Evolution of biological complexity
[2][3][4] Many biologists used to believe that evolution was progressive (orthogenesis) and had a direction that led towards so-called "higher organisms", despite a lack of evidence for this viewpoint.
Consequently, organisms can evolve to become simpler and thus multiply faster and produce more offspring, as they require fewer resources to reproduce.
For example, the parasite Trypanosoma brucei, which causes sleeping sickness, has evolved so many copies of its major surface antigen that about 10% of its genome is devoted to different versions of this one gene.
This tremendous complexity allows the parasite to constantly change its surface and thus evade the immune system through antigenic variation.
The trend towards the creation of some organisms with higher complexity over time exists, but it involves increasingly small percentages of living things.
[15][16] Consequently, in this view, microscopic life dominates Earth, and large organisms only appear more diverse due to sampling bias.
[24] Since the effective population size in eukaryotes (especially multi-cellular organisms) is much smaller than in prokaryotes,[25] they experience lower selection constraints.
These novel entities, although not required for viability, do give the organism excess capacity that can facilitate the mutational decay of functional subunits.
[26] Constructive neutral evolution has also been used to explain how ancient complexes, such as the spliceosome and the ribosome, have gained new subunits over time, how new alternative spliced isoforms of genes arise, how gene scrambling in ciliates evolved, how pervasive pan-RNA editing may have arisen in Trypanosoma brucei, how functional lncRNAs have likely arisen from transcriptional noise, and how even useless protein complexes can persist for millions of years.
Citrullus lanatus is smaller, has more introns and duplications, while Cucurbita pepo is larger with more chloroplast and short repeated sequences.
[41] There was also an attempt to use the hypothesis to explain large nuclear genomes of salamanders, but researchers found opposite results than expected, including lower long-term strength of genetic drift.
[42] In the 19th century, some scientists such as Jean-Baptiste Lamarck (1744–1829) and Ray Lankester (1847–1929) believed that nature had an innate striving to become more complex with evolution.
This belief may reflect then-current ideas of Hegel (1770–1831) and of Herbert Spencer (1820–1903) which envisaged the universe gradually evolving to a higher, more perfect state.
Later scientists regarded biological devolution as nonsense; rather, lineages become simpler or more complicated according to whatever forms had a selective advantage.
In 1985, Morowitz[46] noted that the modern era of irreversible thermodynamics ushered in by Lars Onsager in the 1930s showed that systems invariably become ordered under a flow of energy, thus indicating that the existence of life involves no contradiction to the laws of physics.
