Artificial selection is the selective breeding carried out by humans to alter a population. It is a procedure often used in agriculture: artificial selection has been used to alter the number of eggs laid by hens, the meat properties of bullocks, and the milk yield of cows.
Asexual reproduction is the production of offspring by virgin birth or by vegetative reproduction: that is, reproduction without sexual fertilization of eggs (see sexual reproduction). Dandelion are example of one
Adaptation is the condition of organisms being well designed for life in their environments. Adaptation refers to all the structural, functional and behavioral characteristics that enhance the organism's reproductive success in its natural environment. Classic examples include:
• The beak of the woodpecker and the Galapagos finches.
• The almost faultless camouflage of organisms such as stick insects. Not all evolutionary changes are the result of adaptation. Some are caused by non-adaptive processes such as genetic drift. However, all changes that are the result of adaptation can be explained by natural selection, and the stages in the evolution of the eye provide a good example of this. Adaptations in nature are subject to various constraints.
What is the cost of sex? Put simply, it is the cost of males. Consider two populations: one is asexual, and one is fully sexual. Both have a stable sex ratio of 1:1. For simplicity, suppose that each female produces two offspring. In the asexual case, the population will double every generation, since each member of the population produces two offspring. In the sexual case, the population will remain constant, since half of all offspring are males, and only contribute to the next generation by fertilizing the females. Fifty per cent is a large cost. The problem of explaining sex is to find a compensating advantage of sexual reproduction that is large enough to make up for its cost. The problem is not trivial. Indeed, G.C. Williams has described it as 'the outstanding puzzle in evolutionary biology'. Several possible advantages of sex have been suggested.
Natural selection may be directional: it may favor, for example, smaller individuals and will, if the character is inherited, produce a decrease in average body size. Directional selection could, of course, also produce an evolutionary increase in body size if larger individuals had higher fitness.
Natural selection can sometimes favor extremes over the intermediate types. This is called disruptive selection
evolution as "descent with modification": the change in a lineage of populations between generations. The theory of evolution has four main components:
• Population genetics provides the fundamental theory of the subject. If we know how any property of life is controlled genetically, population genetics can be applied to it directly. • The theory of adaptation; how features of an organism evolve within its environment. • The diversity of life; what a species is, how new species originate, and how to classify and reconstruct the history of life. • Evolution on a grand scale; the fossil record is the main testing ground for large evolutionary events, which occur over the geologic time scale of tens or hundreds of millions of years. Evolution is seen as a branching tree
Biologists call a behavior pattern altruistic if it increases the number of offspring produced by the recipient and decreases the number of offspring of the altruist. The altruism of the sterile 'workers', in such insects as ants and bees (pictured opposite), is one undoubted example; here the altruism is extreme, as the workers do not reproduce...