|Bruce S. Lieberman and Niles Eldredge (2008), Scholarpedia, 3(1):3806.||doi:10.4249/scholarpedia.3806||revision #89065 [link to/cite this article]|
The concept of punctuated equilibria (Fig. 1) was developed to explain a pervasive and intriguing evolutionary pattern: most species change little if at all after they first appear in the fossil record. In many cases, individual species lineages persist for millions of years without showing any significant morphological change. The idea was described in detail in Eldredge and Gould (1972), where the term was coined, although important aspects of the idea were first developed in Eldredge (1971). Punctuated equilibria actually comprises several different and related observations. These include:
- the fossil record contains a rich source of data useful for developing important evolutionary hypotheses;
- speciation typically happens allopatrically, in narrow and geographically restricted populations containing relatively few individuals;
- species are not slowly and gradually adapting and evolving over long stretches of geological time;
- species lineages that show stasis – or an absence of morphological change – dominate the fossil record and provide useful information about the tempo and mode of evolution;
- the first appearance of a new species in the fossil record usually does not represent its point of evolutionary origin but rather the migration of a new geographically isolated species back into its ancestral range, with concomitant expansion in abundance; and
- speciation typically takes on the order of 5,000 to 50,000 years to occur – far shorter than the average duration of species in the fossil record.
Punctuated equilibria’s significance extended beyond explaining patterns of within species evolution. It opened up several new avenues of research and helped spawn a new discipline: macroevolution; it further led to a greater appreciation of the hierarchical structure of nature and its implications for understanding evolutionary patterns and processes. Finally, it helped re-integrate paleontology with the mainstream of evolutionary biology.
The paradigm examples of punctuated equilibria came from Eldredge and Gould’s (1972) own research on trilobites and snails. Eldredge (in Eldredge 1971 and Eldredge and Gould 1972) focused on trilobites of the genus Phacops Fig. 2 which are abundant in Middle Devonian rocks (roughly 385-380 million years old) of eastern North America. Different species closely resemble one another except for subtle differences in the number of rows of lenses in the eye (Fig. 3). New species of Phacops evolved allopatrically along the margins of the pre-existing species range; they then migrated back into the environmental and geographic range of the ancestral species, whence they became abundant. After the initial speciation events, species of Phacops show no significant morphological changes until they go extinct millions of years later. Gould (in Eldredge and Gould 1972) described a remarkably similar pattern involving Pleistocene lands snails (a few hundred thousand years old) from Bermuda referable to Poecilozonites. Again, new snail species, this time differing in subtle shell characteristics, evolved allopatrically, and showed no significant change after the initial speciation event.
Many subsequent tests of punctuated equilibria focused on documenting the evidence for stasis within species. One of the most compelling analyses was conducted by Stanley and Yang (1987) (Fig. 4) who showed that more than a dozen species of Neogene bivalves persisted unchanged over many millions of years. Later, studies by Cheetham et al (1994) on Neogene Bryozoa reiterated the evidence for pervasive stasis, as did work by Lieberman et al (1995) on Devonian brachiopods (Fig. 5), and work by Barnosky (1990), just to mention a few. These and other examples of stasis were comprehensively reviewed in Gould and Eldredge (1977), Eldredge (1985a), Gould (2002), and Eldredge et al (2005).
Challenges to stasis
Although on the whole stasis does appear to be a pervasive pattern in the fossil record, there are exceptions that provide evidence for more infrequent gradual change, with some caveats. Some of the prominent instances of gradualism come from fossil foraminifera (Gould and Eldredge, 1977). When considered over geological time some tiny foram tests do appear to change gradually; however, one potential difficulty with such studies is that they can usually only consider fossil forams from one or a few sediment cores, while the geographic ranges of some of these foram species extended across the entire Pacific Ocean. This made it hard to determine if such morphological changes reflect shifts in the entire species, or in only smaller parts of the species ranges. Ultimately, whether change is consigned to a single geographically restricted population or occurs across the entire species is given added relevance when we consider the mechanisms that cause stasis (discussed more fully below).
Two of the studies that have most prominently challenged stasis are Gingerich’s (1976) work on fossil mammals from the Bighorn Basin of the western United States and Sheldon’s (1987) (Fig. 6) work on Ordovician trilobites from Wales. Both studies concluded that species can show gradual shifts in morphology throughout their history. Each of these studies did only sample a part of the known species’ geographic range; again, this means the changes observed might just be shifts within single populations, not the entire species.
Implications for evolutionary theory
One of the significant aspects of punctuated equilibria was it indicated macroevolution will typically involve cladogenesis, rather than anagenetic modification of populations: in short, speciation is the locus of evolutionary change (Eldredge and Gould 1972). (See Futuyma  for additional discussion and Pagel et al  for a recent empirical documentation of this phenomenon.) In such a framework, evolutionary trends are produced by a series of speciation events, rather than by a single lineage gradually diverging through time (Eldredge 1979; Gould 1982) (Fig. 7). Adaptation and natural selection of course still happen, but at the grand scale the history of life may be more about cladogenesis and the events that play an important role in causing speciation. Punctuated equilibria also implied that species were individuals, in a sense akin to organisms, but at a higher hierarchical level. Each of these led to the formulation of “species selection”: an idea based on the notion that clades might wax or wane through time not because the organisms within those clades were more or less adaptively fit, but rather because of variation in intrinsic rates of speciation or extinction (Eldredge 1979; Stanley 1979; Vrba 1980; Lieberman et al 1993).
The recognition that species are individuals, and that macroevolution is basically about cladogenesis, indicates that species are evolutionarily significant entities, yet much of evolutionary theory has focused on within species patterns of population differentiation (Eldredge, 1985b). In short, punctuated equilibria entailed the need for a hierarchical expansion of evolutionary theory. Further, it suggested that the fossil record was one of the best venues to study evolution, partly because it was the only place where it is possible to directly observe what happens to groups of species over long periods of time.
Although punctuated equilibria was a novel idea, Eldredge and Gould (1972) did pay significant attention to the works of scientists that had come before them, in particular, two of the architects of the Neo-Darwinian Synthesis: Mayr (1942); and Simpson (1944). For instance, Simpson (1944) argued that scientists should treat the patterns of evolution preserved in the fossil record as real, not artifacts of a poor record. Although Simpson (1944) was focusing on evolutionary patterns at high taxonomic levels, Eldredge and Gould (1972) were able to extend this logic to the species-level patterns of evolution preserved in the fossil record. It was also noteworthy that Eldredge (1971) and Eldredge and Gould (1972) were able to show that Mayr’s (1942) favored model of allopatric speciation should actually produce punctuated equilibria rather than gradual change. Thus, one of the central tenets of the Neo-Darwinian Synthesis was in fact more compatible with punctuated equilibria than phyletic gradualism. Given that Mayr was one of the architects of the Synthesis, and that the allopatric model was so universally accepted, this ultimately made the theory more palatable. There were in fact other instances where Neo-Darwinian predictions did not square with paleontological reality. For example, Eldredge and Gould and other authors were able to show that gradual change in species over millions of years would be difficult to explain. This is because such changes would require what amount to consistent, infinitesimally small selection pressures to act over millions of years; it is hard to conceive of environmental pressures that might cause this, or even how such changes could be reasonably distinguished from genetic drift (Lande 1986). Here, two central lynchpins of the Synthesis combined to indicate that punctuated equilibria was a more realistic explanation of evolution than gradualistic alternatives.
Another interesting fact was that stasis had been paleontology’s “dirty little secret” for decades (Eldredge and Gould 1972; Gould and Eldredge 1977), and in practice most paleontologists knew it was omnipresent, yet they were often loathe to admit it for fear of challenging biologists. Eldredge and Gould (1972) took the bold step of dignifying stasis, while providing a mechanism for punctuated equilibria that drew on the theoretical principles of the Neo-Darwinian Synthesis.
Disagreements and controversies
As is often typical of any new idea, punctuated equilibria sparked considerable discussion and generated significant controversy. One aspect of disagreement was the disconnect between what biologists and paleontologists meant by “rapid change.” To a paleontologist, the 5,000 to 50,000 years typical for a speciation event would seem incredibly rapid, especially due to the limits of resolution in the fossil record and in the face of millions of years of otherwise morphological stability. By contrast, to a biologist, the 5,000 to 50,000 years that Eldredge and Gould consigned to speciation events seemed like a tremendous stretch of time: more than long enough to accommodate “gradual evolutionary divergence.” Because of the disconnect between what “rapid” meant to biologists and paleontologists, some biologists were inclined to view punctuated equilibria as necessitating effectively instantaneous evolutionary change (which was incorrect). Also, and in a related vein, Eldredge and Gould (1972) and Gould and Eldredge (1977) were careful to stipulate that only relatively small morphological differences separated closely related species, and in particular that different species were not separated by unbridgeable evolutionary gaps; however, there was also confusion and controversy on this point as well.
Finally, there was some disagreement as to how stasis should be defined. Some who challenged punctuated equilibria held that any amount of change within a species lineage over time was enough to disqualify a particular example as evidence for stasis. Eldredge (1989), Lieberman et al (1995), Lieberman and Dudgeon (1996), Gould (2002), and Eldredge et al (2005) argued, by contrast, that the primary test for stasis should be no net change. The pattern of no net species change could match obdurate morphological stasis or a pattern more akin to morphology following a random walk; such differences would be informative about the types of mechanisms that might cause stasis, and both would be compatible with stasis itself.
Mechanisms of stasis
One of the interesting research areas that punctuated equilibria spawned was the search for mechanisms that might cause stasis. Part and parcel with this issue was the puzzling fact that over historical time scales – decades to centuries – species were capable of showing large amounts of change, yet over millions of years they were basically static (Eldredge et al 2005). A variety of mechanisms for stasis have been proposed and discussed (e.g. Eldredge and Gould 1972; Stanley 1979; Eldredge 1985a; Lieberman et al. 1995; Lieberman and Dudgeon 1996; Sheldon 1996; Gould 2002; Eldredge et al 2005). One idea first suggested by Eldredge and Gould (1972) was that organisms within species might possess certain developmental constraints that act to canalize and restrict the amount and type of morphological change that can occur. This mechanisms is still thought to play some role, but it has not been fully endorsed because it seems that organismal development might not be all that canalized (Gould 2002; Eldredge et al 2005): consider the tremendous range of body type differences that have been produced relatively recently in domestic dog breeds.
Another mechanism that has been proposed is based on the fact that species are usually broken up into different geographic populations (Eldredge 1989; Lieberman et al 1995; Lieberman and Dudgeon 1996; Eldredge et al 2005). Each one of these may undergo a quasi-independent adaptive history. Morphological changes will occur in different populations and the total change within a species through time equals the net sum of the changes in all of its component populations. Given that each of these populations will be adapting to different environmental parameters, morphological change will usually be in different directions of morphospace. Therefore, the sum total of the changes will typically cancel out (Fig. 5). Furthermore, as long as different populations can still interbreed, changes will tend to be homogenized and the net result will be stasis.
Punctuated equilibria: looking forward
Scientists will continue to assess the empirical evidence for punctuated equilibria. This will include tests for stasis in fossil species (e.g. Wood et al 2007). However, it will also involve new avenues of research that use phylogenies to determine if evolutionary change, be it morphological or molecular, typically occurs with or without lineage splitting (e.g. Pagel et al 2006). Another issue scientists will continue to explore is the nature of the allopatric speciation events associated with punctuation. Do these typically involve dispersal over pre-existing geographic barriers or do geographic barriers form within existing species ranges as a result of geological and climatic change (Wiley and Mayden 1985; Brooks and McLennan 1991; Lieberman 2000; Rode and Lieberman 2004, 2005)? Finally, other interesting theoretical questions in macroevolution that have come to the fore thanks to the development of punctuated equilibria will continue to be studied. These include evaluating whether entire groups of co-occurring species show coordinated patterns of stasis and speciation (Vrba 1985; Eldrege 1999; Lieberman et al. 2007) and also what role species selection plays in driving evolution (Lieberman and Vrba 2005).
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