Because he was well aware of the effect his theory of evolution would have on the public and on the Church of England, Darwin delayed publishing his work for several decades while he gathered additional evidence. After invoking selective breeding of domesticated species as evidence that groups of organisms are capable of change, he then proposed that natural populations can change as well. On which two lines of evidence did he base this proposal?

The theory of evolution that states that groups of organisms change over time. In essence, a current population of kangaroos is a product of several changes that have occurred over millions of years, resulting in the kangaroos we currently see.He's perhaps the most famous evolutionary scientist. He published his theory of evolution by natural selection in 1859
Darwin knew a mechanism was behind the change, or evolution, that had occurred, creating different beaks. He called this mechanism natural selection. Darwin proposed that over time, a population of finches could develop different beak shapes in order to best eat whatever food was most available to them, thus allowing them to survive better
He used the term 'survival of the fittest' as a short hand for organisms so well-suited to their environment that they will live long enough to reproduce, passing their traits, like beak shape or eye color, to the next generation
Hence, the right answer is "Organisms within a population vary, and all populations produce more offspring than can be supported by the environment, resulting in competition for survival within the population"