Philosophical Aspects of Genetic Load

The past decade has seen a resurgence of “scientific” eugenics. One scientific concept used in defense of eugenics is genetic load, or the idea that variation imposes a “cost” on a population. The consequences, or costs, of genetic load for actual populations are, however, not well-understood. Genetic load is conceptually heterogeneous and in need of clarification. My aim in the dissertation is to provide that clarification and, in doing so, improve our understanding of natural selection, the sources of diversity, and cast doubt on eugenic arguments depending on genetic load.

In Chapter 1 I take on the argument from increasing genetic load to the necessity of eugenics. Eugenic arguments are frequently dismissed as racist pseudoscience. This may be true of many eugenic arguments, but the genetic load argument cannot be so easily dismissed. Many prominent geneticists, including fore-runners in the field, have argued that the risk to humans from genetic load may be existential.

I show, however, that the simple argument from relaxation of selection to fitness declines is invalid. An increase in frequency of deleterious alleles does not entail a decrease in fitness. Valid versions of the argument, on the other hand do not present an existential threat.

In Chapter 2 I examine two “paradoxes” of genetic load, which continue to be topics of debate in biology. One paradox argues that the amount of genetic diversity in natural populations is inexplicable, as natural selection should act to remove it. The second argues that the well-accepted Nearly-Neutral Theory predicts that the amount of load in natural populations is orders of magnitude too low. Application of this theory to humans, for example, predicts each reproducing female would need to have 16 offspring in order to maintain population size.

These paradoxes both trade on ambiguities in the concept of genetic load. Genetic load is commonly defined as the difference between the maximum fitness in a population and the mean fitness of the population. It is unclear, however, what the maximum fitness in a population is. This maximum might refer to the maximum possible fitness in a population, termed lag load, the maximum actual fitness in a population, termed substitution load, or the largely unstudied maximum likely fitness in a population. Each of these concepts is useful, but confusion among them has led to mistaken inferences.

Central to resolving these paradoxes is “soft,” or density-dependent, selection, in which load and fitness-effects appear to come apart. Discussion of soft selection contributes to the philosophical literature on forms of selection, adding to the existing literature on frequency dependent and balancing selection.

In Chapter 3 I address the effects of genetic load on the process of evolution. Large genetic loads are often thought to have significant negative effects on populations, often suggesting an increased chance of extinction via “mutational meltdown.” Increased numbers of mutations, however, also allow for increased probability of “evolutionary rescue,” i.e., avoiding extinction through adaptive evolution. But this may not be the only way that load can benefit a population. Soft selection may, in some circumstances, increase a population’s probability of survival while, in an apparent paradox, lowering the average fitness of the population. While some have invoked group selection to explain “altruistic” traits (traits that benefit others at a fitness cost to the individual) such as these, this phenomenon is not a case of group selection.

In Chapter 4 I argue that genetic load holds lessons for thinking about laws in biology. The existence of laws in biology is controversial. One recent defense of a law in biology is Brandon and McShea’s Zero-Force Evolutionary Law (ZFEL), which claims that in any evolutionary system there is a tendency for diversity and complexity to increase. Their proposal is not just a law; it is intended as a “gestalt shift,” which implies that diversity and complexity do not, in general, stand in need of explanation. Where others have concluded that research into the sources of diversity supports ZFEL as a law, I argue that this same research undermines it as a gestalt shift.