Abstract
AbstractWhilst the principle of adaptive evolution is unanimously recognised as being caused by the process of natural selection favouring the survival and/or reproduction of individuals having acquired new advantageous traits, a consensus has proven much harder to find regarding the actual origin of species. Indeed, since speciation corresponds to the establishment of reproductive barriers, it is difficult to see how it could bring a selective advantage because it amounts to a restriction in the opportunities to breed with as many and/or as diverse partners as possible. In this regard, Darwin himself did not believe that reproductive barriers could be selected for, and today most evolutionary biologists still believe that speciation can only occur through a process of separation allowing two populations to diverge sufficiently to become infertile with one another. I do, however, take the view that, if so much speciation has occurred, and still occurs around us, it cannot be a consequence of passive drift but must result from a selection process, whereby it is advantageous for groups of individuals to reproduce preferentially with one another and reduce their breeding with the rest of the population.In this essay, I propose a model whereby new species arise by “budding” from an ancestral stock, via a process of inbreeding among small numbers of individuals, driven by the occurrence of advantageous recessive mutations. Since the phenotypes associated to such mutations can only be retained in the context of inbreeding, it is the pressure of the ancestral stock which will promote additional reproductive barriers, and ultimately result in complete separation of a new species. I thus contend that the phenomenon of speciation would be driven by mutations resulting in the advantageous loss of certain functions, whilst adaptive evolution would correspond to gains of function that would, most of the time be dominant.A very important further advantage of inbreeding is that it reduces the accumulation of recessive mutations in genomes. A consequence of the model proposed is that the existence of species would correspond to a metastable equilibrium between inbreeding and outbreeding, with excessive inbreeding promoting speciation, and excessive outbreeding resulting in irreversible accumulation of recessive mutations that could ultimately only lead to the species extinction.
Highlights
Among the myriad of reviews and articles that have been written about “The Origin of Species” by Charles Darwin, a very large proportion underlines the fact that, despite the title of his book, what Darwin established was the mechanism of adaptive evolution by the process natural selection, but that he failed to provide answers to the many questions that surround the origin of species
If the mutation load in the population is M, the probability of any locus being mutated will be M/104, and the probability of carrying two mutated alleles of any given gene will be (M/104)2 = M2.10-8 and the effect on fertility would be (1M2.10-8 )10.000 overall since the threat applies for every single one of the 10.000 essential genes. This is represented as the thick red curve on panel B, and we can see that, whilst the chance of carrying two inactivated copies of the same gene remains extremely low for mutation loads below 20, it starts becoming quite significant for mutation loads over 30, and fertility will drop below 75% when genomes have accumulated, on average, over 50 recessive mutations
One of the reasons that kept me from writing up those ideas for several years was the reasoning that, if this model was even partially correct, one of the many geneticists that have pondered about speciation for the past 150 years should have developed similar ideas before me
Summary
Among the myriad of reviews and articles that have been written about “The Origin of Species” by Charles Darwin, a very large proportion underlines the fact that, despite the title of his book, what Darwin established was the mechanism of adaptive evolution by the process natural selection, but that he failed to provide answers to the many questions that surround the origin of species. The last plant example I have chosen to discuss is that of oaks (Quercus), for their extraordinary capacity to resist speciation, since complete reproductive isolation still has not been reached between many of the approximately 400 ‘species’ of oaks recorded to date Those correspond to very different types which are distributed over very spread and diverse habitats of the northern hemisphere, but many of those ‘species’, and the group of white oaks which are most prominent in north America, can still intercross and yield perfectly fit offspring, with clear signs of hybridisation and gene flow having been documented in wild populations [102,103,104]. A very recent paper looking at wild population of field crickets reported the very unexpected observation that only one in ten of sexually active adults yielded offspring the following year, and this was true for both males and females [114] If it were not the case, we would not see so much variation, so many new characters being selected for in the first place, and selected against later on, and so many species around us
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
