Hierarchy or continuum

Evolutionary tiers

The splitting of evolutionary phenomena into a series of levels leads to a hierarchical view of evolution (cf. p. 99). Gould (1985) proposed a three-tier model of evolution - ecological moments, normal geological time, and periodic mass extinctions - with each tier governed by distinct 'rules and principles'. He claimed that creatures cannot prepare for mass extinctions spaced over tens of millions of years or more, and that their adaptations in the ecological moment at very best provide them with exaptations (characters acquired from ancestors that are co-opted for a new use) for later debacles. To him therefore, catastrophic

Multi-limbed crustacean-like ancestor

Multi-limbed crustacean-like ancestor

Six-legged insect

Figure 6.4 Evolution of trunk Hox gene expression patterns. The crustacean lineage (for example Artemia franciscana) separated from the insect lineage (for example Drosophila melanogaster) about 400 million years ago. Crustaceans retained multiple limbs (shaded) on the trunk, whereas insect limbs reduced to three thoracic pairs. Source: William McGinnis (http://www-biology.ucsd.edu/news/article_020602.html). See also Ronshaugen (2002).

Figure 6.4 Evolution of trunk Hox gene expression patterns. The crustacean lineage (for example Artemia franciscana) separated from the insect lineage (for example Drosophila melanogaster) about 400 million years ago. Crustaceans retained multiple limbs (shaded) on the trunk, whereas insect limbs reduced to three thoracic pairs. Source: William McGinnis (http://www-biology.ucsd.edu/news/article_020602.html). See also Ronshaugen (2002).

events in the third tier reverse, undo, and override accumulations of adaptations in the first tier. Keith Bennett (1997, 176) added a fourth tier to Gould's schema, a tier of individual lifespans (predictable diurnal and seasonal changes).

Gould's idea was warmly embraced by Eldredge, Stanley N. Salthe, and Elisabeth S. Vrba (e.g. Vrba and Eldredge 1984; Eldredge 1985, 1995; Salthe 1985; Vrba and Gould 1986). Eldredge and Salthe (1984) argued that, where biological evolution is concerned, there are two hierarchies - the genealogical and the ecological - which interact to yield evolutionary phenomena. The chief points in their argument are as follows: The genealogical hierarchy supplies the players in the ecological arena. We, as individuals of the human species, see living around us other individual organisms, members of local populations, interacting among themselves and with us as communities. From a genealogical point of view, an ecological individual at the level of the community is a collection of individual organisms drawn from various source species, the species themselves being supplied by monophyletic taxa. In turn, communities combine to form larger units of the ecological hierarchy:

Ecological systems above the level of organisms have their own self-organizing processes - the interactions of various sorts among organisms, among populations, among communities, and so forth. But ecological systems must take what 'central casting' [in the genealogical hierarchy] sends them, there to pick and choose what will fit in and what will not - as in the often dramatic turnover in species composition (membership) often graphically shown in the successional stages of a sere.

(Eldredge 1985, 181)

However, the casting of players by the genealogical hierarchy for the ecological arena is in large measure determined by the ecological game that the players perform. The ecological game determines largely 'what exists in the genealogical hierarchy, which of the particular individuals at the various levels can survive, and in what form' (Eldredge 1985, 182). There are no simple one-way cause-and-effect linkages between the two hierarchies: 'the continued existence and complexion of higher-level ecological entities depend upon what is available in the genealogical hierarchy, just as the nature of those units in the genealogical hierarchy depends very much on past conditions within the ecological hierarchy' (Eldredge 1985, 182-3). Nevertheless, the greatest signal in the linear history of life comes, not from the genealogical 'death' of one species, but from cross-genealogical extinction events caused by biotic or abiotic events in the ecological hierarchy. In other words, the collapse of ecosystems appears not to spring from events within the genealogical hierarchy, but comes from events and processes in the ecological hierarchy itself (Eldredge 1985, 185). Likewise, births of genealogical elements above the level of an organism are largely a reaction to events and processes in the ecological hierarchy. By looking at evolution from the top down - that is, from the coarse-grained perspective of a palaeontologist - Eldredge felt compelled to conclude that evolution is:

a matter of producing workable systems - organisms that (1) can function in the economic sphere and (2) can reproduce. Once the system is up and running, it will do so indefinitely - until something happens. Nearly always, that something is physiochemi-cal environmental change. The economic game is disrupted. Most often, as the fossil record so eloquently tells us, the system is downgraded and must be rebuilt, using the survivors to fashion the workable new version. At other times, new economic situations are simply opened up, as in the rise of O2 tension (through marine photosynthesis). And, yes, occasionally better mousetraps do seem to be built, though the history of adaptation is much more commonly the other way around: the mousetrap is invented that allows a new way of succeeding in the biological economy, and the tens of millions of years of subsequent variation are but themes and variations - a notion developed, for example, by Simpson (1959) as 'key innovations'.

(Eldredge 1985, 213)

This view of evolution does at least provide the architecture for explaining macroevolutionary changes in terms of both internal and external factors, and has much to say about spe-ciation, as well as overturns of entire biota.

The microevolutionary-macroevolutionary continuum

Several pieces of evidence do seem to suggest that there is a continuum between small-scale allele frequency changes in populations and large-scale phylogenetic changes leading to new body plans (Simons 2002). For instance, intermediate forms ('missing links') discovery since about 1985 fill former gaps in the vertebrate fossil record and suggest that the characteristics now used to distinguish five classes of vertebrates (fish, amphibian, reptiles, mammals, and birds) were once not clearly established (Kutschera and Niklas 2004). There again, in grasses, the C4 mode of carbon dioxide assimilation evolved from the C3-mecha-nism some 12.5 million years, but numerous C3-C4 intermediate forms are described from a range of taxa (Kellogg 2000). Such examples notwithstanding, there are several exceptions where microevolution and little-by-little mechanisms of change appear to offer ineffectual explanations. A case in point is the origin of eukaryotic cells from prokaryotic cells by means of endosymbiosis, wherein mitochondria, cilia, and photosynthetic plastids are postulated to have been free-living organisms that were acquired in a particular sequence by host prokaryotes as symbionts (see Taylor 1974; Margulis 1981). Another case where gradualism fails is in the divergence of six-legged insects from crustacean-like, multiple-limbed arthropod ancestor some 400 million years ago (p. 104).

Andrew M. Simons (2002) demonstrated a highly plausible explanation of continuity between microevolution and macroevolution by applying a 'bet-hedging' perspective. This perspective nicely solves the paradox displayed by evolutionary trends over different time-scales, which often go in different directions and seem to indicate a lack of coupling between microevolution and macroevolution. It will be explained here in detail, drawing heavily on Simons's explanation, as it seems a powerful idea that obviates the need for a new and unified theory of evolution based on three (or more) tiers. Figure 6.5 summarizes Simons's theory, showing how a conservative bet-hedging strategy evolves (upper panel) within a population or clade in the face of environmental unpredictability (lower panel). Simons develops his theory by applying the diagram to two different timescales. In the first, which does not include the inset, the time axis covers 463 years up to about 1990 and tracks a bet-hedging phenotype, namely, the onset of reproduction (bolting) in Indian tobacco (Lobelia inflata), a monocarpic perennial herb. He uses this as a model to explain the dynamics of a bet-hedging trait. Tree-ring width defines the environmental variance (data from D'Arrigo and Jacoby 1993). The phenotype ranges from short-year (early final bolting) to long-year (late final bolting) specialists. The basis of the bet-hedging strategy in this plant is the 'decision' when to bolt. Indian tobacco can live through several seasons as a frost-hardy rosette, but it must eventually enter the reproductive mode and produce a o_ Optimal g Bet-.g hedging

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  • Milly
    How is continuum similar to hierarchy?
    2 years ago

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