TAILIEUCHUNG - The Self-Made Tapestry Phần 4

Các sóng này trở thành dịch sang một mô hình không gian cố định bởi sự tương tác của hệ thống phản ứng phổ biến với một chuyển đổi sinh hóa: khi nồng độ của hoạt vượt quá một ngưỡng nào đó tại bất kỳ điểm nào trong không gian, một hóa chất được tạo ra kích thích các tế bào biểu bì tạo hắc tố vào sản xuất melanin. | Page 8 on page 81 the system does not generate stationary Turing patterns but travelling waves rather like those of the BZ reaction. These waves become translated into a fixed spatial pattern by the interaction of the reaction-diffusion system with a biochemical switch when the concentration of activator exceeds a certain threshold at any point in space a chemical is generated there that stimulates melanocytes into producing melanin. Once this switch is thrown it stays that way melanin is produced even if the production of the activator subsequently ceases. The production of activator is assumed to be initiated at several random points throughout the system. Chemical waves of activator then spread outward from these initial points triggering melanin production as they go. But where the wavefronts meet they annihilate each other just as we see in the BZ reaction Fig. . These annihilation fronts define linear boundaries between each domain of activator production and so the system breaks up into melanin-producing polygonal domains separated by unpigmented boundaries Fig. a much closer approximation to the pattern seen on real giraffe pelts. Meinhardt and Koch found that with a little fine tuning of model parameters they could also obtain a better approximation to the leopard s pattern toothese are commonly not mere blobs of pigmented hairs but rings or crescents Plate 10 their model could generate structures like this Fig. . Models of this sort which involve two interacting chemical systems instead of the single reaction-diffusion system considered by Murray are clearly able to produce much more complex patterns. Hard stuff Anyone who is happy to accept with complacency the view that animal markings are simply detennined by Darwinian selective pressures has a surprise in store when they come to consider mollusc shells. The patterns to be seen on these calcified dwellings are of exquisite diversity and beauty and yet frequently they serve no apparent .

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