Speciation and Natural Selection
What is a species? A species is a group of organisms that are theoretically capable of mating to produce viable offspring. We say "theoretically," because members of a species might be geographically isolated (on an island, for example) and yet, if they were able to get together, they could successfully mate. A "population" is a subset of a species, and is comprised of organisms of the same species but which live in close proximity and interact with each other, and therefore have an actual chance of mating. When members of different species attempt to interbreed, the result is usually complete failure. A species, by definition, is reproductively isolated from other species. Occasionally, with closely related species, a sterile hybrid can form. Horses and donkeys are two different, but closely related, species. Horses and donkeys can interbreed, but their offspring, mules, are sterile.
Speciation: The big debate in Darwin's time was over whether there was a fixed number of species and whether species were unchanged throughout history. Newly discovered fossils revealed that some modern species could not be found beyond a certain point in the past. In other cases, the fossils looked somewhat different or dramatically different from any modern form. Finally, there was evidence that some species found in the fossil record, such as the dinosaurs, had gone extinct. While the Great Flood could explain extinctions, the fossil record was fairly conclusive evidence that species are not fixed; they could change over time. During his voyage as ship's naturalist aboard H.M.S. Beagle, Darwin gradually became convinced that the various species of finches of the Galapagos Islands, which didn't closely resemble other birds but were very similar to each other, had originated from a common ancestor. Darwin developed his theory of natural selection to explain how a population of organisms could change over time, gradually resulting in the formation of new species.
- There is individual variation among the members of a species.
- At least some of this variation is heritable, because offspring tend to resemble their parents.
- Because of these inherited advantages and disadvantages, some individuals are better suited for survival than others.
- Organisms can produce more offspring than are necessary to replace themselves.
- Because of the scarcity of resources, there is a competitive struggle for existence.
- Those individuals with advantageous traits are more likely to survive and reproduce.
- The survivors pass down these advantageous traits to their offspring.
- Nature (the environment in which a species lives) selects who lives longer and produces more offspring, and who dies early and produces fewer offspring.
- The appearance of a species can change over generations as disadvantageous traits are eliminated and advantageous traits become more common.
- As a species colonizes diverse environments (hotter or colder, wetter or drier, for example), each population would be selected for traits advantageous in that particular environment.
- Over time, these separate populations would become visibly distinct.
- Populations of a species that are isolated from one another for long enough may accumulate enough differences that interbreeding, when they later come into contact with one another, is impossible or less successful than matings with one's own "type."
- Interbreeding between these distinct types is disadvantageous so those that develop behaviors to avoid interbreeding are favored, creating greater isolation.
- Once hybridization slows, the two sub-species become even more differentiated because there is no further mixing of genes, and one species becomes two.
Evolution: The word evolution is only referenced once in Darwin's Origin of Species. That term is found in the final sentence, where he states that "endless forms most beautiful and most wonderful have been, and are being evolved." He concludes that there is overwhelming evidence that species have changed over time and that new species can arise from old ones. In the most general sense, that is what Darwin meant by "evolution."