David Stove, in “Darwinian Fairytales”, suggests that darwinism was originally built upon the Malthusian foundation that population outruns food supply, and therefore there is a constant battle for survival, in which only those most fit individuals survive.
In discussing this on my blog, there was scepticism that darwinism as she is spoken today still relies on this principle – and that's probably a good thing. It struck me that peacefulness, rather than struggle, and abundance, rather than famine, are the more general experience across nature. We have a cooking apple tree in our garden which is groaning under the weight of apples – far more than we could consume over a winter (although many won't really be suitable for our consumption). No careful eking out of reproductive resources on the part of the apple tree. The same goes for the blackberry bush just behind it. I see plenty of rabbits over the course of the year – not all lying dead by the side of the road – and I feel no overwhelming urge to capture them so that I will have something to eat that evening. And it's not just me – there aren't significant numbers of other predators trying to catch them, either – they just graze lazily wherever they happen to be. Abundance and stasis seem to be more the pattern than conflict and starvation. In any case, geometric population growth (2, 4, 8, 16), as envisaged by Malthus, rarely seems to occur. Even in bacteria with an unlimited food supply, geometric growth would give way to linear after a while.
Sheep, cows and pigs have been domesticated by humans for food and other reasons. Accordingly, they have undergone a highly non-natural selection process – to the extent that in some cases, bred animals are sometimes incapable of surviving in the wild. What is more marked than this is the failure of non-artificially selected species to deal with the rapid changes in environments that have been introduced as a consequence of the spread of humans. Dodos and passenger pigeons were rapidly hunted to extinction; there are many other species that are only being preserved through the serious efforts of humans to act against their apparent inclinations (for example, to kill off large predators, to extract as much food from the ocean as possible and sell it). The inability of species to evolve to deal with human-induced changes in the environment would suggest a limit on the rate at which evolution can occur. No surprise there.
So sudden changes in the environment will overwhelm the developmental ability of a species. And, contra Darwin, the normal state of an interacting population is more one of stasis than struggle. If this is so, then what would drive evolution? It would have to be slower environmental changes – for example, change in climatic temperature, interactions with other gradually changing populations, gradual movement of a population across a land mass or body of water into a new environment. It is possible to envisage genetic drift occurring in two separated populations of the same species, which would lead to the eventual appearance of two separate species. I can conceive of this having been the case, for example, within the horse family, or within the gull family. However, I still really struggle to envisage the evolution of substantial physiological changes. I don't think this is down to failure of imagination. The intermediate forms can be conceived of. It is the fact that these have to be represented by biochemical changes, including at times the addition or deletion of proteins, control mechanisms and so on. It is that sort of detail that is needed to make the case for the plausibility of darwinism. I have no problem with the generation of 500 different sorts of dogs from whatever their ancestors were. I have a lot more of a problem with the generation of dogs and lizards and parrots from the original chordates with no intelligent input.