act very differently on such disjunct populations.
Many species have extensive geographic ranges similar to the American robin; it is difficult to imagine how natural selection under such circumstances can “push” an entire species into a single evolutionary direction over a long expanse of geological time.
Rather, the semi-separate evolutionary histories of local populations imply that no net change will accrue species-wide through geological time.
The phenomenon of stasis — by now empirically documented as typical of most species of Metazoa and Plantae for at least the past half billion years — means that most adaptive evolutionary change actually occurs in conjunction with speciation. This is a rather surprising result on the face of it, and certainly not one anticipated by Dobzhansky, Mayr or other biologists who initially established the importance of species and speciation in the evolutionary process. For why should it be that the origin of species — new reproductive communities — should also entail, as a general rule, most adaptive evolutionary change in general? Yet that is what the fossil record of life’s evolution seems to tell us.
Current thinking on speciation
Light on these crucial evolutionary issues has been shed over the past twenty years. Key to the solution is the documentation, by paleontologists working up and down the geological record of the entire history of life, that evolution occurs in coordinated fashion in many different species lineages living in a regional ecological setting. For example:
- The original example of “punctuated equilibria” involved patterns of stasis and evolutionary change in trilobites of the Phacops rana species group.4
- These trilobites are just one of perhaps as many as 300 such species groups preserved in a 6 to 8 million-year long span of time beginning some 380 million years ago.
- They are found in Middle Devonian rocks that record the history of marine environments, species and ecosystems in all of Eastern and Central North America.
Traditionally, evolutionary biologists have focused on single evolutionary lineages. Though many other species (of brachiopods, mollusks, bryozoans, etc.) also seemed to be showing patterns of stasis, origination and extinction very similar to the trilobites I was studying, I deferred studies of all these very different species to the appropriate experts. This is the main reason why the important pattern of “coordinated stasis” escaped attention for so long: paleontologists by and large must stick to the groups with which they have developed professional expertise.
The term coordinated stasis refers to a pattern10
- where most of the species appear at roughly the same time
- species persist for millions of years, all more-or-less in stasis
- then, abruptly and again in lockstep fashion, a high percentage disappear in a category of ecological/evolutionary event that Elisabeth Vrba refers to as a “turnover pulse.”11
This pattern can be seen in Cambrian trilobites 500 million years ago, marine invertebrate faunas from the mid-Paleozoic through the Mesozoic and Cenozoic, dinosaur faunas of the Mesozoic and in mammalian faunas of the Cenozoic.
In other words, the phenomena associated with “punctuated equilibria” are regionally ecosystem-wide, and involve many different, unrelated species — species whose patterns of evolution, persistence, and extinction occur in near simultaneous fashion. This, perhaps the dominant signal in the evolutionary history of life, is thus profoundly “cross-genealogical” — meaning that such turnover events have causal roots that are deeply ecological — and arise, at base, from large-scale changes in the physical environment. Here, in other words, we finally understand how the physical environment, via ecological systems, impinges on the processes of speciation and extinction.
Here, briefly, are two examples that reveal the nature, and inner dynamic workings, of these ecological/evolutionary patterns:
Brett and Baird have documented some eight successive faunas of marine invertebrates in the Appalachian Basin of the Middle Paleozoic.10
Each fauna survives an average of 5-7 million years.
Ranging from only a few dozen known species to the 300 or more known from the Middle Devonian sequence mentioned above, most of the component species are present at the very beginning of the sequence.
Most persist unchanged throughout the sequence, but then, abruptly, most disappear.
Only, on average, 20% of the species manage to survive to the next successive faunal interval.
The new species that comprise the next succeeding marine regional system are either newly evolved or migrate in from adjacent regions.
Causes of the ecosystem collapse/extinction/new speciation events are incompletely understood, but apparently involve abrupt changes in sea level — most likely reflecting global cooling or warming events, which lower or raise sea level, respectively, by altering the size of the earth’s ice caps.
Vrba’s original example of a “turnover pulse” is based on events culminating at about 2.5 million years ago in Eastern and Central Africa.11
A global cooling event, beginning circa 2.8 million years ago, apparently caused a relatively abrupt reorganization of African ecosystems after about 300 thousand years.
Cooler and drier conditions brought about a radical change in African vegetation patterns, where large expanses of grasslands replaced the formerly dominant wet woodlands.
Ecologically generalized species, such as impalas, managed to survive unscathed, but many wet-woodland-adapted species (e.g., antelope) disappeared — either through habitat tracking or outright extinction.
Concomitantly, animal species adapted to open savannahs soon appeared — either by habitat tracking of existing species into the region or via actual speciation. These included two new hominid species, such as the first members of the genus Homo, along with the oldest known stone tools, which also appear at 2.5 million years ago.
It is Vrba’s special insight that ecosystem decay and fragmentation may lead, not only to habitat tracking in and out of a region, and to true extinction, but to true speciation as well. Recall that fragmentation of a species’ original geographic range, as first developed fully by Dobzhansky and Mayr, is a prerequisite to allopatric speciation. Also, note the date of this African disturbance: 2.5 million years ago — just when the Isthmus of Panama rose — and, according to some geologists, created the Gulf Stream, thought by some to have triggered the global cooling pulse that had such a profound effect on the African biota. Elsewhere, I have also suggested that the patterns of speciation in South America that occasioned Hafner’s “refugium” hypothesis in all likelihood reflect the very same sets of ecological and evolutionary processes - through the very same causes12 — as documented and discussed by Vrba.11
Speciation, then, is integral to the evolutionary process:
- Natural selection shapes most evolutionary adaptive change nearly simultaneously in genetically independent lineages as speciation is triggered by extinction in “turnover” events.
- When physical environmental events that go “too far too fast” start triggering regional, species-level extinction, then evolutionary change — predominantly via speciation — occurs.
- In times of environmental normalcy, speciation and species-wide evolutionary change are comparatively rare.
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