Some stuff I found in my readings today that I may return to at a later stage. This stuff is pretty fascinating, but I haven’t thought about it properly yet. What I have listed here isn’t too coherent, but I think it’s worth putting here so I can come back to it later if I ever decide to collect my biology-related ramblings together into something that makes sense.
The first quote concerns a distinction between convergent evolution and parallel evolution – I had assumed the terms were synonymous – shows what I know! The next three quotes discuss constraints on adaptive evolution and whether there really is a sharp ‘convergent’ vs ‘parallel’ split.
Jan 2008 Convergence and parallelism reconsidered: what have we learned about the genetics of adaptation? Arendt et al.
“Biologists often distinguish ‘convergent’ from ‘parallel’ evolution. This distinction usually assumes that when a given phenotype evolves, the underlying genetic mechanisms are different in distantly related species (convergent) but similar in closely related species (parallel). However, several examples show that the same phenotype might evolve among populations within a species by changes in different genes. Conversely, similar phenotypes might evolve in distantly related species by changes in the same gene. We thus argue that the distinction between ‘convergent’ and ‘parallel’ evolution is a false dichotomy, at best representing ends of a continuum. We can simplify our vocabulary; all instances of the independent evolution of a given phenotype can be described with a single term – convergent.”
Dec 2012: The probability of genetic parallelism and convergence in natural populations. Conte et al.
“the high probabilities of gene reuse estimated from published data indicate that the effective number of genes used in parallel and convergent phenotypic adaptation is typically small. If the causes of this low number can be elucidated, then genetic evolution may indeed be somewhat predictable”
“… the number of genes used and reused in adaptive evolution is a small subset of available genes.”
“… there is no sudden break in the probability of gene reuse between parallel and convergent evolution (figure 2). The distinction is one of degree rather than of kind. ”
Now, these quotes are as I read them, crudely, about microevolution vs macroevolution.
Feb 2009. Stern and Orgogozo. Is genetic evolution predictable?
“The Frigida example is not unique. In many plants and animals, evolution over long periods (variation between species) appears to differ in several ways from evolution over shorter periods (variation between domesticated races and between individuals within a species). Here are three general ways in which long-term and short-term genetic evolution differ.
First, epistasis is commonly found for the mutations that contribute to phenotypic variation within species, whereas it is rarely observed for the mutations that cause differences between species.
Second, null mutations, which arise frequently and often cause pleiotropic and epistatic effects, seem to contribute more to phenotypic variation within species than to phenotypic differences between species. About 55% of the 99 mutations known to cause domestication traits are null-coding mutations, whereas only 7% of the 75 mutations known to cause interspecific differences are null-coding mutations
Third, the frequency of cis-regulatory mutations causing morphological variation differs between taxonomic levels. Morphological changes may occur either through coding changes or through cis-regulatory changes. Because mutations in cis-regulatory regions often have fewer pleiotropic effects than mutations in coding regions, morphological changes are expected to involve mainly cis-regulatory mutations. Within species, most mutations that cause morphological variation have been found in protein-coding regions. In contrast, between species most mutations that cause morphological differences have been found in cis-regulatory regions. Presumably, many of the coding mutations found within species fail to spread through populations, perhaps because of pleiotropic deleterious effects. ”