Environmental Adaptation: How Size, Lifespan, and Plasticity Shape Survival (From Bacteria to Elephants)

The environment includes the physical and biological setting of an organism with which it is coupled and reciprocally interacting. This external milieu is superimposed on the internal environment in which the genes and other cellular machinery function. Environments both drive natural selection by their differential impact on survivorship and are in turn altered by the changing composition of the survivors. The developmental programs of organisms neither simply unfold in mechanical fashion against a placid environmental backdrop, nor do organisms simply create solutions to problems posed by the environment.

Organisms experience the same absolute fluctuations very differently depending on their lifespans. For a bacterial cell (as opposed to the bacterial evolutionary individual or clone) with a generation time of hours to days under natural conditions, absence of nutrients or a slight drop in temperature for several hours would have major physiological and growth rate implications. Either change over the same absolute time frame for an elephant with a generation time of 12 years would be negligible.

How organisms experience and respond to environmental fluctuations also is affected by their size and growth form. Increasing size of the physiological individual is related directly to increasing complexity manifested in various ways: increase in number of cell and tissue types, and hence in the interactions among them; division of labor among cell types; formation of support structures; insulation and homeothermic mechanisms; centralized hormonal and neural control.

Size is inversely proportional to generation time. While a bacterial cell is not as well insulated from its environment as an elephant cell (or, correspondingly, at the genet level, the bacterial clone or the whole elephant), the bacterium can track environmental fluctuations much more rapidly by phenotypic and genotypic changes. The macroorganism responds by virtue of a complex neural network; the counterpart of the neural network for the microorganism, enabling in effect predictive behavior, is the network of regulated biochemical pathways and metabolic controls. Modular organisms, composed of iterated parts and being usually sessile (7Chap. 5), respond differently to the environment than do unitary organisms: For example, resistant propagules (seeds, spores, bulbs, etc.) may outlast local adversity in situ or, frequently, be transported to new sites, while the soma changes by addition or subtraction of modules. Unitary organisms, which are typically mobile, adapt primarily by migration, and by physiological and behavioral mechanisms.

Environments have been classified from the organism’s perspective with respect to resources as fine-grained (experienced in many small doses and not actively sought out or avoided) or coarse-grained (sufficiently large so that the individual chooses among them or spends its whole life in one patch). Though conceptually appealing and useful in a modeling context, the theory of environmental grain is abstract, restricted to behavior in the context of resources, and delimited by the assumptions of the model.

When individuals of a species experience relatively the same environment throughout their lifespan, locally adapted genotypes producing relatively invariant phenotypes known as ecotypes or races, are expected. Where individuals are exposed to an environment that varies principally spatially or temporally, phenotypic plasticity—defined as any kind of environmentally induced phenotypic variation in behavior, physiology or morphology—is expected. Phenotypic plasticity offers the major advantage of dampening the effects of selection by uncoupling the phenotype from the genotype and in so doing provides for adaptation to variable environments. Such plasticity, frequently called ‘phenotypic heterogeneity’ in bacteriology, can occur within a genetically homogeneous cell population in a constant environment triggered by stochastic molecular noise. Noise enables types of dynamic behavior in eukaryotic, multicellular macroorganisms as well as in single-celled microorganisms.

Dormancy, apparent as various manifestations of quiescence, is usually interpreted as a bet-hedging strategy to adverse conditions. Organisms receive environmental cues and transduce these from a physical or chemical mode into an appropriate biological response. Where environmental fluctuations are irregular, the response needs to occur directly as the environment changes. Where fluctuations are predictably cyclic, it is advantageous for organisms to ‘anticipate’ them by recognizing some form of early signal. For example, both macroorganisms and microorganisms exhibit circadian rhythms. In seasonal terms, the message “day length is changing” is transduced via hormones to activate flowering in some plants, or reproductive activity in certain animals. Correlations between environmental parameters allow one form (e.g., photoperiod) to act as a predictor of another (e.g., temperature). Particular phases of life cycles appear to match those environmental conditions for which they are suited but this is a result of natural selection acting on the individual’s ancestors over generations.

Certain bacteria and most fungi exhibit standard forms of dormancy closely analogous to seeds based on production of a resistant, frequently long-lived propagule such as an endospore. Of more interest is that many, if not all, bacteria produce at very low frequency a subpopulation of slow- or non-growing cells called ‘persisters’. Such bifurcation (bistability) of populations has survival value in that the quiescent cell fraction tolerates antibiotic or other environmental adversities that kill active, dividing cells.

Beginning with the classic work of Jacob and Monod on bacterial genetics in the 1960s, changes in gene expression became increasingly recognized as being as evolutionarily important, if not more so, than mutation of structural genes. When changes in gene expression are heritable, they are referred to in current semantics as epigenetic and the phenomenon is well established both in bacteria and eukaryotes. At the population level, epigenetically inherited traits may be functionally indistinguishable from allelic alterations. Events such as gene regulation should be viewed as single-molecule processes and as such are inherently noisy (stochastic), potentially leading to phenotypic heterogeneity within an otherwise essentially identical, isogenic cell population. Molecular noise enables dynamic behaviors in eukaryotic, multicellular macroorganisms, as well as in single-celled microorganisms.

The distribution and abundance of species can be interpreted in large part by the match between organism and environmental pattern. Habitable sites are patches where the organism can develop competitively. Sites are dynamic in time and space due to changes in the organism, the environment, or the organism-environment interaction. Species invasions and successful introductions show that habitable space may exist unfilled.

Distance between habitable sites is a factor driving dispersal and molding life cycle features. Gene flow patterns are quite different for organisms that disperse propagules passively, which decline in numbers logarithmically from a source pool (such as plant seeds, microbes, and clonal benthic invertebrates), and organisms that disperse progeny in directed fashion (most unitary animals), represented generally by a normal distribution pattern of decline. Despite the high mortality that typically occurs during a migratory phase, dispersal is evidently favored by natural selection because in some form it occurs universally and is often facilitated by intricate adaptations. Dispersal is an ancient evolutionary development that has been progressively improved upon by various mechanisms.

Suggested Additional Reading:
Bonner, J.T. 1988. The Evolution of Complexity by Means of Natural Selection. Princeton Univ. Press, Princeton, NJ. How increase in size and complexity of organisms has evolved, and implications for environment-organism interaction.

Carroll, S.B., J.K. Grenier, and S.D. Weatherbee. 2005. From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design, 2^nd ed. Blackwell, Malden, MA. A clearly written, beautifully illustrated book on animal development, including the seminal role of gene regulation as the major creative force underlying morphological innovations.

Lewontin, R. 2000. The Triple Helix: Gene, Organism, and Environment. Harvard University Press, Cambridge, MA. A reminder that organisms both make and are made by their environments. Organisms are not simply constructed from a DNA recipe.