You'e probably come across the terms 'microevolution' and 'macroevolution'. 'Microevolution' is generally taken to mean small-scale changes in a population's gene pool, while 'macroevolution' is evolutionary change at the level of species, or genus, or phylum. This distinction can cause problems with understanding…
… because it can be viewed as implying that the two are quite different processes. (With, in the eyes of some people, quite different probabilities that they can happen.) In fact, the difference between the two is one of degree, and it's a fairly arbitrary distinction, particularly when you remember that the nature of a species isn't always obvious. For example, if one species is gradually evolving into another (something called anagenesis), at what point do you define the new species?
But evolutionary biologists do agree that microevolutionary processes contribute to macroevolution; they just argue about the level of that contribution. For example, David Penny & Matthew Phillips say (2004) that "… it is a basis of darwinian evolution that the mechanisms that we can study in the present are sufficient to account for past evolution", and go on to suggest how this model could be examined in terms of the evolution of birds and mammals.
Another thing to remember is that macroevolution usually (but not always) operates on a much longer time scale than microevolution – we recognise it after it's happened & should not expect (as some folks seem to) to see substantial evidence of macroevolution in the here-&-now.
References: D. Penny & M.J. Phillips (2004) The rise of birds and mammals: are microevolutionary processes sufficient for microevolution? Trends in Ecology and Evolution 19(10): 516-522