The evolutionary significance of mass extinctions

Darwin was firmly of the opinion that biotic interactions, such as competition for food and space - the 'struggle for existence' - were of considerably greater importance in promoting evolution and extinction than changes in the physical environment. This is clearly brought out by this quotation from The Origin of Species:

Species are produced and exterminated by slowly acting causes . . . and the most important of all causes of organic change is one that is almost independent of altered . . . physical conditions, namely the mutual relation of organism to organism - the improvement of one organism entailing the improvement or extermination of others.

The driving force of competition in a crowded world is also stressed in another quotation presenting Darwin's famous wedge metaphor:

In looking at Nature, it is most necessary . . . never to forget that every single organic being around us may be said to be striving to the utmost to increase in numbers; that each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or the old, during each generation . . . The face of Nature may be compared to a yielding surface, with ten thousand sharp wedges packed close together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force.

The implication of the Darwinian view concerning the dominance of biotic competition is that for each winner there is a loser - a kind of zero-sum game. It has been accepted more or less uncritically by generations of evolutionary biologists, but not until the 1970s did it become graced with a name - the Red Queen hypothesis. The story behind the emergence of this name is an interesting one.

At the beginning of the 1970s the rather eccentric University of Chicago palaeobiologist Leigh Van Valen did some interesting research concerning the analysis of survivors of Phanero-zoic taxa which suggested that the probability of a fossil group becoming extinct was more or less constant in time. To account for this, Van Valen put forward his Red Queen hypothesis. Readers of Lewis Carroll's Through the Looking-Glass will recall that the Red Queen explained to Alice that where she lived it took all the running one could do to stay in the same place. Van Valen's hypothesis is based on the assumption that all species within a given adaptive zone compete intensively. A successful adaptive response by one species is assumed to occur at the expense of other species, which must, as the 'quality' of their environment is reduced, either themselves adapt by speciating (i.e. evolving into new species) or becoming extinct. This phenomenon leads to an endless chain of adaptive responses, and in the long run means that fitness and rate of extinction remain constant.

Not unnaturally, Van Valen thought that his work was worthy of the pages of either Science or Nature. Unfortunately, the paper was rejected by both these illustrious journals. This could have discouraged many, but Van Valen simply went ahead to create his own journal, Evolutionary Theory, and published his paper in the first issue. Although his data and conclusions have been disputed, the term 'Red Queen' has had an astonishing success and is now firmly established in the evolutionary literature. (Incidentally, Van Valen also published another, satirical, journal on an occasional basis, called the Journal of Insignificant Research. My preferred part was that devoted to papers in the scientific literature with curious titles and authors' names. Two of my favourites were (1) 'The fertility of witches in the Firth of Clyde', published by the very respectable Journal of the Marine Biological Association, and (2) 'The zoological perspective in social science', by Lionel Tiger and Robin Fox.)

Van Valen has a specialist knowledge of fossil mammals. The high rate of diversification and evolutionary turnover in mammals, which makes them, like ammonites, valuable biostratigraphic indicators, is likely to be the result of a variety of factors, such as strong competitive interactions leading to specialization in feeding methods, limitations on food supply, high mobility and use of energy, interspecies aggression, and territoriality. Such factors will conspire to lower the 'resource threshold' that is needed to prevent extinction, as compared with other animals. Although the Red Queen model might well apply to mammals, there are doubts about its more general validity. Thus, in sharp contrast to mammals, the bivalves, that is, clams and oysters, are nearly all sea-bed suspension feeders which mind their own business, are characterized by weak interactions with other species, exhibit primitive inflexible behaviour, have an uncrowded, largely sedentary, mode of life, and have generalized feeding habits. As a direct consequence of this pattern they have substantially lower rates of evolution than mammals. Limits on bivalve populations are imposed more by predation and fluctuations in the physical environment than by food resources, and biological competition is minimal. What is true for bivalves is without much doubt true of the majority of benthic invertebrates, which make up a large part of the fossil record. If we are to cite Lewis Carroll, Through the Looking-Glass might be less appropriate as a textual source for the relevant evolutionary sermon than The Walrus and the Carpenter.

The alternative to the Red Queen model, stimulated by the study of mass extinctions, has been termed the Stationary model by the doyen of British evolutionary biology, John Maynard Smith, and his Norwegian colleague Nils Stenseth. This implies that, contrary to Darwin's belief, the prime motor of evolutionary change is the physical, not the biotic, environment. Indeed, evolution could conceivably grind to a halt in the absence of abiotic change. Testing these alternative models from the fossil record has not proved straightforward. The most promising material for this purpose is a variety of Ceno-zoic microfossils from deep-sea cores, such as foraminifera, radiolaria, and diatoms. The results unfortunately have proved rather inconclusive so far. One group of workers thought that their evidence marginally favoured the Red Queen model, but others countered this by pointing out evidence of physical oceanographic change that had been ignored.

Since biotic competition manifestly exists today we need to enquire further what sort of competition may have predominated in the past, in the light of what is now understood of biotic turnover through longer and shorter periods of time.

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