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Species, Speciation and the Environment

Niles Eldredge

articlehighlights

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

October 2000

Eldredgepunceekphoto.jpg

Evolution of speciation ideas, from gradualism to punctuated equilibrium. Source: Wikimedia Commons; author: wooptoo.

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 Dobzhansky2 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 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
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.

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.

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 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.

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 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

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 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:

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.

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.amnh.org/exhibitions/darwin/curator/ http://www.nileseldredge.com/

Species, Speciation and the Environment

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Understanding Evolution

Your one-stop source for information on evolution. Learn the facts in Evolution 101, browse the resource library, read about evolution in the news, or discover a wealth of materials to help educate others about evolution and related concepts—it’s all right here! http://evolution.berkeley.edu

Evidence of Evolutionary Transitions

This article by Dr. Michael Benton appears on this site and examines how scientists piece together transitions in the evolutionary history of species.
http://www.actionbioscience.org/evolution/benton2.html

Summary of punctuated equilibrium

Short explanation of the “punctuated equilibrium” theory of Niles Eldredge and Stephen Gould.
http://pespmc1.vub.ac.be/PUNCTUEQ.html

Life in the balance

June 1998 article in Natural History magazine by Niles Eldredge connects the biodiversity crisis to the global ecosystem.
http://www.findarticles.com/cf_0/m1134/n5_v107/20776794/p1/article.jhtml

Eldredge and evolution

This personal web page contains an interesting overview of Eldredge’s position compared to that of evolutionary theorists.
http://home.planet.nl/~gkorthof/kortho10.htm

Speciation

Learn about a variety of ways that speciation happens.
http://evolution.berkeley.edu/evosite/evo101/VSpeciation.shtml

American Museum of Natural History

This museum has an extensive site explaining cladistics, which is a method of determining evolutionary relationships between creatures. The site focuses on the museum’s dinosaur exhibits, hall of vertebrate origins, and extinct mammals.
http://www.amnh.org/exhibitions/Fossil_Halls/fossil-halls2.html

Univ. of California, Berkeley, Museum of Paleontology

This museum’s web site has many resources on the history of evolutionary thought, fossils, phylogenetic relationships between creatures, and geological history. The site is organized by geological time periods, allowing users to explore the fossils as well as the geology and environment of times in the past.
http://www.ucmp.berkeley.edu/exhibit/exhibits.html

Read a book

The Pattern of Evolution by Niles Eldredge offers a fascinating exploration of the way we investigate and understand the evolution of Earth and the life on it, describing how the key issues and events in science over the past two centuries have brought us to the brink of a more holistic understanding of the planet. An interesting and highly readable essay in search of patterns in organic and inorganic nature in support to illustrate the evolutionary process within an ecological context (1999, W.H. Freeman, New York).

American Museum of Natural History

Visit, become a member of, do volunteer work for, or support this incredible museum where collections consist of over 30 million items, including a large array of dinosaur fossils. Second link takes you to the museum’s scheduled field trips, walking tours, and workshops. http://www.amnh.org/
http://www.amnh.org/programs/

Species Survival Commission

Learn how everyone can help the International Union for Conservation of Nature and Natural Resources’ efforts in protecting species worldwide.
http://www.iucn.org/about/union/members/join/

Conservation partnerships

Find inspiration for your community or group by reading how others have taken steps to protect the world’s species.
http://www.nationalgeographic.com/ngm/0106/earthpulse

ActionBioscience.org original lesson

This lesson has been written by a science educator to specifically accompany the above article. It includes article content and extension questions, as well as activity handouts for different grade levels.

Lesson Title: How Do New Species Form?
Levels: advanced level/AP high school - undergraduate
Summary: This lesson explores speciation, stasis, and change with activities that focus on comparing gradualism to punctuated equilibrium. Students can create a 3-D model of punctuated equilibrium, illustrate plate tectonics, use marbles to simulate extinction patterns… and more!

Download/view lesson.
(To open the lesson’s PDF file, you need Adobe Acrobat Reader free software.)

Lessons for middle school to general-level high school

The following links will take you to lessons available on the PBS website:

  • » This lesson illustrates the effect of predators on evolutionary change.
    http://www.pbs.org/wgbh/evolution/sex/guppy/index.html
  • » This lesson looks at adaptive radiation of diverse life forms in an Australian coral reef.
    http://www.pbs.org/wgbh/evolution/survival/coral/
  • » This lesson examines the environmental conditions that prompt the development of new species.
    http://www.pbs.org/wgbh/evolution/darwin/origin/ All of the above simulations of natural selection will allow students to explore the concept of biological change through time (requires free plug-in). Each of these simulations could be conducted by students in groups during an hour or more class period. Completion of work as a class would require access to a computer lab with internet connections.

Useful links for educators

  • » Conservation and Biodiversity
    A hypertext Book by Peter J. Bryant, School of Biological Sciences, University of California, Irvine, examines the origin, nature and value of biological diversity, the threats to its continued existence, and approaches to preserving what is left.
    http://darwin.bio.uci.edu/~sustain/bio65/Titlpage.htm
  • » Discover Life
    Discover Life helps you to identify things, share ways to teach and study nature’s wonders, use maps, report your findings, and contribute to and learn from the Web’s growing encyclopedia of life.
    http://www.discoverlife.org/

Useful links for student research

In addition to the links in the “learn more” section above:

  • » Tree of Life Glossary
    The Tree of Life web site offers a glossary of terms used in constructing phylogenetic trees. http://tolweb.org/tree/home.pages/glossary.html
  • » Geologic Time
    Concise explanation of geologic time, relative time, radiometric time, and major divisions of geologic time.
    http://pubs.usgs.gov/gip/geotime/contents.html
  • » Plate Tectonics
    This exhibit explains the history of our new understanding of Earth and provides a brief overview of the theories behind it. Includes animation of continental drift over time. The second link takes you to the site’s cladistics section.
    http://www.ucmp.berkeley.edu/geology/tectonics.html http://www.ucmp.berkeley.edu/clad/clad1.html
  • » BIOSIS
    This resource contains links to “a simplified classification scheme for the whole animal kingdom,” an extensive Zoological Record Thesaurus (subject/taxonomic hierarchies), and an Index to Organism Names (for identifying the taxonomic group to which a named organism belongs).
    http://www.biosis.org/free_resources/
  • » Biology Browser A free, interactive Life Sciences portal from Biosis.org. Resources include articles and science news, as well as educational resources for students and teachers. www.biologybrowser.org
  • » Images of Life on Earth
    ARKive “is harnessing the latest in digital technology to bring together, for the first time, the world’s most important nature films, photographs, sound recordings, and memories, then using them to build vivid and fact-backed portraits of Earth’s endangered plants and animals.”
    http://www.arkive.org
  1. Darwin, C. 1859. On the Origin of Species by Means of Natural Selection. John Murray, London.
  2. Dobzhansky, T. 1937. Genetics and the Origin of Species. Reprint ed., 1982. Columbia University Press, New York.
  3. Mayr, E. 1942. Systematics and the Origin of Species. Reprint ed., 1982. Columbia University Press, New York.
  4. Eldredge, N. 1971. “The allopatric model and phylogeny in Paleozoic invertebrates.” Evolution 25:156-167.
  5. Eldredge, N. and S. J. Gould. 1972. “Punctuated equilibria: An alternative to phyletic gradualism.” In T. J. M. Schopf (ed.), Models in Paleobiology, 82-115. Freeman, Cooper, San Francisco.
  6. Davis, M. 1983. “Quarternary history of deciduous forests of eastern North America and Europe.” Ann. Missouri Bot. Gard. 20: 550-563.
  7. Coope, G. R. 1979. “Late Cenozoic fossil Coleoptera: evolution, biogeography and ecology.” Ann. Rev. Ecol. Syst. 10:247-267.
  8. Wright, S. 1931. “Evolution in Mendelian populations.” Genetics 16:97-159.
  9. Wright, S. 1932. “The roles of mutation, inbreeding, crossbreeding, and selection in evolution.” Proc. Sixth Int. Congr. Genetics 1:356-366.
  10. Brett, C. E. and G. Baird. 1995. “Coordinated stasis and evolutionary ecology of Silurian to Middle Devonian faunas in the Appalachian Basin.” In Speciation in the Fossil Record, ed. R. Anstey and D. H. Erwin, 285-315. New York: Columbia University Press.
  11. Vrba, E. S. 1985. “Environment and evolution: alternative causes of the temporal distribution of evolutionary events.” S. Afr. J. Sci. 81:229-236.
  12. Eldredge, N. 1999. The Pattern of Evolution. W.H. Freeman, New York.

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