Neo-Darwinism is the name given to the synthesis of Darwinian evolutionary theory with Mendelian genetics that occurred in the 1920's and 30's, and is also known as the modern synthesis. Controversies concerning the relative importance of various evolutionary events are almost exclusively contained within the framework of neo-Darwinism.
Proponents of neo-Darwinism maintain that all evolutionary trends are determined by differential reproductive success of organisms, which pass on traits to their offspring by a particulate genetic inheritance system. Traits acquired during an organisms life-span are not believed to be heritable. Evolutionary innovations are explained by random mutations and recombinations, which may influence an organism's reproductive success. Despite claims that neo-Darwinism does not provide a full explanation of all evolutionary phenomena, it remains the accepted orthodoxy, eg. Charlesworth (1982), Dawkins (1976, 1985, 1986, 1989), Endler (1986), Hoffman (1989), Stebbins and Ayala (1981).
The most publicised of the various controversies concerns the relative importance of speciation and stasis (absence of evolutionary change) to the history of life. Adherents to the "Punctuated Equilibria" view of evolution believe that most evolution occurs through branching speciation events, and not through transformation of entire lineages. Punctuationists believe that macroevolutionary trends result from survival and speciation rates of individual species, or even groups of species. This contrasts with the conventional neo-Darwinian belief that evolutionary trends result from differential survival and reproductive success of individual organisms. Furthermore, they believe that once species are established they do not change in any appreciable way, or only exhibit minor morphological fluctuations. They term this lack of evolutionary change "stasis". Stasis is attributed to difficulties of transformation of species imposed by constraints of history, development and architecture. Speciation is seen as a rapid transformation between stable states. (Eldredge and Gould 1972, Gould 1980, 1982, Gould and Eldredge 1977, Gould and Lewontin 1979, Stanley 1975, 1981).
Punctuationists believe that neo-Darwinian evolutionary mechanisms are only a subset of all evolutionary mechanisms. They believe species have emergent properties due to synergistic interactions of properties that may be of no selective advantage to the individuals that constitute a species, but are only of value to the species as a whole. For this reason they believe that "species selection" is a qualitatively different phenomenon than individual selection. (Gould 1980, 1982, Vrba 1980, Vrba and Gould 1986). Group/species selection has also been discussed outside the context of punctuated equilibrium (Wynne-Edwards 1962, 1986). Gould (1980) has stated: "Evolution is a hierarchical process with complementary, but different, modes of change at its three major levels: variation within populations, speciation and patterns of macroevolution".
Gould (1980) has quoted Mayr (1963, p.586) as stating:
"The proponents of the synthetic theory maintain that all evolution is due to the accumulation of small genetic changes, guided by natural selection, and that transspecific evolution is nothing but an extrapolation and magnification of the events that take place within populations and species"
and continues: "if Mayr's characterization of the synthetic theory is accurate, then that theory, as a general proposition, is effectively dead, despite its persistence as textbook orthodoxy".
It is ironic that Gould chooses to quote Mayr, as ideas first suggested by Mayr feature prominently in Eldredge and Gould's (1972) original formulation of the punctuated equilibrium theory.
Mayr (1954, p. 178) stated: "Rapidly evolving peripherally isolated populations may be the place of origin of many evolutionary novelties. Their isolation and comparatively small size may explain phenomena of rapid evolution and lack of documentation in the fossil record, hitherto puzzling to the paleontologist.". Also see Mayr (1942, p.236 and p.297).
Eldredge and Gould's (1972, p.84) statements are quite similar, stressing the consequences of this theory for the fossil record, ie. that it may be rare to find insensibly graded fossil sequences if new species arise rapidly in small peripherally isolated populations, and not from transformation of the entire ancestral population. Even during the formulative years of neo-Darwinism it had been argued that isolation of populations was an important factor in evolution, eg. Haldane (1930).
In stark contrast to his (1980) claim that the modern synthesis, as a general proposition, is "effectively dead", Gould (1982, p.88) has more recently stated: "punctuated equilibrium is consistent with the standard Darwinian model of the modern synthesis (Mayr 1963)-indeed it was developed in this context". Despite this he still stands by the claim that there are species level properties, not necessarily of selective value to individual organisms, which necessitate a hierarchical approach to evolution. This concept, if demonstrated to be necessary, represents a deficiency in the modern synthesis according to Mayr's (1963, see above) characterization which attributes all evolution to events within populations and species.
Gould (1982, p.92) concedes that the term species selection is often used descriptively and (p.94) "may be powered by traditional natural selection" but recommends the term species selection "be confined to the narrow sense of true group selection-claims for selection among species based on species level properties".
As an example of a species level property Gould suggests (1982, p.95-97) the ability of stenotypic (adapted to a narrow range of environments) marine invertebrates, for females to brood their young "may confer no advantage in terms of natural selection". Instead Gould suggests it may be of value because it enables greater speciation rates through populations being more easily reproductively isolated than would be possible with more mobile planktonic larvae (as used by eurytypic marine invertebrates, adapted to a broad environmental range).
Ability to brood young could quite conceivably evolve by conventional natural selection targeting individual organisms if the species inhabited a heterogenous environment (or an environmental gradient of some type), and as a result it was advantageous for females to leave their offspring in a similar environment to the one already inhabited. Ability to brood young, under these circumstances, would increase the probability of females leaving their offspring in an environment similar to their own, and would confer an advantage in terms of natural selection. Ability to brood young may increase the rate at which species are able to evolve to become specialised.
Increased tendency to speciate would then arise as a consequence of properties advantageous to individual organisms instead of being a property of use only to the species as a whole. Vrba (1980, p.74, 79) has made similar statements concerning greater speciation rates of stenotypes. This species level property is easily attributed to advantages conferred on individuals, rather than being of use only to the species as a whole, and in no way provides evidence that species selection is qualitatively different from conventional selection of individual organisms.
Wynne-Edwards (1962, 1986) believes that animal species regulate their density and reproductive output, to avoid unsustainable exploitation of resources. Group selection is provided as an explanation for this form of altruistic behaviour; adaptations for the common good of a group of individuals not thought to be able to evolve through individual selection (1986, p.13). These phenomena can be attributed to behavioural traits benefiting individual organisms. The most reproductively successful strategy for individual organisms is not necessarily to produce as many offspring as possible. An individual may well maximize its reproductive success by successfully raising a few offspring or by delaying reproduction until more favourable conditions exist, instead of sharing scarce resources between many offspring.
Dawkins (1976, p.197-202), discusses mechanisms for the evolution of altruistic behaviour, even between different species, without necessitating group selection. This is attributed to gene selection. Genes likely to produce behaviour of benefit to other organisms will be favoured, as long as organisms possessing such genes have the ability to discriminate between individuals who reciprocate and others who fail to reciprocate. Genes which produced behaviour of benefit to other individuals regardless of whether they reciprocated would enable other individuals to "cheat", ie. take advantage of the altruistic individual without reciprocating. These genes would not be favoured by natural selection.
Dawkins (1989, p.viii) believes gene selection is just another way of looking at individual selection, both equivalent, but gene selection providing a novel perspective from which to view properties that may or may not be of value to an organism. It is equally possible to attribute altruistic behaviour to differential reproductive success of individuals. Individuals which produce progeny likely to engage in altruistic behaviour, without being exploited by non-reciprocating individuals, would experience greater reproductive success than individuals producing non-altruistic offspring.
Properties which Eldredge, Gould, Stanley, and others attribute to species are equally attributable to individual members of a particular species. A species is successful because the individual organisms within that species are successful. A species may leave daughter species, by whatever mode of speciation, by conventional selection targeting individuals or by random genetic drift. Ability to speciate is due to properties of individual organisms of a species and the ecological circumstances with which they are forced to exist.
Vrba (1984:128) adopts a conventional view that speciation is usually an incidental consequence of genotypic and phenotypic differences accumulating between populations. Vrba believes speciation occurs as an effect of selection causing phenotypic divergence, by promoting adaptations that promote reproductive success.
In order to increase reproductive success, it is entirely possible for selection to promote pre and post mating isolating mechanisms if hybrids between two species/sub-species are inferior to either parent population. These isolating mechanisms would enhance individual organisms reproductive success, and need not be considered of value only to the species as a whole.
Species selection (extinction), and ability to speciate (analogous to reproductive success), are consequences of individual selection. For example consider the extinction of the Tasmanian Tiger (Thylacinus cynocephalus) on the Australian mainland following the introduction of the Dingo (Canis familiaris dingo) several thousand years ago; whether the entire species, or just every individual, was the target of natural selection is irrelevant to the final outcome.
Stanley (1975) believes evolutionary change is concentrated in speciation events and the process of species selection, analogous to natural selection, determines the direction of transspecific evolution. Stanley considers speciation a largely random process providing raw material for species selection to act upon. Species which speciate at high rates and/or survive for long periods, leaving many daughter species, will be favoured by this process.
Mayr (1963, p.621) placed a similar emphasis on the importance of speciation, likening species to "biological experiments" necessary for evolutionary progress.
Stanley (1981, p.96) has stated: "Quantum speciation becomes our logical solution to the problem of the great mammalian radiation - a problem epitomized by the origin of bats and whales from small terrestrial mammals during twelve million years or less".
If this radiation did occur according to the punctuational scheme, favoured by Stanley, the speciation events must have been so frequent and numerous that they were virtually continuous and indistinguishable from each other. None of the puntuationists suggest that whole collections of adaptations arise in single speciation events. The numerous differences between a whale and a small terrestrial mammal imply numerous speciation events, with short lived periods of stasis.
I find this difficult to reconcile with Stephen Gould's (1982, p.84) "operational suggestion" of geologically instantaneous speciation as being "one per cent or less of the species' later existence in stasis". Under this scenario if there were only, for example, one hundred speciation events between an ancestral small terrestrial mammal and a whale, each speciation event would have to occur in an average of 1200 years, followed by 120,000 years of stasis; or alternatively ancestral species would have to give rise to daughter species with great frequency, and yet themselves remain in stasis.
The opposing "gradualist" (a term coined by Eldredge and Gould(1972)) view that the formation of species is not a special class of evolutionary event would maintain that this radiation occurred via differential survival and reproductive success of individual organisms (as opposed to survival and speciation rates of individual species), evolutionary change occurring during both cladogenesis and anagenesis, ie. both during and between the numerous speciation events. Speciation events, giving different groups reproductive isolation, would actually enable separate lineages to undergo subsequent anagenetic change involving the acquisition or modification of adaptations applicable to their ecological circumstances.
Punctuationists would reject this view by pointing to gaps in the fossil record. Aside from plausible imperfections in the fossil record, it has been accepted since Darwin that "the chance of discovering species with transitional grades of structure in a fossil condition will always be less, from their having existed in lesser numbers, than in the case of species with fully developed structures" Darwin (1859, p. 183), just as it has long been accepted that evolutionary rates may vary, eg.Darwin (1859, pp. 313-315), Mayr (1942, p297).
Far from being "the logical solution" to this mammalian radiation, as Stanley (see above) would have us believe, it appears completely illogical that such rapid evolutionary change be confined only to periods of speciation and that the remaining time (ninety nine per cent, if Gould's suggestion be followed) be spent in stasis.
This evolutionary episode indicates that species are malleable, given the appropriate ecological circumstances, and are not as rigidly constrained by their past histories as Gould and others would have us believe. Resistance to change is a common theme in punctuational thinking. Gould (1982, p.90) stresses the stability of structures, the difficulty of their transformation and the idea that change is a rapid transition between stable states.
What one evolutionist might see as change within a lineage, another would see as speciation, possibly accompanied by extinctions. Gould himself (1982, p.85) has pointed this out "what we have called anagenesis is usually no more than repeated cladogenesis (branching) filtered through the net of differential success at the species level". From Gould's perspective, as a paleontologist, it is difficult or impossible to gain an understanding of the intricate details of environmental conditions affecting certain species. For this reason it may be useful to consider speciation as instantaneous, but in reality speciation occurs as a consequence of evolution and not vice versa.
Vrba (1985, p.233) states "Insofar as one observes punctuated equilibria in a given data set one has no choice but to propose that macroevolution is a function of environmental change.". Eldredge (1986, p.142) believes the origin and subsequent modification of adaptations are best explained by conventional neo-Darwinian theory, but timing and context over geological time are yet to be fully understood. It is possible that macroevolutionary patterns, especially extinctions and adaptive radiations, are due in large part to unique, or at least irregular, abiotic events (eg. climate change, continental drift, impact of meteorites, etc.) and as such are not amenable to all encompassing scientific theories.
The Punctuated equilibria and group selection debates have, if nothing else, provided the fuel for much debate, which I believe has only strengthened the modern synthesis, while simultaneously pointing out its limitations, although these limitations have long been recognized, eg. Mayr (1963, p. 587) wrote "The manifestations of transspecific evolution are, of course, in many respects different from those of infraspecific evolution, even though the underlying mechanisms are the same.".
The ideas of Eldredge, Gould, Stanley, Vrba and others may well be useful models for understanding evolution on a larger scale, but they are not in any way inconsistent with the modern synthesis. Stanley (1981, p.186) argues that species selection is valid purely from a point of practicability, as the fossil record is far too incomplete to allow an understanding of details of selection within ancient populations.
Areas where the modern synthesis may prove to be inadequate, or at least in need of expansion, concern the role of symbiosis in the evolution of eukaryotic cells, the non-random generation of mutations and inheritance of acquired characteristics.
It has been suggested (Margulis 1981) that associations between organisms of different species provides the best explanation for the origin of mitochondria, cilia and photosynthetic plastids in eukaryotic cells. These organelles are believed to have once been free living bacteria, acquired symbiotically by prokaryote cells. This mode of acquisition of structures lies outside conventional evolutionary theory. Margulis (p.37) believes nonsymbiotic hypotheses are sufficient to explain the subsequent evolution of plants and animals. For this reason the evolution of eukaryotic cells by symbioses does not provide evidence that neo-Darwinism cannot account for the subsequent evolution of multi-cellular organisms.
It has been reported (Lewin 1990) that Escherichia coli bacteria, may be able to generate non-random mutations in response to altered environmental conditions. This contrasts with the orthodox neo-Darwinian position that mutation is a random process. If non-random mutations do indeed occur, they are unlikely to be a common occurrence, and may only be confined to unicellular organisms.
Cullis (1977) found that heritable changes can be environmentally induced in flax (Linum usitatissimum). Cullis proposes that differing temperature and fertilizer regimes may induce excess copies of a particular chromosome sequence. Growth conditions of subsequent generations then determine how many of the excess copies are incorporated with other chromosome sequences. The altered genotype is then responsible for inheritance of acquired characteristics. Inheritance of acquired characteristics, other than learned behaviour and culture, is not generally considered possible via neo-Darwinian evolutionary mechanisms.
The frequency of occurrence of phenomena similar to those outlined above has not to my knowledge been the subject of any systematic investigation. If these type of phenomena do in fact occur more often than has been believed in the past, they represent deficiencies in neo-Darwinian theory. However, they are unlikely to be important when considering the vast majority of species.
If the modern synthesis (neo-Darwinism) is found to be inadequate, the necessary modifications will be minor in comparison to the bulk of circumstances, where it provides an adequate theory of evolution. The theory itself may evolve, but it will evolve with the fundamental claims remaining intact. Reports of the death of neo-Darwinism have been greatly exaggerated.
Brad Calvert 1991
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