For mammals, the embryo develops within the mother; therefore, the maternal environment influences development. Because one major component of the maternal environment is how well nourished Mom is, embryonic development may respond to changes in maternal nutrition — a position that medical evidence seems to support.
Medical evidence indicates that a fetus deprived of nutrition during key parts of development will develop into a baby with a greater degree of metabolic thriftiness, a group of characteristics that reduce caloric requirements (smaller size and lower metabolism, for example).
Some scientists postulate that these fetal changes may be the result of natural selection; genes that allow a developing human embryo to better prepare for existence outside the mother will be selectively favored. This hypothesis is called the Thrifty Phenotype hypothesis, and it goes something like this: Some of the fetal developmental changes observed in low-birth-weight babies may have been adaptive early in human history. If the mother's condition predicted low food availability when the baby was born (for example, she was deprived of food during pregnancy), genes that allowed for developmental flexibility — like growing slowly, but more efficiently, when food is scarce — may be selectively advantageous.
From an evolutionary viewpoint, what the fetus would be responding to is unclear. The condition of the mother could correlate with any of the following:
^ The environment in which the mother was living
^ The mother's ability to provide resources
^ The environment in which the adult offspring would find itself as predicted by the mother's environment (in those situations in which the environment changes slowly over a time scale longer than the organism's generation time)
But the jury's still out on whether natural selection had anything to do with the fact that human embryos may develop differently when maternal resources are scarce. At this point, the Thrifty Phenotype hypothesis is pure speculation (often the first part of scientific inquiry). What's not speculation is that embryonic developmental changes do occur, which unfortunately are related to other medical problems, such as diabetes and obesity, so being able to identify the cause and the mechanism by which the development pathway is altered is medically important.
The thrifty phenotype hypothesis postulates genes for plasticity, genes that allow the developing fetus to develop to be more or less thrifty based on the maternal environment. This hypothesis is different from the thrifty genotype hypothesis, which states that natural selection for particular genes in some human populations makes these individuals more metabolically efficient. This theory has been implicated in diseases among Native Americans who are now subjected to the modern American diet.
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