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34 35 ConverGent evoLUtion inevitable solutions Variation generally produces evolutionary divergence, as species adapt to fill as many econiches as they can. However, it turns out there are often only a limited number of good design solutions when it comes to solving problems such as how best to fly below or see opposite top, and that is why many species share similar features. When not due to a directly shared ancestry i.e., coded for by the same DNA this is described as convergent evolution, as analogous features have evolved completely independently. Convergent evolution can result in both collective and singular analogous traits. In marsupial and placental mammals, corresponding species that have adapted to fill similar econiches on different continents generally resemble one another despite being genetically distant. An example of a singular analogous trait is the eye of the cephalopod and the eye of the vertebrate. Their eyecoding DNA is different, yet their eyes are virtually identical in both structure and function. Groping around for improvements, evolution again and again finds the same solutions that work best. Left Three out of the many different camera eyes that have evolved completely separately in different species in different parts of the world after Conway Morris. At the top, the human eye focuses by changing the shape of its lens. In the centre, the eye of an octopus a cephalopod mollusk focuses by moving its lens forward and back. At the bottom, the eye of a marine annelid, a relative of the earthworm. Lens eyes have also evolved independently in the brainless cubozoan jellyfish, dinopis spiders and heteropod snails, to name but a few examples. This page, and opposite Examples of convergent evolution. In the matrix of all possible solutions to problems, only some work, and only some of these work well. Whether it is the discovery of a chemical which responds to light, an efficient method of propulsion, or an outer form maximised for lack of drag through water, the best solutions are limited in number, and constrain the outcomes. The idea is not dissimilar to Platos ideal forms, perfect solutions, shadows of which appear on Earth. Above Marsupial mammals have independently evolved similar forms to nonmarsupials. There are marsupial deer kangaroo, squirrels koala, rabbits bandicoots, rats and mice. Above Sharks and crocodiles are perfected forms which have changed only slightly in the last 100 million years. Niche species in Lake Tanganyika have evolved to ressemble ocean counterparts. Optic nerve Retina Pigmented layer Nuclear layer
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