Evolution on Holey landscapes

From: Pim van Meurs <pimvanmeurs@yahoo.com>
Date: Wed Feb 22 2006 - 13:48:05 EST

Fascinating how science is slowly unraveling these details about evolution. Remember that when Darwin formulated his thesis he knew little about genetics, and that when the Neo-Darwinian theory was proposed, little was known about the biochemistry and developmental biology involved. These were all instances which could have disproven Darwin's theory and the fact of common descent (CD) but rather than undermining CD, the new data ended up showing more and more evidence and showed how evolution really is a 'tinkerer' reusing bits and pieces.
 We are now at the beginning of a new era of Evo-Devo which combines evolutionary theory with developmental biology.
 Another example involves the paradox of how evolution could have been so succesful?
 <quote>Hence, the Darwinian solution of optimization problems is possible if and only if the problem is "coded" in a way that makes the mutation-selection procedure an effective one. This fact is known as the "representation problem:" how to code a problem such that random variation and selection can lead to a solution? The representation problem is about the likelihood to obtain an improvement by mutation and/or recombination.</quote>
 We now know that co-evolution of the code, co-evolution of variation all help understand why evolution has been so succesful. One of the most important steps seems to have been the evolution of neutrality. Yes folks, neutrality can not only evolve under selective pressure but is also essential for evolution's evolvability...
 An added bonus is that neutrality not only increases evolvability but also robustness... Imagine that, two apparantly opposite features captured by the same mechanism.
 And things only get better, computer simulations have shown that the more constraints are added to evolution, the valeys in a fitness landscape 'disappear'. Just like scale free networks form a well connected network, fitness landscapes become very evolvable, contrary to expectations.
 Gavrilets, S., ``Evolution and Speciation on Holey Adaptive Landscapes'', Trends in Ecology and Evolution 12 (1997) 307-312
 Extremely high dimensionality of the genotype space results in:
 * redundancy in the genotype-fitness map
 PvM: Redundancy is very important in evolvability
 *a possibility that high-fitness genotypes form networks that extend throughout the genotype space (=> substantial genetic divergence without going through adaptive valleys)
 PvM: For RNA the work by Schuster et al has shown that this is indeed the case.
 * increased importance of chance and contingency in evolutionary dynamics (=>mutational order as a major source of stochasticity)
 These exciting results show that the findings for RNA can be extended and generalized. In other words, evolution seems almost inevitable...
 Gavrilets presents a compelling definition of micro/macro and speciation
 Microevolution and local adaptation ~ climbing from a “hole”
 macroevolution ~ movement along the holey landscape
 speciation takes place when populations come to be on opposite sides of a "hole" in the landscape&#61472;&#61472;
 His definitions of holey landscape mimick the findings in RNA
 *A neutral network is a contiguous set of genotypes (sequences) possessing the same fitness.
 *A nearly neutral network is a contiguous set of genotypes possessing approximately the same fitness.
 *A holey fitness landscape is a fitness landscape in which relatively infrequent high-fitness genotypes form a contiguous set that expands throughout the genotype space.
 and he concludes
 The existence of percolating nearly-neutral networks of high-fitness combinations of genes which allow for “nearly-neutral” divergence is a general property of fitness landscapes with a very large number of dimensions.
 See also this paper
 A dynamical theory of speciation on holey adaptive landscapes
 <quote> The metaphor of holey adaptive landscapes provides a pictorial representation of the process of speciation as a consequence of genetic divergence. In this metaphor, biological populations diverge along connected clusters of well-fit genotypes in a multidimensional adaptive landscape and become reproductively isolated species when they come to be on opposite sides of a ``hole'' in the adaptive landscape. No crossing of any adaptive valleys is required. I formulate and study a series of simple models describing the dynamics of speciation on holey adaptive landscapes driven by mutation and random genetic drift. Unlike most previous models that concentrate only on some stages of speciation, the models studied here describe the complete process of speciation from initiation until completion. The evolutionary factors included are selection (reproductive isolation), random genetic drift, mutation, recombination, and migration. In these models, pre- and post-mating reproduct
 isolation is a consequence of cumulative genetic change. I study possibilities for speciation according to allopatric, parapatric, peripatric and vicariance scenarios. The analytic theory satisfactorily matches results of individual-based simulations reported by Gavrilets et al. (1998). It is demonstrated that rapid speciation including simultaneous emergence of several new species is a plausible outcome of the evolutionary dynamics of subdivided populations. I consider effects of population size, population subdivision, and local adaptation on the dynamics of speciation. I briefly discuss some implications of the dynamics on holey adaptive landscapes for molecular evolution.</quote>
Received on Wed Feb 22 13:48:16 2006

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