Evolving

by John Toscano

Created on: April 12, 2008   Last Updated: August 23, 2008

The question is not so much one of whether humans are still evolving, but we should rather be asking ourselves, "In what direction is human evolution now heading?" What are the selective pressures acting on contemporary human populations, how do they differ from those that drove the evolution of our recent ancestors and in what new ways might we expect humans to evolve? As a matter of semantics, the word devolving, from the verb to devolve, is not an antonym of evolving and refers instead to the act of devolving responsibility or power to a person or group at a lower level in an organisation, such as government.

More to the point, the theory of evolution describes the mechanisms by which organisms change in response to the pressures of natural slection. We may sometimes perceive, especially with the wisdom of hindsight, that changes are in some way retrogressive but this view is judgemental and misunderstands the process of evolution.

The ancestors of the kakapo, the flightless and now endangered parrot of New Zealand, did not degenerate (or devolve, for those who insist on using this word) into becoming flightless, nor did they lose the ability to fly purely by chance. Flightlessness evolved by natural selection because it conveyed a selective advantage on those birds who expressed alleles for this trait. We can surmise that those birds which retained their ability to fly were less well adapted to the habitat in which they found themselves at that time and lost out in competition with their flightless peers. The ability to fly became superfluous and therefore disadvantageous, wasting valuable resources that could better be directed elsewhere. The kakapo ancestors evolved other, more useful, adaptations to their coastal forest habitat, such as an exceptional sense of smell and the ability to accumulate large amounts of body fat. Change by evolution cannot proceed in reverse any more than time can go backwards. Just as the kakapo lost the ability to fly when this ceased to be advantageous, the ancestors of humans changed during their earlier evolution by losing such characteristics as body hair and prehensile tails, all in the cause of adaptation.

What then are the selective pressures driving evolutionary change in modern-day man? Darwin proposed that organisms produce more offspring than their habitat can support but that populations remain relatively constant over long periods. He concluded from this that there is a struggle for existence in which the best-adapted organisms survive and reproduce, passing on characteristics that are favourable to their offspring. Later it was discovered that it was through genes that characteristics are passed from one generation to the next. These simple Darwinian statements generate questions that we can use to examine current human evolution.

Are humans reproducing producing more offspring than the environment can support? That is a difficult question to answer. The World can certainly grow enough food to feed far more than the current population, but only at a great cost to the environment. Burning fuels at a phenomenal rate and therefore altering the world's climate, contaminating the environment with all the different waste products of our activities, damaging ecosystems and precipitating mass extinctions, the list goes on. Rising human population has potentially catastrophic consequences in the future that threaten to seriously reduce the carrying-capacity of our global habitat. So my answer is a tentative yes, we do seem to be reproducing at too high a rate and the chances are that in the near future, natural selection will redress the balance.

Is there a struggle for existence in which those humans that are better-adapted reproduce more? Or put into more modern language, are there differentials in human reproductive success? The answer of course is an emphatic yes. To take two extremes, many modern-day humans choose not to reproduce at all, whilst in many cultures across the world polygamy remains common and it is not unusual for men to have several wives, each with several children. Whatever characteristics he possesses that enable him to achieve this degree of reproductive success will be passed on to his offspring and it would be naive to believe that there is no inherited genetic element in this.

No organism can 'devolve', because evolution is a one-way process that will always be taking place as long as organisms produce offspring. Here we encounter a striking but self-evident truth that every one of our ancestors, going right back to the first life forms of the Pre-Cambrian, reproduced successfully, as otherwise we would not be here today to debate evolutionary theory. That is a quite staggering number of unbroken generations of uninterrupted reproduction. Some of us in the present-day might consider it both ethical and prudent, in view of the uncertainties facing our planet, to opt out of the reproductive process but those who make such a choice will find themselves at the end of a very long evolutionary line as far as their particular genes are concerned.

Human evolution is most certainly still very much in progress but natural selection does not proceed at a constant rate and it may be that change is relatively slow for now. The selective pressures acting on contemporary humans are complex and poorly understood and we are already discovering that certain traits that were advantageous in our ancestors may now prove to be our undoing. Just like the kakapo, many humans evolved an ability to accumulate fat extremely well during times of plenty. This must have helped their ancestors survive periods when food was scarce but now, when abundant food is available at any time in much of the world, we are increasingly concerned about an obesity pandemic that raises new global health issues.

Modern medicine has reduced the selective pressures created by many pathogens. For example, in former times a person with blood group O was far more likely to get seriously ill if infected with the bacterium vibrio cholerae, whilst this may no longer matter very much, at least not for those enjoying good sanitation in the developed world. Other pathogens still exert a strong selective pressure; in much of Africa, where up to three million people die of malaria every year, sickle trait provides a survival advantage over people with normal hemoglobin in regions where malaria is endemic. This provides a salutary reminder that the pressures exerted by natural selection remain much harsher in the Third World, as opposed to the developed world.

We should celebrate how medical advances now offer much better life prospects to people with inherited conditions that might previously have been fatal at an early age, offering the chance of reproductive success to people with such traits. I object to the notion that this represents some kind of backward evolutionary step but it certainly has implications as far as human evolution is concerned. Perhaps much of the evolutionary change currently taking place in humans is an accelerated genetic drift, involving the redistribution of various alleles amongst human populations around the world through increased migration and interbreeding between populations that were previously geographically separated. What are the long-term consequences of individuals carrying the gene for sickle trait migrating to non-malarial regions? Or of changes in the frequency and distribution patterns of many other human alleles? These are very complex questions but the answers, when found, will help develop an understanding of the ways in which humans are now evolving.

Changes may appear slow for the present but the future may prove rather different for mankind. Growing human populations and differentials in reproductive success ought to create the ideal conditions to drive rapid evolutionary change when altered circumstances in the environment demand it. The more extreme the changes are when they come about, the more intense the selective pressures that will be created and the more rapidly they will bring about evolutionary change. At present we are members of a world population going through a phase of steep growth under what may well turn out to be optimal global climatic conditions. Perhaps this has made us grow complacent and unaware that in the longer term our planet could prove to be a somewhat precarious place; it is certainly hard to believe that the current rate of population growth can continue unchecked.

Natural selection does not make long-term plans and I chose the kakapo to illustrate how, with the benefit of hindsight, it can be revealed that solutions found by natural selection to the problems of today may have dire consequences in the future. The kakapo could hardly have "known" that its flightless habit would leave it cruelly exposed to cats, rats and other predators brought to New Zealand by European colonisers. I could have chosen the unfortunate dodo to illustrate the same point, though it looks likely that ultimately the dodo and the kakapo will share the same fate. Not that either species should feel too aggrieved, as 99% of the species ever to have existed reached a point in their history where their environment changed, natural selection found them wanting and they became extinct because they were unable to adapt. As for humans, a natural catastrophe, an alteration in climate patterns, a serious global change caused by human activities, or a new virus that is lethal to 99% of its hosts, any of these could provide us with a dramatic reminder that, as far as our species is concerned, natural selection is still very much in business.

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