Posts tagged Natural selection

The four mechanisms of evolution

Darwin's finches or Galapagos finches. Darwin,...

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Regular readers of the blog will know of my enthusiasm for evolutionary and developmental theories especially the eight stage evo-devo theory.

I have also advocated the four primary problems faced by all creatures undergoing evolution, as delineated by Theodore Millon– the problems of Existence; Adaptation; Replication and Abstraction which lead to polarities of pain/pleasure;  active/passive; self/other and broad/narrow at each of the stages/domains/ solutions.

However, when we pause to look at what the mechanism of evolution actually is, we clearly note that there are a few prerequisites for evolution to take place and unless all the four mechanisms/ preconditions are present it is unlikely that the creatures will evolve.   I have been having this in the back of my mind for quite some time especially as I  have been ruminating on the BVSR (blind variation and selective retention)  theory of Donald Campbell as applied to creativity.

I was recently reading ‘Driven’ and in that book too a lot of emphasis is placed on the V-S-R (Variation, Selection, Retention) mechanism of evolution. I think this popular portrayal of evolution misses another important ingredient required for evolution that of Struggle due to limited resources and excess fecundity. If the problem of limited resources and excess fecundity was not there, probably there would be no pressure to evolve.

 

Thus I would like to frame the four evolutionary prerequisites/ mechanisms as Struggle-Retention- Variation-Selection or S_R_V_S.  To elaborate:

1. Struggle: This is driven by the fact of limited resources and overproduction due to excess fecundity. Here two strategies, leading to 2 polarities,  work; one can either compete for resources or one can cooperate and exist symbiotically.  Also, one can either have a r-strategy (low investment in many) of reproduction or A K-strategy (high investment in few). In the eight stage model, the organisms at first 2 levels would be primarily constrained by this evolutionary mechanisms.

2. Retention:  There must exist some mechanism by which the traits that confer survival/reproductive or selection advantage can be retained over time in the same individual and over generations in the same species. Again the mechanism of stable trait over time as well as  over generations may be in conflict with each other and may lead to a polarity. Creatures at stage 3 and 4 of 8 stage evo-devo theory would likely face issues regarding stability and retention of traits; retaining in individual the same trait is an active process; while retaining in generations is more passively driven.

3. Variation. There must exist some  mechanism that causes minor changes in the stable traits such that variation may lead to deleterious or beneficial effect over the individual having that variation. This is classically implemented using mutations and sexual-recombinations. While mutations confer (dis)advantages at an individual level; recombinations take that to the next level by affecting offsprings variability. The creatures at stages 5 and 6 of evo-devo stages are grappling with these problems of adequate variation in self and other.

4. Selection: There must exist some selection criteria based on which the struggling creatures having stable but slightly varying traits can be selected for or against.  While Natural selection (stage 7) employs the three methods of directional, stabilizing and disruptive selection; the stage 8 deploys a qualitatively different method of sexual selection where the criteria for selection may be  arbitrarily driven by choices of the other gender conspesifics.  While the Natural selection criteria is broad, the sexual selection criteria can be said to be narrow. Another way to look at this is that the selection crteria is either to survive (natural selction) or to reproduce (sexual slection) and those who are sucessful can very well move from one level/species to another (speciation due to sexual selection).

To me this is further  corrobrating evidence of the eight stage evo-devo theory and ABCD model being on the right track.

 

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Altruism as a result of sexual selection

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There is a new article in BPS, that has found some evidence for the fact that altruism may have evolved by the process of sexual selection.

There are many mechanisms that underlie exactly how and why sexual selection takes place- one is the ‘handicap’ /’costly honest signal‘ theory according to which a trait that is actually disadvantageous or a handicap for the host evolves to signal exactly that fact- that despite this handicap I am able to function well and must therefore be of better genetic quality; the most common example being the evolution of peacocks tail which is a handicap and makes the male peacock carrying a big tail more vulnerable for predation, but also is attractive to females and preferred by them.  Another theory is that of ‘runaway selection’ i.e . a trait may evolve in a direction due to some genetic drift and the preference for it may also evolve in tandem such that there is  a slight leaning or preference towards that trait. Now, in a competition, those, typically males, who have that trait will be selected by the females and their progeny will have an advantage as they are more likely to display that trait and be favored by subsequent generations; thus an arbitrary trait may get fixed by this runaway selection where all members of the species want to be part of the new fashion/club in town. I know I am drawing very loose analogy, but just to give an idea. Nakedness or loss of hairs in humans is predicted to have followed this pattern.

Of course sexual selection also differs on whether it is largely intrasexual, driven by competition (selection pressure) between males for eg  having big antlers to defeat and subjugate another male; or is driven by mate preferences and has intersexual selection dynamics- like the peacocks tail.

What the authors of this paper hypothesized was that sexual selection is behind the evolution of altruism or selfless concern with non-kin and that this being the case and this sexual selection dynamics being driven by inetrsexual dynamics, there are bound to be genetic underpinnings to both the trait altruism as measured in males/females and the mate preferences for altruistic trait in both females as well as males. The reason they didn’t anticipate any differences in male sand females was that parental investment, as per them, is roughly equal in case of humans and so both males and females exert equally strong sexual selection pressure son each others traits and ‘choose’ their mates equally.

However, in this study they looked only at females and their genetic basis for altruistic traits as well as mate preference for altruism. The altruistic traits and mate preferential weer measured using self-report instruments.  the genetic components underlying these were estimated using classical twin studies paradigm where correlation between mono-zygotic twins and di-zygotic twins are compared to estimate the genetic contribution. They also calculated the phenotypic correlation between mate preference for altruism and altruistic trait in individuals and tried to calculate how much of this correlation again was genetic in nature or in other words was a result of  mating between those who had the trait and those who preferred the trait. . They hypothesized that in the ancestral environmental this type of mating for trait altruism would have taken place and thus these would be genetically correlated.

This is exactly what they found; they found that both altruistic personality and mate preference for altruism had genetic components and that they both co-varied and that covariance again had genetic component.  the pare itself is full text open access and is written very well, so go ahead and read it yourself. this is an important paper that has come timely when the whole kin-selection paradigm for evolution of eusociallity is being challenged by E o Wilson and team and provides a fresh and alternative perspective of why altruism may have evolved.

Here is a tit-bit from the discussion:

We believe that the sexual selection hypothesis for the evolution of human altruistic traits should now be considered alongside other more established theory (Bshary & Bergmu¨ller, 2008; Lehmann & Keller, 2006), particularly as there is the possibility that multiple mechanisms might underlie a complex behaviour such as altruism. Empirical testing of contrasting theories might even be possible. For example, reciprocal altruism (Trivers, 1971) does not strictly predict the genetic correlation between MPAT and ‘altruistic personality’ found here as ongoing reciprocation towards others would not necessarily result in such a selective process. Indirect reciprocity (Leimar & Hammerstein, 2001) concerns reputation directed towards all other group members while the sexual selection hypothesis focuses solely on altruistic displays that can be evaluated by potential mates (Phillips et al., 2008). A study that examined ‘costly signalling’ of altruistic behaviour through personal donation to a children’s charity found a significant effect on male behaviour when witnessed by a female observer while no such effect was found when male participants were observed by same sex others (Iredale et al., 2008), a finding that could be seen to be at odds with indirect reciprocity. Additional studies could further elucidate the effects of altruistic reputation when directed towards same sex others as opposed to potential mates, thus testing the relative claims of indirect reciprocity against the sexual selection hypothesis.

Tim Phillips1, Eamonn Ferguson2, & Fruhling Rijsdijk (2010). A link between altruism and sexual selection: Genetic influence on altruistic behaviour and mate preference towards it British Journal of Psychology DOI: 10.1348/000712610X493494

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