You ask me to describe a horse; I answer as follows. A horse is a microscopic animal that is incapable of movement. It consists of a rather small number of cells (a few hundred, as opposed to the trillions found in a human). These cells are not organized into sophisticated organ systems. The horse is a parasite of another animal, and so acquires its resources from its host. It is entirely incapable of acquiring energy in any other way. There is no fossil record of its existence, so for all we know there may have been no such thing as a horse before the dawn of the art age in the caves of France, where our forebears drew remarkably good pictures of horses, among other things.
But wait. Their horses don't look like my description. And indeed since my description at first sight looks quite mad you might wish to agree with the cavemen and not with me. There is, however, method in my madness. My description is fine. It just refers to a time-slice in the horse life cycle that is different from the one we normally picture in our minds at the mention of the word 'horse'. We picture the adult, or if not this then perhaps a beautiful but unsteady newborn foal. What I have pictured is the horse as an early embryo, invisible to our view because it is implanted deep within its maternal host.
The point I am getting at here is that animals, and indeed all organisms, are four-dimensional things. The three dimensions of their bodies expand and change as they slide along that slippery and inevitable slope of time. Even as adults we change, albeit more slowly and often not in encouraging ways. As the American biologist John Tyler Bonner has put it, organisms do not have life cycles, rather they are life cycles.1 We tend to picture adults in our minds for all sorts of reasons. Our brains handle three dimensions more easily than four. Adults are bigger and more visible. Even when developmental stages are big and conspicuous, like tadpoles, they are often short-lived compared with the adult. But this is not always so. In some insects, perhaps most famously mayflies, the adult lives a transient life of at most a few days, while the developmental stages through which it was produced lasted much longer. But even in these cases where the rationale for thinking in terms of life cycles is strongest, we still tend to picture the adult in our mind's eye.
The reason for this is rooted in language. Often, at least for familiar creatures, the very word we use may be adult-specific. A tadpole is, arguably, not a frog. But is a foal not a horse? And the same applies in the invertebrate world. A caterpillar is, arguably, not a butterfly (though its genes are identical); but a baby centipede is definitely a centipede.
Whether we should fall into the old familiar groove of picturing the adult, or whether we should be more mentally adventurous and try to force our lazy brains to go 4-D and 'think life cycles' depends on what we are trying to do. For our cave-painting ancestors, the adult was all that was required. But for understanding how horses evolve, this static picture just won't suffice. Every stage in a life cycle only comes into being if the previous one survives. An adult can only come into being if all the earlier stages survive. At the level of the individual, death is all too real an option at every single stage. Therefore at the level of the population there will be natural selection at every stage -because at each stage some individuals will live and some will die. Of course the living and the dying could be genetically identical and the difference merely a matter of chance. But the last century's accumulated knowledge of the huge amount of genetic variation present in nearly all natural populations suggests otherwise.
All this is beginning to sound very conventionally Darwinian. And in some ways, so it is. But Darwinism is all about mechanisms, and we are not quite ready to discuss those yet, or to consider the extent to which Darwinism is acceptable to those who take a developmental approach to evolution. First, we need to complete the mental shift that we have begun towards a four-dimensional view of organisms.
Let's consider a very simple evolutionary tree with just four species: ourselves, a cow, a hen and a fish. There are three ways we can picture this evolutionary tree, as can be seen from Figure 1. First there is the tree of adults; then there is the tree of embryos; finally there is the tree of life cycles. Which is best? The answer is the life-cycle tree. The others are three-dimensional shorthand. The embryo tree is a means to an end, not the end itself. Consideration of the embryo tree is meant to reveal the arbitrariness of using, as most folk do, the adult tree. In fact, since most species experience most mortality at young rather than old ages, it would seem more sensible for those intent on 3-D shorthand to use an early developmental stage as the basis for their tree. That is, they should use an embryo tree rather than an adult tree, because if we want to think in terms of some variants doing better than others in the survival game, this would seem a sensible place to start. Which has the higher mortality rate - tadpoles or frogs? Statistically speaking, there's no contest.
A tadpole, however, is not an embryo. Usually, we restrict the term embryo to those developmental stages that are protected from the elements by virtue of their location within their mother's body or, in some instances, within the casing of an egg. So our embryo tree is too simple. Science is all about generalizing (more on this in Chapter 6), and embryos are special cases of the more general concept of developmental stages. But then again, these stages, like the adult, have no clear boundaries. A life cycle does not operate in discrete stages - rather the process of development is a continuous one. This is even true in those cases, like the tadpole/frog, where major changes occur between one 'stage' and the next. Life flows. So, using the embryo tree as a means of forcing our thoughts out of old and inappropriate habits, we nevertheless end up not with this tree any more than the tree of adults. We end up with the life-cycle tree.
figure i Three ways of picturing evolutionary trees: (a) adult tree; (b) embryo tree: (c) life-cycle tree.
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