The Uses of Diversity. David Ellerman
Чтение книги онлайн.
Читать онлайн книгу The Uses of Diversity - David Ellerman страница 19
Название: The Uses of Diversity
Автор: David Ellerman
Издательство: Ingram
Жанр: Экономика
Серия: Polycentricity: Studies in Institutional Diversity and Voluntary Governance
isbn: 9781793623737
isbn:
Like Darwin, Wright thought it relevant to carefully observe artificial selection. Wright found that breeders do not keep all their animals together in one interbreeding herd. They deliberately break the herd up into subherds, subpopulations, “races,” or “demes” (as in demography). It is a question of balance. The subherds should be small enough so that the variety found in the subherd (through sampling error) or created through mutation, sexual reproduction, and genetic drift will be emphasized through inbreeding. But the subherd should not be so small that inbreeding leads to the quick fixation of ill-adapted genes and the deterioration or demise of the subherd. When a clearly superior example is produced in a subherd, then the seed is crossbred into the other subherds to give them the benefit of the innovation. But seeds could not be constantly crossbred between the subherds as that would defeat the benefits of their semi-isolation. Shifting balances were involved. How small to make the subherds and how much crossbreeding between the subherds?
Seeing these processes at work in artificial breeding and selection, Wright reasoned that Nature might have found some version of parallel “experimentation” with naturally forming subpopulations and cross-fertilization by migration.
Judging from animal breeding, (Wright) thought that natural populations must be subdivided into small-enough partially isolated subgroups to cause random drifting of genes but large-enough subgroups to keep random drifting from leading directly to fixation of genes, for this was the road to degeneration and extinction. Mass selection within subgroups was followed by selective diffusion from subgroups with successful genetic combinations. The final step was the transformation of all subgroups by the immigration of organisms with a superior genotype and subsequent crossbreeding. (Provine 1986, 236)
Using the terminology of the field, a text in population genetics describes the theory as follows:
In the shifting balance theory, a large population that is subdivided into a set of small, semi-isolated subpopulations (demes) has the best chance for the subpopulations to explore the full range of the adaptive topography and to find the highest fitness peak on a convoluted adaptive surface. If the subpopulations are sufficiently small, and the migration rate between them is sufficiently small, then the subpopulations are susceptible to random genetic drift of allele frequencies, which allows them to explore their adaptive topography more or less independently. In any subpopulation, random genetic drift can result in a temporary reduction in fitness that would be prevented by selection in a larger population, and so a subpopulation can pass through a “valley” of reduced fitness and possibly end up “climbing” a peak of fitness higher than the original. Any lucky subpopulation that reaches a higher adaptive peak on the fitness surface increases in size and sends out more migrants to nearby subpopulations, and the favorable gene combinations are gradually spread throughout the entire set of subpopulations by means of interdeme selection. (Hartl and Clark 1997, 259)
The point is that by dividing the population into demes or races, there is more variation and exploration. Since the results can be reaped by the whole population through crossbreeding, the overall rate of advance is increased.
Конец ознакомительного фрагмента.
Текст предоставлен ООО «ЛитРес».
Прочитайте эту книгу целиком, купив полную легальную версию на ЛитРес.
Безопасно оплатить книгу можно банковской картой Visa, MasterCard, Maestro, со счета мобильного телефона, с платежного терминала, в салоне МТС или Связной, через PayPal, WebMoney, Яндекс.Деньги, QIWI Кошелек, бонусными картами или другим удобным Вам способом.