Competition among species, or interspecific competition, can have an even greater effect on selection than competition within species (intraspecific competition). This is often the case in lower density populations. Different species can have positive, neutral, or negative effects on each other's fitness, and the effect species 1 has on species 2 is not necessarily the same that 2 has on 1. The effects that cohabiting species have on each other shapes evolution the same way that selective pressures from within a species or the physical environment shapes it.

Reading assignment:

Cotgreave, Peter and Irwin Forseth. Introductory Ecology, chapter 8