Evolution-Natural and Artificial John Maynard Smith Interdisciplinary Program in Cognitive Science Lee, Jung-Woo June, 7, 1999

Introduction Simulation of Evolution on a Computer –Is it an efficient way of answering practical questions? –Can it contribute to our understanding of evolution? Three general question –Has there been time for natural selection to produce the complex creatures we see around us? –What is the nature of the adaptive landscape? –What features of the genetic system are necessary and what contingent?

Time for natural Selection(1/2) Has there been time for natural selection to produce the complex creatures we see around us? –The genome of higher animals and plants contains 10 8 to 10 9 base pairs of informative DNA. –This is enough, together with the environment and the laws of physics and chemistry, to specify the structure of the adult organism. –But, there has not been time for selection to generate that much information.

Time for natural Selection(2/2) To get a handle on the question –Suppose that we start with a set of random DNA sequence, and try, by selection, to produce a unique optimal sequence. –If in each generation we are allowed to weed out half the sequences, we can specify one base in two generations, or 10 9 bases in 2 x 10 9 generation. –The time available since the origin of life is approximately 4 x 10 9 years, and during most of that time most organisms got through many generations a year

Nature of Adaptive Landscape(1/4) What is the nature of the adaptive landscape? Fisher’s view –evolution by natural selection is a hill-climbing process that can happen only on a smooth landscape. –If mutational steps are small, and populations large and random-mating, it is hard to escape this conclusion. –But, the fitness landscape is determined not only by the physical environment but also by other species, its competitors, predators, prey, which can also evolve.

Nature of Adaptive Landscape(2/4) Wright’s view –If a species is divided into a large number of small and partially isolated populations, a small population may occasionally jump across a local valley, purely by chance. Once a new adaptive peak has been reached, the new type can selectively replace the old. –First difficulty : The valley must be small (ab->AB) The mechanism will not help us to cross large valleys. –Second difficulty : If recombination is common, new genotype, AB, is immediately destroyed by recombination If the ability to make such transition has been important in evolution, it is hard to understand why sex is so widespread.

Nature of Adaptive Landscape(4/4) My suspicion –Evolution has depended on local hill-climbing, and long-term entrapment on local peaks has been avoided because the landscape is always changing. –This does not mean that there are not peaks that cannot be reached. The wheel may be an unreachable peak, but animals have evolved effective alternatives.

Features of the Genetic System(1/2) Two features necessary for any genetic system that is to support adaptive evolution –The system should be digital and –should not permit the ‘inheritance of acquired characters’.

The need for a digital system –arises from the familiar difficulty of maintaining information in a system able to vary continuously. The reason for avoiding ‘Lamarckian’ inheritance –Most changes induced in organisms by the impact of the environment are non-adaptive : they are the effects of injury, disease, and ageing. –A genetic system equipped with a mechanism of reverse translation would lead to deterioration, not to adaptation. Features of the Genetic System(2/2)