The neutral theory explains how many genetic differences arise and spread without improving or reducing an organism’s ability to survive and reproduce. It is important because it distinguishes changes caused by random processes from changes favored by natural selection.
A mutation is a change in genetic material. A mutation is neutral when it does not affect an organism’s fitness—its ability to survive and reproduce. A neutral allele may produce no observable phenotype, or it may produce a visible change that has no effect on fitness. For example, a change in a protein may leave the protein’s function, and therefore the organism’s reproductive success, essentially unchanged.
All flashcards
Neutral theory of molecular evolution
The view that drift of selectively neutral mutations accounts for many genetic differences within and between species.
Functional constraint
A limitation on genetic changes that can be tolerated because a sequence or structure performs an important role.
Infinite sites model
A model in which the supply of new neutral mutations and their fixation probability yield a substitution rate equal to the neutral mutation rate per gene copy.
Nearly neutral mutation
A mutation with a small fitness effect whose fate depends on the balance between selection and genetic drift.
Constructive neutral evolution
A process in which neutral changes create dependencies that can contribute to biological complexity without each step being favored for improved function.
Quiz questions
What does weaker functional constraint generally predict about the rate of sequence divergence?Answer: A higher rate, because a greater fraction of changes can be tolerated.
In the basic infinite sites model, what is the neutral substitution rate per gene copy?Answer: k=μ, independent of population size.
How can a slightly deleterious mutation behave in a small population under nearly neutral theory?Answer: Drift may outweigh weak selection, allowing its frequency to change substantially by chance.
In constructive neutral evolution, how can an initially unnecessary interaction become essential?Answer: A later change impairs independent function, leaving the interaction to preserve the system’s function.
Which pattern is consistent with slightly deleterious changes accumulating over time?Answer: The nonsynonymous-to-synonymous substitution ratio is often greater between species than within species.