Evolution Evolution is change over time. The Theory of Evolution states that all organisms share a single common ancestor and have evolved over time. It also states that this has occurred largely due to changes in environmental conditions that have led to a necessity for organisms to change and adapt or go extinct.

Darwin’s Theory of Natural Selection In 1831, Charles Darwin set sail on the HMS Beagle as the ship’s naturalist. His job was to collect biological and geological specimens. After traveling from England across the Atlantic and collecting specimens along the eastern and western coasts of South America, the ship landed in the Galapagos Islands.

Darwin’s Theory of Natural Selection Although Darwin only spent a few weeks exploring the Galapagos Islands, he later said that he made his greatest discoveries there, which led him to write his book, The Origin of Species. The Galapagos Islands are a volcanic archipelago located 600 miles off the coast of Ecuador.

Darwin’s Theory of Natural Selection In 1859, Darwin publishes his work in which he describes how species originate by means of natural selection. Natural selection is the process where heritable traits that make it more likely for an organism to survive long enough to reproduce become more common over successive generations of a population.

Darwin’s Theory of Natural Selection Natural selection, also known as “survival of the fittest,” refers to the selection by nature for the most fit, or most well adapted members of a species to survive. This selective process allows for only the best traits to be passed on to the next generation.

Darwin’s Finches After studying a variety of birds on the Galapagos Islands, Darwin developed his Theory of Natural Selection. Darwin initially thought he had been studying a variety of birds on the islands, but actually they were all different species of finch, adapted for various environments depending on the available food resources. He decided that specific traits were selected for naturally, depending on which traits were advantageous in a particular ecosystem.

Darwin’s Finches Darwin found that the beaks of finches varied from island to island depending on the food source available for each finch population.

Galapagos Giant Tortoises Darwin missed the opportunity to study Galapagos tortoises but was told by the vice governor that he could tell which island a tortoise was from by the shape of its shell. Dome shelled tortoises feed off the ground while saddleback tortoises can extend their necks to feed from higher vegetation.

Galapagos Giant Tortoises

Evidence of Evolution The fossil record offers some of the most significant evidence of evolutionary change. Fossils have been discovered that give evolutionary biologists clues into what features were present in ancient organisms and how those organisms may have evolved into modern organisms. Archaeopteryx is an extinct organism that shares characteristics of both birds and reptiles.

Fossil Evidence Fossils discovered around the world from the tops of mountains to the bottom of the oceans have helped prove that organisms have changed over time. Fossil teeth of Megalodon, an extinct ancestor of the Great White Shark, have been found on the ocean floor around the globe, proving the shark was once much larger, like most top predators of the time.

Comparative Anatomy Anatomically similar structures inherited from a common ancestor are called homologous structures. Evolution predicts that an organisms body parts are more likely to be modifications of ancestral body parts than they are to be entirely new features. Therefore, homology proves common ancestry, which in turn proves evolution has occurred.

Comparative Anatomy Although homologous structures prove common ancestry, analogous structures prove otherwise. The wings of a bat, bird, and butterfly are analogous, or structurally different; proving that functionally similar features can evolve independently in similar environments.

Vestigial Structures Vestigial structures are structures that still exist in a population but no longer serve a function. Evolutionary theory predicts that features of ancestors that no longer have a function for that species will become smaller over time until they are lost.

Comparative Embryology Comparing embryos of vertebrates during early stages of development shows homology and can therefore show common ancestry. Later stages also show homology which can be used to develop a phylogeny, or ancestral tree.

Comparative Biochemistry Evolutionary theory predicts that molecules in species with a recent common ancestor should share certain ancient amino acid sequences. The more closely related the species are, the greater number of sequences will be shared. Comparing DNA and RNA can yield strong evidence of organisms sharing a common ancestor.

Phylogeny A phylogeny is an ancestral tree illustrating common ancestry. Phylogenies can also illustrate where in evolutionary history a particular trait was acquired by the proceeding species. The diagram is known as a cladogram.

Defining a Species A species can be defined by any one of three ways: Morphological Species Concept: if they look the same they are the same species. Biological Species Concept: if they can mate and produce viable offspring they are the same species. Phylogenetic Species Concept: if they share an immediate ancestor they are the same species.

Speciation Speciation, or the birth of a new species, occurs when a population diverges and becomes reproductively isolated. A population may diverge and become reproductively isolated due to a geographic barrier such as a newly formed mountain range or river or due to the migration of a population into a new area. Geographic isolation is not always necessary for speciation.

Speciation due to Geographic Isolation When a population is divided and reproductively isolated, if enough time passes, the two groups of organisms will no longer be able to reproduce due to genetic variations as a result of meiosis, and are therefore considered to be two new species.

Co-evolution Many species evolve in close relationships with other species. This is known as co-evolution. In some cases, two species will co-evolve to maintain a mutualistic relationship. In other cases a co-evolution arms race will occur, whereby each species is continually evolving to defend itself from the other.