Other challenges from outside biology come from social scientists who suggest that the theory of evolution is a construct of the human mind rather than a valid description of objective reality. Take a look at Ruse, M. 1999. Challenges from outside biology tend to be challenges to the entire theory of evolution by modification by descent. The remainder of my answer will deal solely with challenges to Darwin s theory of natural selection from within biology itself. Microevolution describes what happens to populations over short time-scales measured in generations.
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.