An Introduction to Genetic Analysis. The variational theory of evolution has a peculiar selfdefeating property. If evolution occurs by the differential reproduction of different variants, we expect the variant with the highest rate of reproduction eventually to take over the population and all other genotypes to disappear.
But then there is no longer any variation for further evolution. The possibility of continued evolution therefore is critically dependent on renewed variation. For a given population, there are three sources of variation: However, recombination by itself does not produce variation unless alleles are segregating already at different loci; otherwise there is nothing to recombine.
Similarly, immigration cannot provide variation if the entire species is homo-zygous for the same allele. Ultimately, the source of all variation must be mutation. Mutations are the source of variationbut the process of mutation does not itself drive evolution. The rate of change in gene frequency from the mutation process is very low because spontaneous mutation rates are low Table The mutation rate is defined as the probability that a copy of an allele changes to some other allelic form in one generation.
After yet another generation of mutation, the frequency of a would be increased by 0. It is obvious that the rate of increase of the new allele is extremely slow and that it gets slower every generation because there are fewer copies of the old Source of genetic variation in asexual reproduction new combinations still left to mutate.
A general formula for the change in allele frequency under mutation is given in Box Mutation rates are so low that mutation alone cannot account for the rapid evolution of populations and species.
If "Source of genetic variation in asexual reproduction new combinations" look at the mutation process from the standpoint of the increase
Source of genetic variation in asexual reproduction new combinations a particular new allele rather than the decrease of the old form, the process is even slower. Most mutation rates that have been determined are the sum of all mutations of A to any mutant form with a detectable effect. Any specific base substitution is likely to be at least two orders of magnitude lower in frequency than the sum of all changes.
It is not possible to measure locus-specific mutation rates for continuously varying characters, but the rate of accumulation of genetic variance can be determined. The creation of genetic variation by recombination can be a much faster process than its creation by mutation. This outcome is simply a consequence of the very large number of different recombinant chromosomes that can be produced even if we take into account only single crossovers. If the heterozygous loci are well spread out on the chromosomes, these new gametic types will be frequent and considerable variation will be generated.
Asexual organisms or organisms, such as bacteria, that very seldom undergo sexual recombination do not have this source of variation, so new mutations are the only way in which a change in gene combinations can be achieved. As a result, asexual organisms may evolve more slowly under natural selection than sexual organisms. A further source of variation is migration into a population from other populations with different gene frequencies. The resulting mixed population will have an allele frequency that is somewhere intermediate between its original value and the frequency in the donor population.
Suppose a population receives a group of migrants whose number is equal to, say, 10 percent of its native population size. Then the newly formed mixed population will have an allele frequency that is a 0. If its original allele frequency of A were, say, 0.
Box derives the general result. The change in gene frequency is proportional to the difference in frequency between the recipient population and the average of the donor populations.
Unlike the mutation ratethe migration rate m can be large, so the change in frequency may be substantial. We must understand migration as meaning any form of the introduction of genes from one population into another.
Source of genetic variation in asexual reproduction new combinations We can determine the amount of this migration by looking at the frequency of an allele that is found only in Europeans and not in Africans and comparing its frequency among blacks in North America. We can use the formula for the change in gene frequency from migration if we modify it slightly to account for the fact that several generations of admixture have taken place.
If, as before, P is the allelic frequency in the donor population and p 0 is the original frequency among the recipients, then. For example, the Duffy blood group allele Fy a is absent in Africa but has a frequency of 0.
Among blacks from Georgia, the Fy a frequency is 0. Therefore, the total migration of genes from whites into the black population since the introduction of slaves in the eighteenth century is. When the same analysis is carried out on American blacks from Oakland California and Detroit, M is 0.
In any case, the genetic variation at the Fy locus has been increased by this admixture. Random mating with respect to a locus is common, but it is not universal. Two kinds of deviation from random mating must be distinguished.
First, individuals may mate with each other nonrandomly because of their degree of common ancestry; that is, their degree of genetic relationship. If mating between relatives occurs more commonly than would occur by pure chance, then the population is inbreeding.
If mating between relatives is less common than would occur by chance, then the population is said to be undergoing enforced outbreedingor negative inbreeding. Second, individuals may tend to choose each other as mates, not because of their degree of genetic relationship but because of their degree of resemblance to each other at some locus.
Bias toward mating of like with like is called positive assortative mating.
Mating with unlike partners is called negative assortative mating. Assortative mating is never complete. Inbreeding and assortative mating are not the same. Close relatives resemble each other more than unrelated individuals on the average but not necessarily for any particular trait in particular individuals.
So inbreeding can result in the mating of quite dissimilar individuals. On the other hand, individuals who resemble each other for some trait may do so because they are relatives, but unrelated individuals also may have specific resemblances. Brothers and sisters do not all have the same eye color, and blue-eyed people are not all related to one another.
Assortative mating for some traits is common.
In humans, there is a positive assortative mating bias for skin color and height, for example. An important difference between assortative mating and inbreeding is that the former is specific to a trait, whereas the latter applies to the entire genome. Individuals may mate assortatively with respect to height but at random with respect to blood group.
Cousins, on the other hand, resemble each other genetically on the average to the same degree at
Source of genetic variation in asexual reproduction new combinations loci. For both positive assortative mating and inbreedingthe consequence to population structure is the same: If two individuals are related, they have at least one common ancestor.
Thus, there is some chance that an allele carried by one of them and an allele carried by the other are both descended from the identical DNA molecule. The probability of homozygosity by descent is called the inbreeding coefficient F. Figure and Box illustrate the calculation of the probability of homozygosity by descent.
Individuals I and II are full sibs because they share both parents.
We label each allele in the parents uniquely to keep track of them. Such close inbreeding can have deleterious consequences. If the frequency of the allele in the population is pthen the probability that a random couple will produce a homozygous offspring is only p 2 from the Hardy-Weinberg equilibrium. Now suppose that the couple are brother and sister. If one of their common parents is a heterozygote for the disease, they may both receive it and may both pass it on to their offspring.
As the calculation shows, the rarer the genethe worse the relative risk of a defective offspring from inbreeding. For more-distant relatives, the chance of homozygosity by descent is less but still substantial. Systematic inbreeding between close relatives eventually leads to complete homozygosity of the population but at different rates, depending on the degree of relationship.
Which allele is fixed within a line is a matter of chance. Inbreeding takes the genetic variation present within the original population and converts it into variation between homozygous inbred lines sampled from the population Figure Repeated generations of self-fertilization or inbreeding will eventually split a heterozygous population into a series of completely homozygous lines.
Suppose that a population is founded by some small number of individuals who mate at random to produce the next generation. Assume that no further immigration into the population ever occurs Source of genetic variation in asexual reproduction new combinations.
For example, the rabbits now in Australia probably have descended from a single introduction of a few animals in the nineteenth century.
In later generations, then, everyone is related to everyone else, because their family trees have common ancestors here and there in their pedigrees.
Because the population is, of necessity, finite in size, some of the originally introduced family lines will become extinct in every generation, just as family names disappear in a closed human population because, by chance, no male offspring are left.
As original family lines disappear, the population comes to be made up of descendants of fewer and fewer of the original founder individuals, and all the members of the population become more and more likely to carry the same alleles by descent.
In other words, the inbreeding coefficient F increases, and the heterozygosity decreases over time until finally F reaches 1. The rate of loss of heterozygosity per generation in such a closed, finite, randomly breeding population is inversely proportional to the total number 2 N of haploid genomes, where N is the number of diploid individuals in the population.
As the number t of generations becomes very large, H t approaches zero. Any population of any species is finite in size, so all populations should eventually become homozygous and differentiated from one another as a result of inbreeding.
In nature, however, new variation is always being introduced into populations by mutation and by some migration between localities. Thus, the actual variation available for natural selection is a balance between the introduction of new variation and its loss through local inbreeding.
By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed. Turn recording back on. National Center for Biotechnology InformationU. Variation from mutations Mutations are the source of variationbut the process of mutation does not itself drive evolution.
Box Effect of Mutation on Allele Frequency. Variation from recombination The creation of genetic variation by recombination can be a much faster process Source of genetic variation in asexual reproduction new combinations its creation by mutation. Variation from migration A further source of variation is migration into a population from other populations with different gene frequencies.