Macroevolution describes what happens over long time-scales measured in millions of years. Some biologists, however, believe that natural selection is not the only, or even most important, driving force in evolution. Another force that is widely accepted is genetic drift . This is the idea that random factors have a role in deciding which genes are passed between the generations. It is no good my having some marvellous new adaptation to my environment if I get run over by a bus before I get the chance to pass the gene for this adaptation to my offspring.
Biologists argue about the relative importance of random events in evolution.
It seems likely that genetic drift is very important in small populations. This has implications for conservation. Its role in larger populations, particularly at the macroevolutionary level, remains a matter of debate. You can read about this in any evolutionary biology textbook. In the late 1980 s this was manifest in the debate about directed evolution , also called adaptive mutation .