The harmful effects of close inbreeding have been noticed for many centuries (34, 35, 165). With the rise of Mendelian genetics, it was realized that the main genetic consequence of inbreeding is homozygosis (165, Ch. 2). Two main theories were early proposed to account for inbreeding depression and its converse, heterosis (the increase in vigor observed in an F1 between two inbred lines). These are the overdominance and partial dominance hypotheses, discussed in more detail below. Research into this question has continued up to the present, and this is one of the topics that we discuss. Darwin (35, 36) was the first to point out that the evident adaptations of many plants for ensuring outcrossing could be understood in terms of the selective advantage of avoiding inbreeding depression. We review the evidence that the evolution of breeding systems of animals and plants has been significantly influenced by the occurrence of inbreeding depression. In order to do this, we consider the contemporary genetic theory of inbreeding depression and heterosis, and the experimental data concerning the strength of inbreeding depression. We emphasize data and theory relevant to natural, rather than domesticated, populations as we are chiefly concerned to evaluate the evolutionary significance of inbreeding depression. We do not attempt to give a complete bibliography of this very extensive field but try to concentrate on what seem to be the most significant findings in relation to this aim.

https://www.jstor.org/stable/pdf/2097132.pdf

Inbreeding depression and its evolutionary consequences

Hybridization may influence evolution in a variety of ways. If hybrids are less fit, the geographical range of ecologically divergent populations may be limited, and prezygotic reproductive isolation may be reinforced. If some hybrid genotypes are fitter than one or both parents, at least in some environments, then hybridization could make a positive contribution. Single alleles that are at an advantage in the alternative environment and genetic background will introgress readily, although such introgression may be hard to detect. ‘Hybrid speciation’, in which fit combinations of alleles are established, is more problematic; its likelihood depends on how divergent populations meet, and on the structure of epistasis. These issues are illustrated using Fisher’s model of stabilizing selection on multiple traits, under which reproductive isolation evolves as a side-effect of adaptation in allopatry. This confirms a priori arguments that while recombinant hybrids are less fit on average, some gene combinations may be fitter than the parents, even in the parental environment. Fisher’s model does predict heterosis in diploid F1s, asymmetric incompatibility in reciprocal backcrosses, and (when dominance is included) Haldane’s Rule. However, heterosis arises only when traits are additive, whereas the latter two patterns require dominance. Moreover, because adaptation is via substitutions of small effect, Fisher’s model does not generate the strong effects of single chromosome regions often observed in species crosses.

The role of hybridization in evolution

We develop models of the rates of evolution at sex-linked and autosomal loci and of the rates of fixation of chromosomal rearrangements involving sex chromosomes and autosomes. We show that the substitution of selectively favorable mutations often proceeds more rapidly for X- or Y-linked loci than for the autosomes, provided that mutations are recessive or partially recessive on the average. Selection acting on a quantitative character is expected to result in similar long-term rates of gene substitution for X-linked and autosomal loci, unless there is strong directional dominance. Short-term responses to such selection often preferentially fix alleles at autosomal loci. The fixation of slightly deleterious alleles by random drift and the stochastic turnover of alleles at loci controlling quantitative characters under stabilizing selection usually proceed somewhat more slowly at sex-linked loci. In contrast, the fixation of underdominant chromosomal rearrangements by random genetic drift is faster with se...

The Relative Rates of Evolution of Sex Chromosomes and Autosomes

Although homologous recombination and DNA repair phenomena in bacteria were initially extensively studied without regard to any relationship between the two, it is now appreciated that DNA repair and homologous recombination are related through DNA replication. In Escherichia coli, two-strand DNA damage, generated mostly during replication on a template DNA containing one-strand damage, is repaired by recombination with a homologous intact duplex, usually the sister chromosome. The two major types of two-strand DNA lesions are channeled into two distinct pathways of recombinational repair: daughter-strand gaps are closed by the RecF pathway, while disintegrated replication forks are reestablished by the RecBCD pathway. The phage λ recombination system is simpler in that its major reaction is to link two double-stranded DNA ends by using overlapping homologous sequences. The remarkable progress in understanding the mechanisms of recombinational repair in E. coli over the last decade is due to the in vitro characterization of the activities of individual recombination proteins. Putting our knowledge about recombinational repair in the broader context of DNA replication will guide future experimentation.

Recombinational Repair of DNA Damage in Escherichia coli and Bacteriophage λ

We describe allelic variation at 28 gene loci in natural populations of D. willistoni. Seventy samples were studied from localities extending from Mexico and Florida, through Central America, the West Indies, and tropical South America, down to South Brazil. At least several hundred, and often several thousand, genomes were sampled for each locus. We have discovered a great deal of genetic variation. On the average, 58% loci are polymorphic in a given population. (A locus is considered polymorphic when the frequency of the most common allele is no greater than 0.95). An individual fly is heterozygous, on the average, at 18.4% loci.—Concerning the pattern of the variation, the most remarkable finding is the similarity of the configuration of allelic frequencies from locality to locality throughout the distribution of the species. Our observations support the conclusion that balancing natural selection is the major factor responsible for the considerable genetic variation observed in D. willistoni.

/pdf/enzyme-variability-in-the-drosophila-willistoni-group-iv-1lt5ngx0z9.pdf

Enzyme variability in the drosophila willistoni group. iv. genic variation in natural populations of drosophila willistoni

The most extensive treatment of any living genus thus far attempted, this book deals with the evolution of Drosophila as demonstrated by living forms, and is mainly concerned with isolating mechanisms, genetic systems shown by hybrids and the cytology of over one third of the known species of the genus.

Evolution in the genus Drosophila

Grimshaw (1901, 1902) and Perkins (1910) first focused the attention of dipterists on the aberrant characteristics and abundance of the Hawaiian Drosophilidae. They described 47 species; additionally, Perkins’ extensive descriptions (1913) of the biotic habitat in which the insects live alerted biologists to the unique but unknown ecology of the flies.

The Evolutionary Biology of the Hawaiian Drosophilidae

The Role of Peroxide in the Biological Effects of Irradiated Broth.

There is mounting evidence that Drosophila populations are mlore polymorphic at the level of genetic loci than was formerly thought by many geneticists. There have been many experimental demonstrations of the extensive chromosomal polymorphis^ms, of the presence of considerable quantities of hidden deleterious and lethal alleles in the populations, and of the extensive complexity of the coadapted genetic systems integrated through natural selection.' Nevertheless, the proportion of polymorphisms among the majority of all loci is largely unknown because of inadequate screening techniques. Furthermore, the mechanisms for the maintenance of polymorphisms -remain elusive to experimental verification. The ideal situation of screening populations for genetic comparisons at and between individual loci has been brought a step closer to reality by electrophoretic separation of enzyme molecules. Though enzyme differences are not necessarily related with genetic differences on a one-to-one basis, most may be. At any rate, zymogram analysis of individual flies for electrophoretic mobility or qualitative activity differences gives a good first approximation of the underlying genetic situation for a different class of variation than has previously been studied directly. However, as Lewontin and Hubby3 have pointed out, the randomness of the samples of loci studied by this technique is critical for broader generalizations which might be made from the findings in this class of loci. Sample characteristics are yet to be determined. Lewontin and Hubby3 have found about 40 per cent of all electrophoretically separable enzyme systems and protein bands of no known enzymatic activity of D. pseudoobscura to be polymorphic, having two or more electrophoretic forms of the same protein. One expects the difference in molecular forms to represent at least as much genetic difference, so that the variation of molecular types reflects similar variation of genotypes. If only a moderate bias is involved in estimating this proportion for the genome, then there exists a major discrepancy between the data and the expectation based upon the theory of segregational load. According to the theory, a population may bear, without being jeopardized by reduced average fitness, only a relatively small portion of loci in heterotic polymorphic conditions. In order to establish the meaning of the apparent inconsistency between the theory and observations, it must be established whether or not these isozyme polymorphisms are maintained more or less independently by some form of selection. It is very important to know if these variations are random representatives of a large number of such loci. It has been demonstrated in a laboratory population of D. busckii4 that the polymorphism at the "leucine aminopeptidase" locus is maintained by heterotic selection, though this was not the case for an esterase locus in D. inelanogaster and D. simulans.5

Wilson S. Stone

Papers

Evolution in the genus Drosophila

The Evolutionary Biology of the Hawaiian Drosophilidae

The Role of Peroxide in the Biological Effects of Irradiated Broth.

An analysis of polymorphisms among isozyme loci in dark and light Drosophila ananassae strains from American and Western Samoa.

The Production of Mutations in Staphylococcus Aureus by Irradiation of the Substrate