The role of competition as perceived from the stratigraphic record

The type of competition that Darwin had in mind might be termed displacive competition, implying dynamic behaviour on the part of the newly arrived species or higher taxon in the wedge. How could this be recognized in the stratigraphic record? One reasonable inference is that after the new taxon first appeared, it might progressively expand its abundance and diversity concomitant with the progressive reduction of its biologically most closely related or potentially competitive rival, known as the double wedge pattern (Fig. 10.1a). An alternative type of competition is called pre-emptive competition, which implies that evolutionary success favours the incumbents of a particular ecological niche. The earlier species occupant of a niche would remain there until some physical disturbance caused its elimination. Only then would other biologically or ecologically related forms opportunistically occupy the niche. As portrayed in Fig. 10.1b, there would be very little overlap

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Fig. 10.1a+b What the fossil record in a stratal succession may reveal for (a) displacive and (b) pre-emptive competition.

Taxon diversity

Fig. 10.1a+b What the fossil record in a stratal succession may reveal for (a) displacive and (b) pre-emptive competition.

in time of the earlier and later taxa as portrayed in the strati-graphic record.

What does the record actually show? Let us take the familiar case of dinosaurs and mammals, which both emerged at about the same time in the late Triassic, about 230 million years ago. There is no indication that the 'biologically superior' mammals progressively out-competed the dinosaurs throughout their long history together. Rather it would appear that the two groups coexisted in different ecological niches, the mammals remaining low in diversity and small in size - no larger than rats - but with nocturnal vision, both characteristics making possible the exploitation of niches not in direct competiton with the dinosaurs. Very soon after the dinosaurs were finally killed off by the end-Cretaceous mass-extinction event, however, the mammals greatly expanded their diversity, individual size, and occupation of a variety of ecological niches, some of which, such as the air and the sea, had never been occupied by dinosaurs. Within a few million years of the K-T boundary all the principal mammalian orders had become established, and in the course of time (in the Miocene) huge beasts rivalling some of the larger dinosaurs in size had evolved. This pattern of change seems to be a spectacular example favouring preemptive competition.

Triassic reptiles provide another likely example that has been put forward by Mike Benton. The conventional view among experts had been that the reptiles that earlier evolved had been progressively out-competed by the dinosaurs. This view is not supported by the stratigraphic record, which shows that the older reptiles had become extinct before the earliest dinosaurs were on the scene. All this took place before the end-Triassic mass extinction, but it is associated with at least one extinction event of lesser magnitude. What is true for land vertebrates appears to be more generally true for the marine invertebrate record. To take just the example of the group with the highest rate of evolutionary turnover in time, the ammonoids (including the ammonites), a series of families in the late Palaeozoic and Mesozoic, both radiated and became extinct in a more or less regular succession, with little or no overlap in time. This phenomenon has been termed an evolutionary relay.

It has been claimed that the turnover of plant taxa with time exhibits a fundamentally different pattern from that of animals because plants are less vulnerable to mass-extinction episodes, and displacive competition plays a major role. However, Bill DiMichele of the Smithsonian Institution in Washington D.C. disputes this for the Carboniferous to Permian vegetational transition, which he considers to be replacive rather than dis-placive. Taxa that originated in peripheral, drier habitats in the late Palaeozoic tend to be subgroups of seed plants. The life histories of seed plants indicate a priori a greater resistance to extinction than most groups of 'lower' vascular plants, such as ferns, lycopsids, and sphenopsids. This prediction is confirmed by the nearly continuous expansion of seed-plant diversity since the Palaeozoic, at the expense of lower vascular plants.

Thus the culling by both major and minor extinction events of organisms that evolved earlier throughout Phanerozoic time could have provided the opportunity for organisms that evolved later to take over their ecological niches, a pattern more in favour of the Stationary than the Red Queen model. Stephen Jay Gould has made great play of how much the course of evolution has been interrupted by such contingent historical events as mass extinctions, the bigger events perhaps being so significant as to 'reset the evolutionary clock'. This forms part of Gould's challenge to the conventional view of evolution as a progressive one leading eventually to the emergence of our own species. This is a very large topic that goes beyond the scope of this book, but one or two points are worth making briefly here.

Evolutionary success can be measured in various ways, and by some criteria bacteria, traditionally regarded - rather inaccurately - by us as exceptionally simple organisms, are the most successful in terms of biomass, the variety of metabolisms utilized, and the range of environments occupied. No obvious progress through time is either discernible or likely. So far as the complexity of neural systems culminating in the human brain is concerned there has, however, undoubtedly been progress through time. Simon Conway Morris of the University of Cambridge goes further and points to the ubiquity of evolutionary convergence as providing a strong challenge to Gould's views. The example of the convergent, streamlined shape of active marine predators in different groups of vertebrates is a familiar example, such as tuna and sharks (fish), ichthyosaurs (reptiles), and dolphins (mammals), but convergences of all sorts, including those at the molecular level, can be recognized throughout the organic world. They include progressive tendencies in a variety of evolutionary lineages towards higher intelligence, traditionally regarded as the epitome of evolutionary success.

Evolutionary biologists might be troubled by the general rejection of displacive in favour of pre-emptive evolution as the general pattern to be discerned from the fossil record as a whole. What, they might ask, about co-evolution? The co-

evolution of plants and insects could well be cited as a superb example. One thinks in particular of the marvellous adaptations of orchids for particular insect pollinators, the study of which was pioneered by none other than Charles Darwin. Surely such co-evolution began to evolve as early as the Cretaceous, when the angiosperms became the dominant land flora? No doubt it did, but there is unsurprisingly no fossil evidence for it. There is one example from the fossil record, though, that appears to be an excellent example of co-evolution.

Geerat Vermeij, of the University of California at Davis, known to everyone as Gary, is a remarkable man. He has written several books and many papers, all of which display an immensely wide range of knowledge handled with a high level of intellectual sophistication. However, uniquely for a palaeobiologist, none of his writings contains either pictures or diagrams. This is because he is blind. He has a highly retentive memory and depends on others, mainly his wife, to read relevant scientific literature to him. Not only is he an expert on the taxonomy of snails, his favourite creatures, but he also knows a great deal about grasses, although he has to depend in both cases on his tactile sense. How many of us, even with our visual sense intact, can tell different types of grasses apart?

Vermeij is best known for his theory of evolutionary escalation. Concomitant with the emergence and radiation in the Cretaceous of sophisticated benthic predators, such as teleost fish, carnivorous neogastropods, and crabs, there was a change in the shells of two groups that were part of their favoured molluscan prey: the gastropods and bivalves. Some developed thicker shells or acquired protective spines or strong ribs. (Palaeontologists are apt to speak informally of such features as 'ornament', but there was nothing frivolous in this for the molluscs; it was rather a matter of life and death.) Others, among the bivalves, developed a greater capacity to burrow deeply in the sediment out of harm's way. Yet others retreated to the deep sea. The predators evolved in turn to become more efficient in capturing their prey, something which appears to be equivalent to what Richard Dawkins has called an 'evolutionary arms race'. One thinks in the modern world of the extraordinarily sophisticated use by plants of poisons and spines to resist animal feeding and the corresponding sophisticated response on the part of the animals.

There is no reason, therefore, to deny conventional Darwinian evolution throughout geological history to account for the numerous fine adaptations of organisms, but the more general patterns of change seem more likely on present evidence to be controlled by contingencies relating to perturbations of the physical environment, the most important of which have led to mass extinctions.

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