Views from Understanding Evolution: Parsimonious Explanations for Punctuated Patterns
© Springer Science+Business Media, LLC 2007
Received: 13 December 2007
Accepted: 16 December 2007
Published: 19 January 2008
Antievolution groups have frequently pointed to the debate prompted by Niles Eldredge and Stephen Gould’s (1972) proposal of punctuated equilibria with triumph. “Look,” they’ve claimed, “biologists can’t even agree among themselves how evolution works. The theory must be failing.” In the article included in this issue, “Editor’s Corner: The Early “Evolution” of “Punctuated Equilibria”,” Niles Eldredge (2008) takes on that mischaracterization. He describes the roots of punctuated equilibria and reveals how it builds on (not tears down!) established evolutionary theory.
Such observations are at odds with the pattern we might typically imagine evolution generating: slow and steady change (Fig. 1c)—in which, for example, tracing fossils through rock layers that correspond to a slow cooling in Earth’s climate reveals the gradual evolution of traits suited to colder temperatures. This picture of slow evolution, which Eldredge and Gould termed phyletic gradualism, fits well with one of the keystones of evolutionary theory: natural selection, the process responsible for adapting populations to changes in their environments. It is easy to imagine natural selection, for example, slowly transforming a delicate tropical species into a hardy, well-insulated, cold-tolerant species over many millions of years. In some cases, we do observe such gradualistic patterns in the fossil record, but in many others, we observe evolution in apparent fits and starts. If the evolutionary mechanism of natural selection helps explain apparently gradual evolution, what mechanism can help explain punctuated patterns? Do we need to throw out everything we know about how evolution works or search for an as-yet-undiscovered evolutionary process? No.
The beauty of punctuated equilibria is that the idea explains the apparent jolts of evolution seen in the fossil record with evolutionary processes that we already know to be at work in the world: speciation, migration, natural selection, and genetic drift. Eldredge and Gould simply showed how these mechanisms could work together to produce punctuated patterns in fossils. To see exactly how their idea works, it will help to review two modes of speciation: allopatric and peripatric speciation.
The scene: a population of wild fruit flies is minding its own business on several bunches of rotting bananas, cheerfully laying their eggs in the mushy fruit, when...
Disaster strikes and the population is divided: A hurricane washes the bananas (and the fruit fly larvae they contain) out to sea. The banana bunch eventually washes up on an island off the coast of the mainland. The fruit flies mature and emerge from their slimy nursery onto the lonely island. The two portions of the population, mainland and island, are now too far apart for gene flow to unite them. At this point, speciation has not occurred: any fruit flies that got back to the mainland could mate and produce healthy offspring with the mainland flies.
The populations diverge: Ecological conditions are slightly different on the island, and the island population evolves under different selective pressures and experiences different random events than does the mainland population. Body form, food preferences, and courtship displays change over many generations of natural selection.
So we meet again: When another storm reintroduces the island flies to the mainland, they will not readily mate with the mainland flies because they’ve evolved different courtship behaviors. The few that do mate with the mainland flies produce inviable eggs because of other genetic differences. The lineage has split now that genes cannot flow between the populations.
In the example above, the fruit flies alone were carried to an isolated location where speciation occurred. But, of course, when geographic isolation is caused by mountain ranges rising or by climate change that fragments a habitat into favorable and unfavorable patches, more than one species is likely to be affected. In fact, many different species living in the same area may simultaneously experience allopatric speciation if they are all similarly affected by large-scale physical disturbances.
Double disaster: Not only are the island fruit flies now geographically isolated from their mainland relatives, but only a few larvae have survived the harrowing journey to colonize the island.
Rare genes survive: These few survivors, by chance, carry some genes that are rare in the mainland population. One of these rare genes happens to cause a slight variation in the mating dance. Another causes a slight difference in the shape of the male genitalia. This is an example of the founder effect—a change in gene frequency that occurs because the genes in a newly founded population do not happen to be representative of those in the source population.
Gene frequencies drift: Although rare on the mainland, these small differences become common in the island population over the course of a few generations through the action of genetic drift. In fact, the entire island population ends up carrying these genes.
More changes: As the island population grows, the unique reproductive features on the island result in a cascade of changes caused by sexual selection—for example, improved fit of male and female genitalia to one another and increased female responsiveness to the mating dance. Flies also experience natural selection that favors individuals better suited to the climate and food of the island.
Speciation: After some generations, the island flies become reproductively isolated from the mainland flies. Peripatric speciation has occurred.
Because genetic drift acts more quickly in small populations, full-blown speciation is a more likely result of geographic isolation when one of the isolated populations is small. In this situation, genetic drift, and perhaps strong selective pressure, can cause rapid genetic change in the small population.
Putting the Pieces Together
Stasis: A population of snails is experiencing stasis, living, dying, and getting fossilized every few hundred thousand years. Judging from these fossils, little evolution seems to be occurring.
Isolation: A drop in sea level forms a lake and isolates a small number of snails from the rest of the population. This is the first step in peripatric speciation.
Strong selection and rapid change: The environment in the newly formed lake exerts new selection pressures on the isolated snails. Also, their small population size means that genetic drift influences their evolution. The isolated population undergoes rapid evolutionary change resulting in peripatric speciation.
No preservation: Because of their relatively small population size, the rapid pace of change, and isolated location, no fossils representing transitional forms of the new species happen to be preserved. Even if they had “gotten lucky” and fossilized, these fossils would have been at a different location from those of the parent population.
Reintroduction: Sea levels rise, reuniting the isolated snails with their sister lineage.
Expansion and stasis: The isolated population expands into its past range. Larger population size and a stable environment make evolutionary change less likely now. The formerly isolated branch of the snail lineage may outcompete their ancestral population, causing it to go extinct.
Preservation: Larger population size and a larger range move us back to step 1: stasis with occasional fossil preservation.
That, in a nutshell, is punctuated equilibria: a lot of evolutionary change occurring in a short period of time tied to a speciation event. Eldredge realized that this process would produce a punctuated pattern in the fossil record, in which one species seems to be replaced by a related species (Fig. 1a) or, if the new species does not outcompete the other, a pattern in which a new species “suddenly” appears (Fig. 1b).
There is nothing unusual about the evolutionary process proposed here. Speciation occurs at its normal rate, but it occurs in an isolated location. Evolutionary change only appears to be unusually “jumpy” after the fact because no transitional forms are preserved in the fossil record—but this is to be expected given the small size of the evolving population, its isolated location, and the nature of fossilization. Studies of modern organisms, like the apple maggot fly, have shown that the early stages of speciation can be observed in wild populations in just a few hundred years (Bush 1975). Fossilization, on the other hand, provides us with snapshots of biodiversity taken on much larger intervals—tens of thousands of years, if we are lucky—so it is not surprising that transitional forms are not always preserved.
Punctuated equilibria—large amounts of change in a short period of time tied to a speciation event (Fig. 7a). Just a few hundred thousand years separate the two rock layers, and all of the evolutionary change connecting the ancestor to its descendents took place relatively rapidly—either in this location or nearby. Transitional forms between Ancestor 1 and Species 3 did exist, but they were not preserved in this part of the fossil record.
Phyletic gradualism—slow, steady divergence of lineages (Fig. 7b). Many millions of years separate the two rock layers. In this period of time, Species 3 gradually diverged from Ancestor 1 through a series of transitional forms, but these were not preserved because of poor conditions for fossilization.
Macromutation—a key mutation produces sudden evolutionary change skipping over transitional forms (Fig. 7c). Very little time separates the two rock layers, but that period encompasses a lot of evolutionary change. Species 3 was produced by a mutation that radically changed the offspring of Ancestor 1 in many ways; transitional forms did not exist. Such extreme mutants are sometimes called “hopeful monsters” or macromutations.
As each of these hypotheses is consistent with the fossils, to figure out which is most likely, we must look for other lines of evidence. Studies of genetics turn up no evidence that extreme yet adaptive macromutations of this sort occur, so macromutation seems an unlikely explanation. Punctuated equilibria and phyletic gradualism, on the other hand, are both genetically plausible. However, they lead us to expect that different amounts of time separate the two rock layers. If stratigraphic and isotopic dating studies were to point to a short interval, the data would support punctuated equilibria; a long interval would be consistent with phyletic gradualism.
Punctuated Equilibria and Parsimony
Suggest that Darwin’s theory of evolution by natural selection is wrong
Contradict the central ideas of evolutionary theory, that life is old and organisms share common ancestors
Negate previous work on how evolution by natural selection works
Imply that evolution only happens in rapid bursts
It simply suggests that established mechanisms of evolutionary change (speciation, migration, natural selection, and genetic drift) might work together to help explain a formerly mysterious pattern seen in the fossil record. In this way, Eldredge and Gould’s proposal embodies a key criterion by which all of science operates: parsimony.
The principle of parsimony suggests that when two explanations fit the observations equally well, the simpler explanation—in this case, the one that does not involve proposing an entirely new mechanism of evolution—should be preferred over a more convoluted and complex explanation. After all, why propose a new mechanism when the old ones (which are already supported by many lines of evidence) work perfectly well to explain what we observe?
In science, theories are rejected and radical changes occur when existing ideas cannot account for the available evidence as well as a new idea. Some antievolution groups suggest that evolutionary theory finds itself in this precarious situation—but that is a misrepresentation. In fact, all the available evidence (from biology, geology, and even chemistry and physics) is consistent with evolution. Even punctuated patterns in the fossil record fit well with our understanding of how evolution works. The punctuated equilibria model is not a challenge to evolution, but is rather a deeply connected node in the web of ideas that forms this theory.
Give Me an Example of That
Parsimony applied to phylogenetics. The principle of parsimony has applications in all of science—from paleontology to particle physics. When known processes can explain the available evidence, we have no reason to propose a new, more complex explanation. Here, we have seen that principle applied to patterns in the fossil record, but the same reasoning applies equally well in other areas of evolutionary biology—particularly in the field that reconstructs evolutionary relationships among organisms: phylogenetics. Find out how biologists use parsimony to build evolutionary trees: http://evolution.berkeley.edu/evolibrary/article/usingparsimony_01
Using trees to understand plants. Just as Niles Eldredge observed patterns in the fossil record and came up with a parsimonious explanation for them, other biologists observe patterns in living organisms and try to come up with similarly parsimonious explanations for those patterns. This research profile follows scientist Chelsea Specht as she pieces together the evolutionary history of tropical plants and their pollinators and applies the principle of parsimony to the problem: http://evolution.berkeley.edu/evolibrary/article/specht_01
Punctuated equilibria help explain some sorts of patterns that we can observe in the fossil record—but there is more to be learned from studying these patterns more broadly. Through detailed analysis of the fossil record, scientist David Jablonski reconstructs the rules that helped dictate who lived and died in past mass extinctions. This research profile describes his surprising discoveries and their disturbing implications for the biodiversity crisis today. Learn more online: http://evolution.berkeley.edu/evolibrary/article/jablonski_01
The process of punctuated equilibria is based on the ideas of allopatric and peripatric speciation; however, biologists think that speciation can happen even when subpopulations are not isolated geographically. Review speciation by geographic isolation and get an introduction to parapatric and sympatric speciation: http://evolution.berkeley.edu/evolibrary/article/speciationmodes_01
The history of the concept of allopatric speciation: http://evolution.berkeley.edu/evolibrary/article/history_21
In the Classroom
Fossil Patterns: Gradualism vs. Punctuated Equilibria, from the Evolution and the Nature of Science Institutes. In this activity for grades 9–12, students learn the differences between gradualism and punctuated equilibria by manipulating two sets of simulated fossils (Caminalcules). http://www.indiana.edu/~ensiweb/lessons/peek.html
Getting into the Fossil Record, from the UC Museum of Paleontology. In this interactive module for grades 6–8, students are introduced to fossils and the fossilization process by examining how fossils are formed and the factors that promote or prevent fossilization. http://www.ucmp.berkeley.edu/education/explorations/tours/fossil/index.html
Stories from the Fossil Record, from the UC Museum of Paleontology. This web-based module for grades 6–12 provides students with a basic understanding of how fossils can be used to interpret the past. http://www.ucmp.berkeley.edu/education/explorations/tours/stories/index.html
Sequencing Events, from Modeling for Understanding in Science Education. In this activity for grades 9–12, students attempt to sequence and create a story around a series of cartoon frames to serve as the basis for a discussion about how decisions are made and how arguments are constructed in science. http://www.wcer.wisc.edu/ncisla/muse/naturalselection/materials/section1/lesson1B/index.html
Eldredge and Gould (1972) http://www.nileseldredge.com/pdf_files/Punctuated_Equilibria_Eldredge_Gould_1972.pdf
Gould and Eldredge (1977) http://www.nileseldredge.com/pdf_files/Punctuated_Equilibria_Gould_Eldredge_1977.pdf
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