nio
2010-01-18, 13:09
One of the things about evolution you sometimes hear is that it has "stopped" for humans. ... The key fact that most people latch on to is that infant and child mortality is very low, so the vast majority of humans reach the age of potential reproduction. Random genetic drift aside, evolution via natural selection does not necessarily need differential mortality as a necessary precondition (though this is obviously an efficacious mechanism from the viewpoint of evolution). All that needs to occur is for reproductive fitness to track variation in a heritable trait. Imagine for example that a woman was born with a mutation which prevented her from ever reaching menopause. Assuming that the allele does not go extinct, this is a variant which might actually spread in the modern world, as many women are delaying childbearing until the latter half of their fertility curve. ...
... a few months ago a paper came out which looked at this question. They used the Framingham Heart Study study sample, which goes back to 1948, and compared two generations, both with N's somewhat in excess of 5,000. Natural selection in a contemporary human population:
Our aims were to demonstrate that natural selection is operating on contemporary humans, predict future evolutionary change for specific traits with medical significance, and show that for some traits we can make short-term predictions about our future evolution. To do so, we measured the strength of selection, estimated genetic variation and covariation, and predicted the response to selection for women in the Framingham Heart Study, a project of the National Heart, Lung, and Blood Institute and Boston University that began in 1948. We found that natural selection is acting to cause slow, gradual evolutionary change. The descendants of these women are predicted to be on average slightly shorter and stouter, to have lower total cholesterol levels and systolic blood pressure, to have their first child earlier, and to reach menopause later than they would in the absence of evolution. Selection is tending to lengthen the reproductive period at both ends. To better understand and predict such changes, the design of planned large, long-term, multicohort studies should include input from evolutionary biologists.
Several of these are medical traits, exactly the sort of factors which are presumably greatly palliated by modern medicine. Below are the traits with their heritabilities:
Height (HT) = 0.84
Total Cholesterol (TC) = 0.61
Systolic Blood Pressure (SBP) = 0.53
Weight (WT) = 0.52
Diastolic Blood Pressure (DBP) = 0.49
Age at menopause = 0.47
Glucose level = 0.34
Age at first birth = 0.09
Remember that heritability is simply the proportion of phenotypic variation which can be explained by genetic variation. So for height ~84% of the variation in height is due to variation in genes. By contrast, only 9% of the variation in age at first birth is due to variation in genes. This is important because the higher the heritability, the easier it is for differences in reproductive outcome to translate into natural selection. The response to selection is directly proportional to the product of the heritability and the strength of selection. In other words, tall people tend to have tall offspring. Women who were unusually young when they first gave birth did not usually have offspring who were also unusually young when they first gave birth. This makes some sense since height is a straightforward biological trait, while age at first birth is presumably strongly conditional upon social circumstances and random environmental events.
GNXP (http://scienceblogs.com/gnxp/2010/01/people_evolving_to_be_shorter.php)
... a few months ago a paper came out which looked at this question. They used the Framingham Heart Study study sample, which goes back to 1948, and compared two generations, both with N's somewhat in excess of 5,000. Natural selection in a contemporary human population:
Our aims were to demonstrate that natural selection is operating on contemporary humans, predict future evolutionary change for specific traits with medical significance, and show that for some traits we can make short-term predictions about our future evolution. To do so, we measured the strength of selection, estimated genetic variation and covariation, and predicted the response to selection for women in the Framingham Heart Study, a project of the National Heart, Lung, and Blood Institute and Boston University that began in 1948. We found that natural selection is acting to cause slow, gradual evolutionary change. The descendants of these women are predicted to be on average slightly shorter and stouter, to have lower total cholesterol levels and systolic blood pressure, to have their first child earlier, and to reach menopause later than they would in the absence of evolution. Selection is tending to lengthen the reproductive period at both ends. To better understand and predict such changes, the design of planned large, long-term, multicohort studies should include input from evolutionary biologists.
Several of these are medical traits, exactly the sort of factors which are presumably greatly palliated by modern medicine. Below are the traits with their heritabilities:
Height (HT) = 0.84
Total Cholesterol (TC) = 0.61
Systolic Blood Pressure (SBP) = 0.53
Weight (WT) = 0.52
Diastolic Blood Pressure (DBP) = 0.49
Age at menopause = 0.47
Glucose level = 0.34
Age at first birth = 0.09
Remember that heritability is simply the proportion of phenotypic variation which can be explained by genetic variation. So for height ~84% of the variation in height is due to variation in genes. By contrast, only 9% of the variation in age at first birth is due to variation in genes. This is important because the higher the heritability, the easier it is for differences in reproductive outcome to translate into natural selection. The response to selection is directly proportional to the product of the heritability and the strength of selection. In other words, tall people tend to have tall offspring. Women who were unusually young when they first gave birth did not usually have offspring who were also unusually young when they first gave birth. This makes some sense since height is a straightforward biological trait, while age at first birth is presumably strongly conditional upon social circumstances and random environmental events.
GNXP (http://scienceblogs.com/gnxp/2010/01/people_evolving_to_be_shorter.php)