page_title: Reductive evolution
section_title: Endosymbiotic theory
hierarchical_section_title: Reductive evolution / Endosymbiotic theory
caption_attribution_description: English: Graph of variation in estimated genome sizes in base pairs
context_page_description: Reductive evolution is the process by which microorganisms remove genes from their genome. It can occur when bacteria found in a free-living state enter a restrictive state or are completely absorbed by another organism becoming intracellular. The bacteria will adapt to survive and thrive in the restrictive state by altering and reducing its genome to get rid of the newly redundant pathways that are provided by the host. In an endosymbiont or symbiogenesis relationship where both the guest and host benefit, the host can also undergo reductive evolution to eliminate pathways that are more efficiently provided for by the guest.
context_section_description: Reductive evolution is the basis behind the Endosymbiotic Theory, which states that Eukaryotes absorbed other microorganisms (Eukaryotes and archaea) for their metabolites produced. The absorbed organisms undergo reductive evolution, deleting genes that were deemed nonessential or non-beneficial to the cell in its specific niche in the host. When comparing fossil evidence reductive evolution can be demonstrated.
  DNA found in ancient prokaryotic and mitochondria fossils have been found to have higher levels of cytosine and guanine compared to the DNA found in the same organism today. Different segments of the genome found to be unfavorable have possibly been removed over time due to deletions of DNA causing the genome to be reduced. The amount of cytosine and guanine in an organisms genome is a direct correlation to the overall size of that genome.  
  The genome can become more complex or simplified due to random mutations.
  Chlorella is a secondary endosymbiont that lives within Paramecium species and is an example of obligate intracellular reductive evolution. Moranella is a double membrane gram-negative-like bacteria that lives in another endosymbiont, "Candidatus Tremblaya", which itself lives in the mealy bug.
caption: Genome Sizes of Various Organisms, displaying the reduction of genomes over time to remove inessential genes.
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