between villages are quickly amplified. 

 Shared names mean shared ancestors. If 

 one of those ancestors carried a single 

 copy of one or another harmful recessive 

 gene — as nearly everyone does — then his 

 or her descendants in the village, choosing 

 their mates from a restricted pool, are at in- 

 creased risk of having a child with two 

 copies of the gene. 

 The danger is a real one. Finland, with 

 G its impenetrable forests, has lots of iso- 

 O lated and inbred populations — and many 

 • i-H local centers of inborn disease. Some are 

 ^ almost unknown elsewhere, while others 

 ^ represent isolated clusters of widespread 

 -^ but generally rare illnesses. 

 ^ Social barriers can be just as effective as 

 ^ distance. In Britain, many children of Pa- 

 pL^ kistani immigrants marry among them- 

 selves. Nearly half are married to a cousin. 

 Although only about one British birth in 

 fifty is to this group of closely related par- 

 ents, these children represent about 5 per- 

 cent of all inborn disease. Certain religious 

 communities, such as the Amish, are nat- 

 ural laboratories for genetic disease be- 

 cause they are so inbred. ElUs-van Creveld 

 syndrome, an inherited abnormality of the 

 skeleton, is commoner among the Penn- 

 sylvania Amish than in any other group of 



people. Every sufferer traces his or her an- 

 cestry to Samuel King, a founder of the 

 community. 



Even outside such closed communities, 

 most people have traditionally chosen to 

 mate with those who live close to them. 

 Now this pattern is changing quickly. The 

 tens of thousands of surnames in New 

 York — and the distinct ethnic groups to 

 which many of them are attached — show 

 just how mixed up the world's population 

 is becoming. And cities are not the only 

 places where the pool of potential mates is 

 glowing. Even on the Lipari Islands off the 

 coast of Italy, where in the 1920s a quarter 

 of marriages were between first or second 

 cousins, only about one marriage in fifty is 

 between cousins today. 



A crude but effective way to measure 

 how related one's ancestors may have 

 been is to ask how far apart they were 

 bom. For nearly all the people reading this 

 article, the distance between the places 

 where they and their partners were bom is 

 greater than that separating their parents' 

 birthplaces. And their parents were, in 

 tum, likely to have entered the world far- 

 ther apart than did their grandparents. In 

 parts of nineteenth-century New England, 

 the distance between birthplaces of mar- 



riage partaers was less than twenty miles. 

 Now the average in the United States is 

 several hundred, and most couples are 

 completely unrelated. 



The mixing will not be complete for a 

 long time, if ever — with as much as five 

 hundred years needed to even out the ge- 

 netic differences between England and 

 Scotland alone. But even if global homo- 

 geneity is a long way off, increased move- 

 ment and outbreeding will certainly work 

 to decrease the numbers of children bom 

 with two copies of a defective gene. 



One example of the genetic benefits of 

 outbreeding — albeit one that has its roots 

 in an abhorrent period of history — can be 

 seen in the United States. On average, 

 about a quarter of the genes of North 

 American blacks were contributed by 

 white ancestors — a result of interracial 

 mating during the days of slavery (usually 

 between white males and black females, 

 who had little say in the matter). 



Since the recessive gene for cystic fi- 

 brosis is unknown in Africans and that for 

 sickle-cell anemia unknown in whites, the 

 child of a black-white mating is safe from 

 both diseases. One piece of advice that 

 might be given to someone concemed that 

 his or her child might suffer from genetic 

 disease is to marry someone with a differ- 

 ent skin color. Some geneticists believe 

 that some of the general increase in child 

 health seen in the West over the past cen- 

 tiu-y or so is due to such increased out- 

 breeding. 



Any benefits that genetic mixing will 

 bring cannot last forever. In time the 

 mixed populations of the world will reach 

 a new equilibrium. Many of the genes hid- 

 den in the descendants of mixed marriages 

 will reappear. 



This new uniformity also means that no 

 longer will there be much chance — as 

 there was among the Amish — ^for small 

 and isolated populations to diverge geneti- 

 cally by accident. One of the most impor- 

 tant agents of evolution has lost its power. 



Speculating about what is to come — 

 particularly for a species like our own, so 

 prone to social, political, or ecological dis- 

 aster — is dangerous, but because so much 

 of human evolution has involved random 

 change in small groups, the loss of this 

 agent of change probably means that the 

 biology of the future will not be very dif- 

 ferent from that of the past. Humans may 

 even be almost at the end of their evolu- 

 tionary road. If so, we are as near to our bi- 

 ological Utopia as we are likely to get, al- 

 though it has been reached in ways not 

 dreamed of by Galton. 



74 Natural History 6/94 



