Human Evolution on Trial - Pedigrees
Comparing the breeding of domestic animals to evolution has been criticized since Darwin’s time but sanity tells us there must be similarities. In fact Steve Jones (2000) says “the best place to see evolution is on the farm”. The characteristics selected for may be different but the resulting drift in population genetics and reduction of mitochondrial DNA and Y-chromosome lines are very much the same. Both processes rely on “selection”, that is the culling or extinction of individuals in a population.
Individuals of a species vary. Animal breeders select the individuals they wish to breed from: the ones that exhibit the characteristics they would like to have in their herd for example. In evolution the same process is sometimes called “survival of the fittest” although this expression seems to have been first used to justify a particular economic theory.
In both evolution and breeding the most important process is in fact culling, the elimination of unwanted individuals. The antelope that fails to get away from the lion is more important for evolution than the many that manage to escape. Selection has to work on a population as a whole. The defence will show in “Hybrid Vigour and Inbreeding” [Inbreeding] that, apart from cloning, it is useless to simply breed from the best individuals.
As part of their breeding program farmers usually keep a record of each animal’s ancestry, its pedigree. I now present a very important illustration of nuclear and mitochondrial DNA and the Y-chromosome. This is an important illustration because it starts with my pedigree. We’ll look at yours next. Here is my “whakapapa” or ancestry, as it would be set out in a studbook: - By convention the male from each pair is written on top.
Of course my pedigree doesn’t just stop here but I don’t know all sixteen names of my great great-grandparents even in just the next column. I have one great-grandfather’s surname and his NRY or non-recombining portion of the Y-chromosome (remember the Y-chromosome passes from father to son). But all eight ancestors in the right-hand column provide my genetic or nuclear DNA. Probably roughly in equal portions but I happen to look superficially most like John Putt.
My mitochondrial DNA comes totally from Jeanie Wright who got it from her mother etc. Of course my brothers and sister all have exactly the same pedigree but their name replaces mine. The selection of genes is still 50% from each of our parents even though it is a different assortment of 50% for each of us. We all have the same mtDNA though and my brothers have the same NRY.
You can now construct as much of your own pedigree as you are able to. Remember to put the male of each pair on top: -
You have two parents, four grandparents, eight great grandparents, sixteen great great-grandparents, etc. The number of ancestors doubles each generation. By ten generations there are over a thousand places for names in the right hand column of your pedigree. Therefore if we assume one generation is 25 years your genetic makeup is the product of the contribution of one thousand people living about 250 years ago. Expanding it back only 500 years, or 20 generations, would give a huge list of over a million names in the column at the right; this at a time when the total population of England, for example, was only about three million. Expanding it back 5000 or 50,000 years gives an unimaginably large number of ancestors but we’ll confine it to a more manageable 500 years for now.
In this column of over a million ancestors from 500 years ago your mtDNA comes only from the individual at the bottom of the right hand column. Remember that, although the mitochondrial DNA is passed to all of a particular woman’s children, it is only her daughters who will pass it on to the next generation. If you are a male the main branch of your Y-chromosome will come only from the individual at the top of the column. If your surname goes back that far, and nothing has interrupted the sequence, your surname may also go back to this individual but that is a big ask. Any children you may have will automatically have the million ancestors in your partner’s pedigree added in. Your sons will all have their father’s Y-chromosome but all your children will have their mother’s mtDNA.
The jury can see that genealogies that concentrate only on one line grossly distort reality. I suppose it is theoretically possible half your genes could come from just a single one of these million ancestors and the other half from another one but this is very unlikely. Anyway even if you have as many as 100,000 genes more than 90% of these ancestors from just 500 years ago have no genes surviving in you. Many of those genes could come to you from other ancestors who shared those genes though. Some of those genes lost in your pedigree may survive in other people alive today. Other genes may be extinct.
If you could actually list the million names in your pedigree from five hundred years ago the same pattern of names would appear more than once in that column. Any repeated names on the list represent the level of inbreeding. In fact to guard against inbreeding animal breeders look for common ancestors only four or five generations back in a pedigree, but any population not infinitely large will have some level of inbreeding, i.e. all populations of all species (Falconer 1964). That’s why their members look similar to each other.
If you have been observant you may have noticed the surname Wright appears twice in one column of my pedigree. I actually have no idea how closely related Elizabeth and Jeanie were but let’s say they were sisters. This would mean they had the same mother and father. These two names would therefore appear twice in the next column to the right. Numbers 11 and 15 and numbers 12 and 16 would each be the same name. Of the thirty-two names that would be in my pedigree the next column back, the list from 21 to 24 is repeated as numbers 29 to 32. And so on. So in the column of 1,048,576 names from 500 years ago numbers 655,360 to 720,896 are exactly the same as numbers 917,504 to 983,040. Got that? In the column of over a million names in my pedigree there would be a repeated section of over sixty-five thousand names. Even within these 65,000 names there will be repeated sections and there would be other repeated sections elsewhere in the column. We will return to “Hybrid Vigour and Inbreeding” and how the wave theory of evolution works in Part II.
So far we have been considering pedigrees only from the perspective of the individual on the left, you or me. If we move one step to the right, to your parents, we realise you may not be their only child and they may not be single children. You could now attempt to do a family tree of one pair of your grandparents. They probably had several children and these will be your aunts and uncles. They will share many of their genes with one of your parents and fewer with you. Your aunts and uncles would have married people with completely different pedigrees to yours and their children will be your cousins. They will share even fewer genes with you. In other words if you combine your pedigree with the family trees of each of your million ancestors you can see the population becomes a huge network of genes. If you have ever been to a family reunion you may be able to imagine just how complex that network can be.
This brings us back to the subject of population genetics. Knowledge of what can cause reduction in Y-chromosome and mitochondrial and nuclear DNA lines in a population will be able to help us interpret the evidence for the evolution and distribution of our species.
As an illustration the defence will use the list of your one million different ancestors from 500 years ago and assume population numbers have stayed the same over the generations since that time. This is probably what happens on average for most species although in practice the numbers fluctuate, sometimes wildly, between the generations. For example Steve Jones (2000) gives a brief account of the complex interactions and fluctuations in numbers of snow grouse, lynx, snowshoe hares, willow and birch trees, and squirrels, coyotes and ravens within a small region. The so-called balance of nature is very complex and constantly changing. The more recent steady population explosion in humans has not been our normal state. It is impossible to decide from the evidence exactly how many humans there have been at various times during our history though. The total human population may have been as low as ten million (the size of London today) or even lower (Jones 2000) for much of our past. Anyway we can be sure numbers have fluctuated wildly.
To maintain population numbers of a species each couple, on average, must produce two offspring that survive to breed, that in turn produce two offspring, etc. Let us assume the average number of offspring per pair is four. This is not a large number, even for a human, as many families contain more than this number of children. Four offspring allows a selection rate of 50% but first let’s look at the male / female ratio. Each birth has a 50:50 chance of being a male (or female if you prefer), and so we get the following possibilities:
Just sixteen combinations are possible and Couple No. 1 leaves no female offspring and Couple No.16 leaves no male offspring. Therefore on average one female in sixteen would leave no mtDNA in the next generation and one male in sixteen would leave no Y-chromosome. Of course any close relatives would have the same mtDNA or Y-chromosome in practice but we’ll assume for this illustration that all the couples are unrelated.
At a 50% selection rate the diagram looks the same but what the letters stand for changes to “M” for mate and produce the next generation of offspring and “F” for fail for some reason or other to produce offspring that survive and breed. Once again one in sixteen couples fail to leave any mtDNA or Y-chromosome but this time through lack of any descendants. The combined loss of mtDNA lines (for example) is not simply two out of sixteen each generation. One occasion in sixteen multiplied by sixteen will be the elimination of an all-male family. In the next generation there will also be some families that already have several lines. Because they are less likely to be eliminated the rate of elimination slows. I have read that in a population of a fixed size mtDNA lines will reduce by one each generation (Lewin 1999). But I haven’t seen the maths and probably wouldn’t understand it if I had. If we accept it to be correct in sixteen generations, or four hundred years, the thirty-two survivors from the above small example would all have mtDNA from just one of the original females. And all the males would probably have just one of the original Y-chromosome lines.
Jobling et al (2004) mention that the first studies of the mtDNA of the American Indians showed they basically trace back to only four lines (“MtEve” [Interpretation]). Further studies have shown there are actually a few others although they are rare. But examination of ancient human remains in America show these rare ones were much more common originally (Martin Jones 2001). There has in fact been a reduction in mtDNA lines.
Starting with a closed population any woman who doesn’t produce a female child who survives to reproduce represents the loss of a line of mtDNA. Similarly, any male who doesn’t produce a male child represents the loss of a line of Y-chromosome. It is extremely unlikely all your 500,000 female ancestors from 500 years ago still have mtDNA lines today. Likewise for the 500,000 male ancestors and their Y-chromosomes. In theory, as it is twenty generations ago, twenty lines of each will have been lost. But I would be very surprised if any one of us actually know how many repeated names there are on our list of ancestors from twenty generations ago. Our individual pedigrees would each have far fewer than a million different names from that time.
It is quite easy to see that in time any population of limited size could be reduced to just one line of mtDNA but this may not necessarily descend from a female who had any genetic advantage at the time. Survival could be the result of almost random events. Genes from other members of the original population would still survive and so mitochondrial DNA lines don’t necessarily give a sure indication of nuclear DNA.
Many dairy farmers, for their breeding program, keep track of cow families, usually from the cows in the herd when they take over it or cows they have brought in more recently. They have to have a defined beginning or point of origin. Over time the number of their cow families reduces as they keep more offspring from the better cows, some cows die or are culled, or some cows have mostly bull calves, or dead ones. This leads to extinction of mitochondrial DNA lines in the herd. The way to maintain the line of a good cow in the herd who has had mostly bull calves is to actually keep bulls from that cow. Thus the mtDNA line may be lost but the nuclear DNA is maintained and even usually spread very widely.
Daniel Bradley and his colleagues (Bradley et. al. 1996) have researched mtDNA of Indian, African and European cattle breeds with interesting results. The mtDNA lines readily cluster into the three regions but African cattle have traditionally been divided into two types: humped and humpless breeds. Although it is known the humped, or zebu, cattle were introduced from India centuries ago Bradley’s DNA research indicates mtDNA lines cannot distinguish the humped and humpless breeds in Africa. Their conclusion was that the humped cattle are descended from local humpless cows crossed with humped bull imports. More recent studies of the Y-chromosome have shown this original view to be correct (Martin Jones 2001). The nuclear DNA of these humped breeds is now mostly from humped cattle although the mitochondrial DNA comes from humpless cattle. The defence will present other interesting examples of the independence of mitochondrial and nuclear DNA in “Species”.
Now, if we go back to these hypothetical million people from 500 years ago and imagine their one million hypothetical descendants today it is easy to see that the various genes present in the original population will not be present in the modern population in the same proportions. There will be more of some; less of others, some will have been totally eliminated and new mutations may have appeared. This is called “genetic drift”. For example blood groups other than O are rare in American Indians but A, B and AB were more common in prehistoric America. The ratios have changed, presumably by the elimination of blood groups other than O (Cavalli-Sforza 1995). Genetic drift would happen even if survival was totally random but, of course, genes are usually selected for or against. The greatest selection is usually against undesirable genes but selection doesn’t operate on the genes themselves. Selection can operate only on the individuals that express the genes.
Before we begin to follow this idea back through our collective family history though we should first examine our present ideas about that history.
See next :: Human Evolution on Trial - 'Mythconceptions'
Bradley et al (1996) Mitochondrial Diversity and the Origins of African and European Cattle. Proc. Natl. Acad. Sci. Vol. 93 pp. 5131-5135.
Cavalli-Sforza, Luigi Luca and Cavalli-Sforza, Francesco (1995) The Great Human Diasporas. Addison- Wesley
Falconer, D. S. (1964) Quantitative Genetics. Oliver and Boyd Ltd., Great Britain.
Jobling et al (2004) Human Evolutionary Genetics. Garland Science, New York.
Jones, Martin (2001) The Molecule Hunt. The Penguin Press, London.
Jones, Steve (2000) Almost Like a Whale. Anchor, London.
Lewin, Roger (1999) Patterns in Evolution. Scientific American Library, New York.