When a cell divides, the genomes of ①its two daughters are often not quite identical to each other or to that of the parent cell. On rare occasions, the error may represent a change for the better; more probably, it will cause no significant difference in the cell’s prospects. But in some cases, the error will cause serious damage; for example, by ②disrupting the coding sequence for a key protein. Changes due to mistakes of the first type will tend to be perpetuated, because the altered cell has an increased likelihood of reproducing itself. Changes due to mistakes of the second type — neutral changes — may be perpetuated or not: it is a matter of chance ③whether the altered cell or its cousins will succeed. But changes that cause serious damage ④lead nowhere: the cell that suffers them dies, leaving no progeny. Through endless repetition of this cycle of mutation and natural selection organisms evolve: their genetic specifications change, sometimes ⑤give organisms new ways to exploit the environment more effectively, to survive in competition with others, and to reproduce successfully.