People seem to come in all shapes and sizes. Do any other animals display the same amount of variation in size as humans among healthy adults? Why is there this variety, and what are its consequences for the future?
鈥� Yes 鈥� the dog, for one. Canis familiaris has had an extremely secure niche for a long time, and it has human breeders and carers. This disrupts adaptive evolutionary strategies which would normally enable some individuals to do better than others in the wild. The species does not have its non-adaptive (and arguably useless) variations weeded out by challenges to its survival, so they have room to proliferate.
But of course any old 鈥渦seless鈥� feature could one day prove to be an advantage. This can happen either directly, as with peppered moths evolving in industrial England (darker ones survived better when their world turned black and dirty, a phenomenon called 鈥渋ndustrial melanism鈥�) or indirectly, for example if a gene variant that results in bright pink eyes also gives rise to more flexible blood vessels, protecting against atherosclerosis.
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鈥淚n domesticated animals, breeders can produce unusual traits that would be eliminated in the wild鈥�
Trischa Mann, Ballarat West, Victoria, Australia
鈥� Individual variation in traits like body shape and size is partly to do with differences in lifestyle and environment, but genetics also plays an important role.
In all living things 鈥� including humans 鈥� genetic variants are constantly being introduced through mutations, with a mistake in copying DNA resulting in offspring carrying a modified gene that was not present in the parent(s). In species that reproduce sexually, genes are reshuffled into new combinations with each generation. Because many genes interact in determining traits like body shape, this widens the variations between individuals.
In wild animals, the amount of variation between individuals is likely to depend on how important a particular trait is for survival. If an animal needs to be within a narrow size range to survive, natural selection will weed out any variants outside this range; if not, a greater range of sizes will be seen. Species are likely to have more variation in some traits than others, for example colour over size.
If an animal is domesticated, breeders can produce a far greater variety of unusual traits, such as with dogs. Most of these traits would be eliminated by natural selection in the wild.
In humans, there is clearly variation in body size and shape as well as a range of other features, and we are more likely to notice these because they are in our own species. Relative to animals, the amount of variation in the human species is not considered unusual, and may in fact be a little on the low side. The peoples of Africa are the most genetically diverse compared to those from other continents, which were founded by small migratory populations. These lost some of the variations that had built up in Africa, where our species originated.
In today鈥檚 developed world, it could be argued that humans are no longer subject to natural selection as our ancestors were. Women with body shapes that would once have made childbirth dangerous can now opt for a caesarean, and this increases the range of body shapes that can persist in the population. If this state of affairs continues into the long term, the amount of variation in body size and various other traits will probably gradually increase.
鈥淲ith the advent of tools that allow editing of genes, future variation in humans may be of our own making鈥�
However, with the development of molecular tools like CRISPR that allow editing of genes, it may be that future levels of variation in humans will be of our own making.
Lachlan Jones, Brisbane, Queensland, Australia
鈥� That humans vary in size is surely a tool of evolution. If a species did not vary, then it would become extinct at the first change in the environment. Size 鈥� both height and body shape 鈥� is just one trait that can be selected for. It has been suggested, for example, that the Inuit are short and stocky because this minimises body surface area and thus heat loss in cold climates.
Individual variation, enforced by sexual reproduction and the mingling of parental genes, ensures that people are all different. But we are not that different compared with domesticated animals. For them, human rather than natural selection has driven change, producing dog breeds that range from Great Danes to tiny chihuahuas. We have also bred small Shetland ponies and shire horses, the latter standing anything up to 2 metres tall from ground to withers.
John Davies, Lancaster, UK
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