I am reading the book 'Homo Deus: A breief History of Humankid' by Harari. He mentions the following statement highlighted in the picture below (picture provided for context). I would be appreciated if someone could (1) explain the meaning of it (I have very little background in Biology) and (2) the reason behind it.
Mutations happen all the time when DNA is replicated. These random mutations are the building blocks for evolution under natural selection. So when a single nucleotide is changed, (or deleted or added), there are three possible outcomes. 1. the mutation is neutral, so nothing happens. 2. The mutation is lethal or mal-adaptive for the organism, in which case natural selection will remove the individuals carrying the mutation from the population, or 3. the mutation results in some change in function that helps the organism compete under natural selection. This 3rd kind is the rarest, but with pathogenic viruses and bacteria, with huge populations, the evolutionary test pool is much larger because there are so many new mutations being put to the test with every generation. So, for example, a mutation in a pathogen adapted to spread through chickens, could change the pathogen to be better adapted for spreading through humans.
the mutation results in some change in function that helps the organism compete under natural selection. A) associating change of function with beneficial consequence, not not talking about a change in function in other cases is misleading. Change of function can go both ways. B) summarizing "higher fitness" to "helps the organism compete under natural selection" is misleading because 1) fitness differential does not necessarily require competition 2) NS is not a condition under which ...
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evolutionary test pool are very much undefined. The expression random mutations can also be very misleading (see the post Are mutations random?)
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These random mutations are the building blocks for evolution under natural selection. Mutations are the ultimate source of genetic variance. It fuses evolution whether there is a selective pressure or not.
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adapted to spread through chickens, could change the pathogen to be better adapted for spreading through humans I understand that this goes along the example of the text given by the OP but, IMHO, I think it would be good to avoid giving the misrepresentation to the OP that evolution for a pathogen is an event that necessarily allows a pathogen to jump from one host to the other and that all other evolutionary events would be of no interest or even would not be part of evolution.
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