Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Quantitative trait locus wikipedia , lookup
Human genetic variation wikipedia , lookup
Genetic engineering wikipedia , lookup
Dual inheritance theory wikipedia , lookup
Genetic drift wikipedia , lookup
Polymorphism (biology) wikipedia , lookup
Deoxyribozyme wikipedia , lookup
History of genetic engineering wikipedia , lookup
Group selection wikipedia , lookup
Koinophilia wikipedia , lookup
Population genetics wikipedia , lookup
Evolution-Natural and Artificial John Maynard Smith Interdisciplinary Program in Cognitive Science Lee, Jung-Woo June, 7, 1999 99-06-07 1 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? 99-06-07 2 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 108 to 109 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. 99-06-07 3 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 109 bases in 2 x 109 generation. – The time available since the origin of life is approximately 4 x 109 years, and during most of that time most organisms got through many generations a year 99-06-07 4 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. 99-06-07 5 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 : 99-06-07 • 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. 6 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. 99-06-07 7 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’. 99-06-07 8 Features of the Genetic System(2/2) • 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. 99-06-07 9