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A Critique of the Punctuated Equilibria Model and Implications for the Detection of Speciation in the Fossil Record

January 1980Systematic Zoology 29(2):130-142
DOI10.2307/2412643
Jeffrey Levinton at Stony Brook University
Jeffrey Levinton
43.62Stony Brook University
C.M. Simon
C.M. Simon
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Syst. Zool., 29(2), 1980, pp. 130-142 A CRITIQUE OF THE PUNCTUATED EQUILIBRIA MODEL AND IMPLICATIONS FOR THE DETECTION OF SPECIATION IN THE FOSSIL RECORD JEFFREY S. LEVINTON AND CHRIS M. SIMON Abstract Levinton, J. S., and C. M. Simon (Department of Ecology and Evolution, State University of New York, Stony Brook, New York 11794) 1980. A critique of the punctuated equilibria model and implications for the detection of speciation in the fossil record. Syst. Zool., 29: 130-142.-The evolutionary models of punctuated equilibria and species selection: 1. rely upon a model of species origin from peripheral isolates, and 2. interpret trends as the net result of selection among daughter species, whose morphologies are random with respect to the trend. The punctuational model asserts that gradual morphological change is less impor- tant than the sudden and rapid morphological change which occurs at speciation, and that morphological stasis is the rule through most of the duration of a species' history. Several considerations suggest limitations of these models. 1. Peripheral population model-Evolutionary biologists acknowledge a diversity of speciation models. The extinction- of-intermediates allopatric model and the parapatric model do not require peripheral isolates, and suggest that morphological differentiation may likely arise from typical within-species geographic variation. 2. Anagenesis versus speciation-The punctuated equilibria model, by establishing a dichotomy between evolution at speciation and evolution before and after speciation, obscures more about the tempo and mode of evolution than it clarifies. 3. Stasis- The nature of paleontological species level taxonomy requires the identification of species- specific characters which are invariant with time. This confounds the identification of stasis with species identification. Perhaps a comparison of within- versus among-species character variation might suggest whether within-species variation is the "stuff' of larger scale evo- lution. 4. Species selection-This requires that morphological characteristics of daughter species be random with respect to a long term trend. Ontogenetic and functional morpholog- ical interpretations of phylogeny suggest that trends within species are not necessarily random with respect to trends among species. We describe an example (scallops) where the range of possible daughter species is very restricted. We finally describe a test for the effect of average species duration on rate of anagenesis. Comparing species longevities does not adequately predict the extent of morphological divergence. [Punctuated equilibria; speciation; anagen- esis; gradualism; species selection.] Simpson (1944) championed the uni- formitarian view that speciation in fossil lineages must involve the same genetic factors that were inferred to work in liv- ing populations. Rapid evolution in small isolated populations would thus explain the gaps frequently found in the fossil record. This modernist view of evolution was favored by Rensch (1959), Mayr (1942), Stebbins (1950), Newell (1956), Waller (1969), and Gould (1969). But the alternative view of species gradually transforming into new species through an insensibly gradational series was still, ap- parently, a preferred model for evolution (Raup and Stanley, 1978). In another at- tempt to modernize paleontological thinking, Eldredge (1971) reiterated Simpson's message and suggested ... that the allopatric model ... be substi- tuted in the minds of paleontologists for phyletic transformism as the dominant mechanism of the origin of new species in the fossil record ...." Eldredge dis- cussed the possible consequence of al- lopatric speciation in small peripheral populations: rapid evolution in these populations would appear essentially in- stantaneous in geological time, if pre- served at all. A species which originated as a small population and spread to in- vade the territory of another (likely its parent species) would appear as if de novo in the fossil record. Eldredge and Gould (1972) elaborated on this idea and defined two alternative evolutionary 130 This content downloaded from 129.49.250.35 on Fri, 27 Feb 2015 01:11:39 UTCAll use subject to JSTOR Terms and Conditions
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1980 PUNCTUATED EQUILIBRIA 131 models: 1. a punctuational model which presumes evolutionary change to be con- centrated in rapid speciation events with little morphological adaptation before and after; and 2. a gradual model in which a daughter species is envisioned as originating via the slow gradual trans- formation of a parental species. A corollary of the punctuational model is the phenomenon of species selection, where many new daughter species are produced by the allopatric process, but only some reduced fraction survive (El- dredge and Gould, 1972; Stanley, 1975; Gould and Eldredge, 1977). This process requires that a "set of morphologies pro- duced by speciation events [be] essen- tially random with respect to the direc- tion of evolutionary trends within a clade" (Gould and Eldredge, 1977:139). Thus the generation of species followed by environmental selection among them is analogized with the production and se- lection of random mutations. The impli- cations of this model should be of im- mediate concern to population biologists studying microevolutionary phenomena because it claims to negate the impor- tance of population level phenomena in long term evolution. Eldredge and Gould (1972) imply that the punctuational model is consistent with modern evolutionary theory and that the concept of gradualism is anti- thetical to current concepts of speciation. In this essay we discuss the issues raised by the concepts of gradualism and punc- tuation. We then examine the speciation process and its implications for the fossil record in the light of these issues. Our discussion focuses on speciation via small peripheral isolates, another allo- patric model, parapatric speciation, char- acter evolution and species selection. We particularly wish to discourage the estab-' lishment of a dichotomy between specia- tion events (which are punctuational) and anagenetic changes (which are gradual). We question the belief that microevolu- tion is decoupled from macroevolution, that is, that phenotypic changes associ- ated with speciation are essentially ran- dom with respect to long term trends. GRADUAL AND PUNCTUATIONAL MODELS Phyletic Gradualism was originally characterized by the following require- ments (Eldredge and Gould, 1972:89): 1. New species arise by the transforma- tion of an ancestral population into its modified descendants. 2. The transformation is even and slow. 3. The transformation involves large numbers, usually the entire ancestral population. 4. The transformation occurs over all or a large part of the ancestral species' geographic range. A belief in Punctuated Equilibria (pre- sumably the antithesis of gradualism) re- quires that (Eldredge and Gould, 1972:96): 1. New species arise by the splitting of lineages. 2. New species develop rapidly. 3. A small sub-population of the ances- tral form gives rise to the new species. 4. The new species originates in a very small part of the ancestral species' geographic extent-in an isolated area at the periphery of the range. 5. Species do not change throughout much of their history. Eldredge and Gould (1972) propose that their description of phyletic gradual- ism implies, ". . . the fossil record for the origin of new species should consist of long sequences of continuous, insensibly graded intermediate forms linking ances- tor and descendant . .. ," and since, ac- cording to property 3, the transformation ... usually [involves] the entire ances- tral population ... ," splitting events would be rare or absent. Phyletic grad- ualism, therefore, requires the belief that new species arise predominantly by transformation of ancestral populations into their modified descendants rather than via splitting of lineages, and that splitting, when it does occur, proceeds This content downloaded from 129.49.250.35 on Fri, 27 Feb 2015 01:11:39 UTCAll use subject to JSTOR Terms and Conditions
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132 SYSTEMATIC ZOOLOGY VOL. 29 slowly and gradually, producing progres- sive divergence. To summarize, the Gradualism Model of evolution requires slow and continu- ous anagenetic change throughout the history of a species. The number of spe- ciation events do not control the proces- sion of phenotypic divergence from an- cestral to a derived state. These events may or may not involve splitting. By contrast, the Punctuational Model suggests that slow, gradual, phenotypic divergence does not occur, but rather that "most change ... is concentrated in rapid (often geologically instantaneous) events of speciation and that ". . . the norm for a species during the heyday of its exis- tence as a large population is morpholog- ical stasis" (Gould and Eldredge, 1977:116). Anagenesis, therefore, is the cumulative result of speciation (cladoge- netic) events in the punctuational model. The question is, then, does a belief in gradual anagenetic change require a be- lief in an even tempo of evolution? Stan- ley (1975) claims that gradualism "simply recognizes no increase in rate of evolu- tion with splitting." The "classical" lit- erature suggests not. Rensch (1959:197) championed the widely held belief that, following explosive phases of evolution (i.e., many splitting events), each new branch (species) would continue to evolve, but "with decreasing speed." He pointed out that (p. 268): "gaps are especially frequent in the earliest stages of new structural types as, with the origin of a new type, evolution usually proceeds at a faster rate. It is for this reason that many lines with a rich fossil record proving gradual phylo- genetic transformation ... are missing . ... The quicker evolutionary rates following the forma- tion of new types were due partly to the fact that the new types conquered new habitats by which selection was highly intensified, or that they con- quered the habitats of more primitive types bound to rapid extinction when competing with a new superior animal. Simpson (1944) rightly pointed out that probably the new types repre- sented at first small populations, from which only a faster rate of evolution, but a poorer fossil re- cord, resulted." Simpson (1953) presented evidence that 1. the rate of evolution of a given character varies within and among lin- eages; 2. different lineages evolve at dif- ferent rates and these rates are not con- stant; and 3. within a lineage different characters evolve at different rates. Yet, a repeatedly used argument against grad- ual evolutionary change assumes con- stant evolutionary rates within lineages: "Major morphological evolution must oc- cur by repeated rapid speciation since too little time is available for change by standard, gradualistic rates" (Gould and Eldredge, 1977:120); or, "Rates of phy- letic evolution within the initial lineages of each genus, if projected backward, would require an absurdly long interval for derivation of each genus from the an- cestral genus . . ." (Stanley, 1978:35). Population genetic theory predicts that evolution should be influenced by pop- ulation size, degree of isolation, mutation rate, selective advantage per favorable mutation, and intensity of selection. Ac- cordingly, the rate of evolution in a par- ticular lineage would be the result of the interaction of these variables with the en- vironment of the species in question. Since these factors are variable in effect, we would expect a continuum of evolu- tionary rates of divergence of species' lin- eages. SPECIATION VIA PERIPHERAL ISOLATES-A UNIVERSAL TRUTH? The punctuational hypothesis derives from Eldredge's acceptance of the pre- dominance of speciation events in which ".... Change in, or development of, species-specific characters are envi- sioned as occurring relatively rapidly in peripheral isolates . . . ," especially when population size is small. "The role of gene flow is recognized as the central tenet of allopatric speciation: speciation occurs in peripheral isolation because only geographic separation from the pa- rental species can reduce gene flow suf- ficiently to allow local differentiation -to proceed to full speciation . (El- dredge and Gould 1972:112). The con- cept of peripheral isolates in species for- This content downloaded from 129.49.250.35 on Fri, 27 Feb 2015 01:11:39 UTCAll use subject to JSTOR Terms and Conditions
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1980 PUNCTUATED EQUILIBRIA 133 mation (Mayr, 1963) is important to the punctuational-species selection scenario because small population size would pro- mote rapid differentiation of isolates in many (morphological) directions from the parent species. This process would pro- vide the variation upon which species se- lection could work. It seems to us that a modern approach to speciation would require the consid- eration of a plurality of mechanisms in speciation (White, 1978; Bush, 1975; Endler, 1977; Rosenzweig, 1978; Pimm, 1979). Mayr (1963), who popularized the concept of peripheral isolates, recognizes another mechanism of allopatric specia- tion: the extinction of intermediate pop- ulations in a chain of populations of which the terminal ones had already ac- quired reproductive isolation. Allopatric speciation, therefore, does not require small population size, or peripheral pop- ulations. Complex and congruent vicariant events among unrelated groups support the extinction-of-intermediates model of allopatric species origins. Rosen's (1978) analysis of the biogeography of the poe- ciliid fish genera Heterandria and Xi- phophoros, and box tortoises of the ge- nus Terrapene suggest that all three groups of taxa dispersed into an area ranging from the southeastern U.S. to southern Mexico. Subsequent vicariant events caused congruent speciation pat- terns. Different geographic locations of plesiomorphic sister groups for poeciliid fishes and box tortoises, respectively, strengthen the vicariant hypothesis. If essentially geological-geographical events are the usual progenerative factors in speciation, as suggested by Croizat et al. (1974), then species origins cannot be ar- gued to be typically due to peripheral isolates. Congruence of physical geog- raphy and cladistic analyses of terminal taxa supports the model of extinction of geographically intermediate populations. The process of speciation is therefore a geographically controlled process with no necessary expectation of saltational change within rapidly evolving periph- eral isolates. Indeed, the distinction of species, subspecies or genera is not al- ways simple and may often be a matter of convenience to the practicing system- atist (Rosen, 1978). The most important alternative specia- tion mechanism to allopatry from the point of view of the punctuational model is the process of parapatric speciation (Endler, 1977). Models of parapatric spe- ciation must deal with the question of dispersal versus selection. In the face of dispersal between adjacent populations how can selection counteract gene flow and produce geographic differentiation? Endler (1977) discussed the origin of cli- nal variation and its role in promoting differentiation and perhaps speciation. He emphasized Clarke's (1966) model of differentiation which shows that the in- teraction of modifier genes with a poly- morphic locus affecting clinal phenotypic variation could result in sharp disconti- nuities and eventual strong restriction of gene flow. The parapatric model would predict concomitant clinal change of many seem- ingly unrelated characters. Initially strong selection for local adaptation might cause geographic differentiation over a small part of the genome under relatively strong selection (e.g., local adaptation to predation intensity as a function of lati- tude; adaptation of metabolic processes to temperature). This differentiation would restrict gene flow between sub- populations and thus permit differentia- tion at other parts of the genome that oth- erwise would not initially be under sufficiently strong selection to differen- tiate spatially. This process, termed ac- celerating differentiation (Christiansen and Simonsen, 1978; Levinton and Las- sen, 1978) might eventually result in strong geographic differentiation in di- verse character sets. In the ectoproct, Bugula stolonifera (Schopf and Dutton, 1976) both allozymic and morpholoQgic characters vary concomitantly along the southern coast of Cape Cod. Similar cor- related change occurs in the Eel Pout, This content downloaded from 129.49.250.35 on Fri, 27 Feb 2015 01:11:39 UTCAll use subject to JSTOR Terms and Conditions
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134 SYSTEMATIC ZOOLOGY VOL. 29 Zoarces vivparus, in Danish waters (Christiansen and Frydenburg, 1974). Although differentiation is envisaged by proponents of parapatric differentia- tion to initially evolve in the face of gene flow, an eventual reduction of gene flow would follow. Steep selection gradients are often difficult to distinguish from zones of isolation. For example, breaks in temperature along a north-south trending coastline may coincide with a boundary between relatively isolated water masses (e.g., Pt. Conception, California). It may therefore be operationally difficult to dis- tinguish between the parapatric and "ex- tinction of intermediates" allopatric mod- el. In either case, however, differentiation need not be rapid but may, in fact, appear slow even in geological time. If the en- vironment suddenly changes and favors one of the local races, then we might ex- pect an invasion of this race over the whole range of the species. Such an in- vasion by a morphologically distinct group into another area might appear to be a saltational event in the fossil record (as suggested by Newell, 1956; Simpson, 1961). If the environment changes grad- ually, a shift of the boundary between races might occur, or one race might gradually take over the species range pro- ducing a gradual change in fossil assem- blages. None of these changes require the establishment of peripheral isolates, and some might appear to be punctua- tional if the environment changed sud- denly. The gradual or sudden change in the environment over geologically signif- icant periods of time would elicit corre- spondingly gradual or punctuational re- sponses by the species. Parapatric differentiation and the "ex- tinction-of-intermediates" allopatric models require neither peripheral nor small populations. The environment, its spatial variation and vicariant events pro- vide the framework within which diver- gence must work. Schopf and Murphy (1973) invoke the development of Cape Cod as a vicariant event that resulted in the two extant species of the genus As- terias; one resides to the north, the other to the south. Rosen's (1978) analysis of eastern North America poeciliid biogeog- raphy involves deterioration of climate and elimination of geographically inter- mediate favorable habitats. Whether the speciation process was parapatric or al- lopatric, the points of subdivision in both cases were clearly not peripheral. GENETIC REVOLUTIONS, SALTATIONS AND QUANTUM EVOLUTION: SPECIATION Gould and Eldredge (1977:115) "be- lieve that punctuational change domi- nates the history of life." This idea is not new. According to Rensch (1959:267), pa- leontologists were "accustomed to as- sume ... major evolutionary saltations when a gap appeared in an otherwise complete series of successive fossil stages." Saltations were viewed as the method of origination of higher taxa; i.e., "quantum evolution." Quantum evolu- tion (Simpson, 1944:207) requires that a population pass through three phases: an inadaptive phase "in which the group in question loses the equilibrium of its ancestors ... [a] Preadaptive phase, in which there is great selection pressure and the group moves toward a new equi- librium ... [and finally an] adaptive phase in which the new equilibrium is reached." Grant (1963) argues that diver- gence at the species level "may be and probably is frequently involved in this process [of quantum evolution]." He calls this phenomenon "quantum speciation." Grant (1963:459) insists that quantum speciation differs from other modes of al- lopatric speciation in that "species de- velop from ephemeral local races; the pattern of branching is excurrent rather than dichotomous; and the combination of forces involved is ... [inbreeding in small populations acting upon normal ex- isting variability] instead of ... [selection with wide outcrossing acting upon the existing variability]." Similar models of rapid speciation are discussed by Mayr (1954, 1963), Carson (1959), White (1959), and Lewis and Raven (1958). Mayr (1963:534) prefers the term "ge- This content downloaded from 129.49.250.35 on Fri, 27 Feb 2015 01:11:39 UTCAll use subject to JSTOR Terms and Conditions
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All content in this shaded area was uploaded by Jeffrey Levinton on Nov 13, 2017
Citations44References55CommentsAdd yours
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