It does this of course because natural selection act as a filter letting through the 'fitter' alleles at each generation and so increasing the probability of them occurring in the next generation's gene pool. Provided it conveys an advantage a rare allele quickly becomes common, as I showed with a simple spreadsheet in Playing With Evolution.
This was highlighted for me today in Lisbon when I was sitting enjoying a refreshing água mineral com gás (that's a sparkling mineral water) and custard tarts in the grounds of the Castelo de Sao Jorge today. I saw a small bird which looked like a Great Tit (Parus major) only different. I'm familiar enough with British Great Tits to know when they don't look quite right.
The history of European birds, and especially those sedentary species like the titmouse family, is very interesting because it is intimately associated with the last Ice Age and is a wonderful example of how the environment drives evolution. A word of warning though! If you type "Great Tits in Portugal" into Google, be careful what you click on in the result list!
Portugal, as I'm sure you all know, lies on the western edge of the Iberian Peninsula which consists solely of Spain, Portugal and British-owned Gibraltar. The Iberian Peninsula is separated from the rest of Europe by the Pyrenees Mountains which act as a barrier to the movement of all sorts of species, like butterflies, moths and small birds, who can only normally interact with other members of their species or genus via narrow coastal strips on the Atlantic or Mediterranean coast. In effect, the Iberian members of a species have a almost isolated gene pool and so would be expected to evolve in their own direction more or less regardless of what is happening in the rest of Euro-Asia.
And that is exactly what we find.
Iberia has an abundance of races, subspecies and varieties of very many European species, and all of these have come about since the last Ice Age, when Iberia, Italy and the Balkans acted as refuges for many European species as ice sheets covered Northern Europe. In effect, the rest of Europe was repopulated from these refuges as the ice retreated some 10,000 years ago and the differences we now see either arose during isolation in these southern refuges or has arisen since. For more on how this procees probably drove diversification in Europe, see Creationists' Macro-Evolution Lie.
So, although I couldn't find any particular references to a particular Iberian form of the Great Tit, and it may well have been just a juvenile, an atypical individual or one showing a seasonal colour variation, the chances are good that what I saw was a regional variety, produced by isolation and by change in the frequency of particular alleles due to the effect of natural selection on its ancestors. The probability is that I saw just another example of evolution in progress.