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NATURAL SELECTION The mechanism for biological evolution. Observation 1: Exponential growth Populations tend to produce more offspring than the environment can support. Thomas Malthus Essay on the Principle of Population (1798) Populations in nature cannot continually increase. Sooner or later, food supply is insufficient and famine stops further growth Both Charles Darwin and Alfred Russel Wallace had read Malthus and understood the idea of exponential population growth Population Growth 1 pair of cockroaches could produce 164,000,000,000 in 7 months, if there were no limiting factors. Human Population Growth We see the same pattern in human population growth. We haven’t bumped into our limits yet. Observation 2: Zero growth In a balanced ecosystem, the numbers of individuals in a population remain stable. In terms of population growth the population at its carrying capacity has zero growth Population growth K 3 2 Numbers 1 Time Deduction 1: Competition There must be a struggle for survival Some of the offspring produced in a generation do not survive. What determines who the survivors are? Darwin identified competition as a major factor limiting population sizes Observation 3: Variation and Adaptation The competition for survival tends to be won by individuals that have genetic variations that help them survive. The mechanism of the inheritance of traits was being worked out at this time and remained undiscovered by biologists until 1900 Darwin was however aware that variations were somehow inherited and passed on from parents to offspring. Deduction 2: Survival of the fittest There will be a struggle for survival between the members of the population Individuals with advantageous variations (adaptations) will breed and produce more offspring Over time, the population will become more like the individuals with an adaptive advantage. Natural selection in action As generations pass by, the proportions of the alleles for the different variants will change in favour of those that provide the best adaptations Natural selection has been observed at work in populations of species over the past century Examples include: pesticide resistance in insects, antibiotic resistance in bacteria, industrial melanism in moths, tolerance to heavy metals in plants The Origin of Species by Natural Selection Darwin and Wallace argued that if natural selection proceeded for a long enough period of time it could bring about the evolution of new species Darwin himself favoured a long period of slow changes Recently this has been refined to include the possibility of rapid changes over a short period of time (punctuated equilibrium) Natural selection is not the only way Whether fast or slow, observing the evolution of a new species is unlikely in the lifetime of a scientist That species evolve is a fact but natural selection is not the only mechanism. Genetic Drift In every generation, individuals may survive out of sheer luck. The random survival of individuals will change the makeup of the next generation in random ways. Random survival does not lead to adaptation. Mutation Mutations are changes in the genetic code that happen by chance in the cell. Only mutations that happen in sex cells affect evolution. Mutations are not likely to be adaptive, but sometimes they are. Without mutation, there would be no evolution by natural selection.