Yep, Smidlee, good points. Let's look at them.
First, it seems that we agree that once a beneficial mutation is in a good number of individuals (say, several dozen), that it's average benefit will cause it to spread to the whole population.
The topic now is that earlier stage.
Smidlee wrote:
A beneficial gene must first appear to an individual but it becomes fits in a population. Let say an individual got lucky did have the next evolving gene but got killed in a car accident at age 8 yet their sister was unlucky and had a bad mutation yet survived and had 8 children. natural selection couldn't pick the fittest because of bad luck.
Right. For beneficial mutations with very small benefits (like my 19.9 vs 20% benefit to survival example given previously), individual chance will be more important when looking at the first mutational generation. At the same time, remember that in every generation, a sizeable proportion do in fact survive, and so the mutant as at least those odds (and in fact a touch better odds). So as numerous beneficial mutations appear, many are indeed removed by chance, but also by chance, some will survive, and thus their odds are even better the next round (because they will be in a number of offspring). So yes, it is undoubtedly true that literally billions of mutants with beneficial mutations died due to chance - but at the same time, chance isn't "unfairly tilted" against them either, so over many times that benefical mutations occur, the odds (regression to the mean) dictate that natural selection's influence will still be relevant.
Natural selection has to wait until someone happen to get lucky and survives to reproduce.
Yes, a beneficial mutation has to make to to the next generation, and many no doubt fail, yet their odds are no worse (indeed slightly better) than anyone else's so over many appearances of beneficial mutations, they will, on average, increase in number. Also importantly, don't fall into the trap of thinking that they have to be sequential. You don't have to "wait" for mutation "A" before "waiting" for mutation "B". Genomes are combinatoric, and multiple beneficial mutations are spreading in the population at any given time. For instance, look at skin color. At least a dozen genes affect skin color, so a light skinned group moving into a sunny area will have an overall selective pressure that would favor "darker skinned" mutations in any of those 12 places. A dark skin mutation in locus 8 would be selected for, while three families over a dark skin mutation in locus 2 is also selected for.
You need large population to get lucky enough to find a benefical mutation (small step that Darwinism requires) but then you need a very small population in order to have any chance of the mutation becoming fixed.
No, exponential growth shows that a beneficial mutation can become common in a large population as well, and even in a small population, there are many thousands of births over a number of years.
Then there the problem of marrying someone who probably carries bad mutations. There offspring will has a mix of the beneficial mutation along with the bad ones. ...Evolution has a serious problem with sex.
That is known as the "blending problem". For example - say you had brown rabbits who moved into a snowy region, and one day there was a beneficial mutation that gave one rabbit white fur. That rabbit would be very successful, and say it had 10 kids, which, having a brown mother, would be tan. Those would be less successful, and have kids that (becuase they mated with regular rabbits), be 3/4 dark - not nearly as good anymore. Within a few generations, the beneficial effect is lost. Big problem for evolution, right?
Mendel's work showed that the real world doesn't work like this, but that genes are discrete, so the kids aren't all uniform, and the traits aren't completely blended (as, say, mixing paint cans would be). So in the first generation of 10 kids, some would be as dark as their mother, and some much lighter. NS would mean that the lighter kids would be selected for, and in the next generation (including some interbreeding), some would be white, completely dark and maybe inbetween. NS would again benefit the whitest rabbits, and this would accelerate as the genes for white fur spread in the population.
The "blending hypothesis" was seen as a serious challenge to NS in the late 1800's. However, when actual inheritance was figured out, it was seen that the "blending hypothesis" is incorrect, and thus poses no problem to evolution. In your example, the random assortment in genes mean that some children will have some of the harmful mutations, and some wont', and some kids will have the beneficial ones and some won't - thus NS can work on the various genes, even though all the kids had the same parents. You can see this in your own kids - they aren't clones of each other. I sure can see that in mine.
Blending inheritance - Wikipedia, the free encyclopedia
Gluadys, thanks for also explaining many of these points - I'm sure that some will find gluadys' explainations easier to understand.
Papias