When humans are nursing, we all have an enzyme, lactase, that allows us to break down the milk sugar lactose. However, in our early ancestors, the activity of lactase eventually decreased or stopped entirely. Those modern humans that retain this trait are lactose intolerant as adults. However, as we know, some people are able to safely consume milk (and thus lactose) into adulthood. Sarah Tishkoff and her colleagues summarized their recent findings on lactase persistence a couple weeks ago at the American Association for the Advancement of Science meetings in D.C. From one of Tishkoff et al.’s papers on the subject (see reference below):
These individuals have the ‘lactase persistence’ trait. The frequency of lactase persistence is high in northern European populations (>90% in Swedes and Danes), decreases in frequency across southern Europe and the Middle East (~50% in Spanish, French and pastoralist Arab populations) and is low in non-pastoralist Asian and African populations (~1% in Chinese, ~5%–20% in West African agriculturalists). Notably, lactase persistence is common in pastoralist populations from Africa (~90% in Tutsi, ~50% in Fulani)
What do all these populations with high frequencies of the lactase persistence trait have in common? You guessed it….they all have a long history of cattle domestication. What’s cool about this new study is they show that the genetic mutation that gave rise to lactase persistence in modern Europeans is different from that of modern Africans. So, basically, this trait evolved independently at least twice. It also appears as if the evolution and spread of lactase persistence is consistent with a selective sweep (see this post for more info) that began about 7,000 years ago. So, in other words, it’s spread really fast, which means that it conferred a pretty big advantage to those individuals that had it. For more info see this podcast from Scientific American.
Participate in our poll below…are you lactose intolerant? Can you trace your ancestry back to populations that practiced cattle domestication?
When a mutation arises that confers some sort of advantage, those individuals with the mutation have more kids than those without it. Over time, of course, the mutated gene will become more prevalent in a population (this is simply natural selection). In some cases, other pieces of DNA will hitch-hike along with the advantageous mutant gene because they are linked (i.e., close-by) on the same chromosome and will thus also increase in frequency. A SELECTIVE SWEEP occurs when the positively selected gene and all its neighbors (called a haplotype) become the only variant in a population. So, the result of a selective sweep is a reduction in overall genetic diversity in that region of the genome.
Selective sweeps have certainly occurred in recent human evolution: for example, the genes (and associated DNA neighbors) for skin pigmentation and lactose tolerance appear to have arisen among modern human populations in a manner consistent with a selective sweep.
According to a newly published study in Science, selective sweeps were considered to be a relatively common occurrence among humans. However, the new research suggests that this is not so. From a summary in ScienceNews:
Scientists have favored a model of evolution in which beneficial gene mutations quickly and dramatically sweep through a population due to the evolutionary advantages they confer. Such mutations would become nearly universal in a population. But this selective sweep model may not be accurate for humans, a new study indicates. Human evolution likely followed a more subtle and complicated path, say population geneticists Molly Przeworski of the University of Chicago and Guy Sella of Hebrew University of Jerusalem and colleagues.
It may have been difficult for selective sweeps to take hold in humans because of demographics…[p]eople are scattered throughout the globe, so a beneficial mutation would have a long way to spread. Such a mutation would have to have dramatic effects on evolutionary fitness to go global.