On The Evolution Of Ashkenazi Jewish Intelligence

Ashkenazi Jews pose two mysteries for biological science. First, why do they have so many genetic diseases that fall into just a few categories of metabolic function such as the sphingolipid storage diseases Tay-Sachs, Gaucher, Niemann-Pick, and mucolipidosis type IV? The rates of such diseases are so high that their incidence must be the result of either a recent genetic bottleneck where the Ashkenazi population was very small or natural selective pressures aimed at some other phenotype(s) selected for these genotypes due to advantages that those genotypes offer for other functionality. The second mystery is why are Jews so smart? Granted, a lot of Jews want to argue that they are just studious due to their culture. Also, lots of ideologues - particularly on the political Left - stand ready to attack anyone who argues that ethnic and racial groups differ in average intelligence. But the higher average level of Ashkenazi Jewish intelligence is so glaringly obvious that I figure anyone who tries to argue otherwise is either engaged in intellectual con artistry or is ignorant or foolish. So again, why are Jews so smart?

Well, three researchers at the University of Utah, anthropologist Henry Harpending, Gregory Cochran (a Ph.D. physicist turned genetic theorist), and Jason Hardy put forth a hypothesis that seeks to explain both mysteries simultaneously. Nicholas Wade of the New York Times has written one of the two news stories about it to date. The proposed hypothesis holds that Jews developed their genetic diseases as a side effect of strong selective pressures for higher intelligence during the Middle Ages as they were forced to work mainly in occupations that required greater cognitive ability. (same article here)

A team of scientists at the University of Utah has proposed that the unusual pattern of genetic diseases seen among Jews of central or northern European origin, or Ashkenazim, is the result of natural selection for enhanced intellectual ability. The selective force was the restriction of Ashkenazim in medieval Europe to occupations that required more than usual mental agility, the researchers say in a paper that has been accepted by the Journal of Biosocial Science, published by Cambridge University Press in England.

The Economist has the other article about this research paper. The distribution of the Jewish genetic diseases is clustered too much into a few areas of genetic functionality This concentration of mutations argues for selective pressures as the logical explanation for rate of occurrence of these mutations in Ashkenazi Jews.

What can, however, be shown from the historical records is that European Jews at the top of their professions in the Middle Ages raised more children to adulthood than those at the bottom. Of course, that was true of successful gentiles as well. But in the Middle Ages, success in Christian society tended to be violently aristocratic (warfare and land), rather than peacefully meritocratic (banking and trade). Put these two things togethera correlation of intelligence and success, and a correlation of success and fecundityand you have circumstances that favour the spread of genes that enhance intelligence. The questions are, do such genes exist, and what are they if they do? Dr Cochran thinks they do exist, and that they are exactly the genes that cause the inherited diseases which afflict Ashkenazi society. That small, reproductively isolated groups of people are susceptible to genetic disease is well known. Constant mating with even distant relatives reduces genetic diversity, and some disease genes will thus, randomly, become more common. But the very randomness of this process means there should be no discernible pattern about which disease genes increase in frequency. In the case of Ashkenazim, Dr Cochran argues, this is not the case. Most of the dozen or so disease genes that are common in them belong to one of two types: they are involved either in the storage in nerve cells of special fats called sphingolipids, which form part of the insulating outer sheaths that allow nerve cells to transmit electrical signals, or in DNA repair. The former genes cause neurological diseases, such as Tay-Sachs, Gaucher's and Niemann-Pick. The latter cause cancer. That does not look random. And what is even less random is that in several cases the genes for particular diseases come in different varieties, each the result of an independent original mutation. This really does suggest the mutated genes are being preserved by natural selection. But it does not answer the question of how evolution can favour genetic diseases. However, in certain circumstances, evolution can.

Greg has referred to this hypothesis as "overclocking". The analogy is to overclocking computer processors (computer processing units or CPUs). Some hobbyists turn up the clocks on their desktop PCs to them run faster than they were designed to run. This can cause system instability and other problems. In the case of the Ashkenazis in Europe the hypothesis proposes that selective pressures for higher Ashkenazi intelligence were so high that it caused the propagation of mutations that pushed their intelligence up so quickly (evolutionarily speaking) that the selective pressure overrode the reduction in reproductive fitness caused by the deleterious side effects on some of those who received those mutations. The problem with overclocking is that "Sometimes you get away with it, sometimes you don't."

But I'll hazard a guess: the change accelerates some brain system tied to cognitive functioning - nearly redlines it, leaves it vulnerable to common insults in a way that can cause spectacular trouble. You might compare to overclocking a chip. Sometimes you get away with it, sometimes you don't. More generally, if this is what I think it is, all these Ashkenazi neurological diseases are hints of ways in which one could supercharge intelligence. One, by increasing dendrite growth: two, by fooling with myelin: three, something else, whatever is happening in torsion dystonia. In some cases the difference is probably an aspect of development, not something you can turn on and off. In other cases, the effect might exist when the chemical influence is acting and disappear when the influence does. In either case, it seems likely that we could - if we wanted to - developed pharmaceutical agents that had similar effects. The first kind, those affecting development, would be something that might have to be administered early in life, maybe before birth. while the second kind would be 'smart pills' that one could pop as desired or as needed. Of course, we have to hope that we can find ways of improving safety. Would you take a pill that increased your IQ by 10 or 15 points that also had a 10% chance of putting you in a wheel chair?

Looked at from this perspective many Jews have paid and continue to pay a high price from the effects of mutations that "overclock" their brains.

This hypothesis cries out to be tested because if it is proven then, as Greg points out, these mutations point in directions for research aimed at raising human intelligence. Drugs or gene therapies that raise intelligence would have enormous economic value and one can even put a price tag on the value of higher intelligence. However, such calculations understate the economic value of higher intelligence because most of the value of scientific and technological knowledge produced by high IQ people flows to lower IQ people.

The paper is downloadable as a 40 page PDF (on big PDFs I get better results downloading to a file and then opening rather than running Acrobat Reader from within a browser).

This paper elaborates the hypothesis that the unique demography and sociology of Ashkenazim in medieval Europe selected for intelligence. Ashkenazi literacy, economic specialization, and closure to inward gene flow led to a social environment in which there was high fitness payoff to intelligence, specifically verbal and mathematical intelligence but not spatial ability. As with any regime of strong directional selection on a quantitative trait, genetic variants that were otherwise fitness reducing rose in frequency. In particular we propose that the well-known clusters of Ashkenazi genetic diseases, the sphingolipid cluster and the DNA repair cluster in particular, increase intelligence in heterozygotes. Other Ashkenazi disorders are known to increase intelligence. Although these disorders have been attributed to a bottleneck in Ashkenazi history and consequent genetic drift, there is no evidence of any bottleneck. Gene frequencies at a large number of autosomal loci show that if there was a bottleneck then subsequent gene flow from Europeans must have been very large, obliterating the effects of any bottleneck. The clustering of the disorders in only a few pathways and the presence at elevated frequency of more than one deleterious allele at many of them could not have been produced by drift. Instead these are signatures of strong and recent natural selection.

Their argument against a population bottleneck is key to their larger argument. Dismissal of the bottleneck argument leads inevitably to the argument that the frequency of these mutations that cause genetic diseases must be the result of selective pressure. If they are the result of selective pressure then the next obvious question is what was being selected for? Cochran, Harpending, and Hardy claim higher intelligence increased reproductive fitness for Jews in medieval Europe who were legally prevented from performing in occupations that had lower need for intelligence. Simultaneously Jews were allowed to work in more cognitively demanding occupations involving money handling even as the Catholic Church banned Christians from many of those same occupations.

They take their argument all the way down to the molecular level and argue that the sphingolipid mutations in some of the Jewish genetic diseases boost glucosylceramide which in turn boosts neural axon growth.

The sphingolipid storage mutations were probably favored and became common because of natural selection, yet we dont see them in adjacent populations. We suggest that this is because the social niche favoring intelligence was key, rather than geographic location. It is unlikely that these mutations led to disease resistance in heterozygotes for two reasons. First, there is no real evidence for any disease resistance in heterozygotes (claims of TB resistance are unsupported) and most of the candidate serious diseases (smallpox, TB, bubonic plague, diarrheal diseases) affected the neighboring populations, that is people living literally across the street, as well as the Ashkenazim. Second and most important, the sphingolipid mutations look like IQ boosters. The key datum is the effect of increased levels of the storage compounds. Glucosylceramide, the Gaucher storage compound, promotes axonal growth and branching (Schwartz et al., 1995). In vitro, decreased glucosylceramide results in stunted neurons with short axons while an increase over normal levels (caused by chemically inhibiting glucocerebrosidase) increases axon length and branching. There is a similar effect in Tay-Sachs (Walkley et al., 2000; Walkley, 2003): decreased levels of GM2 ganglioside inhibit dendrite growth, while an increase over normal levels causes a marked increase in dendritogenesis. This increased dendritogenesis also occurs in Niemann-Pick type A cells, and in animal models of Tay- Sachs and Niemann-Pick. Figure 1, from Schwartz et al. (1995) shows the effect of glucosylceramide, the sphingolipid that accumulates in Gaucher disease. These camera lucida drawings of cultured rat hippocampal neurons show the effect of fumonisin, which inhibits glucosylceramide synthesis, and of conduritol B-epoxide (CBE) which inhibits lysosomal glycocerebrosidase and leads to the accumulation of glucosylceramide, thus mimicking Gaucher disease. Decreased levels of glucosylceramide stunt neural growth, while increased levels caused increased axonal growth and branching. Dendritogenesis appears to be a necessary step in learning. Associative learning in mice significantly increases hippocampal dendritic spine density (Leuner et al., 2003), while enriched environments are also known to increase dendrite density (Holloway, 1966). It is likely that a tendency to increased dendritogenesis (in Tay-Sachs and Niemann-Pick heterozygotes) or to increased axonal growth and branching (in Gaucher heterozygotes) facilitates learning. Heterozygotes have half the normal amount of the lysosomal hydrolases and should show modest elevations of the sphingolipid storage compounds. A prediction is that Gaucher, Tay-Sachs, and Niemann-Pick heterozygotes will have higher tested IQ than control groups, probably on the order of 5 points. We do have strong but indirect evidence that one of these, Gaucher disease, does indeed increase IQ. Professor Ari Zimran, who heads the Gaucher Clinic at the Shaare Zedek Medical Centre in Jerusalem, furnished us a list of occupations of 302 Gaucher patients. Because of the Israeli medical care system, these are essentially all the Gaucher patients in the country. Of the 255 patients who are not retired and not students, 81 are in occupations that ordinarily average IQs greater than 120. There are 13 academics, 23 engineers, 14 scientists, and 31 in other high IQ occupations like accountants, physicians, or lawyers. The government of Israel states that 1.35% of Israelis working age population are engineers or scientists, while in the Gaucher patient sample 37/255 or 15% are engineers or scientists. Since Ashkenazim make up 60% of the workforce in Israel, a conservative base rate for engineers and scientists among Ashkenazim is 2.25% assuming that all engineers and scientists are Ashkenazim. With this rate, we expect 6 in our sample and we observe 37. The probability of 37 or more scientists and engineers in oursample, given a base rate of 2.25%, is approximately 4 x 10 -19 . There are 5 physicists in the sample, while there is an equal number, 5, of unskilled workers. In the United States the fraction of people with undergraduate or higher degrees in physics is about one in one thousand. If this fraction applies even approximately to Israel the expected number of physicists in our sample is 0.25 while we observe 5. Gaucher patients are clearly a very high IQ subsample of the general population. Are there Ashkenazi mutations other than these sphingolipid storage disorders that likely became common because of strong selection for IQ? There are several candidates. Ever since torsion dystonia among the Ashkenazim was first recognized, observers have commented on the unusual intelligence of patients. Flatau and Sterling (Eldridge, 1976) describe their first patient as showing an intellectual development far exceeding his age, and their second patient as showing extraordinary mental development for his age. At least ten other reports in the literature have made similar comments. Eldridge (1970, 1976) studied 14 Jewish torison dystonia patients: he found that their average IQ before the onset of symptoms was 121, compared to an averge score of 111 in a control group of 14 unrelated Jewish children matched for age, sex, and school district. Riklan and colleagues found that 15 Jewish patients with no family history of dystonia (typical of DYT1 dystonia) had an average verbal IQ of 117 (Eldridge, 1979; Riklan et al., 1976).

Raising Intelligence Looks Problematic

If this hypothesis is correct (and I believe it is) then it is problematic for efforts to raise human intelligence. How many of the intelligence raising genetic variants bring undesirable side effects? Some scientists speculate that assortive mating of high IQ people is contributing to a rising incidence of autism and Asperger's Syndrome. As smart people become more likely to breed with other smart people the odds increase that pairs of autosomal recessives or other problematic combinations of intelligence boosting genes will be inherited by offspring.

Has human intelligence been selected for so rapidly in the last couple of thousand years that a large portion of all intelligence boosting mutations have undesirable side effects? When a selective pressure is strong early adaptations will have side effects. Henry Harpending explained in the gnxp.com thread on this subject:

Re mechanism: The argument (well known to breeders where there is no argument) goes like this:

In a drastic new environment there is big fitness payoff to IQ. In this new environment there is a payoff to "turning down" BRCA1 to free up early CNS development but at the cost of higher cancer rates later in life. Eventually, especially in a big population, a BRCA1 variant with the optimum activity will show up. Meanwhile carriers of one normal and one broken BRCA1 gene have a big fitness advantage because they have, say, 90% of normal suppression of early CNS development. So the broken BRCA1 allele is favored by selection even though homozygotes for it die. After a long time it would be replaced by the optimum allele but it takes a long time for that optimum allele to show up. Exactly this argument applies to myostatin in several European breeds of beef cattle: it causes muscle hypertrophy and obstetric difficulties. The muscle hypertrophy is good but the obstetric difficulties require veterinarians and in the wild would have been lethal. Re the implications of our model for eugenics, yes, big time, eugenics is IMHO a route to disaster. Well understood engineered gene introductions could be fine but eugenics would be almost certain to bring all kinds of nightmares.

But keep in mind that the human race already has many genetic variations to choose from that contribute to determining cognitive ability. A massive comparison of DNA sequence information between hundreds of thousands of people combined with IQ testing and collection of a lot of life history and medical history information could demonstrate many of the positive and negative effects of each genetic variation which affects cognitive function. Likely some will be better optimized to provide a cognitive boost without much downside.

Advances in biotechnology will provide ways to avoid some of the harmful side effects of these "overclocking" mutations. One way to accomplish this would be to discover regulatory regions in the genome that could be harnessed to selectively turn on the mutated genes only in the nervous system and turn on normal versions of these genes only in cells outside of the nervous system.

But Smart People Having More Babies Will Raise Average IQs

I've got to state the obvious because the obvious is politically incorrect: If smart people have more babies than dumb people the average IQ will rise. If dumb people have more babies than smart people then the average IQ will drop. I'm guessing the latter is currently happening. Bummer dudes.

Testing Of The Hypothesis Should Be An Urgent Priority

Proof of this hypothesis would point scientists in the direction of genes to look at for intelligence enhancement. For example, if the mutation for Gaucher's disease causes an IQ boost then drugs that increase the level of glucosylceramide in neurons might accelerate learning by increasing the rate of axon growth to connect neurons to each other.

The hypothesis could be tested fairly rapidly. Recruit some thousands of Ashkenazi Jews to take IQ tests and to have a few dozen genes tested for assorted genetic variations. Compare the IQ test results to the genetic tests and see if all the known genetic variations in sphingolipid storage metabolism, DNA repair, and several other categories account for a large proportion of Ashkenazi Jewish genetic variations.

We also need to find out whether these various potential intelligence boosting mutations have differing effects from each other on other aspects of cognition. Anyone recruited into testing the hypothesis should also have information collected on their mental health, personality, preferences, values, educational history, occupation, income, criminal record, and anything else that might provide clues as the effects of these mutations on cognitive function. For example, do some IQ-boosting mutations favor a career in law whereas others favor a career in medicine or science or math?

The Future Of Jewish Breeding Practices

Jewish efforts to avoid passing along genes that have harmful effects might be lowering average Jewish intelligence. Some of the genetic variants (e.g. the genes underlying Tay-Sachs, Gaucher, and Niemann-Pick diseases) are autosomal recessive and therefore cause diseases only when a person has two copies of them. If having single copies of these genetic variations boosts intelligence but Jewish couples engage in practices that reduce the number of copies they pass along in general (e.g. by using pre-implantation genetic diagnosis to choose an embryo that has 0 copies of a mutation) then that will reduce the number of Jewish babies born with single copies and therefore if the hypothesis is correct then the resulting babies will be less bright than the average Ashkenazi Jew.

Should the hypothesis be proven then Jewish breeding practices could be adjusted to maximize the benefit of intelligence boosting genetic variations while avoiding the harmful effects. Ideally each child should get one and only one copy of each genetic variation that is autosomal recessive for diseases. Get the intelligence boosting benefit of a single copy while avoiding the diseases that come from having two copies. To execute this strategy a Jewish person would need to get genetically tested and then look for a mate who has complementary mutations for higher intelligence.

If each member of a couple has one copy of an autosomal recessive mutation then, on average, 2 out of 4 pregnancies they start will have the exact 1 desired recessive mutation. But 1 of the other 4 pregnancies will have 2 copies and hence would result in genetic disease. The other 1 of the 4 pregnancies would not have the IQ boosting genetic mutation and hence would not be as smart. If the couple each have the same 3 different autosomal recessives mutations that each boost IQ then the odds of getting a baby that has exactly 1 copy of each of the 3 mutations is only 1 in 8.

The low odds of getting all the desired mutations with the optimal number of copies of each mutation poses a big problem to aspiring eugenicists whether Jewish or non-Jewish. One biotechnological approach to solving this problem would use microfluidics devices to separate and identify each chromosome from a cell to get just exactly the set of chromosomes from each parent chosen for an optimal trade-off of cognitive ability and other qualities. Then somehow insert all those chromosomes back into a cell and kick it into an embryonic state. But we are probably 10 or 20 years away from having such a capability.

For Those Offended By The Idea Of Eugenics

Every time a man or woman chooses someone to mate with they are making choices based on the appearances, status, demonstrated intelligence, and other qualities of that person. Women attracted to rock stars, movie stars, and sports stars are driven by genetically caused eugenic desires.

Use of genetic tests to choose a mate is already done to avoid passing on harmful mutations to offspring. This practice will become much more widespread as the significance of more genetic variations becomes known. The negative connotations associated with the term eugenics are already wearing off. As more people can derive benefits from the use of genetic information to guide reproductive decisions eugenic practices will become very widespread. When that happens the term eugenics may be replaced by a different term that effectively means the same thing. But regardless of what it gets called eugenics will become widely accepted and practiced.

Did Medieval Usury Bans Lead To Israeli Power?

Step back and look at Jewish and European history from the context of this hypothesis. A few things come to mind. First off, Middle Ages bans on Christian money lending created an environmental niche in which high IQ was selected for in Jews. This led to a few important historical consequences. First off, it led to financial and reproductive success of urban Jews and hence resentment against them by both elites and masses in Europe. This resentment of course led to pogroms and Hitler's "Final Solution". There's an old Japanese saying that comes to mind: "The nail that sticks up gets hammered down". Well, smart Jews stood out and the response of jealousy and resentment against the more successful "other" is a recurring theme in human history.

But here's the twist: Catholic usury restrictions, by creating an environmental niche that selected for higher Ashkenazi IQs, therefore made possible the eventual return of Jews to Israel. An ethnic group of much lower intelligence never would have been able to pull off the creation and defense of a state in that location against such hostile neighbors.

On The Persecution Of Market Dominant Minorities

The persecutions of Jews can also be seen in the context of successful minorities around the world. Yale law professor Amy Chua wrote a book about persecution of economically successful minorities entitled World on Fire: How Exporting Free Market Democracy Breeds Ethnic Hatred and Global Instability where she describes attacks in a number of countries (e.g. Indonesia) against Chinese and other groups that are minorities that are economically more successful than the majorities in countries where they live.

Suppose the successful minorities who are persecuted are successful as a result of genetically caused higher intelligence or perhaps due to other genetically controlled cognitive qualities. When this becomes proven scientifically and becomes widely known will the knowledge lead to more or less persecution of cognitively more able and more successful ethnic groups? For example, will Malaysians or Indonesians resent Chinese people even more if genetically caused higher intelligence in Chinese becomes the accepted explanation for greater Chinese economic success in Malaysia and Indonesia? Or will lower class people become more willing to accept their lots in life if group average differences in genetic endowments for cognitive ability are shown to be responsible for the bulk of inter-group differences in incomes, wealth, achievement, and status?

If you are interested in the evolution of human intelligence, the methods by which evolution has changed the human brain to make it smarter, or how changes in human societies can cause changes in natural selective pressure on human evolution then read this paper. If you are interested in the prospects for future intelligence enhancement then, again, read it. If you are interested in the causes interracial conflict or if you are interested in how religious and cultural practices can exert selective pressures on human populations then read it. If you want to dispute the hypothesis then read the full paper and examine their evidence before trying to disagree.

Also see the Gene Expression post on this story and in particular the discussion thread for that post which includes comments by both Greg Cochran and Henry Harpending.

Updates

Update I:Steve Sailer covers the Cochran-Harpending paper.

The savage persecutions suffered by Jews suggest that high intelligence can generate resentment among the masses. No doubt there will be some who will suggest that the Cochran-Harpending paper should have been suppressed to prevent awareness of the secret of Ashkenazi intelligence from seeping out. But you have to be a true-blue intellectual to assume that the only way anybody would ever notice anything as obvious as Jewish brainpower is if it gets mentioned in the New York Times. Political correctness doesn't keep facts from being talked aboutjust from being written about in an intelligent, constructive manner.

Yes, everyone thinks Jews are smarter, even many people who publically deny they believe this. Persecutions of smart minorities happen already. An honest accurate discussion of the causes of resentment smarter and more successful groups would, in my view, make it easier to ameliorate the causes of resentment between ethnic groups. I think people would be less prone to ascribe Jewish successes to conspiracies if Jews were accepted as being smarter for genetic reasons. High IQ genes cause higher intelligence. Higher intelligence increases productivity when learning and working. Hence greater wealth. That's a lot less reason for resentment than the idea that some group is no more productive but engages in conspiracies to take from others.

Steve thinks the Parsis have managed to achieve great success while generating less resentment from other groups.

On the other hand, the happier experience of another ethnic minority that may also have evolved stronger intellectual capacities under similar urban conditionsthe prosperous Parsis of Bombaymay offer clues to mitigating envy.

I wonder if the Parsis were able to do this simply because India was broken up into so many castes that the Parsis had a hard time being noticed by the average Indian.

In any case, the Cochran-Harpending paper offers a fairly new but crucial perspective on the old nature and nurture question. The researchers have demonstrated that it's quite possible for nurture to change nature. Culture can drive heredity. Economics and social customs alter gene frequencies.

This is an incredibly important point. Currently genetic variations for higher intelligence are being selected against in industrialised societies. We've probably changed selective pressures in other ways. But at this point I can only guess as to those chances. Are introverts or extroverts more or less likely to reproduce relative to each other than in the past? I don't know. Are genes for height being selected for? My guess is yes. The genes for obesity might be getting selected for in modern societies. I would have expected heavier weight people to have a harder time finding mates and hence be less likely to reproduce. But perhaps obese people are willing to settle for less desirable mates due to their own perceived lower attractiveness and hence they spend less time searching for for the ideal mate and hence start reproducing sooner and in greater numbers.

Humanity has not escaped from natural selection. The genes that code for the brain are not immune to the pressures of natural selection. Anyone remember the tune "Elvis is everywhere"? Well, "Darwin is everywhere".

Update II: Patri Friedman points out that people genetically engineered to have single copies of all the autosomal recessive IQ boosters could not mate with each other naturally.

Given that modern genetic technology will soon make it easy to ensure that a child has exactly one copy of such a gene, it seems like this sort of thing is a low-hanging fruit for genetic engineering. If such a gene is common, then having one copy is probably an evolutionary advantage - otherwise it would be (mildly) selected against. The main disadvantage is the chance that your kids might end up with two copies of the recessive, so if technology can prevent that, now you just have upside. I wouldnt be suprised if it soon becomes common to give your offspring many such genes - which would have the side effect of making it impossible for them to reproduce with similarly endowed partners without using genetic engineering.

The problem is if you have a lot of recessives boost intelligence when you have a single copy and your mate has them too then the odds would be very low that your offspring would not get two copies of at least one of the genes. Given that these genes cause diseases when you get two copies that would make natural reproduction very risky for offspring.

The obvious solution to this problem is to adjust these genes with additional regulatory mechanisms to make them no longer cause diseases when two copies are present. But development of such regulatory mechanisms requires additional and difficult genetic engineering work.

March 2009 Update: This work has since become part of the basis of an excellent book by Cochran and Harpending entitled The 10,000 Year Explosion: How Civilization Accelerated Human Evolution.