Natural selection
The theory of evolution states that evolution happens by natural selection. The key points are that:
-individuals in a species show a wide range of variation
-this variation is because of differences in their genes
-individuals with characteristics most suited to the environment are more likely to survive and reproduce
-the genes that allow these individuals to be successful are passed to their offspring
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 to the next generation. Given enough time, a species will gradually evolve.
*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.
*Conditions on Earth
Life on Earth today exists because of the conditions that were present when life was evolving. If, in the past, conditions had been different eg hotter, colder, or the Earth more or less massive, then evolution by natural selection could have produced some very different results. Our bodies would have changed to suit the Earth's conditions.