Thursday, August 31, 2006

Are Humans Still Evolving?

Evolution may be defined with or with out the requirements of selective pressure, but in terms of discussing the possibility of current human evolution it is only sensible to accept a definition that is selection inclusive. Accepting this, fact based arguments which suggest the absence of current human evolution may seems valid, but can be easily refuted on the basis of 4 common misperceptions of evolution that lie as hidden assumptions behind such claims. These four classes of error will be outlined below and the relevance to the types of arguments raised that claim humans are no longer evolving will be made apparent.


Originally, evolution meant ‘unfolding’ [1] and was most often used to refer to the process of development - the unfolding of a series of specific events leading to a final product. For instance, an acorn would evolve into an oak tree, a fetus into a baby. As the world view gradually changed during the enlightenment period of the 18th century - from that of a stationary world created by God into a world which gradually shaped by geological change over a considerable period of time - it was natural that this term should be applied to the ‘evolution’ of the world. The association to biology quickly followed as the idea that species may not be immutable gained favour and several possible theories emerged, including Lamarck’s and eventually Darwin’s. Since this time Evolution came to be particularly strongly associated with biology and the ‘unfolding’ of species over time. Although Ernst Haeckel’s famous claim that ‘Ontogeny recapitulates phylogeny’ may no longer be accepted, the word once used to describe ontogeny was quickly adapted so that phylogeny could be described in exactly the same way: Evolution. From our modern standpoint though evolution is much more explicit than just the unfolding of species - it has come into a much more meaningful and exact description, commonly defined as ‘the change in the gene pool of a population over time.’ [2]


Using this definition, the possibility of questioning whether humans are still evolving is not even worth asking. The simple fact is that change in the gene pool over time in any species is completely unavoidable. Eyre-Walker and Keightley claimed in 1999 that humans have had on average 4.2 amino acid altering mutations every generation since humans separated from chimpanzees [3]. This measurement ignores the synonymous substitution of base pairs, and all of the mutations which occur in the non-translated regions of Genome DNA. Since only about 1.5% of the human genome is translated [4], this number is quite incredible. With this introduction of change every generation, the ‘change in the gene pool over time’ is assured. Genetic drift is another mechanism through which gene frequencies are changed overtime, and its occurrence is an undeniable phenomenon. Chance events lead to the increase or decrease of numbers of a particular gene in a population, occasionally leading to fixation of a gene (no other variants exist) or extinction of that gene.


These two prime examples of random changes in the genetic makeup of populations are accepted biological phenomena which apply to humans as much as any other species. To say that these two phenomena are classified as evolution means to say that Humans evolve. Of this there can simply be no question.


It is therefore clear that another more specific definition of evolution must be used in order for any sort of productive inquiry into this subject to take place. To account for the affect of random changes alone being considered evolutionary a definition that requires a selective pressure can be used. By defining evolution as ‘the change in a gene pool over time due to a selective pressure’ we no longer have the random changes problem, and the people claiming that humans are no longer evolving actually have something that they can use: The possible lack of selective pressures. From now on this will be the meaning of evolution for the rest of this paper.


Evolution may be directed by a number of selective pressures, one being sexual selection, and most of the others falling under the general title of natural selection. Natural selection affects the evolution of species in every aspect of their life, from their developmental rate, to their ability to survive to reproductive maturity, their ability to find and copulate with a mate, the viability of those offspring, and how much longer after sexual maturity/copulation that organism may continue living. Natural Selection is the true driving force behind any sort of adaptive evolution.


With selection included we can now describe evolution according to a theory described by Dennett in 1995 based on the earlier work of Lewontin and Brandon [5]. In this definition evolution is said to occur whenever there is variation, heredity and a differential ‘fitness’ (i.e.: allowing a point of action for a selection pressure). In the case of all biological creatures heredity is an absolute standard which goes without saying. The fact that there is replication of the genetic makeup from one organism into the next generation is the backbone of the process of evolution. It is upon this backbone of heredity that variation may build up, brought about by mutations, and that differential fitnesses may arise and in turn be selected. So the question now becomes, “Do humans have variety in their gene pool, and is there a differential fitness to these variants?”


Those who believe that humans are no longer evolving accept that we have variety. As pointed out above with the previous definition of evolution, we have 4.2 Amino Acid altering mutations every generation, and then we have genetic drift; it is entirely unreasonable to claim that we do not have variety between humans. What they do doubt though, is that there is any selective pressure left. They claim that due to the advent of modern medicine, technology, farming techniques, food distribution, heating and cooling systems etc, there are no longer any selective pressures in our lives to separate the fit from the unfit. The fitness differential is irrelevant in the environment that we have made for ourselves because we do everything we can to make sure ‘everyone’ survives. Additionally, even if some people die to unforseen virus or bacterial outbreaks etc, then although they may die, the reaction of medical intervention will be infinitely quicker than that of the evolutionary adaptation to the new selective pressure, and so no net evolution will actually occur. The capabilities within our modern society and the speed with which our culture adapts to change has completely overruled the process of natural selection and so stopped evolution.


There are several problems with these claims. These problems can be placed into several classes of error including;

1. Misunderstanding the nature and power of Natural Selection,
2. Forgetting other forms of selection, such as sexual selection,
3. Assumptions about the entire world from the specific first world lifestyles of the very people claiming this, and
4. Mistakenly taking the term ‘current evolution’ to mean that evolution must happen before our eyes.


Class 1

The first class of error may actually be the most subtle. It comes from the assumption that selection only works on the more obvious phenotypic traits and little else. In its worst form this error is manifest in the claims that humans are de-evolving (an oxymoron in itself) because we are creating easier lives for ourselves, resulting in future generations who have evolved weak skeletons, fat bodies and slow reactions etc. While an easier life may allow for these phenotypic changes to exist, to say that we would evolve in that direction is to either revert to the previous definition of evolution, or misunderstand how evolution due to natural selection works. Evolution due to natural selection occurs only in such a way that better adapted creatures become more prevalent than less adapted creatures. If bodies with weaker skeletons (for example birds), more fat (for example seals), and slower reactions (for example sloths) were advantageous to humans, then that is how we would evolve. If that happened to be the case, then the irony would then be that these phenotypes would be advantageous (direct inference from how evolution works), and the claims of ‘de-evolutionists’ would be shown for exactly what they are, oxymoronic.


The more subtle side of this can be made clear though, in realising that this altering of what is and what is not advantageous from era to era is entirely unpredictable to us. We perceive certain things as ‘good’ attributes (commonly: Sharp teeth, strong muscles, fast runner, intelligence) and other things as ‘bad’ attributes (commonly: obesity, skin prone to sunburn, unco-ordination) and we decide that anything which departs from the good and/or acquires more bad attributes is losing its selective advantage. While this may often be true, the fact of the matter is that our own judgement has been crafted by evolution and we are biased in our judgements towards the things which were adaptive in the past. We have no idea what is going to happen next and so we can’t be sure that our crafted judgements are any longer valid. As well as that we have no way of knowing what hidden benefits may lay under some superficial phenotype. Combine these two consideration and you are faced with a situation in which you may have superficially ‘bad’ (according to our current judgement) phenotypes with underlying attributes which may in the next few hundred generations come to be so advantageous that they create a selective pressure in themselves. Darwin himself observed that “the struggle will generally be more severe between species of the same genus, when they come into competition with each other, than between species of distinct genera,” [6] and so it is with humans already, and probably will continue to become more and more as we reach the limits of our extended niche. What variation it is that holds the key to the adaptive advantage is surely unknown to us, but it seems incredibly unlikely that the advantage will be with those able to outrun or successfully hunt a lion.


On a less subtle level though there is one more element within this class of error that is ignored by people who claim that humans are no longer evolving. Natural Selection is thorough. Amongst all of the variation which we can and cannot discriminate superficially, Natural Selection screens everything. Natural Selection, unlike our ability to pass judgement, is an unrelenting eternal force sifting through every single probabilistic relation within an organism, between members of a community, between organisms in a species, and between an organism and its environment; all at once. To say that modern humans are no longer under a selective pressure is to claim two things: It is claiming that we know what Natural Selection works on; and it claims that we have used this knowledge to control every single instance of potential selective pressure. We certainly do not know this, and we most certainly have not controlled it. Humans are just as subject to selective pressures as every other organism, even if we can’t see them.


Class 2

The second category of error is simply a case of forgetting that there is more to selective pressures than mere survival. The need to procreate is just as important in evolution, and to procreate humans need to find mates. Sexual selection is present throughout nature and is undoubtedly present in Homo sapiens too. One theory even claims that our enlarged brains, our paedomorphic ape appearance, the size difference between males and females, and various other factors are all consequences of sexual selection [7]. Medicines, technology and abundant shelter will never affect the role sexual selection plays in the evolution process, but culture itself may. It is almost impossible to guarantee that our sexual desires, choices and behaviours are guided by our own and our potential mate’s genetic make up, rather than being guided by the culture we live in. To make it a little more complicated, its not even easy to figure out whether our culture is largely guided by our genetic make up, and therefore only an intermediate between our genes telling us what we want and what we actually choose. Whatever the case one fact remains: Sexual selection - on whatever level – occurs. As long as it continues to occur there will be a selective pressure present, and evolution will occur.


Class 3

The third category of error is the belief that the entire world is like the society we are lucky enough to live in. A society where medicine is provided for everyone, where housing is plentiful, where there is available electricity, running water etc. The fact of the matter is that this isn’t the case at all, but instead around 80% of the world’s population lives in developing countries [6]. If we are to talk about the evolution of Humans and we want to focus on one lifestyle, it would in fact make much more sense to focus on developing countries and talk about their way of life. Of course though, if we were to do this, then most of the points raised about medicines, technology, distribution of food, and general ease/comfort of life would no longer apply, and there would be no case. Obviously when the claim ‘Humans are no longer evolving’ is made, the claim is actually meant to be ‘Humans in developed countries are no longer evolving.’


Class 4

The fourth class of error actually intermingles with every other class on some level. Evolution takes many thousands of years to occur and must be discussed accordingly. To talk only about the way things are now and then to try to infer facts about evolution from that flash of existence, is to fall into this error.


This particular error is all encompassing in its nature and is the sort of error that humans are very prone to make. Being organisms that deal with time in units like seconds, minutes, hours, years, and even up to decades, the concept of hundreds of years or several hundred years turns into ‘A long time’ and nothing else. To think of a hundred years, is to think of something only just graspable. To think of a thousand years though, is really something beyond our grasp and we tend to resort to ‘a really long time’ and that is as far as our imagination goes. We may be able to grasp some sort of awe over the length of it, but we do not comprehend it. Tens of thousands of years, millions of years, billions of years all meld into this one conception of ‘a really long time’ and nothing else. There is little meaning in any of it. To then speak of evolution, something which takes thousands of years for any real changes to start being apparent, is to talk of something which we can’t grasp the timeframe of. Falling into this class of error when talking about Evolution is almost inevitable for anyone not consciously aware of this problem.


Understanding now that evolution only works over the course of many thousands of years means that the claim that ‘humans are no longer evolving’ translates into ‘humans will not evolve at all over the next few thousand years’. Realising this, to maintain the claim that humans are no longer evolving is to claim that our control over our environment is so all encompassing and so certain that nothing that happens will break our control. It is a claim that we will never run out of food, that our population growth will never reach maximum capacity, that rising waters will never cause massive loss of farmland or living space, and that no virus or bacterial pathogens will ever break out into a pandemic. It is to claim that humankind has already completely conquered nature in all of its forms.


Whether we accept evolution as something that occurs with or without a selective pressure, the arguments presented to show that humans are no longer evolving tend to become meaningless in light of how evolution actually works. The points may seem valid in some regard, but they all miss a vital point somewhere and so can be easily shown as the empty claim they are. Humans are varied, humans are being selected, and a time will come not so far off in the future when massive selection may be applied as a consequence of our own actions. Humans are still evolving.



References

1. Webster’s 1828 dictionary, http://www.christiantech.com/
2. Chris Colby. Introduction to Evolutionary Biology, Version 2. 1996. http://talkorigins.org/faqs/faq-intro-to-biology.html
3. A. Eyre-Walker, P.D. Keightley. 1999. High genomic deleterious mutation rates in hominids. Nature 397:344-347
4. B Alberts, A Johnson, J Lewis, M Raff, K Roberts, P Walter. 2002. Molecular Biology of the Cell 4th Edition, Chapter 4, p 202, table 4-1.
5. D Dennett. 1995. Darwin’s Dangerous Idea: evolution and the meaning of life. New York. p 343
6. C Darwin. 1859. The Origin of Species, first edition, reprinted in penguin classics 1985, chapter 3:p127.
7. D Brin. 1996. Neoteny and Two-Way Sexual Selection in Human Evolution. Journal of Social and Evolutionary Systems 18(3):257-276
8. http://www.unfpa.org/sustainable/demographics.htm

3 comments:

Shane said...

This was written several years ago by myself as a Uni assignment. It was the written component of a debate we gave. I think that is why I talking about evolution without a selective pressure in it. Because to be honest, I'm not even sure what I mean now. That hardly makes sense to me.

I am pretty sure though, the intention is to say that even if a population is under no specific selective pressure, that doesn't mean 'Evolution has stopped'...it just means the culling section of evolution is not happening. But evolution still persists.

SubJunk said...

Yeah of course evolution doesn't stop, it just doesn't happen in a particular direction/s.
It's a good article man I enjoyed it :)

Kazza said...

Yeah great post. I've thought about human evolution before, but could never really see how it could be at work. Sure sexual selection will lead to some people having more children than others, but since (at least in the western world) almost everyone lives to reproduce I didn't see it as evolution.

I suppose it is since it changes the gene distribution of the population, and pretty soon we'll reach the time when we can no longer grow exponentially. Anyway, great post, lots of food for thought.

Kazza