As a Catholic, I have some advantage when it comes to evaluating the theory of evolution. I have no dogmatic difficulty accepting that God speaks in mythical and metaphorical language in the Bible while using the evolutionary process as a secondary cause to bring about his organic creations. In this way, I can approach evolution, like every other scientific theory, as provisional, and susceptible to revolution.
It is in this spirit of honest inquiry, rather than desperate dogmatism, that the following difficulty, inspired by the Malthusian Trap, occurred to me. I have no idea if the neo-Darwinian model has already considered it or not. There may be a perfectly sufficient explanation, but it is as yet unknown to me.
I call it the Evolutionary Complexity Trap.
The traditional Malthusian Trap is the idea that
“while technological advances could increase a society’s supply of resources, such as food, and thereby improve the standard of living, the resource abundance would encourage population growth, which would eventually bring the per capita supply of resources back to its original level. Some economists contend that since the industrial revolution mankind has broken out of the trap.”
In other words, the conditions that are favorable to population growth are for the very reason that they cause the population to grow, preventative of population growth. Thus the the trap. The only escape is if resource growth always outpaces population growth.
When applied to evolution, the argument is this:
Evolutionary adaptations occur when random mutations are naturally selected because they are advantageous (i.e. they enable an organism to reproduce better than similar organisms)
Those adaptations involve increasing complexity.
Complexity is inherently disadvantageous.
Therefore we should expect there to be a limit on adaptation causing evolution to stall.
Premise (1) is a restatement of the basic idea behind neo-Darwinian evolution. But I must stress it. We must remember that in neo-Darwinian model, the sole metric for natural selection is adaptive advantage. Nothing else. Neither beauty, nor intelligence, nor diversity, nor transcendence, nor any other reason. As Dawkins has explained it, organisms are machines for replicating DNA. That is the purpose of life. The more successful they are at reproduction, the more instances of that organism will exist, and therefore the more they will reproduce, crowding out their less successful relatives in the fight for resources. It is, at bottom, a pure numbers game.
Premise (3) may not be obvious. Complexity is inherently disadvantageous for several reasons. Most basically, complexity is lower entropy than simplicity. As a matter of physics, therefore, simplicity is favored. Complex systems can overcome this by generating disorder to offset their localized low-entropy state. Body heat just is such entropic offset. Second, complexity requires greater resources: to build more complex organisms, you need larger bodies with higher energy requirements to accomplish the same tasks. Third, complexity requires more time: assembly of a complex organism has more steps and more processes than a simple one. Finally, complexity is more fragile--i.e. there more ways to die. Consider how unreliable a computer is compared to an internal combustion engine. This is because computers are orders of magnitude more complex than engines. Complex organisms build in defense mechanisms to offset this, but it remains a basic fact: this is why extremophiles tend to be microbes and not mastodons.
The likeliest place for an evolutionary complexity trap is, I think, at the barrier between single-celled organisms and multicellular organisms. Every cell represents a multiplication of resources: a 4 celled organism, for instance, requires at least 4 times the resources as compared to a single celled organism in order to accomplish the same task: reproduction. There will also be some kind of overhead required for the coordination of the cells--veins and nerves, for instance, are primarily coordination systems. For this reason, a multicellular organism could never be as successful at reproduction as a single celled organism.
Consider how much more successful bacteria are at reproduction than human beings: 7 billion human beings vs. 5,000,000,000,000,000,000,000 000,000,000 bacteria. You might think that the reason there are multicellular organisms is because single celled organisms found that existing in groups allowed them to reproduce more effectively. But even if you consider a human being as a colony of single cells, the bacteria still win: "the average human male is made of 30 trillion cells and contains about 40 trillion bacteria" (source).
The numbers demonstrate my contention: complex, multicellular life, because of its complexity, is fundamentally and inescapably less successful at reproduction than simple, single cellular life.
This brings us back to the trap: why would single cellular life ever adapt into multicellular life? Any adaptive advantage is only obtained at the cost of increased complexity that requires higher resource consumption, longer life spans, and greater fragility, leading to a decrease in reproductive success, as measured by net replications of the organism. And, even if some multicellular organisms evolved, being marginally better at this or that than their single-celled relatives, the rapidly diminishing returns would exert a negative pressure against further complexity. How then, would multicellular life ever reach escape velocity?
We can escape from the Malthusian Trap only if available resources match or outpace population growth. The Industrial Revolution, which economists argue allowed us to escape the trap, was not a blind adaptive process, like natural selection, but an intelligent one. Humanity built technologies to solve for emerging population-dampening pressures. Whereas DNA replicators rely on random mutations to drive their evolution, human populations rely on our already existing intelligence to break free of the trap.
In short, adaptation requires complexity that dampens the process generating the adaptations. Evolution should have stalled. But it did not. The theist has explanations that may better fit the data. Either God directly designs life, or he uses an evolutionary process as a secondary cause. In the latter case, evolution could be imbued with a teleological directionality, the physical basis of which might be mutation, but it need not be entirely random mutation. The dice could be weighted toward certain ends: like the production of the wondrous panoply of life and of rational beings capable of knowing God.
The question for the naturalist is: Does the current neo-Darwinian model of evolution have sufficient explanatory power to account for our escape from the complexity trap?