Mutations tend to be bad, so (not surprisingly) mechanisms exist within cells to reduce the probability of mutation. Biochemical mechanisms repair damaged DNA; proofreading mechanisms catch errors that occur during DNA replication. (Yes, each and every one of us has spell check built in!)
So if mutations can be fixed, why do they exist? For a few reasons.
No repair or proofreading system is perfect. I hope, for example, that this book has no typos; it's gone through several editing and proofreading checks. But every once in a while, you find a typo in a book; maybe you'll find one in this book. No matter how hard you try, being prefect isn't possible, and the same is true of cellular biochemistry.
A trade-off may occur between speed of DNA replication and accuracy of DNA replication. Although mutations tend to be deleterious, slowing down reproduction is also bad for fitness. Genes responsible for a phenotype that reproduces slowly but accurately would be at a disadvantage against genes that generate a phenotype that reproduces more rapidly and is almost as accurate. Each of the individual descendants of the rapid (but sloppy) organism would be more likely to have a couple of extra deleterious mutations, but many more of them would be around, and the overall reproductive success of less error-prone individuals would be lower.
This argument suggests that the mutation rate itself is a character affected by natural selection. And it is. We know from laboratory experiments that mutation rate is a variable heritable character.
It's easy to see how too much mutation would be disadvantageous, so we know that natural selection will keep the mutation rate from getting too high. It's a bit harder to see how natural selection would favor any mutation rate, but there may be a couple of reasons: the short-term trade-offs such as the one described earlier between speed and accuracy, and long-term effects whereby lineages with low mutation rates are eventually eliminated by lineages with slightly higher mutation rates because the slightly higher mutation rate results in at least some favorable mutations. Perhaps the best mutation rate is not too much, but not too little. Consider this the Goldilocks principle: You don't want too many or too few mutations, but just the right amount:
1 Some mutations are advantageous. Mutation is the ultimate source of the variation on which natural selection acts. Natural selection acts to eliminate deleterious mutations and increases the frequency of advantageous ones.
1 Maybe a too-low mutation rate is deleterious: Some scientists speculate that over the long run, a very low mutation rate, while advantageous for individuals in the short-term (because most mutations are bad and you don't want to make mutated kids), may be disadvantageous for the species as a whole. Over long time periods, the selective forces acting on a species are likely to change (climate changes, species move to different habitats, and so on). Without sufficient variation, the species would be unable to respond evolutionarily to these challenges. Such a species could be outcompeted by a species with some higher level of mutation.
This scenario is speculation and involves the tricky subject of selection acting a level other than that of the individual (a topic you can read more about in Chapter 11). Scientists lack a clear understanding of what sets a lower limit for mutation rate — maybe all the biochemistry involved in DNA replication eliminates the possibility of a zero mutation rate — still, the concept is an interesting one.
Bottom line: Mutations tend to be bad, and in the short term, not having any would be good relative to having some. Thinking that a mutation rate is a good thing because you could end up with descendants that are more fit is like thinking that sinking all your retirement funds into the lottery is a good idea because you could end up a millionaire. True, you could get that result, but chances are that you'll end up broke instead.
For asexually reproducing creatures (those with just one parent), reproduction is extremely simple: Make a copy of your DNA and divide. If no mutations occur, parent and offspring are identical.
For organisms that have two parents (called diploid organisms), reproduction is a bit more complicated. Each individual has two copies of DNA, one from each parent, and will pass on a single copy to each offspring (the other parent contributes the other copy). At each location in the genome, a diploid organism has two alleles, and one or the other will randomly end up in each
gamete. When offspring are produced, the relative frequency of different genotypes produced will be a function of the frequency of the different alleles in the population.
Heritable variation is necessary for evolution by natural selection. The pattern of the existing variation (something that can be measured) can tell scientists about whether evolution is occurring.
To understand how allele and genotypic frequencies change under various evolutionary forces, scientists study what happens when none of these forces is at work. Under such conditions, the Hardy-Weinberg equilibrium states that allele frequencies don't change and predicts what the frequency of genotypes should be in a population. This equilibrium states that if you know the frequency of the alleles in a population, you can figure out the frequency of the genotypes in the next generation if (1) mating is random and (2) no evolutionary forces are changing the allele frequencies in the next generation.
To help you understand the Hardy-Weinberg equilibrium, the following information focuses on how allele frequencies correspond to genotype frequencies at a locus where, among all the individuals in the population, there are only two different alleles.
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