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Natural selection and evolution (Natural selection (Summary of Natural…
Natural selection and evolution
Natural selection
Darwin's observations
Organisms produce more offspring than are needed to replace them
eg. a single female salmon can release 5 million eggs per year
Despite over-reproduction, stable, established populations remain the same size
Members of the same species are not identical – they show variation
Deductions:
There must be competition for resources and many individuals must die for the population to stay the same
The organisms best suited for their environment are the ones more likely to survive and reproduce
=> better characteristics are passed on
Summary of Natural Selection
there is variation within the species
changing conditions in the environment (KA
selection pressure
) favours one particular form of the species (which has a selective advantage)
the frequency of the favoured form increases under these conditions
the frequency of the less well adapted form decreases under these conditions
Examples:
Hoverfly
Wasps can defend themselves with a sting
They have a body with black and yellow stripes, KA
warning colouration
Predators learn not to attack them
Hoverflies do not have a sting, but their appearance 'mimics' the wasp
=> predators don't attack them
Selection pressure: predation by birds and other animals
–> hoverflies would have had variation in colour
–> hoverflies with mutations for stripes had a
selective advantage
Polar bear
Polar bears have many adaptations
Thick layer of white fur => reduce heat loss, act as camouflage
Wide, large paws => help walking in snow, used for swimming
Strong, muscular legs => can swim continuously for day
Nostrils that close when swimming under water
Large body mass – largest bears on Earth => small surface area to volume ratio => reduces heat loss
10cm layer of insulating fat
Well-developed sense of smell
Bumps on pads of paws to grip ice
Short, powerful claws => provide grip, needed to hold heavy prey
Selection pressures:
Very cold temperatures => need for insulation and reduced heat loss
White snow => need for camouflage to approach prey unseen
Brown bears with longer and paler fur had a selective advantage –> less likely to die of cold or from lack of food
Antibiotic resistance in bacteria
Antibiotics: chemicals that kill or reduce the growth of microorganisms
Natural antibiotics are produced by bacteria and fungi
=> give microorganisms an advantage over others when competing for resources since the antibiotic kills the competing organisms
Random mutation => bacterium has resistance to an antibiotic
–> this strain survives and multiplies
–> bacteria reproduce very quickly
=> antibiotic is no longer effective
Pesticide resistance in insects
eg. DDT – many species became resistant by the 1950s