Scientists from the Beijing Genomics Institute last month discovered another striking instance of human genetic change. Among Tibetans, they found, a set of genes evolved to cope with low oxygen levels as recently as 3,000 years ago. This, if confirmed, would be the most recent known instance of human evolution.
The difficulty of identifying these shifts is also covered in the article (and the reason this is dismissed by so many…it’s hard).
One of the signatures of natural selection is that it disturbs the undergrowth of mutations that are always accumulating along the genome. As a favored version of a gene becomes more common in a population, genomes will look increasingly alike in and around the gene. Because variation is brushed away, the favored gene’s rise in popularity is called a sweep. Geneticists have developed several statistical methods for detecting sweeps, and hence of natural selection in action.
About 21 genome-wide scans for natural selection had been completed by last year, providing evidence that 4,243 genes — 23 percent of the human total — were under natural selection. This is a surprisingly high proportion, since the scans often miss various genes that are known for other reasons to be under selection. Also, the scans can see only recent episodes of selection — probably just those that occurred within the last 5,000 to 25,000 years or so. The reason is that after a favored version of a gene has swept through the population, mutations start building up in its DNA, eroding the uniformity that is evidence of a sweep.
So as soon as an “upgrade” is available in the gene pool, it changes the color of the pool, so to speak, and immediately new dyes start seeping in, searching for that next true hue.
The theory also makes predictions that have also been observed, such as….
The fewest signals of selection were seen among people who live in the humid tropics, the ecoregion where the ancestral human population evolved. “One could argue that we are adapted to that and that most signals are seen when people adapt to new environments,” Dr. Di Rienzo said in an interview.
To continue the pool analogy, those born in the the deep blue of the tropics and stayed, were good with that color. But you start getting to more extreme environment (cold, altitude) that same color doesn’t cut the mustard anymore.
The second page is a basic discussion on skin color and how there is enough adaptability in the human genome for light skin to have evolved in at least two ways.
The difficulty in comprehending the theory (much less applying it) also lies in the complexity of the systems themselves.
Most variation in the human genome is neutral, meaning that it arose not by natural selection but by processes like harmless mutations and the random shuffling of the genome between generations. The amount of this genetic diversity is highest in African populations. Diversity decreases steadily the further a population has migrated from the African homeland, since each group that moved onward carried away only some of the diversity of its parent population. This steady decline in diversity shows no discontinuity between one population and the next, and has offered no clear explanation as to why one population should differ much from another. But selected genes show a different pattern: Evidence from the new genome-wide tests for selection show that most selective pressures are focused on specific populations.
However, within that complexity, one can expose new insights (again, in keeping with the theory).
One aspect of this pattern is that there seem to be more genes under recent selection in East Asians and Europeans than in Africans, possibly because the people who left Africa were then forced to adapt to different environments. “It’s a reasonable inference that non-Africans were becoming exposed to a wide variety of novel climates,” says Dr. Stoneking of the Max Planck Institute.
The final bit is about the “soft sweet” which continues to occur regardless of outside pressure.
But the new evidence that humans have adapted rapidly and extensively suggests that natural selection must have other options for changing a trait besides waiting for the right mutation to show up. In an article in Current Biology in February, Dr. Pritchard suggested that a lot of natural selection may take place through what he called soft sweeps.
Soft sweeps work on traits affected by many genes, like height. Suppose there are a hundred genes that affect height (about 50 are known already, and many more remain to be found). Each gene exists in a version that enhances height and a version that does not. The average person might inherit the height-enhancing version of 50 of these genes, say, and be of average height as a result.
The article uses a primitive example of this, but I could just link here…and then draw the pictures….taller = more money, more money = more health/breeding partners, = taller species. Although this last (the money/height connection) has only been going on for 20-30 generations and only a couple generations for all people of all genomic heritage (in my country). It will be interesting to see how these studies move forward in the future, as genome databases grow and more cross-testing is available.
It would be quite a thing to get a six-month gene therapy treatment before that next stint on Everest/in the Arctic. Or at least it would be if that kind of stuff isn’t outlawed by people who don’t believe in evolution [search : Gene Manufacturing]