Evolution of Obesity

Abstract

Obesity is the result of a gene by environment interaction. A genetic legacy from our evolutionary past interacts with our modern environment to make some people obese. Why we have a genetic predisposition to obesity is problematical, because obesity has many negative consequences. How could natural selection favor the spread of such a disadvantageous trait? From an evolutionary perspective, three different types of explanation have been proposed to resolve this anomaly. The first is that obesity was once adaptive, in our evolutionary past. For example, it may have been necessary to support the development of large brains, or it may have enabled us to survive (or sustain fecundity) through periods of famine. People carrying so-called thrifty genes that enabled the efficient storage of energy as fat between famines would be at a selective advantage. In the modern world, however, people who have inherited these genes deposit fat in preparation for a famine that never comes, and the result is widespread obesity. The key problem with these adaptive scenarios is to understand why, if obesity was historically so advantageous, many people did not inherit these alleles and in modern society remain slim. The second type of explanation is that most mutations in the genes that predispose us to obesity are neutral and have been drifting over evolutionary time – so-called drifty genes, leading some individuals to be obesity prone and others obesity resistant. The third type of explanation is that obesity is neither adaptive nor neutral and may never even have existed in our evolutionary past, but it is favored today as a maladaptive by-product of positive selection on some other trait. Examples of this type of explanation are the suggestion that obesity results from variation in brown adipose tissue thermogenesis, or the idea that we over consume energy to satisfy our needs for protein (the protein leverage hypothesis). This chapter reviews the evidence for and against these different scenarios, concluding that adaptive scenarios are unlikely, but the other ideas may provide possible evolutionary contexts in which to understand the modern obesity phenomenon. , Abstract Obesity is the result of a gene by environment interaction. A genetic legacy from our evolutionary past interacts with our modern environment to make some people obese. Why we have a genetic predisposition to obesity is problematical, because obesity has many negative consequences. How could natural selection favor the spread of such a disadvantageous trait? From an evolutionary perspective, three different types of explanation have been proposed to resolve this anomaly. The first is that obesity was once adaptive, in our evolutionary past. For example, it may have been necessary to support the development of large brains, or it may have enabled us to survive (or sustain fecundity) through periods of famine. People carrying so-called thrifty genes that enabled the efficient storage of energy as fat between famines would be at a selective advantage. In the modern world, however, people who have inherited these genes deposit fat in preparation for a famine that never comes, and the result is widespread obesity. The key problem with these adaptive scenarios is to understand why, if obesity was historically so advantageous, many people did not inherit these alleles and in modern society remain slim. The second type of explanation is that most mutations in the genes that predispose us to obesity are neutral and have been drifting over evolutionary time – so-called drifty genes, leading some individuals to be obesity prone and others obesity resistant. The third type of explanation is that obesity is neither adaptive nor neutral and may never even have existed in our evolutionary past, but it is favored today as a maladaptive by-product of positive selection on some other trait. Examples of this type of explanation are the suggestion that obesity results from variation in brown adipose tissue thermogenesis, or the idea that we over consume energy to satisfy our needs for protein (the protein leverage hypothesis). This chapter reviews the evidence for and against these different scenarios, concluding that adaptive scenarios are unlikely, but the other ideas may provide possible evolutionary contexts in which to understand the modern obesity phenomenon.