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Evolutionary Progress

We might ask whether there is any underlying direction in evolution, such as a tendency for organisms to become more complicated. Although this might at first sight appear to be obviously so, the concepts of evolutionary progress and of the evolutionary increase in complexity are in fact the subject of great debate. (Note that an increase in complexity would be just one possible example of evolutionary progress; in other words, the concept of evolutionary progress is broader than that of complexity increase, and could be used in relation to many other measures.)

Countless putative examples of evolutionary progress exist in the literature, such as an increase in the maximum sizes of organisms [Bonner 88], an increased resistance of taxonomic groups to extinction [Raup & Sepkoski 82], and net progress in the expansion of life (e.g. increase in the number of species, the number of individuals, and in biomass and energy flow) [Ayala 88]. Daniel McShea criticises the evolutionary progress literature for concentrating on theorising rather than empirical inquiry, but he also warns that ``marshaling cases does not document a pervasive trend either. The many increases could well be offset by an equal (or greater) number of decreases'' [McShea 96] (p.477). Referring to the question of whether there has been an increase in the complexity of multicellular animals (metazoans) over the Phanerozoic Era, McShea concludes that ``the evidence so far supports only agnosticism, indeed it supports an emphatic agnosticism'' (ibid. p.489).

Almost as numerous as the number of examples of claimed evolutionary progress are the number of measures that have been used to track this progress. These measures are often intended to indicate the complexity of an organism, and include the length of coding DNA in an organism's genome [Cavalier-Smith 85], the number of different cell types in an organism [Bonner 88], and the number of species in a community (Bonner, ibid.).

Among the mechanisms which have been proposed to drive evolutionary progress, a common theme is the selective pressure and ecological niche opportunities that arise from other species in the ecosystem. Coevolution and symbiosis are general principles here, and have been applied to explain evolutionary progress in a number of specific ways. Examples include Van Valen's Red Queen hypothesis, in which it is postulated that even in a constant physical environment there will be indefinite evolutionary change as each species evolves to meet changes in others in the ecosystem [Van Valen 73]. A similar scenario was suggested by Waddington ([Waddington 69] pp.115-6). On a larger scale, consideration of population sizes and spatial heterogeneity are also common factors in explanations of differentiation between populations, such as allopatric models of speciation,2.18 and Wright's ``shifting balance'' theory.2.19

Although many different mechanisms have been proposed to explain apparent evolutionary progress, McShea notices that most are based upon a small number of underlying principles [McShea 91]. These include some kind of ratcheting process, where a progressive step, once it occurs, is not easily lost (perhaps because the new development has become very tightly integrated with the rest of the system) and is thereby able to act as a stable base for further progression. Another common principle is the repetition-and-differentiation-of-parts theme (as noted by Darwin ([McShea 91] p.308) and Maynard Smith [Maynard Smith 86] (pp.45-47) among others).

However, many would claim that there is no consistent progress in evolution. D'Arcy Thompson, in his seminal book On Growth and Form, remarked ``That things not only alter but improve is an article of faith, and the boldest of evolutionary conceptions ... I for one imagine that a pterodactyl flew no less well than does an albatross, and that Old Red Sandstone fishes swam as well and easily as the fishes of our own seas'' [Thompson 17].2.20 George Williams doubted that there was any consistent notion of complexity which can be said to have increased from the Paleozoic to the present day [Williams 66] (pp.42-43), and Stephen Jay Gould certainly agrees with these sentiments (e.g. [Gould 89]).

That Darwin's theory does not in itself predict evolutionary progress is emphasised by Maynard Smith: ``our theory of evolution does not predict an increase in anything. At first sight, Fisher's [Fisher 30] `fundamental theorem of natural selection' might seem to predict an increase in `mean fitness', but it would be a mistake to think that there is any quantity that necessarily increases'' [Maynard Smith 88] (p.220). Whether Darwin himself believed in the idea of evolutionary progress is itself a matter of debate (for an overview, see [Nitecki 88]). This is not surprising when some people quote passages such as: ``as natural selection works solely by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection'' [Darwin 59] (Chapter 14), and others quote: ``After long reflection, I cannot avoid the conviction that no innate tendency to progressive development exists'' [Darwin 72].2.21

At the root of much of this controversy lie the facts that the notion of progress is poorly defined, it can be used in a variety of different ways, and, above all, it is a value-laden notion. In an analysis of the concept, Francisco Ayala notes that progress ``contains two elements: one descriptive, that directional change has occurred; the other axiological (= evaluative), that the change represents betterment or improvement'' [Ayala 88] (p.78). It is the latter aspect of the concept that is the source of much of the disagreement. Many argue that we, as human beings, are not impartial observers, and that our value-judgements are easily influenced (even subconsciously) by cultural, political and religious considerations (e.g. [Ayala 88], [Hull 88], [Ruse 88], [Nitecki 88], [Gould 88], [McShea 91]). In stark terms, Gould says ``progress is a noxious, culturally embedded, untestable, nonoperational, intractable idea that must be replaced if we wish to understand the patterns of history'' [Gould 88] (p.319). The use of the term complexity is also problematic. McShea says ``Complexity is more specific, more concrete, and therefore more tractable, but on account of its historical and still-commonplace association with progress, it carries the axiological taint'' [McShea 91] (p.320).

next up previous contents
Next: Different Types of Complexity Up: The Pattern of Life Previous: The Pattern of Life
Tim Taylor