MACROEVOLUTION: TEMPO AND MODE. II

Niles Eldredge and Steven Gould stirred up the mud of Tempo and Mode in Evolution with their paper in 1972 on so-called "punctuated equilibrium". The traditional view of evolution was one of phyletic gradualism. This encompassed slow, gradual change in phenotype and speciation by gradual change from one species into another. The alternative - punctuated equilibrium was put forward as a means of accounting for the ever present "gaps" in the fossil record (see figs. 20.4-20.5, pp. 561-562). Eldredge and Gould argued that the gaps were not artifacts of incomplete representation, but that there were essentially no intermediate forms. The general notion is that long periods of stasis or morphological equilibria are punctuated by periods of rapid morphological change.

This issue was a bit of a blow to the traditional "Darwinian" approach to evolution which largely focused on slow gradual change. This affiliation with "non-Darwinian" evolution is misguided and mislabeled because the original and updated versions of punctuated equilibrium invoked speciation in small isolated populations which fits squarely with Mayr's peripatric model of speciation. Moreover, Darwin described in the Origin of Species a pattern that is entirely consistent with stasis; Darwin did believe that the evolution of complex adaptations was gradual (the eye was built adaptively from preexisting parts in ancestors and did not pop into being quickly in evolutionary time).

The stratigraphic phenomena would be observed from 1) morphological stasis in a large population 2) an unrecorded founder event to a peripherally isolated population 3) speciation, perhaps through a "genetic revolution", where a new equilibrium morphology would be assumed and 4) Range expansion of this new form back into the range of the original form (see diagram below). These events, entirely consistent with "Darwinian" or "Modern Synthesis" phenomena, would be observed as a punctuational pattern (see fig. below). Note that there are other sequences of events that might give rise to the punctuational pattern.

Several questions arise:

1) What is rapid? 10,000 - 100,000 years can be an instant in geological time (especially in the context of some deposition rates) but is ample time for evolutionary events in populations. Recall that the shift from the peppered to the dark form of Biston betularia occured within the span of 100 years by a completely "Darwinian" mechanism.

2) Is rapid morphological evolution associated with speciation events? Answer: not always (many species of insects [lacewings, fruit flies] are "good species" but are very difficult to tell apart). It can be: there are convincing examples of punctuated patterns in fossil record: Williamson's mollusks.

3) How do we explain stasis? stabilizing selection, developmental constraints, absence of selection? Eldredge and Gould claim that stasis is data, i.e., the absence of change is interesting. If stasis is due to stabilizing selection, then there is perfectly good evolution going on: selection against individuals at the tails of the distributions within populations. If stasis is due to developmental constraints then there is an interesting "battle" going on between the environment and the homeostasis of the organism. The issue of punctuated equilibrium has contributed a lot to the science of paleontology since it has focused new attention on 1) changes at speciation in the fossil record (see pp. 567-570), and 2) the notion that stasis is interesting and important and needs explanation.

The publication of the idea of punctuated equilibria set off a bit of a challenge among paleontologists to show that their "own" mode of evolution was the correct one. Thus gradualists came out with papers showing convincing evidence of gradual evolution (figure 20.7, pg. 565) and the Punc. Eq. types came out with papers showing rapid shifts in phenotype in the fossil record. The absurd example is a data set by Gingerich which he interprets as gradual and is reinterpreted by Gould and Eldredge as punctuational! (see below). Like any polarized debate, there are two kinds of intermediates where reality lies: 1) some data sets show one mode, others show the other and 2) documented cases of punctuated gradualism: periods of stasis punctuated by short periods of gradual change. What remains to be confirmed is whether different lineages tend to show one pattern and others the other: the relative frequency of the two alternative modes in the fossil record will ultimately settle the debate.

The punctuation debated focused a lot of interest on the notion of hierarchical phenomena (sensu units of selection). One important hierarchical issue is Species Selection: differential rates of increase or decrease in species diversity among different lineages due to differences in rates of speciation and/or extinction. The basic principles of species selection are 1) speciation is random with respect to phenotype, 2) most changes occur at speciation, 3) different extinction and speciation rates are due to some biological properties of the different taxa.

Some consequences: 1) species selection can introduce evolutionary trends and 2) differences in morphological or taxonomic rates of evolution among different lineages can be due to species selection. The important point is that it is the pattern of speciation that drives such trends, not the direction of morphological changes.

An excellent example of the dynamics of species selection (or how one might interpret data from the fossil record in light of differences in extinction and speciation rates) is provided by Hansen's studies of planktotrophic vs. non-planktotrophic gastropod (snails). Planktotrophic lineages last longer in the fossil record (lower extinction rate) See fig. 23.3, page 643. However, the proportion of planktotrophs decreases in the fossil record (see figure 23.4, page 645 and note typo in figure caption). How can one account for this apparent paradox? If one invokes a higher speciation rate among non-planktotrophs, then this might do it; i.e., species selection might account for the patterns of diversity changes. Read the text for this section (pp. 641-644).

A general question about species selection: is it a pattern or a process? Following the parsimony of G. C. Williams, can we explain species selection by differential survival of individuals within populations, and if so is species selection just a by-product of individual selection., or do higher level processes operate? (thus the hierarchical issue in species selection). If the latter is true, the big question remains: is macroevolution decoupled from microevolution?? (i.e., are population-level processes insufficient to account for evolution above the species level? If you talk to a population geneticist they would say NO! If you talk to a paleontologist some would say OBVIOUSLY!