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The principal forces of evolution in prokaryotes and their effects on archaeal and bacterial genomes. The horizontal line shows archaeal and bacterial genome size on a logarithmic scale (in megabase pairs) and the approximate corresponding number of genes (in parentheses).The effects of the main forces of prokaryotic genome evolution are denoted by triangles that are positioned, roughly, over ...
Large-scale gene transfer has also occurred between the ancestors of eukaryotic cells and prokaryotes, during the acquisition of chloroplasts and mitochondria. [48] Gene flow is the transfer of alleles from one population to another. Migration into or out of a population may be responsible for a marked change in allele frequencies.
Genetic variability is either the presence of, or the generation of, genetic differences. It is defined as "the formation of individuals differing in genotype , or the presence of genotypically different individuals, in contrast to environmentally induced differences which, as a rule, cause only temporary, nonheritable changes of the phenotype ."
Genetic changes include mutations, which are caused by damage or replication errors in organisms' DNA. As the genetic variation of a population drifts randomly over generations, natural selection gradually leads traits to become more or less common based on the relative reproductive success of organisms with those traits.
The rate of evolution is quantified as the speed of genetic or morphological change in a lineage over a period of time. The speed at which a molecular entity (such as a protein, gene, etc.) evolves is of considerable interest in evolutionary biology since determining the evolutionary rate is the first step in characterizing its evolution. [1]
The evolution of biological complexity is one important outcome of the process of evolution. [1] Evolution has produced some remarkably complex organisms – although the actual level of complexity is very hard to define or measure accurately in biology, with properties such as gene content, the number of cell types or morphology all proposed as possible metrics.
There are two different ways in which adaptation can arise in experimental evolution. One is via an individual organism gaining a novel beneficial mutation . [ 2 ] The other is from allele frequency change in standing genetic variation already present in a population of organisms. [ 2 ]
The fact that levels of genetic diversity vary much less than population sizes do is known as the "paradox of variation". [66] While high levels of genetic diversity were one of the original arguments in favor of neutral theory, the paradox of variation has been one of the strongest arguments against neutral theory.