Evolution has produced some exquisite examples of biological machinery, from bat echolocation to the human brain. But are there things that cannot ever evolve? And why?
To answer this question, we first need to understand a couple of things. The first is that natural selection is about preserving what works, not what is in principle the best “design” for something. We can envisage more efficient designs than those found in nature, but given that evolution is a population phenomenon that compares how well contemporaries do against one another, it will favour those designs that happen to work best, even if they are shoddy solutions (relative to a universal optima that we can imagine) to a problem.
Secondly (and related to this first point), is that what gets selected has to be built upon what’s already there. This means that selection will result in things that are ad hoc and convoluted. As Richard Dawkins has said about the laryngeal nerve (a nerve that exists in the pharynx or throat region of vertebrates), it is easy to imagine a more efficient, less resource-wasting design that has the nerve pass straight through the pharynx rather than under it and back up again, but the cost in embryological upheaval of such a change would likely be prohibitive because of the negative effects on other morphological arrangements as a side-consequence. The intermediates would never be selected for, and hence the less efficient, sub-optimal version is what gets preserved. Of course, it needn’t have been so if organisms had been designed afresh by a deity or some other intelligent agent. What we actually see, though, are solutions to life’s problems that clearly show the imprint left behind by history, because the solutions are often a patchwork of prior solutions that are “stuck” at local rather than universal (effectively imaginary) optima. Organisms carry, as Charles Darwin said of human beings (the most exalted of all organisms), the indelible stamp of their origins.
There will probably never be cheetahs pursuing gazelles on the African savannah at Mach 2.0. There will probably never be sharks that fire torpedoes at seals. There will probably never be birds with electronic radar. But we can find analogues of these; we find fish that spit water at insects to knock them out, and tarantulas that shoot hairs at adversaries. We find bats that use sound waves to determine the location of prey, and dolphins that use sonar to find food hiding beneath the sea bed. Natural selection is all about the allocation of resources in direct proportion to the suite of demands imposed by the environment that is pertinent to the organism in question – the ease of acquiring nutrients, the ease of finding a mate, the risk of being predated upon, abiotic factors like temperature, and so on. All these factors impose selection pressures of their own – often in opposition to other pressures – and what results will normally be a compromise between them, a design that represents a solution to the average set of pressures in an environment over a period of time. There will be a lot of historical baggage that will be dragged along, and if there are no efficient means of getting rid of it, it must be tolerated, as it were.
Lineages can change in dramatic ways, but there are definite limits on what evolution can do, defined, first of all, by the laws of chemistry and physics, and by the immediate needs of the organisms that comprise populations.