Tuatara: Volume 25, Issue 2, January 1982
Darwinian Views and Patterns of Speciation in New Zealand or What's Adaptive about a Radiation?
Darwinian Views and Patterns of Speciation in New Zealand or What's Adaptive about a Radiation?
Robin Craw's recent analysis of biogeographical studies in New Zealand (1980) is timely. Like Craw, I am impressed with Sir Charles Fleming's immense contribution to this field. However, I too believe that healthy debate about the interpretation of data is vital to the life of any science. It seems to me that the alternative given by Croizat (1964) and others to many contemporary ideas in biogeography is useful, because it is only when one takes a stance outside a paradigm that its substance can be seen clearly. I further believe that the discussion of Darwinian and Vicarian approaches to biogeography is a manifestation of an even more general phenomenon which is occurring in biology today. I refer to a renewed critical analysis of neo-Darwinian theory.
Sir Charles Fleming has consistently championed neo-Darwinian interpretations to explain the characteristics of the New Zealand flora and fauna. One principal tenet of neo-Darwinism advocated by Fleming, Simpson, Mayr, Stebbins and others, is that speciation occurs as large populations slowly adapt to different environmental circumstances (see for example Fig. 16, Mayr 1942). This assumed intimate connection between natural selecton and the origin of species gave added significance to the early population genetics of Fisher, Haldane and Wright. Evidence that natural selection can cause changes in gene frequencies in large populations was considered verifying evidence that natural selection can incidentally result in the origin of new species (see Fig. 1a).
Such a model of speciation is central to Darwinian, as well as neo-Darwinian theory. In more recent times it has been referred to as the “Dumbell” model of speciation (Stebbins, 1969) and in paleontology as “phyletic splitting” (Simpson, 1947). Fleming has invoked this model to account for the origin of a number of species within the cockle genus Bassina (Fleming, 1958) and, 17 years later, of the endemic parrot genus Nestor (Fleming, 1975). In keeping with such a theory Fleming (1958) describes speciation as “… the splitting of one species into two separate species …” and this is “… preceded by some form of spatial, generally geographic, isolation, such as is entailed in the formation of geographic races”. Darwin (1859) himself said that species are “… only strongly page 42 marked and permanent varieties, and that each species first existed as a variety”. Darwin felt strongly that this was a fundamental aspect of his theory. He reacted angrily to those who accused him of arguing that man evolved from apes, responding that his theory maintained that man and apes had a common ancestor. He saw as central to his theory the hypothesis that species arise by slow accumulation of differences as a result of natural selection. He maintained that the extreme imperfection of the geological record largely explains why intermediate forms which would make up the complete sequence are not found. Hence he remarked “He whorejects these views on the nature of the geological record, will rightly reject my whole theory” (1859, p. 342).
An alternative model of speciation (Fig. 1b) argues that species arise from small populations which have become geographically separated from an ancestral species. Like gradualism this also can properly be described as an allopatric model. This founder model of speciation differs from the gradualist Darwinian model in other ways apart from the size of the population involved. The founder model maintains that species remain stable through time and hence the two species do not diverge, but the ancestral species remains the same and the small population undergoes rapid change, and a speciation event results. It is possible, of course, that the small population involved is not a peripheral isolate of an ancestral population but the ancestral population itself. That is the species range and population size is reduced drastically and a small relic population survives and a speciation event results.
Let us return to the Nestor parrots, the Kaka and Kea to exemplify the alternative model. Fleming (1975, 1979) postulates that during the early Pleistocene glaciation a sea barrier separated two Proto-kaka populations in the North and South Islands. The two populations became differentially adapted to the alpine South Island conditions and the more temperate North Island. These populations consequently speciated into the Kea and North Island Kaka, and the South Island subspecies of the Kaka is the result of reinvasion from the north.
An alternative view, and one which has not been previously considered is that both the Kea and the South Island “subspecies” of the Kaka may be derived from small isolates of the Kaka, which is now restricted to the North Island but was once distributed over both islands. It seems plausible to suggest that a small population of such a species was trapped on the western side of the Southern Alps and consequently speciated. We now recognise this form as the Kea. As evidence for this, subfossil deposits of Keas are much less common than Kaka (P. Millener, pers, comm.), indicating that the former has always been restricted to alpine areas. A population from the same ancestral species may have been isolated in the northern parts of the South Island and have given rise to the South Island Kaka population of today. Alternatively, as Fleming suggests, it may represent a founder population derived from the North Island. These relationships are represented in Fig. 2 and illustrate the founder effect hypothesis.
Another entirely distinct line of evidence indicates that, at least in some groups, species arise by the chance separation of a few individuals (perhaps even a single female carrying male sperm) from an ancestral population. The work of Carson and his co-worker (1970, 1976) provides compelling evidence that a large number of species of Drosophila endemic to the Hawaiian Islands have arisen in this way.
I believe therefore that there is good evidence that species can arise via small isolated populations. Equally I find the available evidence that large populations can speciate vague, and open to the possibility that founder-effects are involved. Species possess various systems which result in little genetic change through time and which are responsible for the characteristic stasis of species (Lerner, 1954; Carson, 1975; Paterson, 1978). My own view can be well summarised by Powell's (1947) comment when discussing the evolution of New Zealand land snails. “It would seem therefore that the numerically superior populations more or less isolated by topographic boundaries preserve their individuality by pressure of numbers, and that new forms arise primarily and under exceptional conditions through the accidents of small numbers becoming isolated from the main areas of distribution”.
In view of the intimate Darwinian connection between adaptation and speciation the question must be asked:
Is speciation itself adaptive?
In keeping with the view that speciation is a goal of evolution, species are described as groups of organisms which are reproductively isolated from other groups. The isolation is “achieved” by the creation of “Isolating Mechanisms” and these are responsible for “protecting the integrity of species”. Since “Biological Isolating Mechanisms” (Mayr, 1942) need not be called into action if the populations involved are geographically separated, the question of the status of such populations is largely academic. Dobzhansky (1950), in fact referred to Isolating Mechanisms as belonging to two major categories geographic and reproductive.
This “Isolationist” view of species has recently come under critical attack. Paterson (1978, 1980, 1981) has argued that this is inappropriate and misleading and termed it the “Isolation Concept”. He has introduced an alternative Recognition Concept which views a species as a group of organisms which are tied together by a common communication system between males and females. Paterson has termed this the Specific-Mate Recognition System (SMRS). Hence, according to this view, the so-called “protection of species integrity” is an incidental consequence or effect (sensu Williams, 1966) of a change in the SMRS of a population. Speciation, according to this concept, is not a goal or purpose of evolution page 45 but an incidental consequence. The question of specific-status is meaningful regardless of whether the populations being considered are geographically separated or not. In contrast, Dobzhansky (1951) argued that reproductive isolating mechanisms are not necessary, and hence will not have developed, when the populations are geographically isolated.
Fleming's (1975) study of divergence in the scrub cicadas (Genus Kikihia) appears to use the same reasoning as Dobzhansky. It is argued that subspeciation occured in South Island populations of these cicadas which were restricted to coastal areas of scrub vegetation during glacial stages. Apparently these populations are almost exclusively allopatric and they are described as subspecies or races (Fleming, 1975; 1979), despite the fact that they differ in song. Since as Fleming (1975) states “The complex and stereotyped ethology of cicadas, in particular the specificity of the males' songs and of the females' response to them, play a notable role in speciation”, a cogent argument could be advanced for the specific distinctness of these populations in terms of the Recognition Concept. One cannot help but feel that in dealing with such allopatric populations authors consider their status is a matter for arbitrary designation, or that Isolating Mechanisms are “not necessary” since the populations are allopatric.
In New Zealand many species of indigenous birds are described as having separate North and South Island subspecies. Again, this appears to be a matter of convention rather than a considered biological opinion based on evidence. I believe that the status of these bird populations is of considerable importance both for evolutionary biology and for the conservation of threatened species.
The Recognition Concept might be usefully employed here. This concept is predictive in that, if one can determine the nature and characteristics of the SMRS of these geographically separated populations, it is possible to predict whether males and females of these groups would recognise each other as mates. I would suggest that studies of the details of courtship behaviour including analyses of male songs, etc. may enable a determination of the status of these populations. This does require, however, a knowledge of which characteristics of calls are important in mate recognition and which are not.
Consideration of the Recognition Concept also leads to an alternative view of the nature of speciation. If a species is essentially a group of organisms which is tied together by a common communication system, the SMRS, then speciation results when the SMRS of individuals of a daughter population so changes that these individuals no longer recognise members of the parental population as mates. The SMRS and meiosis are then recognisable as the two major components of sexual reproduction. Meiosis results in the production of haploid gametes from a diploid organism, while the SMRS has the function of achieving the efficient meeting of these gametes (Fig. 3).
A significant change in the SMRS results in the production of a new sexual cycle and speciation. Seen in this way speciation, in any form of sexual organisms, is not itself adaptive but an incidental consequence of the existence of sex.
I thank E. C. Young, A. Harper, N. Henderson, E. Slooten, S. Dawson, M. McLea, P. F. Jenkins, C. White and M. Ford for their comments on the manuscript. The author especially thanks A. J. Hughes for much useful discussion of adaptationist arguments.
Carson, H. L., 1975: The genetics of speciation at the diploid level. Am. Nat. 109: 83-92.
Carson, H. L. and Kaneshiro. K. Y., 1976 : Drosophila of Hawaii : Systematics and Ecological Genetics. Ann. Rev. Ecol, Syst. 7 : 311-345.
Carson, H. L., Hardy, D. E., Spieth, H. T. and Stone, W. S., 1970 : The evolutionary biology of the Hawaiian Drosophilidae. Pages 437-543 in Hecht and Steere (Eds), 1970, Essays in Evolution and Genetics in Honor of Theodosius Dobzhansky. Appleton-Century-Crofts, New York.
Craw, R. C., 1980 : How to be a good biogeographer in 1979. Tuatara 24 (2) : 81-87.
Croizat, L., 1964 : Space, Time, Form : The Biological Snythesis. Published by the author, Caracos.
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Eldredge, N. and Gould, S. J. 1972 : Punctuated equilibria : An alternative to phyletic gradualism. Pages 82-115 in Schopf, T. J. M. (Ed), Models in Paleobiology. Freeman, Cooper and Co., San Francisco.
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Tuatara 25(1), p. 18 (Winstanley)
Fig. 2 has been rotated 180°.
As the figure is printed, the caption should read:
Terminal appendages of the New Zealand Corduliidae. Females above, males below.
Left to right: Procordulia smithii, Procordulia grayi, Hemicordulia australiae, Antipodochlora braueri.
Scale line represents 1mm.