Evolution is the process by which a species develops changes to inherited traits over time. Evolution occurs when a particular population of a species includes individuals that exhibit those changes. The variation in genetic traits can be caused by the flow of genes during reproduction, the recombination of genes, and genetic mutation.
The necessary factors for evolution to occur are a means of expanding the gene pool, which happens through genetic mutation and recombination of genes, and a means of selecting the best combinations of genes, which is known as natural selection.
Evidence for evolution
Evidence for evolution comes from a variety of fields. In the fossil record, transitional species exist between fish and amphibian, amphibian and reptile, reptile and bird, and reptile and mammal. The first fish lacked jaws and dominated the Silurian period. Ichthyostega is the earliest documented amphibian and was preceded by coelocanths, such as Eusthenopteron. Early amphibians had weak arms, probably preferring the water. Seymouria was an early mammal and there were mammal-like reptiles before them, such as Cynognathus. The number of jaw bones decreased from five in reptiles to one in mammals while the number of ear bones increased from one to three. Few transitional species are probably more famous than Archaeopteryx, the first bird, which is documented by the famous fossil showing the imprint of its feathers in the mud.
Charles Darwin and the Galapagos Islands
Charles Darwin, the father of evolution, was jolted into developing the theory by way of a trip to South America and the Galapagos Islands off the coast of Ecuador. Darwin, a naturalist, noticed that the animals on the mainland differed from related animals on the islands. Moreover, there were even different species on the different islands of the Galapagos such as finches, tortoises, and iguanas. Finally, Darwin asked himself, "Why did recent fossils like glyptodonts and ground sloths resemble current species, but still were different?" Darwin realized that life must have evolved and wasn't created all at once. Darwin published his Origin of the Species in 1859.
Just two years later, the first missing link was discovered: Archaeopteryx. Archaeopteryx was half bird and half small theropod dinosaur, a creature like the velociraptor of fame from the Jurassic Park movie. And it too lived during the Jurassic Period, roughly 140 million years ago. Since 1861, seven more Archaeopteryx have been discovered. Furthermore, paleontologists discovered seven more dinosaur-like birds that are 30-40 million years more recent than Archaeopteryx.
Certain elements like uranium, potassium, and carbon have radioactive isotopes that decay in a very predictable manner. Some isotopes of uranium change to lead, potassium-40 changes to argon, and carbon-14 changes to carbon-12. By measuring the level of these isotopes, the age of rocks and fossils can be measured. These three isotopes can be found in rock of varying ages with uranium good for dating the oldest and carbon-14 best for dating the youngest.
Younger layers of rock are deposited on top of older layers of rock. Old fossils are never found with younger fossils. The only way an older rock formation can be found on top is through an occurrence like the Llano Uplift near Austin, Texas. Forces pushing from below made the older rocks go to the earth's surface. Really famous fossils are called index fossils because the fossil can be used to date the rock in which it was found.