Philosophical Aspects of Genetic Load

The past decade has seen a resurgence of “scientific” eugenics. Genetic load—the difference between the mean and maximum fitness in a population—is one scientific concept that has been used in defense of eugenics. The consequences of genetic load for actual populations are, however, not well-understood. Genetic load is conceptually heterogeneous, in need of clarification, and interacts with natural selection in surprising ways. My aim in the dissertation is to provide that clarification and, in doing so, improve our understanding of natural selection and the sources of genetic diversity, while casting doubt on eugenic arguments that depend on genetic load.

Chapter 1 challenges the conclusions of several prominent geneticists that genetic load poses an existential risk to the human population. These geneticists have argued that the increasing effectiveness and availability of medical interventions has reduced—and will continue to reduce—the effectiveness of natural selection, leading to increased genetic load, and a consequent decline in human fitness. This decline in fitness, they argue, may be so severe that future humans may not be able to survive and reproduce. As modern medicine is the source of the problem, they claim that eugenic means are the only way to combat the decline. I show, however, that the simple argument from relaxation of selection to fitness declines is invalid. An increase in genetic load does not entail a decrease in fitness. Valid versions of the argument, far from an immediate risk, suggest a slow 14,000+ year decline in fitness which would not lower fitness below historic levels.

Chapter 2 resolves two “paradoxes” of genetic load that are topics of debate in biology. The first paradox is that the amount of genetic diversity in natural populations seems inexplicable, as natural selection should act to remove it. The second is 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 that each reproducing female would need to have 16 offspring in order to maintain a stable population.

The key to resolving these paradoxes is soft selection. In soft selection, whether a trait is good for its bearer depends both on how many competing individuals there are and how common that trait is—the trait is both density- and frequency-dependent. In natural populations, many traits are dependent in both ways. Assuming that selection is hard (i.e. not soft) leads to overestimating the actual selective effects of these traits. This, along with disambiguating several genetic load concepts, resolves the paradoxes.

Chapter 3 argues that soft selection and genetic load may help explain some traits that increase the probability of survival and reproduction for others while reducing the probability of survival and reproduction for the trait bearer. These “altruistic” traits pose a puzzle for evolutionary biology as it does not seem that natural selection could favor such traits. The most common explanation among philosophers for altruistic traits is selection for the good of the group or species (group selection). Many biologists, however, think group selection cannot be the explanation as it is too rare and ineffective or, worse, is conceptually confused as selection acts on individuals, not groups. Soft selection provides an alternative explanation for some cases. An increase in genetic load may sometimes increase a population’s probability of survival while lowering the average fitness of the population. This can happen, for example, when variation in fitness due to over-crowding causes the culling of some individuals, similar to a gardener culling some seedlings to make room for others to grow.

Chapter 4 argues that the historical puzzles that led to genetic load research hold lessons for thinking about laws in biology. One recently proposed biological law is Brandon and McShea’s Zero-Force Evolutionary Law (ZFEL): in any evolutionary system there is a tendency for diversity and complexity to increase. Their proposal does not just purport to be a law; it is intended as a “gestalt shift,” which implies that diversity and complexity do not, in general, stand in need of explanation. Brandon and McShea argue that, in the same way that Newton’s First Law removes the need to explain inertial motion in physics, ZEFL removes the need to explain diversity and complexity. Some have concluded that research into the sources of diversity, such as Kimura’s development of Neutral Theory, which posits that most genetic variation is selectively neutral, supports ZFEL as a law. I argue that even if this research supports ZFEL as a law, the very same research undermines ZFEL as a gestalt shift. The gestalt shift holds that the puzzles that led to the development of these theories do not stand in need of explanation, but the debates about and theories developed to explain genetic diversity continue to be relevant.