One of the fundamental questions in evolutionary thought is how change take place within a species. How does it happen that the organisms that make up the population today are different in important ways from the organisms that made up a similar population a thousand or a million years ago?
For biologists, that question can be rephrased in terms of changes in gene frequencies. Suppose one could make a list of all the genes in a population are muskrats, they will share a great many genes in Common with each other. However, all muskrats in the population will not have identical genes. There will be some variability among those genes.
Read next
From @temibejay