Evolution in the Fast Lane
Behavior changes to suit differing environments.
Posted May 21, 2020
Evolutionary scientists confront a paradox. Animals must benefit from current opportunities to survive, whether to feed, to evade predators, or to mate. Yet, the process of genetic adaptation is painfully slow with mutations occurring only about once per 100,000 years.
Fortunately, there is behavioral evolution which means that we do not need long to react to change. Indeed, adaptive changes in behavior have little to do with genetic evolution. Gene selection is better at accounting for anatomical changes such as the lengthening of a giraffe's neck and the whole suite of adaptations through which its heart spurts blood up to its lofty head.
There are three different kinds of adaptation as laid out in my new book, Evolution in the Here and Now. First, there is gene selection. Second, genes are expressed differently depending on early experiences (epigenetics). Third, there is learning, individual and social.
Genetic evolution can operate at the level of an entire species, or population. For example, some species are monogamous and others are not.
How Evolution Shapes Monogamy in Voles and Humans
Prairie voles live in family groups consisting of a monogamous mated pair and their offspring. Mountain voles are mostly solitary due to a relative scarcity of food. Adult males and females meet only to mate and form no attachment.
Prairie vole couples copulate often. This experience gets them hooked on each other in much the same way that people get addicted to drugs (1,2).
So why do mountain voles not get addicted to each other? Experiments found this is due to brain receptors (for oxytocin and angiotensin (3). Similar mechanisms are implicated in human sexual behavior, although some individuals are more monogamously inclined than others for genetic reasons (4).
On the other hand, genetic variation offers a weak explanation of personality differences.
Specific genes account for small fractions of personality variation. Some personality traits, such as extroversion, have no specific underlying genes.
Extroverts are more physically imposing, and more physically attractive, than introverts both in modern societies and among indigenous residents of the Amazon region (5).
They are evidently better equipped to assert themselves in relationships with others because their previous life experiences have been generally positive.
So, genetic variation can be a poor explanation for individual differences, even if the trait runs in families, as extraversion does.
Parental behavior offers another cautionary tale about the fallibility of genetic determinism in social life.
Parental Behavior and Epigenetics
The majority of abusive parents were abused themselves as children (6). The cycle of violence in families has parallels in the intergenerational transmission of maternal behavior in other species, specifically rats and monkeys.
Some rat mothers are more attentive to their offspring than others are. The amount of maternal care in rats influences the attentiveness of females to their own offspring when they become mothers themselves.
Maternal licking affects the pattern of DNA methylation in rat pups' brains so that there is greater expression of glucocorticoid receptors for pups that are licked more (7). Behaviorally, this is associated with greater capacity to deal with stressful situations while remaining calm. Rats receiving a lot of maternal licking are less fearful in novel situations.
For humans, childhood abuse is associated with abnormal methylation in the adult brain also, affecting glucocorticoid receptors (7).
Instead of referring to parental “abuse,” it is possible that offspring are raised to be appropriately risk-averse in a dangerous environment. Either way, behavior is predictably altered by changing early environments.
A more obvious example of rapid adaptation is provided by social learning of what to eat and whom to mate with.
Social Learning of Food Preferences and Mate Choice
In addition to brain biology and formative early experiences, natural selection acts on socially-learned behavior that is far more important for the adaptive flexibility of nonhuman species than previously believed.
Moose are not born afraid of wolves—as many adaptationists had assumed—but acquire their adaptive fears from observing their mother's response to various species (8). By the same token, the food an individual chooses to eat is largely a function of the foods preferred by their mothers.
Behavioral adaptation is a highly flexible process and many adaptations occur within the lifetime of an individual.
1. Young, L. J., Murphy Young, A. Z., & Hammock, E. A. (2005). Anatomy and neurochemistry of the pair bond. Journal of Comparative Neurology, 493, 51-57.
2. Vacek, M. (2002). High on fidelity. American Scientist, 90(3), 225-226.
3. Insel, R., and Hulihan, T. (1995). A gender-specific mechanism for pair bonding: Oxytocin and partner preference formation in monogamous voles. Behavioral Neuroscience, 109, 782-789
4. Walum, H., Westberg, L., Henningsson, S., Neiderhiser, J. M., Reiss, D., Igl, W., et al. (2008). Genetic variation in the vasopressin receptor 1a gene (AVPR1A) associates with pair-bonding behavior in humans. Proceedings of the National Academy of Sciences, 105, 14153-14156.
5 Lukaszewski, A. W., and von Rueden, C. (2015). The extroversion continuum in evolutionary perspective. Personality and Individual Differences, 77, 186-192.
6 Maestripieri, D. (2005). Early experience affects the intergenerational transmission of infant abuse in rhesus monkeys. Proceedings of the Nationmal Academy of Sciences, 102, 9726-9729.
7 Franklin, T. B., and Mansuy, I. M. (2010). Epigenetic inheritance in mammals: Evidence for the impact of adverse environmental events. Neurobiology of Disease, 39, 61-65.
8 Berger, J., Swenson, J. E., & Persson, I. L. (2001). Recolonizing carnivores and naïve prey: Conservation lessons from Pleistocene extinctions. Science, 291, 1036-1039.