Biological Evolution in a Nutshell
Theory of Evolution
Darwin—among others—observed that every creature is unique. Each organism has its own set of attributes that are transmitted to offspring. These lead to unique children, but with common features to the parents. Humans have managed to harness these changes by selective breeding (for instance, by mating pets that have desirable attributes).
Evolution is the result of nature's effect on this process, causing entire species that change slowly. According to Darwin, evolution is a consequence of the universal struggle for life. There are three major forms of pressure on each individual:
Darwin suggested that such constraints affect the behavior of each and every creature, as well as its chances of survival. Creatures unsuccessful in different aspects of life (for instance, hunting or mating) have fewer chances of having offspring. On the other hand, slightly more effective creatures are more likely to transmit their attributes to the next generation. This is the "preservation of favorable individual differences."
This is made possible by the fact that breeding happens relatively quickly; therefore, there are more creatures born that can survive. The constraints of the environment imply that the fittest survive. This is known as natural selection.
Natural selection happens on a short time scale—a daily or even hourly basis. However, evolution is a very slow progress, requiring generations for favorable attributes to spread throughout the species.
What makes evolution possible is the underlying biology of reproduction, allowing creatures to transmit beneficial attributes to their offspring. This happens on a cellular basis with the genetic code contained within chromosomes. Let's take humans as an example.
Each cell has 23 pairs of chromosomes. The chromosomes can be seen as the structure carrying the genetic code. Reproductive cells (sperm and ova) have just 23 single chromosomes. One normal cell divides into two reproductive cells during a phase called meiosis. Before the cell splits, the chromosomes pairs cross over: Their genetic code is mixed up a bit. Each reproductive cell has genes from both members of a chromosome pair.
During fertilization of the ova by the sperm, these chromosomes recombine together to reform the 23 pairs. The resulting cell is the first cell of the offspring, and contains half the genetic code from the father and half from the mother.
The entire process is extremely complex, because many different molecules are involved. In some cases, mutation can happen. Mutation is an error in copying the genetic code, either at the gene level or at the chromosome level. This can turn out beneficial in some cases, but often causes irrevocable problems in the reproduction.
Evolution can be seen as an optimization process. The attributes of the creatures are optimized over time with respect to the environmental constraints. John Holland introduces these ideas in his book Adaptation in Natural and Artificial Systems [Holland75]. Computer scientists have managed to simulate evolution and apply these ideas to optimize problems such as genetic algorithms.
The genetic code at the roots of most life forms on earth makes evolution possible, by allowing successful attributes to be transferred to the next generation. For genetic algorithms, representation is also the most crucial issue.