Sometimes, evolution is the result of natural selection. Other times, it's the result of random factors (genetic drift). Populations have variability; not all the individuals are the same, and sometimes individuals with particular genetic traits leave more descendant than others. That's evolution in a nut shell: The next generation is genetically different from the last one because not everybody's genes made it! These changes can have big effects on populations. Sometimes they end up with altered proportions of different variants (more fast cheetahs than slow ones, for example). Sometimes, they lose genetic variation, and sometimes, just sometimes, populations speciate (that is, form a new species).
You can consider this part to be the nuts-and-bolts section of the book, because it explains that biological variation exists, where this variation comes from, and the different ways it can change through time. Plus this is the part where I explain how scientists can watch evolution happen both in laboratory experiments and in nature, as well as how they can use data about species today to come up with strong hypothesis about evolution in the past.
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