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Continuing my highly unauthorised and unofficial attempt to summarise 29+ Evidences for Macroevolution: The Scientific Case for Common Descent.
Next up are transitional forms. If common descent is correct, then there's just one family tree for living things; if there's just one family tree, then if you have a gap in that tree, you should be able to predict what the creature occupying that gap looked like - for example, an intermediate between reptiles and birds (but never an intermediate between birds and mammals - there's no such branch on the tree).
In the case of the transition from reptile to bird, there's a complete set of fossils of various species which show all the stages in the change - all the possible intermediates. The same is true for the transition from reptiles to mammals, including fossils which show how the mammal inner ear bones evolved from reptilian jawbones. Other examples include intermediates between the ancestors of whales, which lived on land, and modern whales; the land-dwelling ancestors of dugongs, and modern dugongs; and intermediates between humans and our closest relatives, chimpanzees.
Also, these intermediate forms - and indeed, all organisms - should appear in the right chronological order in the fossil record. They match very well. The whole tree would be blown apart by, for example, a mammal fossil turning up in Devonian rocks (before reptiles had appeared), or even a reptile-bird fossil that was older than reptile-mammal fossils.
(Kate adds: more of these intermediate forms are turning up all the time as more fossils are found and analysed. The common ancestor of humans and great apes turned up last year.)
Next up are transitional forms. If common descent is correct, then there's just one family tree for living things; if there's just one family tree, then if you have a gap in that tree, you should be able to predict what the creature occupying that gap looked like - for example, an intermediate between reptiles and birds (but never an intermediate between birds and mammals - there's no such branch on the tree).
In the case of the transition from reptile to bird, there's a complete set of fossils of various species which show all the stages in the change - all the possible intermediates. The same is true for the transition from reptiles to mammals, including fossils which show how the mammal inner ear bones evolved from reptilian jawbones. Other examples include intermediates between the ancestors of whales, which lived on land, and modern whales; the land-dwelling ancestors of dugongs, and modern dugongs; and intermediates between humans and our closest relatives, chimpanzees.
Also, these intermediate forms - and indeed, all organisms - should appear in the right chronological order in the fossil record. They match very well. The whole tree would be blown apart by, for example, a mammal fossil turning up in Devonian rocks (before reptiles had appeared), or even a reptile-bird fossil that was older than reptile-mammal fossils.
(Kate adds: more of these intermediate forms are turning up all the time as more fossils are found and analysed. The common ancestor of humans and great apes turned up last year.)
