scientists have not seen any evidence as far as literally an organism changing (i guess i could have used the word adapting instead) to its environment. so with this sense, evolution cannot be proved
It's easy to undertstand. Say there are 5 hermaphroditic organisms who reproduce sexually (for reasons of making this easier to understand). Now, of the five organisms, they each have different mutations. Say one has a better sense of smell, one is missing two limbs, one is fast, one is slow, and one is overall superior to them all. Of all the organisms, the one missing limbs will most likely die off, because they aren't adapted. Then the slower one will most likely die off, because they can't do this, or that. Then there are three left. Smell, speed, and superior. Most likely, these three organisms will survive, and unless one dies off, there will be a bunch of organisms with good smell, a bunch with good speed, and a bunch that are superior. From there, they may become three different species, creating 3 species, from what all started with 5 mutations, where natural selection occured.
(Sorry if that's confusing. I try.)
Here are 4 different finches. Of these 4 finches, they are each suited for eating a different type of thing.
The remaining land-birds form a most singular group of finches, related to each other in the structure of their beaks, short tails, form of body and plumage: there are thirteen species, which Mr. Gould has divided into four subgroups. All these species are peculiar to this archipelago; and so is the whole group, with the exception of one species of the sub-group Cactornis, lately brought from Bow Island, in the Low Archipelago. Of Cactornis, the two species may be often seen climbing about the flowers of the great cactus- trees; but all the other species of this group of finches, mingled together in flocks, feed on the dry and sterile ground of the lower districts. The males of all, or certainly of the greater number, are jet black; and the females (with perhaps one or two exceptions) are brown. The most curious fact is the perfect gradation in the size of the beaks in the different species of Geospiza, from one as large as that of a hawfinch to that of a chaffinch, and (if Mr. Gould is right in including his sub-group, Certhidea, in the main group) even to that of a warbler. The largest beak in the genus Geospiza is shown in Fig. 1, and the smallest in Fig. 3; but instead of there being only one intermediate species, with a beak of the size shown in Fig. 2, there are no less than six species with insensibly graduated beaks. The beak of the sub-group Certhidea, is shown in Fig. 4. The beak of Cactornis is somewhat like that of a starling, and that of the fourth subgroup, Camarhynchus, is slightly parrot-shaped. Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends. In a like manner it might be fancied that a bird originally a buzzard, had been induced here to undertake the office of the carrion-feeding Polybori of the American continent.