Conditional Development Strategy and Environmental Cues
In the scenario discussed above, individuals develop a trait for addressing a specific niche, depending on whether they carry or not the allele or alleles which predispose for this trait. Those who carry it will develop the trait, while those who do not will not develop the trait. There is still another possibility, namely, that individuals carry the allele, but whether it is expressed or not depends on the environmental conditions during an individual’s development. For instance, an allele that predisposes for aggression that would enable a man to address the male-male reproductive niche may be expressed if the individual finds himself in a violent context at a young age, which hints that the male-male niche is large, and by becoming aggressive, he will be readier to exploit it. On the other hand, if the individual finds himself in a peaceful and nonviolent setting, which hints that the male-male reproductive niche is small, then this allele may not be expressed as the man will not reap considerable reproductive benefits from becoming aggressive.
It has to be said also that the expression of alleles may be contingent on the presence of other traits, sometimes referred to as “reactive heritability” (Lukaszewski, 2011; Lukaszewski & Roney, 2015). In the example above, if alleles that predispose for aggression are expressed irrespectively of the traits that an individual has, they are likely to be expressed in men who are physically strong as well as in men who are physically weak. But in the latter case, the fitness of these alleles would be compromised, as individuals who are aggressive but physically weak will frequently end up in fights with physically stronger men, fights which are likely to lose. It would pay then for these alleles to remain silent and be expressed only when they are found in a physically strong male body. Thus, alleles for aggression, the expression of which is contingent on other traits, may be favored over alleles that are expressed irrespectively of the presence of other traits.
This argument adds some complication because the presence of these other traits may be contingent on environmental clues or on the presence of yet other traits. Therefore, a sexually selected trait, the expression of which is contingent on the presence of another trait, may be triggered by environmental factors, if the expression of the other trait is contingent on environmental factors. An environmental cue triggers the expression of the other trait, and the presence of this other trait triggers the expression of the sexually selected trait. Thus, many sexually selected traits may appear to be contingent on the environment, although in reality, they are contingent on the presence of other traits, the expression of which is contingent on the environment.
We can now ask in which case selection forces would favor alleles that are always expressed and in which case they would favor alleles the expression of which depends on environmental factors or the presence of other traits. The answer is that the former would be favored when the specific aspects of the environment that these alleles address are static, while the latter would be favored, when they are not static or when the other traits which are required to address a specific environmental niche are not always present. Starting with the static case, oxygen in the air and solid land are properties which do not vary (i.e., there are always there) in the specific ecological niche that humans occupy. Accordingly, people have evolved adaptations such as legs to walk around and lunges to extract oxygen from the air. Individuals without these adaptations cannot survive, so all individuals are endowed with genes which code for them, the expression of which is not contingent on environmental conditions or the presence of other traits. Alleles, that say code for lunges, the expression of which is contingent on environmental cues, will not be better off than alleles that are expressed irrespectively of environmental conditions, as humans cannot survive without lunges. Thus, selection forces will not favor these alleles, which are not expected to exist in the gene pool.
Moving on to the non-static case, we can use the male-male competition reproductive niche as an example. This niche may not be static; there may be, for instance, prolonged periods of conflict and war, followed by prolonged periods of piece. Thus, alleles that predispose for traits such as aggression, that enable a man to address effectively the male-male reproductive niche, and that are expressed irrespectively of the environmental conditions are worse off than the respective alleles whose expression depends on environmental conditions. The reason is that men who carry the former will be aggressive in times of conflict, gaining reproductive advantages, but will also be aggressive in times of peace, suffering reproductive and social exclusion costs. On the other hand, men who carry the latter will be aggressive in times of war, gaining reproductive advantages, and not aggressive in times of piece, not suffering reproductive costs.
Moreover, traits such as physical strength, which are required to exploit the male-male competition niche, vary across individuals, even if they are genetically related. For example, a man whose father is physically strong may not be physically strong because he got the genes for physical strength from his mother and not from his father. As these traits are expected to vary, it would pay for the expression of the alleles for aggression to be contingent on the presence of physical strength. In particular, even if the male-male competition niche is large, it would not pay for an allele that predisposes for high aggression to be expressed, unless it is found in a physically strong man. If it is expressed in a physically weak man, it would suffer reduced fitness as this man would get into fights he would lose. Thus, if men did not vary in physical strength, alleles whose expression is contingent on physical strength would not be better off than alleles whose expression is independent of the presence of physical strength and most probably would not exist in the population.
Another way to see it is that since the size of male-male competition reproductive niche and the physical strength trait which is required to exploit it vary, selection forces would favor alleles that code for an aggression mechanism which adjusts its developmental trajectory on the basis of environmental cues and the presence of physical strength. If environmental cues indicate a large male-male competition niche, and the alleles are found in a physically strong male body, then the man may turn to become aggressive; if any of these two conditions are not satisfied, the man may become less or nonaggressive.