Although some people may try to refute the theory of evolution, nobody can deny that natural selection occurs. We can demonstrate this quite easily within a single human lifetime, and humans have been inadvertently using natural selection to our own advantage for over 10,000 years. The processes I’m discussing, of course, are artificial selection and domestication.
When Darwin first began to think about evolution, one area of greatest interest to him was domesticated species, in particular the pigeon. The pigeon exists in around 300 of varieties, which have been selected for by pigeon fanciers for at least 5,000 years. The similarities between domestic pigeons and their wild counterparts are clear, however it seems that humans have, over many pigeon generations, been able to shape many aspects of their appearance including plumage colour and shape, body size, and beak shape. Other domesticated species such as dogs, cattle and even crop plants, have undergone significant changes in their appearance and internal anatomy since humans first began breeding them.
We began using natural selection to our advantage before we had even the slightest comprehension of its existence or its mechanisms. Early hunter-gatherer societies were associating increasingly with other animals, such as wolves and ungulates such as sheep and aurochs (ancestors of domestic cattle). These associations began the process of domestication – as time went on each species adapted to gain the most benefit from the association and the partnership became closer. Much more recently, people became aware of the selective force they were able to impose upon their animals, and conscious domestication was implemented to increase the speed with which traits changed.
One of the most important relationships humans have is with dogs. Archaeological evidence suggests that there may have been associations between humans and the domestic dog (Canis lupus familiaris) as much as 30,000 years ago, and the association was definitely present by 14,000 years ago. However, genetic evidence indicates that the domestic dog diverged from it’s ancestor the grey wolf (Canis lupus) around 100,000 years ago. How can these vastly disparate estimates be reconciled? Very early associations between humans and the grey wolf, which left evidence of divergence in the domestic dogs genes, was with an animal still very much like the grey wolf. There is ample fossil evidence of human association with wolves as far back as 100,000 years ago. Behavioural changes may have occurred more rapidly than physical ones, leaving early human fossils associated with what was essentially a tame wolf. Domestication, and early associations between man and wolf, are believed to have begun in the Middle East. Farming began around 15,000 – 10,000 years ago in this region, and changes in human diet and food gathering practices, the tame wolves were put to work at new jobs. This in turn imposed new selective pressures on the wolves, and lead to the appearance of modern domestic dogs.
Subsequent human selection over the last 15,000 years or so has lead to a huge variety of different dog breeds, all of whom are, technically speaking, the same species. The diversification of dog breeds has happened over a remarkably short period of time. Of course, whether all dog breeds match the reproductive-isolation definition of a single species is debatable; for logistically reasons alone it is hard to imagine a Chihuahua being able to mate with a great dane! It may not have all been up to the humans in this relationship, however. Some people believe that domestication is a two-way process, and that wolves self-selected themselves through a tendency for naturally tamer wolves to be more likely to associate with humans and benefit from that relationship. Wolves were benefiting from this relationship too; by working together with humans they sought to gain access to a more regular food source and larger kills.
A domestication that makes less sense, superficially, is that of cats. Cats (domestic or wild) do not possess many of the traits that are thought to aid the process of domestication, such as a strong, but modifiable social hierarchy. The hierarchical structure of a wolf pack can easily be moulded into a master-dog relationship without many major changes. By contrast, cats are largely solitary hunters and fiercely independent. It is thought that the relationship between cats and humans began slowly, around the time that people started needed to store grain. It was at this time that they also suddenly found themselves with a mouse problem, and the wildcat (Felis sylvestris) found a plentiful source of delicious mice.
The pigeon is one domesticated animal who has been particularly well loved, at least by certain factions, for a long time. The domestic pigeon (Columba livia domestica) was derived from the Rock Pigeon around 10,000 years ago. The impressive variety of domestic pigeons, and the obvious control that pigeon fanciers were able to exert over their particular breed, featured heavily in Darwin’s “On the Origin of Species”. Pigeons have been invaluable as communicators on the behalf of their human partners because of their superb navigational skills.
As well as for protection, hunting, pest control and communication, animals have been domesticated simply as a source of food. Sheep and goats were domesticated in the middle east between 11,000 and 8,000 years ago, and proved to be good hearding animals. Cattle and pigs followed shortly after. These animals preferred to live in large groups, grazing and could be easily kept in a fairly small area.
Pre-adaptation and Domestication
Animal domestication has been extremely widespread, and some have suggested that many of the animals involved display a certain set of ‘pre-adaptive’ characteristics; for example flexible diet, fast growth-rate (and therefore short generation time), pleasant disposition and willingness to breed in captivity. These animals proved easier to take care of, faster to adapt to human control and offered humans the opportunity to interfere with their mate choices. All of this made the early associations with humans less likely to fail, but the domestication of animals like cats indicates that this is not the full picture.
If it seems to the casual observer that we have been rather successful in domesticating animals, our successes with plants are even more impressive. Not only in terms of the number of species and number of varieties produced but in the extreme changes we have been able to achieve; many of our common crops and vegetables would be unrecognisable were you to unwittingly encounter their wild ancestor.Domesticating plants has often involved a major change in their reproductive biology. In many examples humans have completely taken over control of the plant’s reproduction. Domestic bananas are triploid, meaning they don’t contain large unappetising seeds, but it also means they are totally incapable of propagating themselves independently of humans. Wheat (Triticum spp) is another excellent example. Wild wheat drops its grain onto the ground – these are its seeds! By contrast, domesticated wheat keeps its grain, and this enables easier harvesting. Early in the domestication of wheat, at some point a rare mutation arose which caused the plant not to drop the grain. If this mutation occurred in the wild, the plant would not sow its seeds and not produce any offspring. However, under human cultivation, this was advantageous, and this plant was preferentially used to plant next years crop. And thus, wheat became dependent on humans. Now, many of us are almost certainly dependent on wheat.
Different plants were domesticated in different regions; the bottle gourd (Lagenaria siceraria) was domesticated in Europe for use as a container prior to the invention of ceramics, maize, beans and cassava were domesticated in the Americas and rice and soy in East Asia. What characteristics of plants predisposed them to domestication? For starters, since plants can’t run away, or panic or generally attack their potential domesticator in fear, plants inevitably put up less resistance to domestication. However, one key component of their developmental biology may have predisposed them to the dramatic physical changes we have achieved. As well as small, single nucleotide mutations that are comparatively common, there are more serious mutations which affect whole regions or even whole chromosomes. These can be deletions or duplications, as well as other even more complex rearrangements. Duplications of whole chromosomes (known as trisomy) are quite rare, but can result from a failure to for cells to divide in egg or sperm (or pollen) development. Even rare is the case of polyploidy, where a whole extra set of chromosomes is present. Animals are extremely intolerant of chromosome duplications and especially polyploidy. They almost always kill the offspring before it even develops. In humans, only one case of trisomy is viable to birth; downs syndrome – the duplication of chromosome 21. Polyploidy is never viable in humans.
By contrast, polyploidy is easily tolerated in plants. Their development is even more segmental than ours, and it is thought that polyploidy is less likely to have negative effects on plants because it simply results in more of the same – more segments, longer branches, bigger petals, etc. And this has been extremely useful in domestication. Humans often wanted to select for larger plants with bigger flowers and more potent aromas; these provided more food and wooed more women. Although some have noted an increased occurrence of polyploidy in domesticated species, recent research suggests this may not have been the case. Nevertheless, polyploidy is certainly responsible for some of the more extreme alterations in physical appearance achieved by domestication, such as the cabbage plant.
All of this is extremely powerful evidence for the action of natural selection. But it isn’t really proof. It is co-relational, historical data, for which we cannot be sure of cause and effect. Not to worry, though, because over the last 30 years or so, scientists have been working hard to provide us with a wealth of experimental evidence that natural and artificial selection are capable of modifying a huge variety of physical and behavioural traits across the animal kingdom.
In 1950, Dmitri Belyaev began breeding Silver Foxes (Vulpes vulpes), and over many generations, by picking the tamest animals that responded most positively to humans, he created a domesticated silver fox. This new creature showed no fear towards humans and exhibited many dog-like traits, such as wagging it tail and licking it’s human. They also physically resembled dogs more than their ancestor, with floppy ears and smaller heads.
Much more rigorous examples come from species which are more suited to a laboratory, and whose generation time is shorter still. Several species have featured heavily in experimental evolution, including the fruit fly (Drosophila melanogaster), E.coli, stickleback fish (Gasterosteus aculeatus), mice (Mus musculus) and yeast (Saccharomyces cerevisiae. In 3 generations, marine sticklebacks were able to match the cold-tolerance of their fresh water relative. Horned beetles (Onthophagus acuminatus) have been selected for longer and shorter horns (7 generations), salamanders (Ambystoma talpoideum) have been selected to undergo or not undergo metamorphosis (5 generations) and mice have evolved to be more or less aggressive (11 generations).
Longer and more complex experiments have been performed with E.coli and also with mice. Over a 50 generation experiment, that is still running today, mice have been selected to increase their tendency to want to run on a wheel. This simple selective pressure has resulted in changes to their aerobic capacity, heart size, hind limb bone structure, and even to the complexities of their brain chemistry. Mice selected to run more have evolved to enjoy running more; the motivation and reward system in their brain has changed through alterations to dopamine function and the endocannabinoid system.
In E.coli, one long-term experiment has been running since the 1990s. The particular beauty both of the biology of this study species, and in turn the design of this experiment, is that at any given time a sample of the current population may be drawn out, frozen and kept for later. When thawed, that generation is resurrected and can be directly compared with later or earlier generations in a variety of different environments. So, you can take a sample of a normal, unselected population, just bumbling along in laboratory conditions. Then impose a certain selective regime for many generations, such as providing a source of a key nutrient which usually has to be manufactured by the cell, or adding a poisonous substance, and wait. At any given time in the selective process, the current population can be compared to the past population, and a quantitive measure of the changes that have occurred is provided. Over 2,000 generations, this experiment was able to achieve a 37% increase in fitness! A wealth of information has come from these experimental lines, detailing how evolution progresses and more recently studies at the genetic level to detect where changes have occurred.
The process of evolution is clear to observe within the span of one or just a few human lifespans, and evidence is plentiful from domestication, artificial selection and experimental evolution. Animals with generation times much shorter than our own are capable of displaying quite rapid changes that can be observed and documented in extraordinary detail. Long before Darwin explained the theory of natural selection, humans were utilising natural selection to domesticate livestock and companions, and to make improve the quality of our food. That these relatively small-scale processes scale up to large-scale evolution of whole genera and families is not hard to imagine over the vast amounts of evolutionary time available. And what is illustrated constantly, throughout the animal kingdom and the fossil record is that evolution is not a discrete process. The reason we find it so difficult to reconcile our need to categorise things with the evidence we see in the natural world is because in reality, the concept of a ‘species’ or a ‘genus’ is merely a human characterisation in a world of grey-scale. Nothing is clear-cut. Evolution is gradual, and it is caused by the exact processes we can now observe in the lab, just over unimaginable periods of time.
Articles in this Series:
- Intro: Reasons Why Evolution is True
- Part One: The Panda’s Thumb
- Part Two: Parasitoid Wasps
- Part Three: Ring Species
- Part Four: Galapagos Finches
- Part Five: The Quirky Human Eye
- Part Six: Homology
- Part Seven: Coevolution
- Part Eight: PreCambrian Rabbits
- Part Nine: DIY Evolution
- Part Ten: Convergent Evolution
Want to know more?
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- Wikipedia List of Domestication Dates and Locations
- Price (1984) Behavioural aspects of animal domestication The quarterly review of Biology 59; 1 – 32
- vonHoldt et al (2010) Genome-wide SNP and haplotype analyses reveal a rich history underlying dog domestication Nature 464; 898 – 902
- Pigeon Breeds
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- Sniegoski, Gerrish and Lenski (1997) Evolution of high mutation rates in experimental populations of E.coli Nature 387: 703 – 705