the main body of evidence for evolution comes from the following areas of science:
- nested hierarchy of life
- vestigial characters
- embryology
- homology
- fossil evidence
twin nested hierarchy
a nested hierarchy is a system of classification. it takes the structure of a tree with higher levels of generality near the root node and higher levels of specificity at the leaf nodes. for example, when you go to a library, to find a book you want, you have to search through several layers of classification. say we were trying to find the book "the blind watchmaker" by richard dawkins. the first thing we would do is choose between the three major sections of the library, fiction, non-fiction, and reference. the book we are looking for is non-fiction, so we choose the non-fiction section and descend one level of the tree. once we are in non-fiction, we must determine what bookshelf our book is in. to do this, we use the dewey decimal system. 500 is the classification for the natural sciences and mathematics, so we again descend one level. the next step is 575, for evolution and genetics. and the next step is 'D' for dawkins, and finally 'B' for "the blind watchmaker." the classification tree for finding a library book is a type of nested hierarchy.
however, it is not what is known as a "true" hierarchy. that is, books do not fall into this classification scheme on their own accord. humans must assign weights to particular classification levels. for example, we could have chosen to start with 'D' for dawkins, and then go to non-fiction, followed by 'B', followed by 575. sounds strange, but the fact is that you would find books just as easily either way.
an example of a true nested hierarchy would be languages. languages can be grouped into hierarchies without placing arbitrary uneven weights on certain characteristics. we can classify languages like french, italian, spanish, and latin together, and likewise we can put english, german and dutch together. with those 2 groups, we can then link them to other languages like polish, slavic, hindi, etc. what makes this a true nested hierarchy is that no matter what order you choose for your grouping, you get the same (or at least extremely an similar) tree.
one of the founders of modern biology, linnaeus, was the first person to comprehensively apply a nested hierarchy classification system to living organisms. what he discovered was that living forms make a true nested hierarchy. all organisms that have milk glands also have four limbs. all organisms with four limbs also have a backbone. all organisms with a backbone also have a jaw, and, unbeknownst to linnaeus, all organisms with a jaw also have mitochondria. groups always completely contain their subgroups.
the reason that this provides astounding evidence for evolution is that true nested hierarchies are always produced by processes of inherited descent. a designer could have chosen any pattern at all for the forms of life - including no pattern whatsoever. but the one pattern we do see is exactly the one we would expect for evolution.
vestigial characters
a vestigial character is one which is diminished in function or form. vestigial characters can either be completely useless, or have some secondary use that is unrelated to or less than the functional abilities of the character. vestigial characters include the eyes of blind cave fish, the wings of an ostrich, and the plantaris tendon in human feet. vestigial characters defy explanations of design, but fit perfectly within evolutionary theory. a designer has no reason to design useless eyes on a cave-dwelling fish that cannot use them. a designer could easily have given the ostrich a true sail, instead of giving it wings that are clearly meant for flight. plantaris tendons serve no purpose whatsoever in humans, and in fact many people are born without them and never know it.
vestigial characters show that organisms include traces of their past histories in which these characters were once used for their functional purposes. ostriches have wings because they descended from flying birds. blind cave fish have eyes because they descended from normal fish with working eyes. humans have plantaris tendons because we evolved from tree-swinging apes that needed to grasp branches with their feet.
embryology
embryology is the study of how new organisms form from the sex cells of their parents. since it has been determined that the genetic code for building us is encased in our DNA, and evolution proposes that modifications of DNA has led to the diversification of all life, embryology is an excellent test for the validity of evolution.
our DNA codes for the synthesis of proteins, the building blocks of life. during embryonic growth, we can correlate how portions of DNA will affect the adult organism, and we can measure the effects of changing that DNA through genetic engineering. some modifications induce slight changes, such as eye or hair color, but other changes induce more drastic and cascading effects that can render an organism almost unintelligible as members of their species. as evolution proposes that mutations of DNA are the underlying source of evolution's variation, we should be able to see evidence of this by studying the embryology of various organisms and seeing if they conform to the nested hierarchy.
and in fact, this is exactly the case. for example, fish embryos and mammal (and everything in between) embryos display features in early development that are indistinguishable from each other. it is only later in development that the pharyngeal pouches in fish become gills, while in land-dwelling animals they go on to form structures such as the eustachian tube, the middle ear, the tonsils, the parathyroid, and the thymus.
there is absolutely no reason from a design standpoint to build these structures from the same embryonic beginnings, especially since they serve immensly different functions. these quirks of embryonic development only make sense if we infer that the history of an organism's ancestry is represented by them.
homology
homology is the recognition that structures with different functions nonetheless have similar structures. for example, all four-limbed organisms share the same limb structure. they all have a single main limb bone, which radiates at a joint into two supporting bones, which again radiate at a joint into further bones. whether these bones become legs, arms, claws, wings, or flippers, they all have this same structure.
again, these structures do not make sense in terms of design. a designer would apply designs based on how well suited they were for their functions. the homology of characters only makes sense if it represents patterns of common ancestry.
fossil evidence
ever since fossils were discovered, men have been fascinated by the many forms that are extremely different from today's life. dinosaurs, pterosaurs, and trilobytes demonstrate that life long ago was not the same as current life. since evolution proposes that these life forms are related through genealogy, at least some of these extinct fossil forms should show these transitions in progress. we should expect to see transitions that reflect our nested hierarchy tree, and never find fossil forms that show transitions that contradict the tree.
again, this is exactly what we find. we have fish-amphibian transitions like icthyostega and acanthostega, amphibian-reptile transitions like hylonomus and paleothyris, reptile-bird transitions like dromeosaur, caudipteryx, and archaeopteryx, and reptile-mammal transitions like dimetrodon, procynosuchus, and thrinaxodon. we also have an extremely rich set of fossils demonstrating the ape-human transition, like australopithecus afarensis, australopithecus africanus, homo habilis, homo erectus, and cro-magnon man.
if these creatures were all unique designs, it is an extremely odd coincidence that they fit within our already specified nested hierarchy. a designer could have made a fish-bird transitional, for example. but we only have transitionals that fit within the nested hierarchy.
these are only very basic introductions to only five out of many mountains of evidence for evolution. no scientific explanation has ever come close to explaining these features of the world like evolution has. when we take into account the millions of other facts of biology, evolution still remains standing.