It is an incontrovertible fact that organisms have changed, or evolved, during the history of life on Earth……………
Biological evolution results from changes over time in the genetic constitution of species. Genetic changes often, but not always, produce noticeable changes in the appearance or behavior of organisms. Evolution requires both the production of variation and the spread of some variants that replace others.
· Genetic variation arises through two processes, mutation and recombination. Mutation occurs when DNA is imperfectly copied during replication, leading to a difference between a parent’s gene and that of its offspring. Some mutations affect only one bit in the DNA; others produce rearrangements of large blocks of DNA.
· Recombination occurs when genes from two parents are shuffled to produce an offspring, as happens regularly in sexual reproduction. Usually the two parents belong to the same species, but sometimes (especially in bacteria) genes move between more distantly related organisms.
· The fate of any particular genetic variant depends on two processes, drift and selection. Drift refers to random fluctuations in gene frequency, and its effects are usually seen at the level of DNA. Ten flips of a coin do not always produce exactly five heads and five tails; drift refers to the same statistical issue applied to the transmission of genetic variants across generations.
· The principle of natural selection was discovered by Charles Darwin (1809-1882), and it is the process by which organisms become adapted to their environments. Selection occurs when some individual organisms have genes that encode physical or behavioral features that allow them to better harvest resources, avoid predators, and such relative to other individuals that do not carry the same genes. The individuals that have these useful features will tend to leave more offspring than other individuals, so the responsible genes will become more common over time, leading the population as a whole to become better adapted.
Distinct species diverge from one ancestor and can no longer interbreed.
· The process that many people find most confusing about evolution is speciation, which is not a separate mechanism at all, but rather a consequence of the preceding mechanisms played out in time and space. Speciation occurs when a population changes sufficiently over time that it becomes convenient to refer to the early and late forms by different names. Speciation also occurs when one population splits into two distinct forms that can no longer interbreed. Reproductive isolation does not generally happen in one generation; it may require many thousands of generations when, for example, one part of a population becomes geographically separated from the rest and adapts to a new environment. Given time, it is inevitable that two populations that live apart will diverge by mutation, drift, and selection until eventually their genes are no longer compatible for successful reproduction…………
Evolutionary biology is a strong and vigorous field of science. A theoretical framework that encompasses several basic mechanisms is consistent with the patterns seen in nature; and there is abundant evidence demonstrating the action of these mechanisms as well as their contributions to nature. Hence, evolution is both a theory and a set of established facts that the theory explains.
Like every other science, there is scientific debate about some aspects of evolution, but none of these debates appear likely to shake the foundations of this field. There exists no other scientific explanation that can account for all the patterns in nature, only non-scientific explanations that require a miraculous force, like a creator. Such super-natural explanations lie outside of science, which can neither prove nor disprove miracles.
http://www.actionbioscience.org/evolution/lenski.html