In general evolution happens so slowly that it cannot be watched.

The four letter code that constitutes the DNA of all living things changes over time; for example individual or several letters can be copied incorrectly [substitution], lost [deletion] or gained [insertion]. Such changes can lead to functional and structural changes in genes and proteins and ultimately to the formation of new species.

Insertions are much more common whilst deletions appear to be rare.

The Pope, Catholic Church, Church of England and mainstream churches all accept the big bang and evolution!

Lord Carey the former Archbishop of Canterbury put it rather well – “Creationism is the fruit of a fundamentalist approach to scripture, ignoring scholarship and critical learning, and confusing different understandings of truth”!

Nice that christians and atheists can agree and laugh together even if it is at fundie expense!

But behind the laughter is the despair at the fundamentalists striving so hard to destroy christianity by turning it from a religion to an ideology!

"When they say evolution happens to a population what do they mean, how does that happen?"

See, if we ignore the last three words, you actually have a very common and sensible question.

To say that evolution happens, we first have to define what evolution is. Evolution is the name applied to the phenomena of certain genes becoming more or less common within a population. This clearly does happen and you don't have to look very far or very long to see that the genes of cattle, dogs, cats, fruits, pigs, rabbits, and many other types of organisms have changed very dramatically in a very short period of time. This is because we have bred them selectively; purposefully inhibiting the transferring of certain genes from one generation to the next, while encouraging the transference of others. The result of these genetic changes is that the current generations of these organisms are very different to what their ancestors were a few hundred generations ago.

Selective breeding isn't the only mechanism by which this happens though. The environment itself can influence which genes are transferred by allowing those who posses the genes that make them able to reproduce more within it do so, and inhibiting those who posses genes which make them less able to reproduce within it from doing so. We call this natural selection. The individuals themselves can influence it by choosing to mate with those individuals with genes that make them sexually attractive, and not mating with those who are less attractive to them. We call this sexual selection. We also see this effect without any external influences since every individual has genetic mutations unique to them; in other words each child is slightly different from its parents, even more different from its grandparents, even more different from its great-grandparents and so on. We call this genetic drift.

So far I've only given examples of the emergence of new sub-species, but we have also seen the genetic differences between current generations and their ancestral ones vary to such an extent that they are now considered to be a different species (these changes over the generations have been directly documented in some species of gulls, robins, flies, salamanders, warblers, finches, moths, mosquitoes and bacteria to name only some of the most commonly known and least contested observed examples). We can also see these changes easily with comparative genetics and the fossil record, which in some respects are more reliable than our own created records, in species which we haven't kept records of, or species that emerged before the ability to observe them doing so.

A popular misconception among creationists is that evolution proposes that one individual can give birth to something fundamentally different to what it itself is. This is not true. The offspring can only be slightly different from its parents, as all offspring are - the "macro-evolution" is just the fact that many successive generations become too different to the ancestors to be considered the same species. The more generations you have the more different they will become from their ancestors. For example look at the visible light spectrum (http://en.wikipedia.org/wiki/File:Linear… and try to pinpoint where violet turns to blue, blue turns to green, green turns to yellow, yellow to orange and orange to red. You can't. Each progression is so subtly different that you can only say there is a fundamental difference after many minute changes, and the more minute changes you get, the more different the colour becomes from where you started.

I hope that answers your question, but if not, just let me know what you want clarifying.