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Species, Speciation and the Environment
Niles Eldredge
An ActionBioscience.org original article
The environment plays a major role in the evolution of species by:
dramatic environmental changes triggering extinction as well as speciation
species arising after splitting from an ancestral species when they acquire new adaptations to a changing
environment
species stabilizing for millions of years followed by abrupt disappearance when their ecosystem is
disrupted
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October 2000
Evolution of ideas on speciation
Darwin’s idea of evolution: it’s a slow and gradual process.
Darwin
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 Dobzhansky 2 and
systematist Ernst Mayr3 developed the idea that:
Species must adapt to environmental change to survive.
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.
When members of a species become separated by geography, they will eventually become separate
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 formally continuous species distributions. A seaway, for example, might develop between
two formally connected areas of land; conversely, land might emerge separating formally connected
oceans — as 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
There is an ecological pattern to how species arise and die out.
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.
Species can survive and remain unchanged for millions of years.
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.
Examining stasis
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:
View #1
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:
In environmental upheaval, some species migrate to habitats to which they are adapted.
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.
View #2
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:
Natural selection acts differently on related species living in different habitats.
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 Wright 8,9) 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.
Speciation is often the result of environmental adaptation.
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.
Punctuated equilibrium theory: long periods of stability followed by abrupt extinction of species.
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 millionyear 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
New, unrelated species often appear at about the same time after an extinction event.
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 ecosystemwide, 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:
Example #1
Brett and Baird have documented some eight successive faunas of marine invertebrates in the
Appalachian Basin of the Middle Paleozoic.10
Marine invertebrate pattern: about 20% survive after each major extinction.
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.
Example #2
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
New species either appeared or migrated to grasslands after an extinction event in Africa.
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.
Global cooling triggered new ecosystems and new species 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
Conclusion
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.
© 2000, American Institute of Biological Sciences. Educators have permission to reprint articles for
classroom use; other users, please contact [email protected] for reprint permission. See
reprint policy.
Paleontologist Dr. Niles Eldredge, is the Curator-in-Chief of the permanent exhibition “Hall of
Biodiversity” at the American Museum of Natural History and adjunct professor at the City University
of New York. He has devoted his career to examining evolutionary theory through the fossil record,
publishing his views in more than 160 scientific articles, reviews, and books. Life in the Balance:
Humanity and the Biodiversity Crisis is his most recent book.
http://www.gc.cuny.edu/directories/faculty/E.htm