Theories of evolution
The theory of evolution states that evolution happens by natural selection. The key points are that:
Individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed on to the next generation. Given enough time, a species will gradually evolve.
You need to remember that variation can be caused by both genes and the environment. But it is only variation caused by genes that can be passed on to the next generation.
Life on Earth today exists because of the conditions that were present when life was evolving. If in the past the conditions had been different, evolution by natural selectionnatural selection: the natural process whereby the best-adapted individuals survive longer and have more offspring, thereby spreading their characteristics through a population. 'Survival of the fittest'. could have produced some very different results. For example, if the Earth had been hotter, colder, bigger or smaller, our bodies would have changed to suit those conditions.
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