Selection Pressures

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The basis of Natural Selection

Intraspecific competition = competition between organisms of the same species

Intraspecific competition occurs because organisms produce more offspring than their environment can support.

Phenotypic variation = total variation in characteristics of an organism

Phenotypic variation means that certain offspring will die early or never reproduce. This could be due to susceptibility to disease or the inability to attract a mate, obtain food or escape predators.

Selection pressures = environmental force altering allele frequency in a population

Selective advantage = a variation that gives an organism an advantage, making it more likely to survive

Those with phenotypic variations which give them a selective advantage, by being better adapted to the selection pressures around them, are more likely to survive, reproduce and pass their successful alleles to the next generation.

Resistance in antibiotics

Binary fission = asexual reproduction where one cell divides to produce two cells which can then both grow to the size of the original cell

Micro-organisms (eg.bacteria) reproduce rapidly, often dividing by binary fission as often as every 20 minutes. This means that the allele frequency in a population of bacteria can take place over a relatively short amount of time.

Many bacteria carry genes for resistance to certain…


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