Symbiogenesis refers to the merging of two separate organisms to form a single new organism. The idea originated with Konstantin Mereschkowsky in his 1926 book Symbiogenesis and the Origin of Species, which proposed that chloroplasts originate from cyanobacteria captured by a protozoan. Today both chloroplasts and mitochondria are believed to have such an origin; this is the endosymbiotic hypothesis. Konstantin Sergivich Merezhkovsky (1855-1921) (also transliterated Konstantin Sergeevich Merezhkovsky, Constantin Sergeevič Mérejkovski, Constantin Sergejewicz Mereschcowsky, Konstantin Sergejewicz Mereschkovsky and Konstantin Sergejewicz Mereschkowsky) was a prominent Russian biologist and botanist active mainly around Kazan, whose research on lichens led him to propose the theory of symbiogenesis - that larger, more complex... Orders The taxonomy of the Cyanobacteria is currently under revision. ... Protozoa (in Greek protos = first and zoon = animal) are single-celled eukaryotes (organisms with nuclei) that show some characteristics usually associated with animals, most notably mobility and heterotrophy. ... The inside of a chloroplast Chloroplasts are organelles found in plant cells and eukaryotic algae that conduct photosynthesis. ... Mitochondria are visible as thread-like structures in the light microscope. ... The endosymbiotic theory concerns the origins of mitochondria and chloroplasts, which are organelles of eukaryotic cells. ...

In Acquiring Genomes: A Theory of the Origins of Species, Lynn Margulis argued that symbiogenesis is a primary force in evolution. According to her theory, acquisition and accumulation of random mutations are not sufficient to explain how inherited variations occur; rather, new organelles, bodies, organs, and species arise from symbiogenesis. Whereas the classical interpretation of evolution (neo-Darwinism) emphasizes competition as the main force behind evolution, Margulis emphasizes cooperation. Lynn Margulis. ... A phylogenetic tree of all living things, based on rRNA gene data, showing the separation of the three domains, bacteria, archaea, and eukaryotes, as described initially by Carl Woese. ... In biology, mutations are permanent, sometimes transmissible (if the change is to a germ cell) changes to the genetic material (usually DNA or RNA) of a cell. ... Schematic of typical animal cell, showing subcellular components. ... In biology, an organ (Latin: organum, instrument, tool) is a group of tissues, which perform a specific function or group of functions. ... The modern evolutionary synthesis (often referred to simply as the modern synthesis or the evolutionary synthesis), neo-Darwinian synthesis or neo-Darwinism, generally denotes the combination of Charles Darwins theory of the evolution of species by natural selection, Gregor Mendels theory of genetics as the basis for biological...

Many ecologists agree, but this idea has little support from other evolutionary biologists. They see little evidence that symbiogenesis has had a major impact on eukaryotic life, or that much of its diversification can be attributed to it. Other than the two examples of mitochondria and chloroplasts, there is no clear evidence of other major traits or transitions that can be attributed to symbiogenesis.

It is a fundamental principle of classical neo-Darwinism, or population genetics theory, that mutations arise one-at-a-time and either spread through the population or not, depending on whether they offer an individual fitness advantage. There is a major body of scientific work, both theoretical and experimental, based on this paradigm. Those who have worked in the field tend to regard its foundation as unassailable. Population genetics is the study of the distribution of and change in allele frequencies under the influence of the five evolutionary forces: natural selection, genetic drift, mutation, migration and nonrandom mating. ...

Nevertheless, the neo-Darwinist perspective remains vulnerable to challenges like that of Margulis because its experimental support comes overwhelmingly from the laboratory, not from the wild. We understand clearly how artificial selection works in the laboratory, but there is legitimate controversy over whether nature's laboratory works in just this way. Indeed, genome mapping techniques have revealed that family trees of the major taxa appear to be extensively cross-linked—possibly due to lateral transfer of genes carried by bacteria, as Margulis predicted. This Chihuahua mix and Great Dane show the wide range of dog breed sizes created using artificial selection. ...