Tag Archives: disruptive selection

Deletion – notes

“trait deletions create novelties by subtractions”

  1. “a complex trait lacking an element may immediately have an altered function”
  2. “the loss of a trait may have correlated developmental effects that the remaining phenotype elements into new configurations”
  3. “a deletion, if it is repeatedly produced, makes the resultant phenotype subject to divergent evolution under selection, simply because it is different”
  4. “deletion of a phenotype subunit can release other, genetically and developmentally correlated traits from the evolutionary constraints, freeing the remaining traits to evolve more rapidly and independently”

“Deletions can evolve gradually, by change in regulation to reduce the frequency of expression of a trait, as in the loss of an alternative phenotype, or by change in form such that elements of the phenotype are gradually lost”

“A deletion occurs every time an alternative phenotype evolves to fixation, which means that its former alternative is no longer expressed”

“deletions occur at different levels of organization, from pieces of genes to elements of behavior and whole life stages of individual development”

“[in] seasonally polyphenic animals and plants, extensive switching occurs between different extreme states of physiology, behavior, and sometimes morphology that is similar to life stage switching but may occur repeatedly or cyclically during the lifetime of a single individual”

plasticity of development / lability of evolution

“ontogenetic events do not necessarily have to occur in a particular order but can be shuffled”

“A common kind of variation is a bimodal frequency distribution of some continuously variable trait”

host size association

“Bimodal associations can also result from disruptive selection against intermediates, or from frequency-dependent selection favoring different discrete forms”

Current theory suggest a “allelic switch, or genes of major effect on a developmental switch,” in other words a “bimodal distribution of genotypes underlying a bimodal distribution of phenotypes”

“Another kind of evolutionary response to disruptive selection is possible, and the study of alternative phenotypes indicates that it may be the most common one.”

“Disruptive selection can lead to the evolution of polymorhisms, polyphenisms, and alternative behavioral and physiological phenotypes, with the alternative adopted by an individual dependent on conditions or on a small number of loci influencing regulation.”

“the result of disruptive selection can be the evolution of disruptive development-alternative developmental pathways. This produces a bimodal distribution of phenotypes in the population without a bimodal distribution of genotypes. Intermediates are developmentally deleted by selection that favors ability to switch between extremes, rather than by elimination of intermediate genotypes from a continuously variable array.”

can disruptive selection lead to a plastic phenotype response?

“how switch mechanisms, likeother novel aspects of form, are constructed during evolution by tinkering with preexisting responses, sometimes capitalizing on different pathways to the same end”

“Selection against intermediates have been demonstrated in studies of alternative phenotypes in males”

“it would be of interest to see if there are any bimodal distributions not accompanied by disruptive selection” [Moran, heat shock switch]

“It is clear that a developmental threshold effect is sometimes involved because in at least some species the two modes show different allometric relationships when plotted against size, even though size itself is continuously variable”

“This indicates a change in epigenetic tragectory governed by a switch”

“the developmental genetic architecture of the bimodal distribution is most clearly revealed by the finding of a hormonal basis for the switch”

“JH for horned beetles”

“Alternative decelopmental pathways can arise when some environmental circumstace deletes intermediates, leaving only the extremes of what had been a continuous distribution.”

“Extensive phenotype deletions have raised the question of the fate of the underlying genes

  • are they immediately lost?
  • are they eventually modified by unselected mutations until they become disfunctional?
  • are tehy carried intact although simply unexpressed. If so, how long can an unexpressed gene persist and still maintain the potential for functional expression?”

“Lost traits do not necessarily mean lost genes”

Ohno “The indispensability of a gene can only be judged by the performance of homogenious deficiency individuals” “and tests for the effects of complete absence of the gene would have to include performance of young and adults, males and females, and a range of different environmental situations in nature, not just in the laboratory”

“But healthy survival, especially in the laboratory, is not an adequate test of the fitness effect of a gene, omitting as it does effects on social, mating, and other untested biotic interactions.”

“How long unexpressed or unselected genes persist evidently depends importantly on the accuracy with which they are replicated and repaired”

Marshal, et al, 1994 “silenced genes and developmental pathways have a significant probability of over 0,4-6 million years of being reactivated and concluded that this is unlikely to occur after >10 million years without having function maintained by selection in some context”

“The many examples of atavisms and reversions having remarkably detailed resemblance to those expressed in related taxa show that both form and regulation can recur intact, sometimes after millions of years of subunit loss.”