Reworded 'strength parameter' as 'rate of adaptation', and 'selection…

[kopperud]

…' to 'adaptation'

Hello, this is my first pull request in the hackathon.

Thanks to Michael May for writing such a nice tutorial on how to fit OU models in revbayes. I have one suggestion, and that is to reword the description of the alpha parameter from "strength of selection" to "rate of adaptation", in line with the terminology used in Hansen 1997.

The motivation for this is to signify that the comparative OU model does not really measure selection toward an optimum directly, as is done in other fields such as quantitative genetics.

If you look at virtually any artificial selection experiment, be it weak or strong selection, it is common to see that the phenotype adapts to the new optimum in a time frame on the order of generations. With most phylogenetic comparative studies of adaptation, however, the estimated alphas/phylogenetic half lives are usually on a timescale of millions of years. As an example, arguing that Darwin's finches needed millions of years for selection to operate on a new fitness optimum of beak sizes sounds absurd. Since there is such a strong inconsistency in terms of different timescales, it is perhaps better to acknowledge that the comparative OU model does not describe the same phenomenon as we observe in selection experiments. Further, not all types of selection will fit well as an OU model - fluctuating directional selection, for example, may result fit better as a Brownian motion than an OU process.

In order to conceptually separate the OU process by what is understood by "stabilizing selection" in the quantitative genetics literature, it may be better to use a more vague term such as "macroevolutionary adaptation".

Best regards Bjørn Tore Kopperud

[mikeryanmay]

Totally agree with the change! Thanks for the well-reasoned comments!