Friday, 22 April 2011

The fast runners of evolution


In the deer family, genetic variability is greater within some species than between some genera. Does Fst tell us what we think it tells us?

At almost any genetic marker (blood types, serum proteins, enzymes, mtDNA, etc.), a typical gene varies much more within than between human populations. And this is true not only for large continental populations but also for small local ones. The geneticist Richard Lewontin found that 85% of our genetic variation exists among individuals and only 15% between ‘races.’ He concluded:


It is clear that our perception of relatively large differences between human races and subgroups, as compared to the variation within these groups, is indeed a biased perception and that, based on randomly chosen genetic differences, human races and populations are remarkably similar to each other, with the largest part by far of human variation being accounted for by the differences between individuals. (Lewontin, 1972)


Was Lewontin right? Some geneticists have remained unconvinced, their doubts focusing on three points:

1. A small genetic difference can still make a big cultural difference

Even if human populations differ only slightly at certain gene loci, these slight differences can still have big effects.

For instance, the historical economist Gregory Clark has argued that the slow but steady demographic expansion of the English middle class from the 12th century onward gradually raised the population mean for predispositions to non-violence, deferment of pleasure, and other future-oriented behavior. Although the nascent middle class was initially a small minority in medieval England, its descendants grew in number and gradually replaced the lower class through downward mobility. By the 1800s, its lineages accounted for most of the English population.

There then came the triumph of Victorian morality—a relatively sudden cultural change due to a genetic change that had slowly reached a point of critical mass. The English middle class could now impose its behavioral norms on the whole population, thereby abandoning the ‘two-tier morality’ of other class-stratified societies (Clark, 2007, pp. 124-129, 182-183; Clark, 2009).

2. Lewontin’s finding is true only if we look at one gene at a time

Genes vary much more within than between human populations only if we look at one gene at a time. The pattern reverses if we aggregate variation at several gene loci. The more we aggregate, the more the genetic variation will exist between populations and not within them. This point was first made by Cavalli-Sforza back in 1966 and later by Mitton (1977, 1978), Edwards (2003), and Sesardic (2010).

3. A big chunk of inter-individual genetic variation is actually intra-individual

Although only 15% of human genetic variation is composed of population differences, the remaining 85% is not necessarily between individuals. Since we are diploid organisms, some genetic variation is actually intra-individual—the differences between the genes you inherited from your mother and the genes you inherited from your father. If we factor out this kind of variation, population differences actually account for a third of all human genetic variation (Sarich and Miele, 2004).

How valid are these three points?

The first one was true historically and, presumably, prehistorically. A slight genetic advantage could indeed leverage very disproportionate benefits. “Winner takes all.” This kind of dynamic, however, is no longer legitimate in modern societies, at least not to the same extent. Although we accept that losers should lose, we don’t accept that they should lose everything. Our societies provide a wide array of redistributionist mechanisms to ensure that slight advantages don’t snowball into big ones.

The second point is certainly true. Clearly, two groups are easier to tell apart with several criteria than with one. With enough criteria, any overlap will shrink to zero and all individuals can be unambiguously assigned to either group. This is basic logic. But all this proves is that human populations are identifiable. It doesn’t prove that the differences between them are greater than the differences within them.

The third point invites the same reply of “So what?” If our intra-population variation is inflated by intra-individual variation, the same would be true for all species, and not just our own. Remove intra-individual variation, and you’ll certainly get a higher estimate of inter-population variation. But this will be true across the board. Human races will still look relatively unimportant.

In all this, a more fundamental criticism is being ignored. How meaningful is the ratio of inter-population to intra-population variation? Just what exactly does it tell us?

This ratio, called Fst, is not as meaningful as one might think:


Fst isn’t a good measure of genetic-phenotypic mediation. As a case example, Long and Kittles (2003) found a between human population Fst of 11% based on their sample; when they added chimpanzees, the between population Fst increased only to 18% [3]. Mountain and Risch (2004), citing this example, note that ‘‘a low FST estimate implies little about the degree to which genes contribute to between-group differences.’
(Occidentalist, 2011)


Indeed, some sibling species show the same kind of genetic overlap that we see between human races. And yet these species are anatomically, physiologically, and behaviorally distinct (Frost, 2008).

Remember, when two populations differentiate under the impact of diverging selection pressures, this differentiation concerns only a tiny fraction of the genome. Why? There are two reasons:

(a) Much genetic variation is of low selective value, often being little more than "junk" variability, and thus responds weakly to changes in selection pressure.

(b) Much genetic variation is equally adaptive in both of the new adaptive landscapes. There are many cases of genetic polymorphisms that widely occur not only among different populations of one species, but also among related species (Klein et al., 1998).

Fst cannot tell us how much populations really differ from each other within a species—and by ‘really’ we’re talking about adaptive differences that show up in anatomy, physiology, and behavior. It basically tells us how long these populations have been separated from each other, with some adjustment for ongoing gene flow. In our case, Fst tells us that human races are young, very young.

But this we know already. The past 40,000 years have seen our ancestors spread into a multitude of natural environments—from tropical rain forest to arctic tundra. And the past 10,000 years have seen humans enter an even greater variety of cultural and social environments—from simple horticulture to complex societies with class differentiation, State formation, urbanization, systematized religion, and the ability to store, accumulate, and exchange information via writing.

We also know that these same years have seen an accelerating pace of genetic change. Natural selection has altered at least 7% of our genome over the last 40,000 years. In particular, the speed of genetic change rose over a hundred-fold with the advent of agriculture some 10,000 years ago (Hawks et al., 2007).

The correlation is very weak between the passage of time and the degree of evolutionary change. Some organisms have remained virtually the same for millions of years. Others have changed very quickly. We, humans, are the fast runners of evolution.

References

Cavalli-Sforza, L.L. (1966). Population Structure and Human Evolution, Proceedings of the Royal Society of London. Series B, Biological Sciences, 164, 362-379.

Cavalli-Sforza, L.L., P. Menozzi, and A. Piazzi. (1994). The History and Geography of Human Genes, Princeton: Princeton University Press.

Clark, G. (2007). A Farewell to Alms. A Brief Economic History of the World, Princeton University Press, Princeton and Oxford.

Clark, G. (n.d.). The indicted and the wealthy: surnames, reproductive success, genetic selection and social class in pre-industrial England,
http://www.econ.ucdavis.edu/faculty/gclark/Farewell%20to%20Alms/Clark%20-Surnames.pdf

Edwards, A.W.F. (2003). Human genetic diversity: Lewontin’s fallacy. BioEssays, 25, 798-801.

Frost, P. (2008). The 85% truism, Evo and Proud, January 4

Hawks, J., E.T. Wang, G.M. Cochran, H.C. Harpending, and R.K. Moyzis. (2007). Recent acceleration of human adaptive evolution, Proceedings of the National Academy of Sciences USA. 104, 20753-20758.

Jorde, L.B., W.S. Watkins, M.J. Bamshad, M.E. Dixon, C.E. Ricker, M.T. Seielstad, and M. A. Batzer. (2000). The Distribution of Human Genetic Diversity: A Comparison of Mitochondrial, Autosomal, and Y-Chromosome Data, American Journal of Human Genetics, 66, 979–988.

Klein, J., A. Sato, S. Nagl, and C. O’hUigin. (1998). Molecular trans-species polymorphism, Annual Review of Ecology and Systematics, 29, 1-21.

Lewontin, R. (1972). The apportionment of human diversity, Evolutionary Biology, 6, 381-398.

Long, J.C. and R.A. Kittles. (2003). Human Genetic Diversity and the Nonexistence of Biological Races, Human Biology, 81, 777-798.

Mitton, J.B. (1977). Genetic differentiation of races of man as judged by single-locus and multilocus analyses, American Naturalist, 111, 203-212.

Mitton, J.B. (1978). Measurement of differentiation: reply to Lewontin, Powell, and Taylor, American Naturalist, 112, 1142-1144.

Mountain, J.L. and N. Risch. (2004). Assessing genetic contributions to phenotypic differences among ‘racial’and ‘ethnic’groups, Nature Genetics, 36, S48 - S53.

Occidentalist (2011). Did Sarich Get It Right? Occidentalist, April 14
http://occidentalascent.wordpress.com/2011/04/14/did-sarich-get-it-right/

Sarich, V. and F. Miele. (2004). Race: The Reality of Human Differences, Basic Books.

Sesardic, N. (2010). Race: a social destruction of a biological concept, Biology and Philosophy, 25, 143-162.

10 comments:

Dragon Horse said...

"For instance, the historical economist Gregory Clark has argued that the slow but steady demographic expansion of the English middle class from the 12th century onward gradually raised the population mean for predispositions to non-violence, deferment of pleasure, and other future-oriented behavior. Although the nascent middle class was initially a small minority in medieval England, its descendants grew in number and gradually replaced the lower class through downward mobility. By the 1800s, its lineages accounted for most of the English population.
"

Yes, what genes were these again? I think I missed that. What he said was a lot of indirect conjecture, nothing more.

Insightful said...

Most of the world's genetic variation is still locked up in Africa. Simple fact..

Chuck said...

"If our intra-population variation is inflated by intra-individual variation, the same would be true for all species, and not just our own. Remove intra-individual variation, and you’ll certainly get a higher estimate of inter-population variation. But this will be true across the board. Human races will still look relatively unimportant."

Pete,

Thanks for addressing this. With regards to the point about inter/intra individual variance, the issue of contention is the importance of the between member, between subspecies variance relative to the between member, within subspecies variance. This applies across species. (Though, my guess is that members of other species don’t debate this rather arcane point). The issue comes up since some see Fst --incorrectly, as you note -- as a measure of possible heritable phenotypic variance. And since others use low Fst values to imply that there are few practically significant heritable differences between individuals of different races. As noted before, Barbujani and Colonn (2010) state:

"The remaining 85% represents the average difference between members of the same population. One way to envisage these figures is to say that the expected genetic difference between unrelated individuals from distant continents exceeds by 15% the expected difference between members of the same community. (Barbujani and Colonn, 2010. Human genome diversity: frequently asked questions)

Barbujani and Colonn (2010) go on to argue that race is a worthless category since there could be few heritable differences between individuals of different races. One way to reply is to just point out what you said about Fst and its meaning. Another way to reply is to grant, for the sake of argument, the presumed meaningfulness of fst and to challenge the validity of the claim that “The remaining 85% represents the average difference between members of the same population....”

Putting aside the issue of meaningfulness, is the above statement true or false?

Tod said...

The differences that make a difference (genetic propensities for 'non-violence, deferment of pleasure, and other future-oriented behavior') may make the elite (upper-middle class people) of all races more similar to each other than the ordinary people of their own races.

That would explain a lot!

Beyond Anon said...

Dragon Horse says:

Yes, what genes were these again? I think I missed that. What he said was a lot of indirect conjecture, nothing more.


Well, Clark's description does not mention genes because he does not need to. He was simply describing what happend.

Let me see, the best sprinters and long distance runners in the world are of African origin (albeit, different parts of Africa). What I have just stated is a fact based on the statistics we have available. We don't know much about the genes involved, but you can be sure that it is genetic. Do you dispute either of these? Do you claim that the sprinting and long distance running abilities of those two African groups is cultural? Is it because their parents convinced them that they could run fast or for a long time?

Peter Frost said...

Dragon Horse,

Darwin developed his theory of evolution by natural selection without any understanding of Mendelian genetics.

You cannot look for something unless you understand what you're looking for. If gene-culture co-evolution is more than just a hypothesis, we need to understand the relevant historical and social processes. This is what Gregory Clark has done.

Insightful,

The "simple fact" is correct. But it doesn't mean what you (apparently) think it means. Modern humans have existed longer in Africa than on any other continent. They have therefore accumulated more genetic variability. But most of this variability is of little or no adaptive value. Much of it is 'junk.'

From Africa, modern humans have budded off and spread to other continents through successive founder effects, which have trimmed away much of this variability.

Tod,

Yes, there has been some convergent evolution (be it cultural, genetic, or both). But there have also been different trajectories of cultural/genetic development. This is one of the points made by Francis Fukuyama in his upcoming tome.

Beyond Anon,

Well put. We have to understand phenotypic differences before we can dig down into the genotypic substratum.

Tod said...

The process Clark documents is no longer at work. In the modern world what Harpending and Cochran call cad dads can do quite well reproductively but they can not increase their socio economic status.

So within Europeans for example the 'differences that make a difference' may be causing the classes to diverge even further genetically.

Kiwiguy said...

***The process Clark documents is no longer at work. ***

No, it probably isn't.
'Double relaxed Darwinian Selection' Nyborg 2011

Anonymous said...

So, where are the proof that English middle-class predispositions to non-violence, deferment of pleasure, and other future-oriented behavior are genetic rather than cultural ? Why assume that these values are genetic, rather than learned, by virtue of having been raised in an upper-class environment ?

In my opinion, downwards mobility of upper/middle class values in England have more to do with the Protestant Reformation and English laws of inheritance than any genetic factor. These values of thrift and deferement of pleasure are typical Protestant values, which have contributed to the rise of the Industrial Revolution in England and more generally in Northern Europe, as shown by Max Weber, contrasting to the stagnation of Catholic Southern Europe, when industrialization only begun under the rule of nationalistic authoritarian regimes, such as Napoleonic France and Fascist Italy, with a voluntarist impulse from the State.
While in France or Italy the law of primogeniture only applied to the nobility, so a tiny fraction of the population, in England laws of primogeniture applied to the commoners as well, so a landlord's or merchant's youngest sons were relegated at the lower end of the society, despite their higher-class education and background, and have to claim back their social position all by themselves. In southern Europe, the nobilty youngest sons typically became members of the Catholic clergy and thus didn't produce any further descendance and actually became evolutionary dead end, while in England, under the Reformation, celibacy and monastic orders were abolished and priestly charges also became hereditary and governed by primogeniture.

Also bear in mind that during the Little Ice Age, agricultural productivity was higher in warmer Southern Europe, which ensured better survival rates for the lower classes.

Harmonious Jim said...

Peter Frost said: "But there have also been different trajectories of cultural/genetic development. This is one of the points made by Francis Fukuyama in his upcoming tome."

I'm reading that tome now. It's informative on cultural/political divergence, but I see nothing at all (so far) on genetic divergence. Sadly, he doesn't even consider a few Frostian hypotheses.