Evolution

When used in a biological context, the word evolution refers to the change in the population of a species' inherited traits from generation to generation. These traits are the expression, or "phenotype", of genes which, at least on earth, are stored in DNA. All species on earth are believed have come about by the mechanism of evolution. Evolution occurs as random genetic changes build up over time and those that prove to be negative, or harmful, are weeded out as the organisms that have them are rendered unable to compete with the general population. Inversely, the organisms that had received beneficial alterations to their genes would be more suited for survival and thus more likely to live long enough to pass on their genes to another generation.

Origins of the Theory

The theory of evolution was first proposed in 1859, by Charles Darwin in his book On the Origin of Species. A somewhat similar theory had been porposed by Lamark, but was quite different in that it said an organism could acquire traits during its lifetime and pass them on to its offspring.

Patterns in Nature

The field of evolutionary biology seeks to provide explanations for four conspicuous patterns that are manifest in nature. The first three concern living species, whereas the fourth relates to fossils.

Genetic variation

Genes are linked to how organisms look and behave.

There is tremendous genetic diversity within almost all species, including humans. No two individuals have the same DNA sequence, with the exception of identical twins or clones. This genetic variation contributes to phenotypic variation - that is, diversity in the outward appearance and behavior of individuals of the same species.

Adaptation

Organisms must adapt to their environment to survive.

Living organisms have morphological, biochemical, and behavioral features that make them well adapted for life in the environments in which they are usually found. For example, consider the hollow bones and feathers of birds that enable them to fly, or the cryptic coloration that allows many organisms to hide from their predators. These features may give the superficial appearance that organisms were designed by a creator (or engineer) to live in a particular environment. Evolutionary biology has demonstrated that adaptations arise through selection acting on genetic variation.

Divergence

Species evolved along different paths from a common ancestor.

All living species differ from one another. In some cases, these differences are subtle, while in other cases the differences are dramatic. Carl Linnaeus (1707-1778) proposed a classification that is still used today with slight changes. In the modern scheme, similar species are grouped into genera, similar genera into families, and so on. This hierarchical pattern of relationship produces a tree-like pattern, which implies a process of splitting and divergence from a common ancestor.

Mechanisms of Evolution

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.

Offspring with genetic mutations are different from their parents.

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.

Genes can be shuffled between organisms.

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.

Not all mutations become fixed in a population.

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.

Natural selection guarantees that the fittest are most likely to pass on their genes.

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.

Evidence for Evolution

Main article: The incontrovertible evidence of common descent

Fossil record

Main article: Fossil record

Section in progress

Similarities of embryos

Main article: Similarities of embryos

Section in progress

Distribution of related species

Main article: Distribution of related species

Section in progress

Observed genetic adaptation

Main article: Observed genetic adaptation

Section in progress

See also

• Natural Selection
• The incontrovertible evidence of common descent
• Disproving Evolution
• Young Earth Creationism
• Intelligent Design
• Vandalism to the this page

References

1. Wile, Jay L. Exploring Creation With General Science. Anderson: Apologia Educational Ministries, Inc. 2000
2. Understanding Evolution