In 1878 the above sequence of images was taken by Eadweard Muybridge to settle a debate among horse racers--when a horse was galloping full speed, did they ever have all four legs off the ground? The human eye alone was not fast enough to resolve the dispute. Muybridge, the photographer who was called in, set up a sequence of cameras that would be set off by trip wires and found that horses are in fact completely air born at certain points within a full gallop. (Watch the video of this horse here.)
Based on these images alone, though, how do we know that the horse was actually running? Couldn't the pictures have been of different horses? Couldn't they have been jumping and not running? This is more than just a thought experiment. We're asking if it's justified to tie different pieces of information together to form a progression, just like we do in evolution. Below we will explore several examples of biological progressions and ask what evidence there is that they represent evolutionary change over time.
Claim: Species have changed over time--descent with modification
Prediction: If evolution is true and species have changed over time, then we would expect there to be examples of this preserved in rock strata, across geography, in a lab, or in recorded history.
Falsification: Progressions of change across time do not exist. Species are shown to be unchangeable.
Observations: Fossil progressions, microevolution, biogeography (see examples below).Corroboration: Dating techniques confirms the progression’s sequence. Multiple traits are held in common by the organisms showing relatedness.
Inference: Provided with this gradual gradation in a trait we can infer that the species are related (though not necessarily direct descendants--evolution tends to meander).
The fossil record demonstrates not only large scale changes across time, phyla, but also within individual species. Three particularly important features are represented within the fossil record. Firstly, the species represented within the fossil record are often vastly different from the life of today. Secondly, is that the the fossils are separately and distinctly layered. Meaning, that we don’t find rabbit fossils in the Pre-Cambrian layers, nor dinosaurs mixed in with mammoths. Lastly, both on the macro and micro scale of the fossil record we see trends of change that progress through time.
The Larus Gulls of the Northern Hemisphere: Notice the darkening of the feathers in a clockwise direction starting at the bottom. Gull species near each other can interbreed, except between the Herring Gull and the Lesser Black-Backed Gull that diverged from each other long the longest ago. This inability to breed tells the story of how the species likely spread overtime around the world and met back up, now no longer able to interbreed.