The geometry of Macro-evolution: links between Evo-Devo and adaptive dynamics
Adaptive dynamics (AD) is a recently developed framework geared towards making the transition from micro-evolution to long-term evolution based on a time scale separation approximation. This assumption allows defining the fitness of a mutant as the rate constant of initial exponential growth of the mutant population in the environment created by the resident community dynamics. This definition makes that all resident types have fitness zero. If in addition it is assumed that mutational steps are small, evolution can be visualized as an uphill walk in a fitness landscape that keeps changing as a result of the evolution it engenders.AD arguments are largely local in character. As such they can only deal with what might be called meso-evolution. For longer timescales (macro-evolution) it becomes necessary to look at general morphological and developmental arguments that bear on the larger scale geometry of fitness landscapes. From this enlarged perspective the low dimensional fitness landscapes studied in AD can be seen as the surfaces at the top of ridges in a much higher dimensional landscape over potential morphologies, with the abyss around the ridges created by the lack of a proper development, or functioning. The location of the ridges and abysses is grossly the same for large sets of possible environmental conditions. Biological parlance expresses this constancy by referring to the corresponding selective processes as internal. High dimension and ridgyness turn out to conspire in a number of ways:
1. Developmental systems leading to mutation distributions that are in some way aligned with the ridges evolve much faster than systems where such is not the case.
2. The stabilizing selection in the off-ridge directions has a great robustness of the developmental system as evolutionary consequence. Yet, the high dimension of genotype space makes that this robustness can never lead to a lack of suitable mutational variation, and thereby to the conservation of features. Hence, the fact that evolution largely proceeds through the quantitative variation in the size and shape of homologous parts should be due to the stabilizing internal selection that arises as a consequence of organismal organization.
3. So-called allopatric speciation supposedly occurs by separated populations wandering around along the high fitness maze, so that after a while their mixed offspring, having intermediate properties, ends up in the abyss. As random fitness landscapes almost never engender allopatric speciation the question arises whether, and if so for what reason, evolved genotype to phenotype maps may be more speciation prone.
4. Large mutational steps far more often than not make an individual land in the fitness abyss, and only the much rarer very small steps keep it on the top. This may provides theoretical justification for the assumption made in AD.
The CEES seminar room has a coffee-machine – it is therefore recommended that you come a bit earlier and get yourself a good cup of coffee (for the price of 3 NOK).
Published Feb. 6, 2012 2:40 PM
- Last modified Mar. 8, 2021 10:20 AM