A common creationist argument against evolution is that chance could not provide a morphological change, i.e. speciation/macroevolution, often backed up with a bizare misinterpretation of thermodynamics. The flaw in this is, of course, that evolution is not based upon chance, but upon natural selection. This is why the frequency of alleles in a population remains relatively stable unless a selection pressure occurs.
E.g. 1:
Here, both extremes of variation are selected against, producing a less varied gene pool, the "normal" is selected for, so variation decreases somwhat.
E.g. 2:
Here, one extreme has been selected against, so you will see a shift towards the opposite extreme in the normal.
E.g. 3:
Here, the original normal is selected against, this produces a curve with two peaks, in such a situation, speciation can occur, as the original species splits into two new species over time.
Source
E.g. 1:
Here, both extremes of variation are selected against, producing a less varied gene pool, the "normal" is selected for, so variation decreases somwhat.
E.g. 2:
Here, one extreme has been selected against, so you will see a shift towards the opposite extreme in the normal.
E.g. 3:
Here, the original normal is selected against, this produces a curve with two peaks, in such a situation, speciation can occur, as the original species splits into two new species over time.
Source
