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All populations shows some sort of variation. Natural selection has led to great biodiversity. For many centuries humans have also altered the characteristics of organisms by deliberately choosing which individuals to breed. This is known as selective breeding. First the desirable characteristic is identified. For example size of cob in maize ( corn ) It is obviously better for the farmer if the cob is large and gives a high yield. Since there is always variety in a population some plants will produce larger cobs than others. For breeding purposes the farmer will select the plants with the desired characteristic. In this case plants with large cobs. X The next generation will have a higher percentage of plants with large cobs. The farmer will again select only those plants with the desired characteristic for breeding. X The next generation will have a higher percentage of plants with large cobs. The farmer will again select only those plants with the desired characteristic for breeding. X The next generation will have a higher percentage of plants with large cobs. The farmer will again select only those plants with the desired characteristic for breeding. X In this way over a number of years the farmer will have improved his crop. This is known as Selective Breeding as the farmer selects the characteristic to improve and then selects the individuals in the population which he will allow to breed. It is not just plants which can be improved in this way. Animals are also selectively bred for a number of characteristics. For example race horses are bred for speed or stamina. = Fast Horse = Slow Horse In a normal population of horses there will be some fast horses but mostly slower ones. X The horse breeder will only allow his fastest horses to breed X X X X Agriculture has produced many examples of both animals and plants which have been improved by the use of selective breeding. Selective Breeding In Animals Animal Characteristic Improved Cattle Beef and Milk Production Pig Growth Rate and Litter Size Sheep Wool Quality and Quantity Chicken Growth Rate and Egg Production Behaviour, Appearance and Abilities Dog Selective Breeding In Plants Plant Characteristic Improved Maize Oil Content and Size of Cob Tomato Size, Colour and Flavour Wheat Apple Height ( being the same height makes it easier to harvest ) Size, Colour and Flavour Beans Colour of Bean The Limitations of Selective Breeding Difficulty in getting selected individuals to breed. It may take months or years to obtain just one generation. Hybrids may turn out to be inferior due to the new combination of genes. The genes for a desired characteristic may not be present. Inbreeding can lead to the increase in the frequency of recessive genes. Often recessive genes give inferior qualities