Editing Quasispecies model (section)

===Quasispecies, fitness, and evolutionary selection===
In a species, though reproduction may be mostly accurate, periodic mutations will give rise to one or more competing genotypes.  If a mutation results in greater replication and survival, the mutant genotype may out-compete the parent genotype and come to dominate the species.  Thus, the individual genotypes (or species) may be seen as the units on which selection acts and biologists will often speak of a single genotype's [[fitness (biology)|fitness]].<ref>{{cite web|url=https://www.khanacademy.org/science/biology/her|title=Evolution and the tree of life {{!}} Biology {{!}} Science|website=Khan Academy|language=en|access-date=2019-02-20}}</ref>

In a quasispecies, however, mutations are ubiquitous and so the fitness of an individual genotype becomes meaningless: if one particular mutation generates a boost in reproductive success, it can't amount to much because that genotype's offspring are unlikely to be accurate copies with the same properties.  Instead, what matters is the ''connectedness'' of the cloud.  For example, the sequence AGGT has 12 (3+3+3+3) possible single point mutants AGGA, AGGG, and so on.  If 10 of those mutants are viable genotypes that may reproduce (and some of whose offspring or grandchildren may mutate back into AGGT again), we would consider that sequence a well-connected node in the cloud.  If instead only two of those mutants are viable, the rest being lethal mutations, then that sequence is poorly connected and most of its descendants will not reproduce. The analog of fitness for a quasispecies is the tendency of nearby relatives within the cloud to be well-connected, meaning that more of the mutant descendants will be viable and give rise to further descendants within the cloud.<ref>{{cite journal | first = Francis | last = Heylighen | url=http://pespmc1.vub.ac.be/books/Complexity-Evolution-old.pdf |title=Complexity and Evolution | journal = Entropy | date = 2023 | volume = 25 | issue = 2 | page = 286 | doi = 10.3390/e25020286 | doi-access = free | pmid = 36832653 | pmc = 9955364 | bibcode = 2023Entrp..25..286V | quote = Lecture notes 2014-2015 }}</ref>

When the fitness of a single genotype becomes meaningless because of the high rate of mutations, the cloud as a whole or quasispecies becomes the natural unit of selection.