At this point things become a little technical; it’s where we make some important conclusions. Stay with us – it’s interesting.
”Plasticity resists the threatening effects of dangerous changes to environments in two ways.”
Phenotypic plasticity and assimilation therefore enable small but beneficial changes when animals face environmental stress. In the above examples the animals remain wood frogs or tiger snakes, but are adapted variants of their recent ancestral population. In other words, in a small, incremental way, they have evolved. Plasticity resists the threatening effects of dangerous changes to environments in two ways. First, where environments are unstable, it is a mechanism of continuous adjustment to environmental fluctuation. Second, when environmental change is more severe and / or permanent, plasticity can act via assimilation as the basis for a mechanism of permanent, genetically innate adaptation, which may contribute in the longer term to the emergence of new species.
”These adaptations have not originated from random genetic changes.”
By its nature, plasticity is a special form of phenotypic variation, deployed as pliable traits responsive to environmental change. In the wood frog and tiger snake examples, since the permanent adaptations take the form of now fixed points on previously variable ranges in characteristics, the plasticity has acted as the source of variation on which natural selection has acted. The adaptations we have described (a biochemical one in wood frogs, a morphological one in tiger snakes) have therefore not originated from random genetic changes, rather, they began with organisms exhibiting specific forms or extent of plastic traits (the enabling phenotypes) that had greater fitness in changed environments. Plasticity therefore channels biological responses to environmental change in specific, narrow directions. Then, if the altered environment is maintained (the consequences to the animals, though moderated by the plastic response, still being significant) additional variations may cause assimilation. In some cases these variants may complement the enabling phenotype (e.g. by giving it constitutive expression or tuning it in other ways), and continuing natural selection will cause these to spread through and dominate the population.
”The involvement of genetic change has been led by this plasticity-enabled process.”
This route to adaptation and evolution certainly involves mutation, but the contribution mutation makes is highly dependent on the nature of the plasticity. The adaptive approach to the environmental challenge has been enabled by variation present in the form of the plasticity. The involvement of subsequent genetic change has been led by this plasticity-enabled process, and has caused permanence or enhancement of the adaptation. This influencing of adaptive direction by plasticity has a powerful, indeed profound consequence: adaptive solutions to threatening environmental change are frequently not random in their origin or, consequently, their nature. Rather, plasticity directs them, guiding the adaptive mechanisms specifically to the nature of the stress. Instead of responses to environmental problems having to emerge from chance events, plasticity offers existing phenotypes that are already applicable.
”It is possible to see examples in which plasticity has played a distinctive role in the emergence of a new species.”
Adaptations are the basis of evolution. A single fitness-improving change in a biological trait will not usually cause the animal’s species to change; new species arise when combinations of changed characteristics make an animal population significantly and permanently distinct from its ancestors. The island populations of those tiger snakes that migrated thousands of years ago, and which have a permanently large rather than plastic head size, have adapted but not changed to an extent that they are now new subspecies. If new environmental challenges appear, perhaps the resulting additional adaptations will cumulatively cause them to speciate. The few thousands of years since their first island occupation is a very short time on the evolutionary scale and, currently, they remain merely a variant within the species Notechis scutatus. It is, however, possible to see examples in which plasticity has played a distinctive role in the emergence of a new species.
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