Developmental Plasticity

Origins

Humans exhibit developmental plasticity in morphological, psychological, and behavioral traits, and these often change in concert. For example, given equal access to energy, females who grow up in harsh environments (e.g., violent neighborhoods) tend to mature faster, experience menarche earlier, develop an interest in having children at a younger age, engage in sex earlier, and have their first child at a younger age. This suite of correlated responses might be adaptive: It shifts an individual to earlier reproduction in conditions where life expectancy is low. Humans share this life history response with other mammals—including certain species of monkeys and rodents—suggesting ancient evolutionary roots. However, more recently evolved traits, such as human language, also exhibit developmental plasticity. Thus, the origins of developmental plasticity might be different for different traits. Despite variation in origins, in each case, similar selection pressures might have favored developmental plasticity.

Function

Developmental plasticity entails long-term effects of earlier experiences. The adaptive value of such lasting effects depends, among other things, on the stability of environmental conditions. If the environment is stable generation after generation, so that all individuals experience the same conditions, there is no value to plasticity: Natural selection might favor fixed phenotypes that are specialized for the prevailing conditions (e.g., all polar bears are exposed to cold climates, therefore all polar bears develop fur coats). In contrast, if conditions are variable between individuals but are stable within a single lifetime (e.g., some people are consistently exposed to safe conditions, others to dangerous conditions), natural selection might favor developmental plasticity. Early experiences predict later experiences; hence, individuals can adjust their development based on them. If, however, an environment is variable even within lifetimes (e.g., the same person experiences safe and dangerous conditions over time), natural selection can favor lifelong flexibility, as opposed to lasting effects of earlier experiences. In humans, some traits are fixed (e.g., number of eyes), others are developmentally plastic (e.g., life histories), and still others exhibit lifelong flexibility (e.g., the ability to learn new skills).

The adaptive value of developmental plasticity depends also on the stability of somatic conditions. For example, if early-life stressors (e.g., famine, toxins) hinder the construction and maintenance of a healthy body (e.g., cell repair, immune functioning), individuals are likely to face a faster somatic decline and may tailor their development based on their poor prospects (e.g., by accelerating their life histories). This response does not depend on autocorrelation in the external environment (e.g., if there is drought today, there will be drought tomorrow) but on autocorrelation of an organism’s internal condition (e.g., if my body is in a poor state today, it will also be tomorrow, irrespective of the external environment). The external and internal accounts of developmental plasticity are mutually compatible and their [Page 587]predictions often converge, but not always. For example, the internal account predicts that within a given environment, individuals who have incurred lasting somatic damage (e.g., due to parasite exposures) will develop faster life histories than individuals who have not incurred such damage. The external account would not predict this, as all individuals in this environment will be equally likely to suffer in the future from death and disability caused by external conditions.

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