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6 7 LivinG Proof and dead ends In support of his evolutionary theory Darwin found important examples of evolution in action. One was artificial selection, or eugenics. Darwin argued that humans had created domesticated plants and animals by imposing selective processes on captive populations. Careful breeding, he suggested, created desirable characteristics in dogs, cats, horses, pigeons and chickens opposite top in much the same way that nature did in the wild. Traveling on the Beagle 183136, Darwin had noticed groups of closely related species that appeared to have adapted to slightly different environmental demands. Examining the exotic reptiles and birds of the Galapagos archipelago in 1835, he found that each island had its own idiosyncratic species of tortoise opposite and finch below, demonstrating that isolation events had allowed natural selection to take populations in different evolutionary directions on different islands from a common ancestor. Two problems remained Firstly, Darwin had only demonstrated lateral evolution, not longitudinal evolution species could adapt and vary on a theme, but a tortoise appeared to remain a tortoise and a bird a bird, so his theory didnt yet explain how wholly new types of animal and plant came about. Secondly, he lacked a provable mechanism behind this variation. Above Selective human breeding of cattle has created many hundreds of breeds, each with specific qualities. Some are bred for milk, others for meat, some for hot climates, others for freezing hillsides. Chickens too are selectively bred, some for eggs, others for meat. Darwin bred pigeons at his home to better understand the process of artificial selection and get a firsthand grasp of how quickly small variations in individuals could be passed into entire populations. Left Giant tortoises on the Galapagos Islands. There are 11 existing species of tortoise spread over the islands, all probably descended from a single ancestor. On dry islands, where little grows except cacti, taller tortoises with longer necks had the advantage over their shorter counterparts. Taller cacti also tended to survive being eaten and are found on these islands, an example of an evolutionary arms race. The tortoises allow the finches on the islands far left to peck ticks from their skin, which suits both parties well, and is an example of a mutualistic symbiotic relationship.
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