Although Charles R. Darwin is the most famous individual when learning about evolution, there were many other individuals studying evolution before and since Darwin. His book "On the Origin of Species" published in 1859 helped lead to wide acceptance of the theory of evolution. Much of his research and observations began in 1831 while he was on a 5 year voyage aboard the HMS Beagle. The Galapagos Islands are still great living laboratories to study the process of evolution. Gregor Mendel, another famous scientist in the field of genetics, discovered that traits were inherited in a very predictable manner. Before Mendel discovered how traits were inherited, farmers and herders were breeding for traits, albeit haphazardly. Mendel's discovery led to the understanding of the basic mechanisms of inheritance and dominant and recessive traits.
Evolution has been occurring since the
beginning of life forms on earth, about 3.5 billion years, and is
still occurring today. Evolution is not something out of the past,
but is an ongoing mechanism that occurs in all living organisms.
Life has descended from a common ancestry through a long series of
events throughout the history of life on earth. The life changing
events can be seen on the "tree of life" or the phylogenetic
tree, which is a diagram that shows the evolution of life through
time as mapped by humans.
From the phylogenetic tree you can see that many species have become extinct over time. Extinction is the death of the last living organism of that species. Many extinctions occur due to the loss of habitat (throughout the history of earth the climate has fluctuated). Mass extinctions are considered rare events that are precipitated by epic events. Extinction of one species may lead to extinction of other species, which is called "chain of extinctions". Extinctions have been occurring just about as long as there have been living species, however, the accelerated rate of extinctions caused by humans is alarming to many and many environmental groups and governments have programs to address the problem.
Evolution is the change in inherited traits of a population over time and through generations. A trait is a characteristic such as eye color, skin color, height, or behavior that is expressed by the individual because of their genes. There are two main mechanisms of evolution; they are natural selection and genetic drift. Natural selection is the process where certain genes are favored over other genes. Genetic drift is caused by a random change in population alleles, within a relatively small population the changes may completely eliminate a trait. Natural selection is more predominant in larger populations while genetic drift occurs in smaller populations that may have become isolated from others.
Genetic variation occurs naturally within a population either by natural selection or by mutations. Although most genetic mutations are harmful a small number may be beneficial to a species and are naturally selected for during sexual reproduction. Natural selection tends to favor beneficial mutations that increase an organisms chance of survival, while disfavoring those mutations that do not increase chance for survival.
Evolution allows for adaptation of a species or population to occur and allows them to be ultimately more suited to life in the environment. The adaptation may be the selection of a particular trait or group of traits that allow the species to be better able to survive it's environment. Adaptations are an outcome of natural selection among individuals within a population. A simplified example of adaptation is the emergence of antibiotic resistant strains of bacteria. Antibiotics may work on 99.9% of a bacteria, however, the 0.1% of the bacteria that survive can reproduce making a strain of bacteria resistant to antibiotics.
Human selection for genetic traits has been practiced for hundreds and thousands of years. Farmers and herders breed their stock to produce the best traits. We continue to do so to this day. Plants are bred for insect resistance, drought resistance, etc. Animals are bred to produce bigger eggs, more muscle mass, whiter wool, etc. Although it is not the same as natural selection, in some cases it may have the same results. Man's best friend, the dog, is a textbook example of selective breeding, to the point of registering their dogs as pure bred and setting standards for the breed. Yet all breeds of dogs can reproduce, therefore they are still the same species. Fish breeders also select for traits they want in their guppies, such as long tails, colors, designs, and etc.
This article is intended as a brief overview of evolution. There are entire series of academic textbooks on the topic, there are individual's that spend their entire life studying evolution. Evolutionary biology is an interdisciplinary field consisting of biologists, paleontologists, geologists, and other individuals from other science fields.