The Powers of Natural Selection

An interesting form of directional change is the process called by Sir Alister Hardy behavioural selection. This means a change in behaviour that alters the relationship of animals with their environments, so that different characters become advantageous, different characters become disadvantageous, the conditions of natural selection are altered, and it selects for structural, physiological or biochemical changes. The process was described independently by Ewer, Hardy himself and Sevenster. (Russell and Russell, 1990) For instance, suppose a few members of a bird population adopt a new feeding habit, and this habit is socially transmitted to other members by imitation and to the next generation by tradition (both processes well known in mammals and birds). Natural selection then changes the structure of their beaks, to adapt better to the new food habit. Obviously the reverse sequence could not occur.

Hardy gives Lamarck credit for realising that behaviour change could lead organic evolution. The mechanism he proposed – the inheritance of characters acquired by the use or disuse of organs – turned out to be wrong. But this mechanism (accepted by Darwin) remained perfectly plausible long after his death. ‘Much had to be learned before Weismann, in the 1880s, could proclaim the integrity of the germ plasm’ (Russell and Russell, 1990). Since we wrote this, it has appeared that Weismann’s proclamation was mistaken, though, like Lamarck’s mechanism, it was a reasonable idea at the further stage our knowledge had reached in Weismann’s day. It is now known that the barrier between the soma (non-reproductive cells) ‘and the germ line is not absolute. In some organisms it does not exist’, and this may have consequences at the cellular or molecular level. (Jablonka and Lamb, 1999) Such consequences would not affect any matters discussed in my series, and there remains no evidence for Lamarck’s mechanism based on the use or disuse of organs. But his awareness that behaviour change can lead to organic change was the insight of a great scientist.

Behaviour selection becomes very important in man, because cultural evolution is constantly changing the relationship of human populations with their environment. Claire Russell (1978) pointed out that the discovery of fire must have led to natural selection for reduced hairiness, because of the extreme danger of being very hairy when working with fire. In 1393, the crazy Charles VI of France, with five of his nobles, had the very foolish idea of turning up at a wedding celebration in very hairy fancy dress, like hairy beasts or savages, to surprise the guests. (Autrand, 1986) The Duc d’Orleans approached them with a torch to see who they were, and all six caught fire. The Duchesse de Berry saved the king by smothering the flames with her long train. One of the nobles rushed into the neighbouring kitchen and jumped into a tub of water. The other four burned to death. Poe used the incident as the basis for his short story ‘Hop-Frog’.

The art works of the Upper Palaeolithic show that human beings had certainly ceased to be very hairy by then. But the making of stone scrapers suggests the making of clothes (and by inference reduced hairiness) as early as the Riss Ice Age. The control of fire was discovered some time earlier. ‘In the early and, so to speak, ‘experimental’ stages of fire control’, coarse hair would have been extremely dangerous, and selection for reduced hairiness ‘would be powerful and rapid’. (Russell, 1978) Claire Russell notes that all this explains the different distribution and fineness of hair in men and women. ‘In a survey of 224 modern tribal societies, it was found that fires were made and tended always or usually by men in only 24, whereas they were made and tended always or usually by women in 84 societies’. (Russell, 1974) ‘Lord Raglan has shown that the ritual maintenance of sacred fires has also been generally entrusted to women’, for instance the Vestal Virgins and the nuns of St. Bridget. (Russell, 1974) ‘If women were primarily responsible for fire control in its earliest stages, they would have been particularly vulnerable to having their fur catch fire, and their faces, breasts and limbs would have been most exposed to the danger. Hence the reduction of hair, or the presence of particularly fine hair, on just these parts of the female, as opposed to the male, human body.’ (Russell, 1978)

Human teeth provide further examples of behavioural selection. (Brace, 1962) There is a considerable reduction of the size of the molar crowns in the later specimens of Homo erectus compared to the Australopithecines. Brace ascribes this to the cooking made possible by fire control, which reduced the need for such a large grinding surface. At this stage the incisors were enlarged, probably for biting off chunks of meat when bigger game could be caught (for instance with fire-hardened wooden spears). But after fine flint blades were developed in late Mousterian times, they could be used for cutting slices of meat, and the incisors were reduced again.

‘The first evidence of cattle domestication comes from Europe (Greece) in the seventh millennium BC.’ (Russell and Russell, 1983) Natural selection has enabled the Europeans to digest cow’s milk, unlike many human populations, since they continue to have the enzyme lactase (which digests lactose) after weaning. This useful character was obviously selected after cow’s milk became regularly available, therefore since the seventh millennium BC. (Russell and Russell, 1983; Cavalli-Sforza, 2000)