Genetic Load and its Importance

The past decade has seen a resurgence of scientific eugenics. In 2016, for example, geneticist Michael Lynch published an article in the well-respected journal \emph{Genetics}{=tex} arguing that human mental and physical performance is declining at a rate of 1% per generation (Lynch 2016). This estimate was not based on measurements of physical and mental performance, but rather on an argument from genetic load: because medical interventions, such as “surgical procedures, pharmaceuticals, nutritional supplements, and physical and psychiatric therapies” have mitigated the effects of selection on “bad genes,” the incidence of deleterious alleles (genes variants) in the human population has risen and will continue to rise.

The problem, Lynch argues, lies in the fact that natural selection can only act on existing variation in a population. This variation, however, imposes a “cost” on a population. A population is less fit if it contains fitness variation than if it were made up of only the optimal phenotype, or set of traits. Suppose, for example, a population made up of two phenotypes: red and blue. Red is fitter—it produces more surviving offspring on average—than blue. A population made up entirely of red individuals will, then, have a higher average fitness than a mixed population of reds and blues. Moreover, if selection is relaxed, as Lynch and others argue it has been in humans, this will lead to a higher proportion of low-fitness individuals in the population.

This cost, known as genetic load, has been deployed in biological arguments for eugenics since Muller (1950) and its use continues in the works of pro-eugenics philosophers such as Powell (2015) and Gyngell et al. (2019). The consequences of genetic load for actual populations are not as well-understood as these authors imply, however. The concept of genetic load is conceptually heterogeneous and in need of clarification. It has given rise to several false predictions and puzzles whose solutions are, as yet, unclear.

In this project I aim to make clear what genetic load is, what we do and do not know about it, and criticize its use in arguments for eugenics.

Outline of Chapters

The Genetic Load Argument for Eugenics

It is commonly accepted that one cannot argue from solely empirical premises to a moral conclusion. However, empirical premises are often key in moral arguments. In the present case, genetic load and its apparent consequences for populations have convinced some biologists and philosophers that eugenic measures are necessary to preserve the health of and prevent the extinction of the human population. While there has been much attention to the moral premises of eugenic arguments by philosophers such as Barnes (2016) and Wilson (2017), the empirical premises have been subject to less scrutiny.

Eugenic arguments are frequently dismissed as racist pseudoscience that relies on “a prejudiced and incorrect understanding of Mendelian genetics” (Eugenics and Scientific Racism, n.d.). This may be true of many historical and contemporary eugenic arguments, but the genetic load argument cannot be easily dismissed on these grounds. The argument’s originator, Hermann Muller (Muller 1950), was an architect of modern evolutionary theory and outspoken critic of race-based eugenics. The argument’s current defender, Michael Lynch, is a well-regarded and widely-cited geneticist.

In this chapter I evaluate the empirical work and show that the simple argument from relaxation of selection to fitness declines is invalid. An increase in frequency of deleterious alleles does not necessarily correspond to a decrease in fitness.

Two Puzzles of Genetic Load

In this chapter I examine two puzzles of genetic load, which continue to be the topic of debate in biology. The first puzzle is that there exists any significant amount of genetic diversity in natural populations (see Crow 2008 for discusion). Because genetic diversity decreases the fitness of a population, natural selection should act to remove any fitness-affecting genetic diversity in a population. At mutation-selection equilibrium, then, the amount of fitness-affecting genetic diversity should be minimal. However, there does appear to be a significant degree of fitness-affecting diversity in natural populations.

A second, and more recent, false prediction, pointed out by Kondrashov (1995), is that the well-accepted Nearly-Neutral Theory (Ohta 1992) predicts there are many very small deleterious mutations in the genome and that these ought to lower expected fitness to a degree that is not empirically supported. Lesecque et al. (2012), for example, note that application of this theory to humans predicts that 88% of individuals should fail to reproduce and each reproducing female would need to have 16 offspring in order to maintain population size.

These puzzles trade on ambiguities in the concept of genetic load. I explore two recognized kinds of load, lag load and substitution load, as well as a largely ignored third kind—expected load—identified by Bruce Wallace (1991). Central to this discussion is Wallace’s concept of “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 (discussed in Sober (2024)).

Genetic Load and Evolution

Large genetic loads are often thought to have significant negative effects on populations, often suggesting an increased chance of extinction via “mutational meltdown” (Lynch and Gabriel 1990; Lynch et al. 1995a, 1995b). Mutational meltdown occurs when some deleterious mutations become fixed through drift, lowering population fitness, which leads to the fixation of further deleterious mutations through drift, and so on, until extinction results. It is too quick, however, to conclude that genetic load is necessarily a burden on a population.

As Haldane (1937) points out, increased numbers of mutations also allow for increased probability of “evolutionary rescue,” i.e., avoiding extinction through adaptive evolution. This chapter connects with recent philosophical work on evolvablility (see [Sterelny et al.]{.nocase} 2007; Brown 2014; Bourrat et al. 2024) and discusses the role of diversity in a changing environment.

Wallace (1991) further argues that genetic load can be a benefit to a population through the phenomenon of soft selection, increasing the 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 such as these (Sober and Wilson 1999), Wallace holds that the phenomenon he has identified is not a case of group selection. This mechanism for the evolution of altruistic traits may thus avoid biologists’ longstanding skepticism about group selection.

ZFEL and the Genetic Load Research Project

The search for laws in biology has been widely criticized. Some, such as Beatty (1995) hold that there are no laws, while others, such as Sober (1997) hold that there are laws but they are a priori and mathematical, not empirical.

McShea and Brandon (2010), the first a biologist, the second a philosopher, propose that biology has an empirical first law, akin to Newton’s first law of motion. Like Newton’s first law, it aims to give an account of what happens when no forces are acting. McShea and Brandon’s first law is the zero-force evolutionary law, or ZFEL. In brief, they claim that in any evolutionary system there is a tendency for diversity and complexity to increase.

According to McShea and Brandon however, ZFEL is not just a law; it is a “gestalt shift.” ZFEL puts diversity and complexity in the background while foregrounding selection and constraints on diversity. This means that diversity and complexity do not, in general, stand in need of explanation, just as inertial motion in Newtonian mechanics does not stand in need of explanation. It is this “gestalt shift” which I think has the most significant consequences for biological theory and stands most in need of defense.

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. If increases in diversity had not stood in need of explanation then, in their estimation, there would have been no need to study load in the ways that were done.

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