Macroevolution
Contents Updated: Monday, September 13, 1999
The Human Brain
Assuming that nerve connexions, the synapses, in the brain work like the elements of a computer and correspond to one of two states, on or off, then the brain, which has a hundred million million synapses, has two to the power of a hundred million states, a number greater than the number of elementary particles in the universe. It is capable, in this computer analogy, of storing and processing immense amounts of information.
Not only that, the brain also arranges its synapses into tiny microcircuits that increase still further the total number of possible states of the brain and add to the efficiency of processing information. The human brain has prodigious amounts of memory and the potential for even higher performance.
Besides this huge amount of brain capacity, there is the split brain. In most animals both parts do the same things but, in humans, they have begun to specialize in various ways. We are developing two brains working in parallel on different types of problem.
Our brains have apparently overshot their optimum size. Like a computer with four megabytes of memory but which is only able to address 640 kilobytes, they have incorporated excess capacity and, at present, most of it is redundant. But sooner or later a mutation will arise that is able to make use of all that power. One need not expect subsequent evolutionary change to be slow.
Evolution often uses redundancy. Redundant parts of an organism are found new uses by evolution. Fish taking to the land no longer needed their swim-bladder, an air sac that kept the fish buoyant in the water. The swim bladder was redundant but found an excellent new use as a rudimentary lung.
Higher organisms often have large amounts of apparently redundant DNA (called introns) between the coding sequences that contain the instructions for the growth of the organism. The introns contain bits of DNA with odd properties. Some are mobile, acting as though they are hitching a ride on the main sequence of the DNA molecule but cannot make up their mind where to sit. Some are "decayed" genes, no longer functional but subject to mutation. Others seem to be immune to mutation.
There are repetitive sequences apparently made by bits of the code that are conceited, duplicating themselves at random places in the introns and even from one chromosome to another. Genetic information is increased commonly by a mutation causing part of the genetic code to double. The redundant excess part is then able gradually to take on a new role.
The introns seem to be one place where mutation and role adaptation can occur with ease. They are just the place to look for the causes of fast evolution of new species.
Rapid Evolution
Even ordinary evolution by selection of the fittest can be extraordinarily fast. There are examples of new characteristics evolving observably such as the growth of resistance to antibiotics in bacteria, the resistance of insects to DDT and the resistance of rats and mice to warfarin rat poison. In each case resistant strains were selected in only a few generations, taking only a few years.
Every schoolchild will know of industrial melanism in the peppered moth. A rare dark variety of the moth began to outnumber the common speckled variety because of industrial pollution. Normally the speckled variety was adequately camouflaged on clean lichen-covered tree bark but pollution killed the lichen and blackened the bark making the speckled variety conspicuous. Natural selection was effected by foraging birds. The dark mutant found the blackened bark excellent camouflage and the birds missed them.
The house sparrow, introduced into North America in the middle of the 19th century has evolved into several distinct sub-species in only about 110 generations. Some plant species have separated in only 50-100 generations. Experimenters with fruit flies claim to have shown speciation to occur in only 12 generations.
Lake Nabugabo became separated from Lake Victoria by a sand bar only 4000 years ago. Today the sand bar is still only three km across yet it has enabled five species of haplochromis to evolve. They are amongst the newest species we know and illustrate how quickly speciation can occur even in vertebrates when a population gets isolated.
Closely related species are nearly identical in the protein coding parts of their DNA but differ enormously in the repetitive sequences in the introns. G.A.Dover proposed that it is the differences in the apparently functionless repetitive sequences that determine the species. When these satellite sequences differ two animals cannot successfully mate. They, at best, produce sterile hybrids like the mule.
The satellite sequences can change very rapidly compared with the stability of the protein coding sequences. They copy themselves rapidly to random locations, even in other chromosomes, thus providing a means of rapid speciation. There is, in humans, a repetitive sequence amounting to three per cent of all DNA. By changing this sequence in a newly fertilized egg it might be possible to produce a different species of human being almost overnight.
Though the repetitive sequences seem to be functionless, could they express themselves somehow at the macro level, perhaps in a way that could only be sensed by others with the same sequence? Could they account for forms of rapid speciation? Are these mechanisms adequate to account for profound changes like advanced intelligence?
Saltation
In gradual evolution there are always countervailing factors. Can any form of gradual evolution lead to really revolutionary new physiological structures? The human brain seems to have three levels of structure, each one overlaying the previous one. What led to the adding of a new and apparently superior part to the brain on two occasions?
Some biologists have always felt that there were problems in the Darwinian view of evolution by accumulation of small changes even allowing for isolation and rapid evolution. How, for example, could it account for the major divisions in taxonomy such as that between reptiles and mammals? There seem to be too many fundamental differences between such groups of creatures. How could all these vital distinguishing features have evolved simultaneously?
One solution that has always been controversial, and still is, is saltation. Saltation is macromutation—major changes occur in a single mutation not via the accumulation of many small changes (called micromutation).
The argument against saltation is that large changes in physiology caused by mutation must be harmful because they amount to a gross deformity. The chances that a deformity on this scale would be beneficial to a creature are considered to be vanishingly small.
R.A.Fisher used the analogy of a microscope to show that macromutations cannot lead to viable changes. Imagine a microscope almost in focus. The focus is the evolutionary equilibrium state and the microscope is nearly at it. Suppose a tiny small random movement of the microscope adjustment were made to represent a micromutational change. The microscope barrel can only move inwards or outwards so there is a 50 per cent probability that the change will improve the focussing and a 50 per cent probability that it will make it worse. Eventually, through accumulating such small mutations, the microscope could become fully focussed.
What though if the random change to the adjustment were large? Moving the barrel in the wrong direction would obviously worsen the focus; but moving in the correct direction would also worsen the focus, because a macromutation, a large change, would considerably overshoot it.
A random mutation could be in the right direction and exactly the size needed to drop the microscope into focus, but this is so unlikely compared with all the other possibilities that it can be safely ignored. In practice a random large change cannot improve the focus of the instrument.
The alert reader will notice that this analogy is a terrible example of evolution. Evolution is not random in its overall effect precisely because natural selection tends to eliminate the bad variations. The microscope would tend to become focussed because the micromutations away from the focus would die out leaving those that were tending towards the focus. The macromutations would die out anyway because they must always be further from the focus.
Two Valleys in the Landscape
What, now, if this microscope had two foci? If, for example, it had two eyepieces which were themselves not equally adjusted so that when one was in focus, the other was not, and vice versa. Now there are two chances of getting a focus, one with each eye. If the microscope were adjusted close to one focus, the micromutational argument of Fisher would still apply, but what of the macromutational argument. There must be a chance that a random macromutation would put the other eyepiece into better focus than it was—perhaps, even put it into better focus than the instrument had before. In this case a macromutation could improve the overall focus of the instrument. Though it is still unlikely, it is more likely than before.
How does this translate into arguments about evolution? In the analogy of a multidimensional evolutionary space, a hyperspace, stable forms correspond to depressions in the landscape. The flow of evolution can be imagined as the flow of a river down the valleys of this landscape into a depression, forming a lake corresponding to a stable species or a developed feature.
Translating the microscope analogy into the landscape analogy, we find a lake at the microscope focus, the species equilibrium position, but on either side of it we are moving uphill away from the focus. The uplands are bleak. Individuals here are badly adapted, the result of mutation: species here are badly adapted, the result of a changed environment. Finding themselves there they had better quickly head downhill towards the lake (the focus of the microscope in Fisher’s analogy) by evolving rapidly or die. Death is, of course, extinction for the species or the effect of a disastrous mutation on an individual.
The microscope with two foci is a landscape with two valleys separated by a hill. A macromutation, a large jump from the edge of the lake in one valley could land you in, or close to, the lake in the other valley. If the other valley were deeper and steeper (representing a highly specialized species), merely landing on the other side of the watershed might lead to rapid evolution to the new species—the flow down the steep hillside would be rapid. The mutants, though ill-adapted, must become rapidly fitter by micromutational (Darwinian) selection or perish anyway.
Far from macromutation being always unsuccessful in a multidimensional space, there are rare occasions when it is successful and provides the sudden jump that the fossil record needs to explain the distinctions between major groups of organisms like families.
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