Editor’s note: In his new book Evolution: Still a Theory in Crisis, Michael Denton not only updates the argument from his groundbreaking Evolution: A Theory in Crisis (1985) but also presents a powerful new critique of Darwinian evolution. This article is one in a series in which Dr. Denton summarizes some of the most important points of the new book. For the full story, get your copy of Evolution: Still a Theory in Crisis. For a limited time, you’ll enjoy a 30 percent discount at CreateSpace by using the discount code QBDHMYJH.
Beyond what I have said already, I am going to discuss another piece of evidence for the structuralist claim that internal causal factors have played a critical role in evolution.
In the fossil record, there are the many long-term “orthogenetic” trends. These trends manifest themselves as continuous, unidirectional change in all the successive members of particular lineages, sometimes over millions or hundreds of millions of years. To explain long-term unidirectional trends in strictly Darwinian terms would necessitate the highly implausible postulate of constant selective constraints operating on all the successive and often diverse members of the lineage over millions of years in diverse environments.
One of the most stunning cases of an apparently long-term non-adaptive trend is the so-called “reduction of the gametophyte generation” in land plants. The challenge it poses to any functionalist interpretation of the evolution of plants is self-evident.
The temporal succession of land plants started in the Ordovician period some 450 million years ago with the appearance of simple non-vascular plants without lignified internal transport mechanisms, roots, stems, or true leaves, like the mosses and liverworts. These were followed by the emergence of the various classes of vascular plants, including the club mosses, the horsetails, and ferns, and the true seed-bearers, first the familiar conifers (gymnosperms), and then the flowering plants (angiosperms).
Successive forms show increased development of water transport mechanisms, the development of woody trunks, the rise of complex leaves, and advances in reproduction involving the evolution of the seed and (in the case of the flowering plants) the fruit. The evidence suggests that each “more complex” group arose from the most complex pre-existing group at the time.
While many of the new features that emerged during this succession, namely water-transport systems, leaves, seeds, etc., can be viewed as adaptive, at the same time a mysterious trend was occurring in the reproductive cycle that has never been satisfactorily accounted for in adaptive terms, and that continued for 400 million years.
To understand the enigmatic nature of this trend, recall that in all land plants from mosses to angiosperms the reproductive cycle is divided into two multicellular stages: the sporophyte phase and the gametophyte phase. In flowering plants, the sporophyte is the main body of the plant, comprising the branches, leaves, flowers etc., while in the mosses the gametophyte phase forms the main and conspicuous part of the plant. The sporophyte in all plants from mosses through ferns and gymnosperms (conifers) to angiosperms (flowering plants) produces the sex cells or gametes (by a reduction division or meiosis). And in all plants the gametes undergo ordinary cell divisions, producing a multicellular gametophyte, which eventually gives rise to the gametes that fuse to form a fertilized egg cell, which grows into the sporophyte.
But although all terrestrial plants consist of these two multi-cellular stages, in the earliest plants the haploid phase or gametophyte formed the main body of the plant and the sporophyte was a mere reproductive appendage, as in extant mosses where the green fleshy fronds that we see is the gametophyte. But in the case of an angiosperm or flowering plant what we see, i.e., the stems, leaves, and flowers, make up the sporophyte. As we move from mosses to horsetails to ferns to conifers to flowering plants, the gametophyte becomes reduced, step by step from being the main part of the plant in the mosses to a small but still independent part of the plant in the ferns to only a handful of the cells in the angiosperms.
To account for this trend in terms of incremental selection of day-to-day variants in a population of plants is simply to defy reason. One would have to suppose that over a period of 400 million years, in plants as diverse as lycopods, tree ferns, redwoods, and Australian eucalypts, the survival value to individual plants in a population was in some way enhanced by the pull of this mysterious reduction.
But what improvement in reproductive fitness could account for such an abstract and mysterious trend over 400 million years in such vastly different plant types? There is no commonality of structure, environment, or function uniting all these diverse plants that might have imposed this remarkable 400-million-year trend on their evolution. Moreover, the reduction is by no means gradual and continuous, but occurs in jumps from ferns to gymnosperms to angiosperms.
Darwinian evolution, with its short-term vision, tests only the immediate adaptive value of mutations, and is impotent to impose such an extraordinary trend on the major plant lineages for 400 million years. To my knowledge, no convincing explanation based on Darwinian assumptions has ever been given for this trend. Some causal factor other than selection for immediate environmental fitness must surely have been involved throughout. And this is not the only example. Others include the reduction of forelimbs in therapod dinosaurs, the loss of aortic arches in vertebrates, and the reduction of the hind limbs in the species leading to modern whales.
The existence of long-term trends in the history of life irreducible to any credible functionalist explanation represents another important strand in the consilience of structuralist evidence against Darwinian functionalism, evidence showing that the functionalist paradigm cannot in itself provide a comprehensive explanation for the development of life on earth.
Image credit: © tolgatezcan / Dollar Photo Club.