The evolution of biological complexity is one important outcome of the process of evolution. 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.
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. This idea of "progression" and "higher organisms" in evolution is now regarded as misleading, with natural selection
having no intrinsic direction and organisms selected for either
increased or decreased complexity in response to local environmental
conditions. Although there has been an increase in the maximum level of complexity over the history of life, there has always been a large majority of small and simple organisms and the most common level of complexity appears to have remained relatively constant.
Selection for simplicity and complexity
Usually
organisms that have a higher rate of reproduction than their
competitors have an evolutionary advantage. Consequently, organisms can
evolve to become simpler and thus multiply faster and produce more
offspring, as they require fewer resources to reproduce. A good example
are parasites such as Plasmodium - the parasite responsible for malaria - and mycoplasma; these organisms often dispense with traits that are made unnecessary through parasitism on a host.
A lineage can also dispense with complexity when a particular
complex trait merely provides no selective advantage in a particular
environment. Loss of this trait need not necessarily confer a selective
advantage, but may be lost due to the accumulation of mutations if its
loss does not confer an immediate selective disadvantage. For example, a parasitic organism
may dispense with the synthetic pathway of a metabolite where it can
readily scavenge that metabolite from its host. Discarding this
synthesis may not necessarily allow the parasite to conserve significant
energy or resources and grow faster, but the loss may be fixed in the
population through mutation accumulation if no disadvantage is incurred
by loss of that pathway. Mutations causing loss of a complex trait occur
more often than mutations causing gain of a complex trait.
With selection, evolution can also produce more complex
organisms. Complexity often arises in the co-evolution of hosts and
pathogens, with each side developing ever more sophisticated adaptations, such as the immune system and the many techniques pathogens have developed to evade it. 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.
More generally, the growth of complexity may be driven by the co-evolution between an organism and the ecosystem of predators, prey and parasites
to which it tries to stay adapted: as any of these become more complex
in order to cope better with the diversity of threats offered by the
ecosystem formed by the others, the others too will have to adapt by
becoming more complex, thus triggering an ongoing evolutionary arms race towards more complexity. This trend may be reinforced by the fact that ecosystems themselves tend to become more complex over time, as species diversity increases, together with the linkages or dependencies between species.
Types of trends in complexity
Passive
versus active trends in complexity. Organisms at the beginning are red.
Numbers are shown by height with time moving up in a series.
If evolution possessed an active trend toward complexity (orthogenesis), as was widely believed in the 19th century, then we would expect to see an active trend of increase over time in the most common value (the mode) of complexity among organisms.
However, an increase in complexity can also be explained through a passive process.
Assuming unbiased random changes of complexity and the existence of a
minimum complexity leads to an increase over time of the average
complexity of the biosphere.This involves an increase in variance,
but the mode does not change. The trend towards the creation of some
organisms with higher complexity over time exists, but it involves
increasingly small percentages of living things.
In this hypothesis, any appearance of evolution acting with an
intrinsic direction towards increasingly complex organisms is a result
of people concentrating on the small number of large, complex organisms
that inhabit the right-hand tail
of the complexity distribution and ignoring simpler and much more
common organisms. This passive model predicts that the majority of
species are microscopic prokaryotes, which is supported by estimates of 106 to 109 extant prokaryotes compared to diversity estimates of 106 to 3·106 for eukaryotes. Consequently, in this view, microscopic life dominates Earth, and large organisms only appear more diverse due to sampling bias.
Genome complexity has generally increased since the beginning of the life on Earth. Some computer models have suggested that the generation of complex organisms is an inescapable feature of evolution.
Constructive neutral evolution
Recently work in evolution theory has proposed that by relaxing selection pressure, which typically acts to streamline genomes, the complexity of an organism increases by a process called constructive neutral evolution. Since the effective population size in eukaryotes (especially multi-cellular organisms) is much smaller than in prokaryotes, they experience lower selection constraints.
According to this model, new genes are created by non-adaptive processes, such as by random gene duplication.
These novel entities, although not required for viability, do give the
organism excess capacity that can facilitate the mutational decay of
functional subunits. If this decay results in a situation where all of
the genes are now required, the organism has been trapped in a new state
where the number of genes has increased. This process has been
sometimes described as a complexifying ratchet. These supplemental genes can then be co-opted by natural selection by a process called neofunctionalization.
In other instances constructive neutral evolution does not promote the
creation of new parts, but rather promotes novel interactions between
existing players, which then take on new moonlighting roles.
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, and how pervasive pan-RNA editing may have arisen in Trypanosoma brucei.
History
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.
This view regarded the evolution of parasites from independent organisms to a parasitic species as "devolution"
or "degeneration", and contrary to nature. Social theorists have
sometimes interpreted this approach metaphorically to decry certain
categories of people as "degenerate parasites". Later scientists
regarded biological devolution as nonsense; rather, lineages become
simpler or more complicated according to whatever forms had a selective
advantage.
