Population biology is a study of populations of organisms, especially the regulation of population size, life history traits such as clutch size, and extinction. The term population biology is often used interchangeably with population ecology, although 'population biology' is more frequently used when studying diseases, viruses, and microbes, and 'population ecology' is used more frequently when studying plants and animals.
In October 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long- continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The results of this would be the formation of a new species. Here, then I had at last got a theory by which to work; but I was so anxious to avoid prejudice, that I determined not for some time to write even the briefest sketch of it. In June 1842 I first allowed myself the satisfaction of writing a very brief abstract of my theory in pencil in 35 pages; and this was enlarged during the summer of 1844 into one of 230 pages, which I had fairly copied out and still possess.
Charles Darwin, from his autobiography. (1876), pp34–35.