|Evolution: Gradualism vs. Punctuated |
Charles Darwin, author of On the Origin of Species by Means of Natural Selection (1859), believed that morphological change was inevitable and proceeded slowly, encompassing slight, successive, and gradual changes within lineages.
Speciation, therefore, was the result of the gradual accumulation of changes within ancestral populations over time, ultimately leading to the formation of recognizably new and different species.
According to Darwin, sudden, large-scale changes were improbable or impossible—an idea epitomized by the phrase Natura non facit saltum, or “Nature never makes leaps.” Darwin’s concept of the slow and gradual transformation of a species’ entire ancestral population into distinct descendant species over time has been termed “phyletic gradualism,” or anagenesis.
If true, the expectation of anagenic transformation leads to the supposition that the fossil record for any lineage should contain an “inconceivably great” number of intermediate forms.
Darwin, however, realized that the fossil record is, in fact, not littered with an “interminable” and “enormous” number of intermediate forms. Darwin’s solution to this problem with his theory was presented in a chapter of his book On the Origin of Species by Means of Natural Selection titled “On the Imperfection of the Geological Record.”
Here Darwin persuasively argued that the paleontological record of the past is extremely imperfect because of degradation of fossiliferous deposits and differential rates of deposition and fossilization among lineages.
He also noted that, on geologic time scales, persistent, long-lived and widespread species are more likely to appear in the fossil record than short-lived species or species confined to narrow geographic ranges.
The “Modern Synthesis,” Microevolution, and Species Formation
In the 1930’s and 1940’sDarwin’s theory of natural selection was melded with then-current knowledge of genetics, heritability, and mathematics to produce the modern synthesis theory of evolution.
Under this paradigm, evolution came to be defined as a change in gene frequency over time. The microevolutionary processes of mutation, migration, random genetic drift, and natural selection were recognized as the primary mechanisms that alter gene frequencies.
At this time other authorities began to carefully consider the mechanism(s) by which species arise. One particularmode of species formation, and one supported by considerable empirical evidence, is termed the allopatric model of speciation. Allopatric species are formed as subpopulations of a more widespread ancestral species become geographically isolated and, in time, reproductively isolated from one another.
Paleospecies and Punctuated Equilibria
These ideas, particularly those of gradual, inevitable change by microevolutionary processes and the imperfection of the fossil record, and were accepted by paleontologists for more than a century. However, in 1972 Niles Eldredge and Stephen J. Gould published a paper in which paleontologists were challenged to examine fossil sequences (and gaps) more objectively.
Eldredge and Gould drew three important conclusions about the fossil record. First, some gaps in the fossil record are real and cannot be attributed to other factors; gradual series of transitional forms do occur, but they are extremely rare.
Second, paleospecies often persist for millions of years without substantial morphological change. Third, they recognized that paleospecies found in older strata are sometimes rapidly replaced bymorphologically different taxa in younger deposits.
Thus, a literal interpretation of the fossil record requires the acknowledgment of short, rapid bursts of evolution. That is,within the paleontological history of a lineage, morphological stasis is occasionally interrupted by near-instantaneous (in geological time) formation of morphologically different species.
If these gaps are real, how are they to be explained? How is one species suddenly (in geologic time) replaced by a morphologically modified form in the fossil record?
To answer these questions, Eldredge and Gould developed the theory of punctuated equilibria that, in essence, fused a literal interpretation of the fossil record with the mode of speciation most often observed in extant populations (such as the allopatric model of speciation).
Eldredge and Gould’s theory of punctuated equilibria is based on the following postulates or observations. First, speciation typically occurs via allopatric speciation.
Second, the origin of descendant species is, in geological time, rapid and occurs in a limited geographic area. Third, most adaptive change occurs at the time of speciation. Fourth, speciation is typically followed by long periods of morphological stasis, particularly within large, widespread species.
Fifth, the abrupt appearance of a new species within the range of the ancestral species is a result of ecological succession, immigration, or competition. Finally, apparent adaptive trends (such as macroevolutionary trends) observed in the fossil record are the result of species selection within lineages over time.
The authors reasoned that as long as a species is capable of successfully exploiting its habitat, adaptations that originated at the time of speciation are unlikely to be altered, and morphological stasis is the result.
Thus, in terms of the geologic time scale, most species seem to persist unchanged over long periods of time. The origin of a new species, its growth in numbers, and the extension of its geographic range are, therefore, determined by reproductive and ecological characteristics.
In short, under the punctuated equilibrium model new species can be successful if they are sufficiently distinct in their habitat requirements and if they are able to compete with or outcompete close relatives should they come into contact with one another. This theory of speciation and diffential reproduction and survival of species produces well-documented, large-scale evolutionary patterns or “macroevolutionary” trends.
Punctuated Equlibria and Plants
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It is true that in extant plants rapid changes in physiological or morphological characteristics associated with speciation are not uncommon. However, such phenomena are rarely documented in the fossil record.
For example, interspecific hybridization or polyploidization may (almost instantaneously) form new, reproductively isolated species whose niches may differ from those of their immediate progenitors.
Thus, in plants, rapid species formation is empirically known. Stasis is also known from the fossil records of some plant lineages. For example, in terms of floral structure, extant members of the Loraceae, Chloranthaceae, Nymphaceae, and themagnoliids scarcely differ fromtheir early to mid-Cretaceous ancestors.
Morphological stasis is also observed in gingkos, Metasequoia, cycads, lycopods, sphenopsids, and ferns as well as the genus Pinus, which arose in the Jurassic. Rapid radiations are also documented in the plant fossil record, as occurred after the rise of angiosperms during the early Cretaceous.
Thus, the plant fossil record provides clear evidence for stasis in some lineages and rapid diversification in others. Despite these observations, paleobotanical examples of lineages whose evolution fits the predictions of the punctuated equilibrium model are few.