Variation is the slight individual differences within populations. All living things
change and evolve from one generation to the next. As they do so, more variation
is produced.
Some variations is inherited (passed on from parents) and some is acquired (developed during life).
Animals and plants produced by sexual reproduction will show variation from their parents, for example in the size of the muscles in the legs of lions.
When
new organisms are produced, not all of them are likely to survive because of
competition for resources such as food, water and shelter. The same is true for
plants (they compete for resources such as nutrients, light, water and space).
The
individuals with the most favourable characteristics are most likely to survive.
The process of natural selection follows a sequence, as listed below.
- Some of the variations within a population may give some individuals an advantage over others in the population. Bigger muscles in the legs of a lion would enable it to run more quickly and get food more successfully.
- In an environment where there is food shortage, the lion with the biggest leg muscles is most likely to survive to adulthood.
- The weaker individuals die before having the chance to breed, but the surviving adults breed and pass on the advantageous genes to their offspring.
- More of the next generation carry the advantageous genes, resulting in a stronger population, better adapted to a changing environment.
Slow
changes in the environment results in adaptation in a population to cope with
the change. Failure to adapt could result in the species becoming extinct. This
gradual change in the species through natural selection over time, in response
to changes in the environment, is a possible mechanism for evolution.
Examples: antibiotic-resistance strains of bacteria
Bacteria
reproduced rapidly - a new generation can be produced every 20 minutes by binary
fission. Antibiotics are used to treat bacterial infections: an antibiotic is a
chemical that kills bacteria by preventing bacterial cell wall formation.
Mutations
occur during reproduction, which produce some variation in the population of bacteria.
Individual bacteria with the most favourable features are most likely to survive
and reproduce.
A mutation may occur that enables a bacterium to resist being killed
by antibiotic treatment, while the rest of the populating is killed when
treated. This bacterium would survive the treatment and breed, passing on the
antibiotic - resistant gene to its offspring. Future treatment of this population
of bacteria using the antibiotic would be ineffective.
Video GCSE BBC Science Bitesize - Variation, Inheritance and Evolution
Video The Animation of Antimicrobial Resistance
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