The modern theory of evolution is the result of such a process. The theory (and its assumptions and corollaries) has been tested extensively by scientists from many disciplines since it was first proposed by Darwin and Wallace in 1858[#](Darwin & Wallace 1858). As a result, evolutionary theory is a synthesis, incorporating facts from many fields in biology, including geneticsThe study of heredity and variation in living organisms., systematicsThe study of phylogenies and the evolutionary relationships among living things. , population biologyThe study of populations of organisms, especially in terms of demography (life history, birth and death rates)., molecular biologyThe branch of biology that studies the structure and activity of molecules essential to life, in particular the biochemistry of DNA., paleontologyThe study of prehistoric life forms on Earth through the examination of fossils., and physiologyThe study of the mechanical, physical, and biochemical functions of living organisms..

Perhaps the most important modification to the theory of evolution came from the fields of genetics and molecular biology. Darwin and Wallace knew that offspring inherited traits from the parents, but did not know the mechanism. At the time, Mendel’s work was unknown to them (and nearly everyone else). What’s more, Darwin thought that acquired characters (changes to an individual as it grows, such as the loss of a tail to a predator) could be passed on to offspring.

Over the next few decades, and after the (re)discovery of Mendel’s work, others modified Darwin’s original concept, determining that acquired characters could not be passed on. During the period around 1930 - 1950, the theory of evolution was further elaborated[#](Dobzhansky 1937)(Fisher 1930)(Mayr 1942)(Simpson 1944), as evolutionary scientists uncovered how genetic and phenotypic variation is generated, and showed that sexual reproduction (recombination) creates in every generation a new, variable population of individuals for natural selection to act on.

The discovery of the chemical structure of DNA[#](Watson & Crick 1953) was especially important because it provided a copying mechanism for the genetic material, and opened a very productive field of study. For example, molecular geneticists subsequently learned that changes in the base-pair composition of DNA are translated into changes in protein structure or developmental regulations, and that no change in a protein or other cellular constituents other than nucleic acids can alter the information encoded in DNA[#](Watson, et al. 1994).

The “modern synthesis” of evolutionary biology represents a sort of consensus that forms the core of the modern theory of biological evolution. There is still some healthy scientific debate about some of the details (e.g., is evolution slow and steady or is it punctuated with periods of rapid change?), but nearly every life-scientist accepts evolution as fact.

• Natural selection acts on individuals, but individuals do not evolve. Evolution occurs within a population of organisms that breed with one another. The evolutionary transitions in these populations are usually gradual - new species evolve from preexisting varieties by slow processes and maintain at each stage their specific adaptation. There are some exceptions to this general rule. Immigration of individuals between populations can prevent evolutionary divergence.
  

• Genetic and phenotypic variability in plant and animal populations is brought about by independent assortment, genetic recombination and random mutations that occur in the production of gametes. The amount of genetic variation that a population of sexually reproducing organisms can produce is enormous.
  

• Phenotypic traits are the outward expression of genes.  Genes are segments of the DNA molecule, which are translated into proteins which in turn regulate cell functioning, metabolic and developmental processes, growth and behavior.  DNA is the molecule of inheritance.  Gametes (sex cells) contain parental DNA.  After fertilization the offspring's cells contain a combination of parental genes, which determines the offspring's phenotype.  No change in a protein or cellular constituents other than nucleic acids can alter the information encoded in DNA.
  

• Natural selection is the most important force that shapes the course of phenotypic evolution in most taxa. It causes a shift in the population towards a novel phenotype that is better adapted to altered environmental conditions. Other phenomena that can cause evolution include polyploidy and, in small populations, genetic drift (random loss of genes from the gene pool).
  

• Speciation is typically defined as the point in evolution at which differences between forms become profound enough to prevent successful mating. A number of pre- and post-mating isolation mechanisms have been identified. Allopatric speciation (divergence of populations that are geographically isolated from each other) likely accounts for the origin of many species. However, sympatric speciation (the occurrence of new species without geographic isolation) is also documented.
  

• Macroevolution is a gradual step-by-step-process that is simply an extrapolation of microevolution.

Kutschera, U & KJ Niklas. 2004. The modern theory of biological evolution: an expanded synthesis. Naturwissenschaften 91:255–276

Gould, SJ. 1994. The evolution of life on earth. Scientific American. 271: 85-91