Latitudinal variation in genetic divergence of populations and the potential for future speciation.

Evolution Int J Org Evolution. 2004 May;58(5):938-45.
Martin PR, McKay JK.

The increase in biological diversity with decreasing latitude is widely appreciated but the cause of the pattern is unknown. This pattern reflects latitudinal variation in both the origin of new species (cladogenesis) and the number of species that coexist. Here we address latitudinal variation in species origination, by examining population genetic processes that influence speciation. Previous data suggest a greater number of speciation events at lower latitudes. If speciation events occur more frequently at lower latitudes, we predicted that genetic divergence among populations within species, an important component of cladogenesis, should be greater among lower latitude populations. We tested this prediction using within-species patterns of mtDNA variation across 60 vertebrate species that collectively spanned six continents, two oceans, and 119 degrees latitude. We found greater genetic divergence of populations, controlling for geographic distance, at lower latitudes within species. This pattern remained statistically significant after removing populations that occur in localities previously covered by continental glaciers during the last glaciation. Results suggest that lower latitude populations within species exhibit greater evolutionary independence, increasing the likelihood that mutation, recombination, selection, and/or drift will lead to divergence of traits important for reproductive isolation and speciation. Results are consistent with a greater influence of seasonality, reduced energy, and/or glacial (Milankovitch) cycles acting on higher latitude populations, and represent one of the few tests of predictions of latitudinal variation in speciation rates using population genetic data.

Genghis Khan, Father of Millions?

June 22, 2004 —Genghis Khan left a legacy shared by 16 million people alive today, according to a book by a Oxford geneticist who identified the Mongol emperor as the most successful alpha male in human history.

Regarded by the Mongolians as the father of their nation, Genghis Khan was born around 1162. A military and political genius, he united the tribes of Mongolia and conquered half of the known world with a cavalry riding on grass-fed ponies.

By the time Genghis died in 1227, his empire stretched from the Pacific coast of China to the Caspian Sea.

Bryan Sykes, professor of human genetics at Oxford University and author of "Adam's Curse," a study of the Y chromosome, believes Genghis's "super Y" chromosome survived and proliferated as far as the British Isles. He has just begun to check it at Oxford Ancestors, a leading provider of DNA-based services for use in personal ancestry research.

"We will offer British men genetic tests to see if they are Genghis's descendants. It is possible that the Mongol emperor's Y chromosome spread as far as the U.K. through gradual immigration from further East over the centuries," Sykes told Discovery News.

The genetic testing follows another Oxford study, which involved a survey of the Y chromosome — which is passed unchanged from father to son — from all over Central Asia.

The researchers found one Y chromosome fingerprint that was identical in eight percent of the male population.

"This was highly unusual and suggested that they may all have descended from one man living in the fairly recent past. By seeing what small changes had occurred, it was possible to estimate the time at which this common ancestor lived, and it was consistent with an origin in the 12th or 13th century," Sykes said.

Matching that evidence with the overlap between where the chromosome was abundant and the geographical extent of the Mongol empire established by Genghis Khan in the 12th century, the researchers concluded it was Genghis' chromosome.

The Mongol emperor's habit of killing the men and inseminating the women when his army conquered a new territory, coupled with handing the Empire and other wealth to his sons, and their sons, would explain how the chromosome came to such prevalence today, said Sykes.

The final piece of evidence came from the Hazara, a hill tribe in Pakistan who had a strong oral history of being descended from Genghis Khan.

"The Y chromosome was present in the Hazara, but not in the surrounding tribes, who did not have this oral history. Though the evidence is circumstantial, it is, I believe, very strong," Sykes said.

Finding Genghis Khan's tomb, one of the great secrets of all time, could provide the definitive evidence, leading to a direct comparison of Genghis' Y chromosome with those of modern men.

Sykes' hypothesis seems to be consistent with history, according to David Morgan, a Mongol history specialist at the University of Wisconsin.

"There's no reason to doubt that Genghis Khan fathered a good crop of children, if one is to believe the testimony of contemporaries," Morgan told Discovery News.

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Shy blue eyed boys

In book `survival of the prettiest'

psychologist Jerome Kagan has found that children with pale pigment, in particular children with blue eyes, are far more likely to be shy and inhibited than dark-eyed children. They are the most likely to be fearful of new situations, hesitant in approaching someone, quiet with a new person, and the most likely to stay close to their mothers. Brown-eyed children are bolder. Kagan speculates that fear of novelty, melanin production and corticorsteroid levels share some of the same genes.

His theory is speculative, suggesting that when people migrated to northern Europe they were faced with the problem of keeping up a body temperature that was used to a warmer climate. A mutation that increased the efficiency of the sympathetic nervous system and upped the level of norepinephrine...would have also raised the body temperature and offered a survival advantage. Unfortunately, it would have left them with a more reactive nervous system and a more timorous temperament. Where does the pigment come in? High levels of norepinephrine can inhibit the production of melanin in the iris and can increase the level of circulating glucosteroids that can inhibit melanin production as well. So blond hair and blue eyes and shyness may be a common biological package....
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Another study:

Physical and physiological correlates of behavioral inhibition.

Rosenberg AA, Kagan J.

University of California, Berkeley 94708.

Previous investigations have suggested that the temperamental quality of inhibition is related to the threshold of reactivity to unfamiliar events within certain limbic structures. In earlier work, children in three independent samples who had been selected to be inhibited were more likely to have blue than brown eyes, whereas uninhibited children were more likely to have brown eyes. The present study, which selected two-year-old children on the basis of eye color (blue or brown) rather than behavior, found a significant association between blue eyes and behavioral inhibition, and between brown eyes and an uninhibited style. Although the inhibited children were more likely to have a high and stable heart rate than were the uninhibited children, there was no relation between eye color and these cardiac measures. Several interpretations of the association between these temperamental categories and iris pigmentation are proposed.

PMID: 2636201 [PubMed - indexed for MEDLINE]

Another again:

Shyness and little boy blue: iris pigmentation, gender, and social wariness in preschoolers.

Coplan RJ, Coleman B, Rubin KH.

Department of Psychology, Carleton University, Ottawa, Ontario, Canada.

In recent years, researchers have uncovered a link between iris pigmentation and inhibition/social wariness among young children (e.g., Rosenberg & Kagan, 1987, 1989; Rubin & Both, 1989). In the present study, 152 Caucasian preschool-aged (Mage = 54.09 months, SD = 5.84) children (77 males) with either blue (n = 84) or brown (n = 68) eyes, were compared in terms of parental and teacher ratings of social wariness, social play, and aggression. A significant Eye Color x Gender Interaction was found in terms of indices of social wariness; blue-eyed males were rated as more socially wary than brown-eyed males, while blue- and brown-eyed females did not differ in this regard. These results supported the notion that eye color is a marker variable for social wariness in young children.

PMID: 9452906 [PubMed - indexed for MEDLINE]

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