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An organism is only as "evolutionarily strong" as it's "weakest" gene, regardless of how great other genes might be - since an organism either passes on half or more of it's genes, or none at all.
Right - and through this mechanism weak genes will be selected against, and the stronger ones will be selected for. Sure, some strong genes are lost in those phenotypes - but there are hopefully plenty of copies still in existence. We'd expect the proportion of good genes to go up in the population compared with the bad ones. There is a positive selective pressure on the good ones, and a negative pressure on the bad ones.
One thing that is easy to miss here (simply because we are humans who didn’t evolve to deal with massive parallel experiments), is the fact that even a small advantage can cause a single gene to increase in frequency, despite random factors and it’s being always included in an aggregate of alleles.
Now, that was hard to understand. Let me explain. As a scientist, I often do large “design optimized experiments”, where many variables (say, 8) are run in many (say, 160 of them) experiments. Then the data from all those experiments is statistically calculated to see if any of the variables shows an effect on the outcome. If even a small effect is present, it often shows up in this process (called “linear regression”).
So what does that have to do with alleles? It means that if a gene is present in hundreds of individuals (which it will be within a few generations, unless it is very strongly selected against), then it can slightly affect the reproductive success wherever it appears. If this effect is very small - say just a 5% chance of producing one more offspring than an individual lacking the gene, then it still has an effect over those hundreds of cases. Now, multiply that effect by hundreds of more generations, each with hundreds of individuals, and it becomes a huge effect, just as lightly pushing an asteroid will eventually radically change it’s orbit.
It’s easy for us, as humans looking at a few individuals in a few generations, to forget this. This shows why the fact that alleles are always in groups of alleles isn’t really that important, and why it can be misleading to focus too much on the fact that an organism is only as strong as it’s weakest allele.
Have a great day everyone-
™ Version 4.2
