Diversification of life

Key points

• There may be 5-50 million species on Earth today, a level of diversity almost certainly higher than at any time in the past.

• There is a debate about whether life diversified according to a logistic model, reaching a global equilibrium level, or according to an exponential model in which diversity continues to expand without reaching a global carrying capacity.

• The classic logistic/equilibrium explanation for the diversification of animal life in the sea is hotly debated at present.

• Many examples of evolutionary trends, one-way changes in a feature or features, are in reality more complex.

• The idea of progress in evolution, change with improvement in competitive ability, is hard to demonstrate.

• It is important not to confuse pattern with process; too often scientists and the public assume such processes as competition, adaptation and progress without testing for alternatives.

• Major steps in evolution (e.g. evolution of wings and feathers in birds, evolution of limb loss in snakes) are well documented by fossils and evolutionary trees.

• An alternative, biological, view of the major steps focuses on fundamental subcellular systems, replicators and genetic systems.

We will move forward, we will move upward, and yes, we will move onward.

Vice-President Dan Quayle (1989)

Figure 20.1 Two models for the diversification of marine invertebrate life over the past 600 myr of good-quality fossil records. (a) The empirical model, in which the data from the fossil record are plotted directly, and (b) the bias simulation model, in which corrections are made for the supposedly poor fossil record of ancient rocks. (Based on information in Valentine 1969; Raup 1972.)

Figure 20.1 Two models for the diversification of marine invertebrate life over the past 600 myr of good-quality fossil records. (a) The empirical model, in which the data from the fossil record are plotted directly, and (b) the bias simulation model, in which corrections are made for the supposedly poor fossil record of ancient rocks. (Based on information in Valentine 1969; Raup 1972.)

Perhaps Mr Quayle was a little confused about his direction, but many have interpreted the evolution of life as a long story of progress. Others argue that evolution has probably been a little more like Mr Quayle's progress: one step forward, one sideways, one back, two forward . . . We will explore this theme further.

The record of fossils gives a rich and spectacular picture of the history of life. Paleontologists have been as successful as archeologists and historians in piecing together a detailed picture of the events of the past, even though paleontologists have a very much longer time scale to deal with and a more patchy record. It is likely that the last 200 years of paleon-tological research have given a broadly correct picture of the order of appearance of major groups of plants and animals through geological time, their distributions over the continents and oceans of the past, their life strategies and adaptations, and their patterns of evolution (see Chapters 2-7), despite the many gaps and inconsistencies in the fossil record (see pp. 70-7).

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