Are We Still Evolving?

When we were told about evolution in biology classes, we were introduced to it as a process that species underwent over great periods of time. We were taught that evolution requires a selective pressure, something that causes the preferential retention of a heritable trait within the gene pool leading ultimately to the delineation between species observed in modern times. However, most of us would have discussed evolution with the detachment characteristic of the classroom, such that it would have seemed like a process that only other species underwent. What we did not consider was whether evolution was currently happening to us, Homo sapiens.

The first part of this story comes from a gene called Microcephalin, which is thought to be involved in determining brain size in humans.

In September 2005, Bruce Lahn, a professor of human genetics at the University of Chicago, announced that a variant of the Microcephalin gene had increased in frequency in our gene pool at a rate that was too rapid to be ascribed to chance (1, 2). Tantalisingly, the only other explanation apart from chance that could have caused this change is a strong positive selection in favour of the gene. Is this evidence that our species is still evolving?

Those who believe that human evolution ceased with the emergence of modern man in the form of Homo sapiens point to what they perceive as the cessation of natural selection. Natural selection works by ensuring that the individuals who are best adapted to their environment are most likely to survive and reproduce leading to a greater representation of their genes in subsequent generations. Modern medicine and state provision of welfare have removed these selection pressures by allowing those who are less well-adapted to survive and reproduce successfully. According to Steve Jones, a geneticist at University College London, survival in the developed world no longer depends on genes. “Just 500 years ago - yesterday in evolutionary terms - a British baby had only a 50 per cent chance of making it to reproductive age. Now, the fi gure is around 99 per cent,” Jones says (3). Furthermore, he adds, “No longer, as in the Middle Ages, do a few rich men have many children while many of those in poverty are forced into the army or into monasteries.” This levelling of reproductive success across the board has led to fewer opportunities for natural selection to act.

Moreover, it is argued that cultural evolution outstrips its genetic counterpart as the dominant force in recent evolutionary history (4). Darwinian mechanisms have led to a complex human brain, capable of creating a sophisticated culture, which in turn enlarges the pool of ‘memes’ – Richard Dawkins’s neologism that designates a ‘unit’ of transmission of culture (5). This has provided a non-genetic means to adapt to change, via a culture-mediated, memetic mechanism. Hence with time, our brains and subsequently our culture have become increasingly complex, strengthening the memetic mechanism whilst paradoxically vitiating that of Darwinian natural selection. In modern times, memes and/ or technology may even dominate to the extent that they force genetic evolution to counter natural selection.

This, however, confuses the issue: for even with the intervention of memes and technology, evolution in the genetic sense can still be occurring, albeit perhaps in a direction opposite to that which traditional selection pressures would have chosen. Culture itself, then, can be regarded as a selection pressure. In fact, the vast disparity between the complexity of the human brain and that of other species can be attributed to a cycle of positive feedback between our culture and genes (6). As culture became more sophisticated, society increasingly favoured the witty, articulate and intelligent, whose genes were keyed to complex brains and hence the creation of culture. Evolutionary psychologist Geoffrey Miller points out that such individuals were likely to have been more attractive sexually (7), leading to the propagation of these genes throughout the species’ gene pool - assuming that traits such as wit are inheritable. Culture in turn would have become even more complex, reinforcing the reproductive success of some individuals, and so on. This self-reinforcing cycle thus led to the runaway complexity of the human brain. Given that our culture is more complex now than it ever was, in all likelihood it is as potent a selection pressure, if not more so, as it was inthe recent past when it played its crucial role in augmentingour intelligence. Similarly, the evolutionary power of sexualchoice is likely to be undiminished.

Further evidence for continuing human evolutioncomes from several studies. Lahn’s study on Microcephalin isan example, as is one by Voight et al which scanned newlyavailable single nucleotide polymorphism data from theInternational HapMap Project, and found widespread signalsof recent positive selection in genes such as those thatgovern morphological attributes like skin pigmentation (8).

So, are we still evolving? Firstly, it is important to clarifythat complete stasis of the gene pool is not possible, giventhat genetic drift – the fl uctuation in allele frequency due tostatistical variation operates under all circumstances, with orwithout natural selection. We need only consider, then, thenet effect of changes to selection pressures.

These may have diminished due to modern medicineand welfare schemes, but as we have seen, there is clearevidence that they have not ceased entirely. In other waysthey may even have increased in importance. The globalpopulation has burgeoned in the last century, providingmore genetic material for natural selection to work on –due in part, paradoxically, to medicine itself. Culture andsexual choice continue to operate as selection pressures, asdo infectious diseases such as malaria that have not beenentirely brought to heel by medicine. It is clear, then, thatwe continue to evolve.

Other studies demonstrate that we are not justevolving, but evolving in different directions according toour geographical location. For example, alleles conferringresistance to malaria are strongly selected in Africanpopulations (9-11). The widespread lactose intolerancefound in Asia and Africa but not in Europe can also beexplained by the differing selection at the lactase gene (12),which could have been heightened in the latter populationsdue to the greater use of dairy products in their diets.

The differential evolution of intelligence between races,a very controversial topic, has also been researched in thesame way. A recent paper by Cochran et al was publishedto favourable coverage in the press (13), and presenteda scientific argument for the increased intelligence ofAshkenazi Jews relative to other ethnic groups, via selectionpressures largely cultural in origin (14). Conversely, thepaper has been criticised as relying on a hypothesis whichis unsupported by non-circumstantial evidence (15). Thissuggests that it is diffi cult to attribute particularly sensitivegroup differences – such as intelligence – to genetic andevolutionary origins, even in the best of papers.

It does seem, however, that this paper is a very shortway from showing that there is a racial difference inintelligence. With this in mind, is this a question we shouldbe exploring? The risks seem obvious: science used to justifyracist prejudices and agendas would be science misused.Even with this in mind, the genetics of groups remains animportant frontier in research. It provides medicine withstatistical data for the development of treatments specifi cto certain groups, which can be more effective. Given thatresearch in this area is important and likely to continue, wecan expect to have to grapple with more discoveries, likeCochran’s, that will be jarring to our egalitarian values.

It is therefore comforting for us to bear in mind thateven where the evidence for group differences as a resultof continued human evolution is seemingly undeniable – aclear instance of which can be found in DNA analyses likeTishkoff’s (11) – it is instructive to keep in mind that groupdifferences, where proven, will apply only to group averagesand not to individuals. As Steven Pinker, a professor inpsychology at Harvard, points out, “There are geniuses anddullards, saints and sinners, in every race, ethnicity, andgender” (15). Accepting the evolutionary and genetic basisof group differences would not be incompatible with theequable treatment of individuals on the basis of who andwhat they are, and not on the basis of which group theybelong to.

On a divergent note, even if we acknowledge ourcontinued evolution as a species, there are questions left tobe answered. Are the genetic differences demonstrated bythe said studies transient or would they persist as permanentgroup-distinguishing characteristics? For even at very lowrates of inter-breeding between different populations, genepools would tend to average out their differences overseveral generations. In other words, without reproductiveisolation – which is unlikely given our globalised society– genetic differences, if any, would be transient on anevolutionary timescale. If they are, all groups of humanscan ultimately be said to be equal, vindicating those whobelieve in equality.

However, if interbreeding was discouraged by aselection pressure, such as social prejudice, differentpopulations would only reproduce amongst themselves. Ina simple scenario of two populations, both with markedlydifferent levels of a trait, intelligence for instance, this wouldover time result in an absence of individuals with moderateintelligence – if we were to make the simplistic assumptionthat a child’s intelligence is the average of her parents’.Instead, we end up with two groups of people separatedby a vast gulf in their intellectual capabilities. In geneticterminology, disruptive selection would have occurred,polarising the distribution of a level of a trait. If this wereto occur, genetic differences between groups of humanswould persist and we could ask ourselves: in which directionare we evolving? Are we diverging into separate branches,as in disruptive selection, or is our species as a whole movingin a unifi ed direction? Or does some other evolutionary fateawait us?

Given that there is little to no reproductive isolationon Earth, and that there are conceivably few selectionpressures preventing interbreeding, we are unlikely toevolve into distinct new species. But if humankind were tocolonise space, colonists of distant planets could becomereproductively isolated from those on Earth, leading tothe evolution of new species adapted to alien climes.Moreover, all species are fated to either die out or evolveinto something else, but Homo sapiens may be unique inhaving an alternative: a “transhumanist” future, attainedby harnessing technology to transcend the limitations of ourbodies and brains (16). This is speculation, however, whichwe will leave to science fi ction. For the time being, it is clearthat wherever we may end up, the story of human evolutionis far from complete.

Victor Chong is a 2nd year student at Gonville & Caius College reading Medicine.

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