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Natural Selection Mrs.jackie Natural Selection • 1858- Charles Darwin and Alfred Rusell Wallace published a theory of evolution by natural selection Theory of Natural Selection • Individuals within a species are all slightly different from each other. These differences are called variation. • IF the climate or food supply changes, some of these variations may be better able to survive than others. • If one variety lives longer than others, it is also likely to leave behind more offspring. • If some offspring inherit the variation that helped the parent survive better, they too will live longer and have more offspring. • In time, this particular variety will outnumber and finally replace the original variety. Theory of Natural selection • Also called Survival of the fittest • Thomas Malthus in 1798, suggested that – The size of the human population would outstrip the rate of food production. – The number of people will be regulated by famine, disease and war. Darwin apply Malthus ideas to other population of living organisms Darwin´s ideas • He observed that – Animals and plants produce vastly more offspring than can possibly survive to maturity. – Therefore there must be a struggle for survival Example: A pair of rabbits had eight offspring which grew up and formed four pairs, eventually having 8 offspring per pair, in four generations the number of rabbits stemming from the original pair would be 512. The population of rabbits however remains more or less constant, therefore have failed to survive the reproductive age. Competition and selection • There will be a competition between the members of the rabbit population for food, burrows and mates • If food is scarce only the healthiest rabbits will survive and breed • Competition does not necessarily involved direct conflict – Able to run faster from predators – Better camouflage Competition and selection • If the offspring inherit the advantageous characteristics of their parents, they may give rise to a new and more adapted race. • This new variations are called survival value • Selection pressures- better adapted varieties are selected by the pressures of the environment • For natural selection to be effective, the variations have to be heritable. The peppered moth example • The common form is speckle but there is a variety which is black. • The black variety was rare in 1850 but in 1895 in the Manchester area its number had risen to 98% • Observations showed that the light variety was concealed better than the dark when they rested on tree-trunks covered with lichens. • IN Manchester Pollution had caused the death of lichens and the darkening of tree trunks • The dark variety was able to camouflage from birds. The peppered moth example • The selection pressure: predation by birds • Adaptive variation: dark color. Sickle cell anemia Sickle-cell anemia • A person with this condition has inherited both the Hbs and Hbs allele for defective hemoglobin resulting in the destruction of red cells • IN many African countries sufferers have a low change of reaching reproductive age. • Selection pressure remove the homozygous recessive from the population • However the heterozygous HbA Hbs have virtually no symptoms of anemia but have the advantage of been resistant to malaria. • The selection pressure of malaria favors the heterozygous over the homozygous HbA HbA Artificial selection • When humans breed animals or plants with specific characteristics – Examples: dogs, crop plants, cows. Homework • What features of a bird's appearance and behavior do you think might help it compete for a mate? • What selection pressures do you think might be operating on the plants in a lawn?