Robert Carroll, in the opening chapter of Vertebrate Paleontology and Evolution, states: “Although fossil vertebrates were very incompletely known in the nineteenth century, remains of dinosaurs, giant marine reptiles, and mammals without modern descendents provided Darwin and other biologists with irrefutable evidence of extinction and, less directly, of the process of evolution. Evolution might have been accepted without fossil evidence, but fossils now seem inexplicable without evolution. Yet, the foremost vertebrate paleontologists of the nineteenth century—Cuvier, Owen, and Agassiz—did not accept evolution as put forth by Darwin, but argued for a succession of creations and extinctions.
(This text re-introduces that argument.) Until the 1940’s, vertebrate paleontologists remained outside the mainstream of evolutionary thought…. Perhaps we should not be surprised that vertebrate paleontologists did not support the prevailing view of slow, progressive evolution but tended to elaborate theories involving saltation (jumps), orthogenesis, or other vitalistic hypotheses. Most of the evidence provided by the fossil record does not support a strictly gradualistic interpretation….
Few contemporary paleontologists would deny that natural selection (genotype bias of the phenotype) controls the direction of evolution, but many would seek additional factors (like variation in environmental conditions), to account for the rapid evolution that characterized the early diversification, and radiation of groups, and the early stages in the elaboration of new structures. The great longevity of many groups and the minor evolutionary changes they exhibited pose another problem.”
Incorporating variable environmental conditions, along with considerations of environmental selection, extinction, creation, and adaptation, fundamentally solves all of these significant problems. In the last chapter, R. Carroll states: “The search for special evolutionary factors Environmentally-Determined Evolution? 175Retrospective 175
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must be centered on the conditions that facilitate the subsequent radiation of a group (environmental conditions- involving environmental selection, extinction, creation, and adaptation), not on the initial appearance of a species. The most spectacular evolutionary changes involve the emergence of new structures and ways of life (like adaptation to a new environment)….
The most dramatic evolutionary events are those that combine a major change in habitus (newly created environmental conditions- involves selection) with the appearance of a new structural-functional complex (adaptation). Examples include the origin of amphibians, birds, pterosaurs, and ichthyosaurs. The appearance of these groups is made all the more striking by the rarity of intermediate forms in the fossil record.” And while evolutionary leaps to reptile pterosaurs and reptile ichthyosaurs may await specific intermediates, neither requires a great leap for the imagination to contemplate. There are lizards that fly (Draco volan), snakes that fly (Chrysopelia ornate, C. paradisi, and C. pelias), and mammals that fly (bats—
1100 species); there are freshwater and marine crocodilians (23 species), marine iguanas (Amblyrhynchus cristatus) and marine mammals (123 species—whales, dolphins, manatees and seals); and, all are present today.
As for the spectacular evolutionary change from fish to amphibians, there is no longer any great leap to go from a modern lungfish like Protopterus dolloi and/or a lobe-finned fish like the modern coelacanth Latimeria chalumnae, to a tetrapod-transition like the previous chapter’s fossil discovery of Tiktaalik roseae. And as for a spectacular evolutionary change from dinosaurs to birds, the leap is even smaller yet to go from an animal like the fossils of the feathered dinosaur Protarchaeopteryx robusta, or the closest known dinosaur relative of birds, Caudipteryx zoui, to an animal like the bird that is closest relative to dinosaurs, Archaeopteryx macura. The rarity of intermediates has recently become less rare. At the present time, Michael Benton, in the opening chapter of When Life Nearly Died, notes that “Evolution works to hone the fine details of the adaptations of organisms against commonly encountered problems, such as droughts, floods, predators and diseases, but rare events that happen perhaps once every few million years just cannot be accommodated. The phenomenon is… ‘bad luck, not bad genes.” So there was good reason that so many believed for so long that something other than the Neo-Darwinian view (Darwin’s Theory of Natural Selection integrated with Mendelian genetics) was involved. The perfect Neo-Darwinian Hypothesis was just ruined by the facts. As long as the geneticists kept their heads in the sand, everything made sense, one way or another. And as long as the paleontologists kept their heads in the sand, their views of catastrophic extinctions and inexplicable creations, which were followed by radiations and baffling randomly variable periods of stability, worked for them. Both were a little bit wrong; yet, both were a little bit right. It was simply just not an “either-or” situation. While
environmental extinction events certainly make a case for environmentally-determined evolution, the environmental influence in evolution really cannot stand alone beyond this. There is really no “environmentally-determined evolution,” nor is there a “genetically-“environmentally-determined evolution.”
Evolution is environmentally AND genetically-determined.
Overemphasizing environmental influence in environmental adaptation is no better than overemphasizing the role of genetics and the latter error has recently been in excess. The geneotype-determined view is incomplete.
The environmentally-determined view has always been incomplete.
Evolutionary novelty does not arise from an “either-or” situation. It originates from a “both” situation. Neither an environmentally-determined view (alone) nor a geneotype-determined view (alone) can really explain the changes in types of life over time. That being said, the environment may not directly control genetic mutation, but it does influence the initiation of random display of phenotypes in response to stress, phenotype stabilization of genetic expression, and the survival of the individual. The role of the variable environment in extinction and adaptation has been well-documented so far, and there is more to come.
Kirschner and Gerhart believe that facilitated (genetic) variation enables the expression of a mutation. The mutation does not express the trait by itself;
it only needs to change something, like a threshold, and this change may or may not make an observable change. Mutations in the genome are usually silent or neutral. If the genome expresses a previously silent mutation (e.g., with homozygous recessive alleles, it usually expresses a change; however, the observed change itself may even be neutral (no environmental selection advantage or disadvantage). And if it does not produce an environmentally-sensitive change in phenotype, the newly expressed neutral mutation effectively remains silent. But a mutation expression that is not neutral undergoes environmental selection, which promotes or diminishes any inheritable bias from that individual: “The main accomplishment of the theory of facilitated variation is to see the organism as playing a central part in determining the nature and degree of variation…. It is the capacity of the core processes to support variation that we see as the main factor in generating phenotypic variation and in minimizing the lethality of phenotypic variation.
It is the nature of these processes, which are poised to generate physiological variation within the organism, that allow genetic variation to be so effective in generating phenotypic variation on which (environmental) selection acts.”
This is a genetically-determined view, with an indirect link to early developmental processes. Like any genetically-determined view is incomplete, it is still incomplete (but it is less incomplete).
Is there an environmentally-determined view with an indirect link to early developmental processes? And would it also be incomplete? Mary Jane West-Eberhard (K&G chapter—Phenotype and Environment) Environmentally-Determined Evolution? 177
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emphasized the broad applicability of an adaptation-accommodation hypothesis in explaining the origin of complex phenotypic variations. “In West-Eberhard’s view, evolution of a novelty proceeds by four steps. In the 1st, called trait origin, an environmental change or genetic change affects a preexisting responsive process, causing a change of phenotype (often reorganization). At this initial step, she regards environmental stimuli as being more important to evolution than genetic variation. These traits may or may not be adaptive…. In the 2nd phase, the organism adapts or accommodates to its changed phenotype by compensating in part for the perturbed condition by using what we would say are its highly adaptive core processes. In the 3rd phase, recurrence, a subset of the population continues to express the trait, perhaps owing to the continued environmental stimulus. In the 4th phase, genetic accommodation, selection drives gene frequency changes that increase fitness and heritability, although the phenotype change is not necessarily ever completely under genetic control.
While having a heritable component, it could retain an environmental dependence. Thus… a phenotypic accommodation phase (is) followed by a genotypic accommodation phase.” (Over time, repeatability and predictability of an external signal should favor the developmental incorporation of the environmentally-induced developmental pathway by favoring genotypes that reliably develop a consistent phenotype across generations.)
In an article entitled: Phenotypic Accommodation: Adaptive Innovation Due to Developmental Plasticity, West-Eberhard discusses a general model for the origin of adaptive phenotypic novelties: “The following model is intended to describe the evolutionary origin of all kinds of adaptive traits—
morphological, physiological, and behavioral, whether induced by a mutation or an environmental factor—at all levels of organization. This is a brief summary of concepts presented in more detail and with more complete supporting evidence elsewhere (Developmental Plasticity and Evolution text).
(a) A novel input occurs which affects one (if a mutation) or possibly more (if environmental) individuals. Individuals may experience novel inputs due to evolution in another context (e.g., which moves them into a new environment, or has novel pleiotropic, i.e., many different, effects on the phenotype via other pathways).
(b) Phenotypic accommodation (without any change in the geneotype): Individuals developmentally responsive to the novel input immediately express a novel phenotype, for example, because the new input causes quantitative shifts in one or more continuously variable traits, or due to the switching off or on of one or more input sensitive traits (causing a reorganization of the phenotype). Adaptive phenotype adjustments to potentially
disruptive effects of the novel input exaggerate and accommodate the phenotypic change without genetic (genotype) change.
(c) Initial spread: The novel phenotype may increase in frequency rapidly, within a single generation, if it is due to an environmental effect that happens to be common.... Alternatively, if it is due to a positively selected mutation, or is a side effect of a trait under positive selection, the increase in frequency of the trait may require generations.
(d) Genetic accommodation (change in genotype, i.e., gene frequencies, under selection): Given genetic variation in the phenotypic response of different individuals, the initial spread produces a population that is variable in its sensitivity to the new input, and in the form of its response. If the phenotypic variation is associated with variation in reproductive success, natural selection results; and to the degree that the variants acted upon by selection are genetically variable, selection will produce genetic accommodation, or adaptive evolutionary adjustment of the regulation and form of the novel trait.
In an article entitled: Developmental Plasticity and the Origin of Species Differences, she further elaborates on environmental determination: “A large body of evidence indicates that regardless of selective context (genetic or environmental variation) the origin of species differences (new traits) under natural selection occurs as follows:
1. The origin of a new direction of adaptive evolution starts with a population of variably responsive, developmentally plastic organisms. That is, before the advent of a novel trait, there is a population of individuals that are already variable, and differentially responsive, or capable of producing phenotypic variants under the influence of new inputs from the genome and the environment. Variability in responsiveness is due partly to genetic variation and partly to variations of the developmental plasticity of phenotypic structure, physiology and behavior that arise during development and may be influenced by environmental factors, including maternal effects that reflect genetic and environmental variation present in previous generations. Genetic variation and developmental plasticity are fundamental processes of all living things: all individual organisms with the exception of mutation-free clones, have distinct genomes, and all of them have phenotypes that respond to genetic and environmental inputs. By
‘responsiveness’ and ‘developmental plasticity,’ I do not mean just phenotypic plasticity in the way the term is usually used, to mean only responsiveness to the external environment. Rather, I include Environmentally-Determined Evolution? 179
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responsiveness to the action of genes, which may modify the internal environment of other genes and phenotypic elements within cells, with effects that extend outward to higher levels of organization and responsiveness. Any new input, whether it comes from the genome, like a mutation, or from the external environment, like a temperature change, a pathogen, or a parental opinion, has a developmental effect only if the preexisting phenotype is responsive to it. Without developmental plasticity, the bare genes and the impositions of the environment would have no effect and no importance for evolution (good point).
2. Developmental recombination occurs in a population of individuals because of a new, or newly recurrent, input. A new input from the genome, such as a positively selected mutation, or from the environment of the affected individuals, causes a reorganization of the phenotype, or ‘developmental recombination.’ Given the variable developmental plasticity of different individuals, this process produces a population of novel phenotypes, providing material for selection.
3. Genetic accommodation may follow. If the resultant phenotypic variation has a fitness effect, that is, it correlates with the survival or reproductive success of the affected individuals, then selection (differential reproduction of individuals or other reproducing entities with different phenotypes) occurs. If the phenotypic variation has a genetic component, selection leads to ‘genetic accommodation;’ that is, adaptive evolution that involves gene-frequency change. Genetic accommodation of regulation adjusts the frequency, timing, and circumstances of the novel response (e.g.
by adjusting the threshold for its expression), and genetic accommodation of form refines the characteristics and efficiency of the newly expressed trait.”
Unlike Kirschner and Gerhart in the previous chapter of this text, West-Eberhard believes that core processes support an environmentally-initiated, norm-of-reaction based, physiologically-supported, plastic phenotypic change of the developing phenotype; genetic (genotype) accommodation then occurs at a later time. In the same article entitled:
Developmental Plasticity and the Origin of Species Differences, she states: “I argue that the origin of species differences, and of novel phenotypes in general, involves the reorganization of ancestral phenotypes (developmental recombination), followed by the genetic accommodation of change. Because selection acts on phenotypes, not directly on genotypes or genes, novel traits can originate by environmental induction as well as mutation, then undergo selection and genetic accommodation fueled by standing genetic variation or subsequent mutation and genetic
accommodation. Insofar as phenotypic novelties arise by adaptive developmental plasticity, they are not ‘random’ variants because their initial form reflects adaptive responses with an evolutionary (norm-of-reaction constrained) history, even though they are initiated by mutations or novel environmental factors that are random with respect to (future) adaptation. Changes in gene frequency involve genetic accommodation of the threshold or liability for expression of a novel trait, a process that follows, rather than directs, phenotypic change. Contrary to common belief, environmentally initiated novelties may have greater evolutionary potential than mutationally induced ones. Thus, genes are probably more often followers than leaders in evolutionary change.”
The environmental importance for both generating and maintaining the integrity of genetic expressions is strongly supported in West-Eberhard’s environmentally-integrated developmental view; and, it is complete because it also allows genetic integration. In the Adaptive Evolution chapter of Developmental Plasticity and Evolution, West-Eberhard emphasizes that “selection for a trait does not (necessarily) mean selection for the spread of an allele that specifies the trait; it means selection for a change in regulation or behavior (including habitat or even diet selection) such that the trait is more readily, and therefore more commonly, produced.” This would be a norm-of-reaction-limited genetic expression;
here, there would be no newly-expressed mutation. Little genetic change would be needed for an inheritable change of expression (e.g. regulatory variation). However, the genotype change would occur afterwards, due to environmental selection favoring the genotype that reliably develops a consistent phenotype across generations. In West-Eberhard’s Gradualism chapter in Developmental Plasticity and Evolution, she reaffirms that there is no primacy of selection over variation as the architect of design (i.e., all selectable variation is not just genetic, or mutational in origin): “New selectable variation can originate due either to mutation or environmental change. Variation originating in both ways can have evolutionary consequences and can lead to adaptive evolution, due to genetic accommodation under selection. This means that both variation and selection always contribute to the evolution of adaptive design. Neither can be assigned a dominant role because development is the source of all selectable variation, and selection determines which variations among these produced by development spread and persist.” In the above, environmental and genetic variation and their contribution to adaptive design are clearly recognized, as is the role of a selective process (which I also maintain is very much an environmental selection process-resulting in preserved natural selection genetic bias). I strongly support the importance of her focus on development, but am not sure I totally agree with the statement “development is the source of all selectable variation.”
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Development can be a process that incorporates selectable variations (traits), but the variation is sourced from genetic variation and environmental variation. Development can also be the result of selectable variation, but the variation source is genetic variation and environmental variation.
Development is variable, but it only exhibits variations; it is not the source of variation itself. Rather, development is the source of the adult phenotype, which also exhibits variations.
The environmentally-genetically-integrated views have much in common. We both agree that there is no primacy of genetic influence, or primacy of environmental influence in forming an adaptation, but a range of environmental and genetic interactions, utilizing plastic cellular processes to form the new phenotype. Yet, West-Eberhard’s view may be somewhat more of an “either-or” view for environmental or heredity roles for initial formation of the phenotype. If the environment causes the formation of a new phenotype from the genotype norm-of-reaction, choices are limited to the norm-of-reaction plastic range of expression, which includes any ancestor-possible recombination (novelty may arise from physiologic change, heterochrony-timing, or regulatory gene functional variation). And, it is followed by a deemphasized genetic accommodation. And while her view is not that different (genetics and environment always contribute), I prefer the view that relies more heavily on genetic expression to form any phenotype, and requires little genetic accommodation after-the-fact. In this, as phenotype formation occurs, development incorporates indirect feedback from both genetic and environmental sources at the time of formation. This view of adaptation not only includes the preceding but also expands the options into greater epigenetic change, where an old mutation could be newly-expressed from the existing genotype variation, without any new mutational change in the genotype. Genetic expression (not mutations, but the expressions of mutations) can be modified by the environment. A mutationally-modified norm-of-reaction response could allow even greater evolutionary change, but would require enough time to allow formation of a random phenotype that is favored by adverse or supportive environmental selection.
The environmentally-genetically-integrated views have much in common. We both agree that there is no primacy of genetic influence, or primacy of environmental influence in forming an adaptation, but a range of environmental and genetic interactions, utilizing plastic cellular processes to form the new phenotype. Yet, West-Eberhard’s view may be somewhat more of an “either-or” view for environmental or heredity roles for initial formation of the phenotype. If the environment causes the formation of a new phenotype from the genotype norm-of-reaction, choices are limited to the norm-of-reaction plastic range of expression, which includes any ancestor-possible recombination (novelty may arise from physiologic change, heterochrony-timing, or regulatory gene functional variation). And, it is followed by a deemphasized genetic accommodation. And while her view is not that different (genetics and environment always contribute), I prefer the view that relies more heavily on genetic expression to form any phenotype, and requires little genetic accommodation after-the-fact. In this, as phenotype formation occurs, development incorporates indirect feedback from both genetic and environmental sources at the time of formation. This view of adaptation not only includes the preceding but also expands the options into greater epigenetic change, where an old mutation could be newly-expressed from the existing genotype variation, without any new mutational change in the genotype. Genetic expression (not mutations, but the expressions of mutations) can be modified by the environment. A mutationally-modified norm-of-reaction response could allow even greater evolutionary change, but would require enough time to allow formation of a random phenotype that is favored by adverse or supportive environmental selection.