4.3.9 natural selection & evolution
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theory of natural selection
• individual has characteristic which helps it survive in environment & lives long enough to reproduce • evolution = selects individuals with adaptations who survive & reproduce • adaptations passed on generations • more & more have adaptive characteristic
steps 1-6 of natural selection
• mutation creates alternative versions of gene (allele) • creates genetic variation between individuals in species - intraspecific variation • now variety exists, the environment can ‘select’ - when resources are scarce, the environment selects those variations (characteristics) which provide an advantage = selection pressure • individuals with advantageous characteristic survive/reproduce • thus, they pass on advantageous characteristic = inheritance • next generation will have higher proportion of individuals with successful characteristics - over time, group of organisms become well adapted to environment = adaptation
what must occur before evolution takes place
genetic variation
pesticides
designed to kill pests
insecticides
designed to kill specifically insects
what does an insecticide apply
very strong selection pressure - if insect susceptible = die - if form of resistance = may survive & reproduce to pass on resistance characteristic so it quickly spreads
when was resistance to pesticides first documented & describe
1914 - scale insects resistant to inorganic insecticides
how short can resistance arise in
little as 2 years
problem of resistance to pesticides by insects
• can accumulate in food chain • if many resistant insects eaten by predator, they receive much larger dose • can reach humans
what pesticide has been banned in many areas
DDT
problems associated with antibiotic resistance
• possible that some bacteria are resistant & survive • many people stop taking antibiotics before end of course = resistant bacteria survives & reproduce creating resistant strain
what has led to bacteria practically resistant to all antibiotics
overuse & incorrect use of antibiotics
what does prescribing multiple antibiotics do
greatly reduces chance that any bacteria will survive
selection of resistant microorganisms - before selection - directly after selection - final population
before selection: variety of levels of resistance - variation is caused by mutations which create genetic variation within species directly after selection: least resistant individuals killed - all that remains is individuals possessing some resistance final population: partly resistant individuals reproduce - next generation contains much higher proportion of resistant individuals
example of bacteria which have gained wide range of resistance
MRSA - developed resistance to increasing range of stronger drugs - ‘evolutionary arms race’ = medical researchers struggling to develop new/effective drugs, but the bacterial populations rapidly become resistant to them