Variation, selection and selective breeding
Biology · GCSE · The School
Natural selection, stated carefully
Variation already exists in a population, by chance. Some variants survive and reproduce more successfully in the conditions of the moment. Those variants become more common in the next generation. Nothing is trying, and no individual changes to suit its surroundings — the population changes because of who leaves offspring. Say it in that order and the answer is complete.
Evidence you can point at
Antibiotic resistance in bacteria is the clearest case and it happens fast enough to watch: a few bacteria already carry resistance, the antibiotic removes the rest, and the resistant ones multiply into the space. Darwin's finches show beak shape matching food source across neighbouring islands. Dog breeds show what selection does when people do the choosing rather than the environment.
Selective breeding and its cost
Choose the individuals with the characteristic you want, breed them, choose again from the offspring, and repeat for many generations. It gives high-yield wheat and docile cattle. The cost is a narrowed gene pool: closely related animals share more alleles, so harmful recessive conditions appear more often, and a population with little variation is vulnerable to a new disease that a varied one would survive.
GOAL: wheat that resists a fungus. BY NATURAL SELECTION, with no one involved: a few plants happen to resist, the fungus kills the rest, and resistant plants dominate the field within a few seasons. BY SELECTIVE BREEDING: a farmer finds the plants that survived, keeps their seed, sows only that, and repeats — many generations, and it only works if some resistance already existed in the crop. BY GENETIC ENGINEERING: the resistance gene, possibly from a wild grass or another species entirely, is inserted directly, in one generation, and it does not require the gene to have been present in wheat at all. Same trait, and the third route is the only one not limited by what the species already carries.