Mutation Day 2024 is on Sunday, September 29, 2024: How neutral mutations can be important in the evolution of populations?

Sunday, September 29, 2024 is Mutation Day 2024. TMNT Master - HAPPY MUTATION DAY!! Teenage Mutant Ninja... HAPPY MUTATION DAY!

Neutral mutations are neither harmful nor beneficial.

Therefore, they are invisible to natural selection. (Since they neither improve nor worsen one individual's chances of survival and reproduction over another.)

However neutral mutations can still spread into the population by just random replications and matings. This is called genetic drift.

In other words, they are 'silent'. They are mutations that exist and propagate in populations, but seem to have no effect at all.

The reason they can become important to evolution is that a day can come when they *do* have an effect. In other words, even though an individual mutation may have no immediate effect on survival or reproduction, a *combination* of neutral mutations may provide some new benefit or harm ... at which point natural selection *will* act on that combination.

These mutations may be

(a) additional mutations on the same gene that occur generations apart; or

(b) separate mutations on different genes that occur generations apart; or

(c) separate mutations on different genes that occur in different parts of the population, but get combined when carriers of these separate mutations meet and mate.

Neutral mutations thus become important because they can exist "silently" in populations for thousands of generations, with significant percentages (10%, 20%) being carriers without any effects. But when two or more such neutral mutations are combined in the same individuals, then if this provides some benefit, then this combination can spread *rapidly* into the population (because they are already held by significant percentages).

An example of (a) is gene duplication. This is a kind of mutation where a gene gets duplicated on the same or a different chromosome as the original gene. The individual now has two copies of the same gene. These two copies may be identical, or may have slight diffferences that have no immediate effect (neutral). This can be harmful (producing an excess of some protein), or beneficial (producing more of a beneficial protein), or neutral (producing a redundant copy of a protein, which is regulated by other proteins so that the individuals with and without the mutation have no apparent difference).

But generations later, a *second* mutation on one of those copies may result in new properties of that second protein. The result is that the individual now gets the benefit of having two proteins with slightly different functions.

An example of gene duplication in action is the three-color vision we find in apes (including humans) and Old World monkeys (the monkeys of Africa and Asia). Genetically this is due to two genes on the same chromosome (the X-chromosome) that have all the markers of being the result of a gene duplication. That gene duplication by itself would be neutral. Having two copies of a gene that codes for the exact same pigment in the eye is neither harmful nor beneficial.

But one of those duplicates ended up with a *second* mutation that changed the peak wavelength that the protein responds to. By itself this mutation may also have been neutral (changing the peak wavelength providing no benefit), or it may even have been harmful (because it would have lost the ability to distinguish the original wavelength). But the fact that it occurred on a *duplicate* copy means that individuals ended up with the ability to distinguish colors that those without this configuration were unable to distinguish. This provided *benefit*, and natural selection took over ... to the point where most humans have this same three-color vision.

What are mutations and how are they caused?

Mutations happen every day. It just so happens that most are caught or they do not effect the body. Mutations are just a change in the base sequence of DNA in a gene: these mutations you are referring to are probably ones that are significant enough to have a impact on appearance or function of the organism. Cancer is a great example of a mutation in the human body after birth. Downs Syndrome is an example of a mutation in the genes before birth/during cell division.

Evolution- present day species?

Earlier species splits into distinct populations through mechanisms such as geographical separation. That population has different mutations - variation - arise within it's genome and is subject to different selective pressures, different environment, different competitor species, etc. These selective pressures favour individuals that possess advantageous variations, they have longer necks, or can run faster or have bigger brains or whatever. Those favoured individuals have more offspring than the others and the variation becomes general within the population. The cumulation of these changes leads to a sufficiently distinct genome that when the populations of the original species intermingle they are no longer able to produce fertile offspring. Hence new species.

Now consider that there will be situations where isolation amongst simple and relatively immobile organisms is easier to have, and it requires "fluke" circumstances to transport a breeding population from one environment to another, the classic being rafting Iguanas in the Pacific/Caribbean. Once that population becomes isolated and is offered a "different" environment, or an opening intro another way of life then selection will favour individuals with variations that wouldn't have been selected in the original population or were just neutral in that population.

Cumulative change over time without gene flow between populations.