Non-Darwinian evolution

Deviations from compositional randomness in eukaryotic and prokaryotic proteins: the hypothesis of selective-stochastic stability and a principle of charge conservation

Eight proteins of diverse lengths, functions, and origin, are examined for compositional non-randomness amino acid by amino acid. The proteins investigated are human fibrinopeptide A, guinea pig Insulin, rattlesnake cytochrome c, MS2 phage coat protein, rabbit triosephosphate isomerase, bovine pancreatic deoxyribonuclease A, bovine glutamate dehydrogenase, and Bacillus thermoproteolyticus thermolysin. As a result of this study the experimentally testable hypothesis is put forth that for a large class of proteins the ratio of that fraction of the molecule which exhibits compositional non-randomness to that fraction which does not is on the average, stable about a mean value (estimated as 0.32 plus or minus 0.17) and (nearly) independent of protein length. Stochastic and selective evolutionary forces are viewed as interacting rather than independent phenomena. With respect to amino acid composition, this coupling ameliorates the current controversy over Darwinian vs. non-Darwinian evolution, selectionist vs. neutralist, in favor of neither: Within the context of the quantitative data, the evolution of real proteins is seen as a compromise between the two viewpoints, both important. The compositional fluctuations of the electrically charged amino acids glutamic and aspartic acid, lysine and arginine, are examined in depth for over eighty protein families, both prokaryotic and eukaryotic. For both taxa, each of the acidic amino acids is present in amounts roughly twice that predicted from the genetic code. The presence of an excess of glutamic acid is independent of the presence of an excess of aspartic acid and vice versa.

A deterministic model of cyclical selection

A deterministic model of cyclical selection in randomly mating populations is studied. Sufficient conditions for a protected polymorphism, which are for the special case of alternating selection also necessary conditions, are obtained using a simple graphical approach. The most important condition requires ‘marginal overdominance’ (Wallace, 1968); the other conditions seem hard to satisfy in a natural situation. Furthermore it is shown that the cyclical selection model can be regarded as a special case of a frequency-dependent selection model (Cockerhamet al.1972). Using this property, a mean fitness function for the cyclical selection model is derived. Generally, the mean fitness will not be maximized under cyclical selection. The relevance of the model to the problem of the role of cyclical selection in the maintenance of genetic polymorphism in natural populations is discussed. It is concluded that this relevance is probably rather limited with regard to the creation of protected polymorphism, but that the influence of cyclical selection on transient polymorphisms might be more significant. An approximate formula for the time needed for a given change in gene frequency under cyclical selection is derived. It appears that cyclical selection can extend considerably the time during which a transient polymorphism persists, especially if the selective differences in the different environments are of the same order of magnitude and of opposite sign.

Entropy of the genetic information and evolution

The entropy of the amino acid sequences coded by DNA is considered as a measure of diversity of variety of proteins, and is taken as a measure of evolution. The DNA or m-RNA sequence is considered as a stationary second-order Markov chain composed of four kinds of bases. Because of the biased nature of the genetic code table, increase of entropy of amino acid sequences is possible with biased nucleotide sequence. Thus the biased DNA base composition and the extreme rarity of the base doublet CpG of higher organisms are explained. It is expected that the amino acid composition was highly biased at the days of the origin of the genetic code table, and the more frequent amino acids have tended to get rarer, and the rarer ones more frequent. This tendency is observed in the evolution of hemoglobin, cytochrome C, fibrinopeptide, immunoglobulin and lysozyme, and protein as a whole.

Gene frequency comparisons between taxa: support for the natural selection of protein polymorphisms

The hypothesis has been advanced that the pervasive protein variation found in natural populations of many organisms is adaptively neutral, and thus not subject to natural selection. This neutrality hypothesis predicts that at polymorphic gene loci different configurations of allelic frequencies will occur in different species. Results of an extensive study of protein variation in several species of Drosophila show that any two species have very similar allelic frequencies at a substantial proportion of all gene loci, while at many other loci the species have very different sets of alleles. Genetic distances have been calculated between pairs of subspecies, morphologically similar species, and morphologically different species. The distribution of genetic distances is strikingly different from the predictions of the neutrality theory. Protein variation appears to be maintained by natural selection.

Fitness of allozyme variants in Drosophila pseudoobscura. II. Selection at the Est-5, Odh and Mdh-2 loci

We have studied the effects on fitness of allelic variation at three gene loci (Est-5, Odh, andMdh-2)coding for enzymes inDrosophila pseudoobscura. Genotype has a significant effect on fitness for all six parameters measured (female fecundity, male mating capacity, egg-to-adult survival under near-optimal and under competitive conditions, and rate of development under near-optimal and under competitive conditions). No single genotype is best for all six fitness parameters; rather, genotypes with superior performance during a certain stage of the life-cycle may have low fitness at some other stage, or in different environmental conditions. Heterozygotes are sometimes best when all fitness parameters are considered. There are significant interactions between loci. The various forms of balancing selection uncovered in our experiments may account for the polymorphisms occurring in natural populations ofD. pseudoobscuraat the three loci studied.

Convergent morphological evolution detected by studying proteins of tree frogs in the Hyla eximia species group

Protein studies have uncovered an apparent case of convergent evolution among North American tree frogs. The species Hyla eximia and Hyla regilla are so similar in external morphology that the "wrightorum" subspecies is assigned by some authorities to H. eximia and by others to H. regilla. Yet microcomplement fixation experiments show that "wrightorum" albumin, though virtually indistinguishable from authentic H. eximia albumin, differs as much from H. regilla albumin as from albumins of species outside the genus Hyla, such as Acris crepitans. The morphological resemblance of "wrightorum" to H. regilla is thus probably due to convergence.

Two types of molecular evolution. Evidence from studies of interspecific hybridization

To assess the significance of macromolecular sequence differences among species, we compared the serum albumins of 81 pairs of vertebrate species capable of producing viable hybrids. Micro-complement fixation experiments showed that the average difference between the albumins within such pairs was only 3 immunological distance units for placental mammals (31 pairs), but 36 units for frogs (50 pairs). Albumin immunological distance is strongly correlated with other measures of genetic distance, including those made with DNA annealing techniques. It therefore seems likely that mammalian species pairs capable of hybridization are far more similar at the macromolecular sequence level than is the case for most hybridizable frogs. We think the most likely explanation for the marked molecular restriction on hybridization among mammals is that the ratio of regulatory evolution to protein evolution is higher for mammals than for frogs. Mammals may have experienced unusually rapid regulatory evolution; indeed, this could be the factor responsible for their unusually rapid anatomical evolution.

On some principles governing molecular evolution

THE FOLLOWING FIVE PRINCIPLES WERE DEDUCED FROM THE ACCUMULATED EVIDENCE ON MOLECULAR EVOLUTION AND THEORETICAL CONSIDERATIONS OF THE POPULATION DYNAMICS OF MUTANT SUBSTITUTIONS: (i) for each protein, the rate of evolution in terms of amino acid substitutions is approximately constant/site per year for various lines, as long as the function and tertiary structure of the molecule remain essentially unaltered. (ii) Functionally less important molecules or parts of a molecule evolve (in terms of mutant substitutions) faster than more important ones. (iii) Those mutant substitutions that disrupt less the existing structure and function of a molecule (conservative substitutions) occur more frequently in evolution than more disruptive ones. (iv) Gene duplication must always precede the emergence of a gene having a new function. (v) Selective elimination of definitely deleterious mutants and random fixation of selectively neutral or very slightly deleterious mutants occur far more frequently in evolution than positive Darwinian selection of definitely advantageous mutants.

Intragenomal distribution of DNA repair synthesis: repair in satellite and mainband DNA in cultured mouse cells

DNA repair synthesis was examined in mouse satellite and mainband DNA derived from confluent Balb/c 3T3 cells damaged with ultraviolet radiation or N-acetoxy-2-acetylaminofluorene. Two different approaches were used: (i) Contact-inhibited cells were treated with hydroxyurea to reduce replicative synthesis to low levels; and (ii) bromodeoxyuridine was used to label newly replicated DNA in cells that had escaped contact inhibition. DNA was separated into mainband and satellite fractions in Ag(+)-Cs(2)SO(4) gradients. After treatment with either ultraviolet radiation or N-acetoxy-2-acetylaminofluorene, repair synthesis occurred to the same extent in mainband and satellite DNA. Repair synthesis increased over an ultraviolet radiation dose range of 30-200 erg/mm(2), and the extent of repair in the two DNA species was similar at each dose level. An analysis of the separated strands of satellite DNA from ultraviolet-irradiated cells indicated that the extent of repair is closely correlated with the availability of pyrimidines for cyclobutyl dimer formation and provided evidence that repair synthesis occurs at the site of damage. Within the precision of our experiments the results suggest that at least one group of highly repetitive, nontranslated DNA sequences is repaired to about the same extent as the rest of the genome.

Are recommended daily allowances for vitamin C adequate?

Recommended daily allowances, U.S.A., for ascorbic acid are several times the minimum daily requirement of about 10 mg to prevent clinical symptoms in adults. The recommended daily allowances reflect the levels of ascorbic acid required to produce tissue saturation in human beings, so that higher doses are promptly excreted in the urine. In various investigations, responses to ascorbic acid were not obtained under conditions where the intake was considerably lower than the recommended daily allowances, but higher than the level needed to prevent clinical symptoms of ascorbic acid deficiency. The human ascorbic acid requirement cannot be calculated by a linear extrapolation from guinea pigs per kg of body weight because guinea pigs are much smaller than human beings and, hence, their metabolism is more rapid per unit of body weight. Evolutionary loss of ascorbic-acid-synthesizing ability may be a neutral change that was incorporated by genetic drift.

Genetic differentiation within and between species of the Drosophila willistoni group

We describe allelic variation at 28 loci in six Caribbean populations of four sympatric species of Drosophila. Within any one species the allelic frequencies are very similar from population to population, although there is evidence of local as well as regional genetic differentiation. The genetic distance is greater between populations from different islands than between populations of the same island. When the allelic frequencies are compared between different species, a remarkable pattern appears. In any pair of species nearly half of the loci have essentially identical allelic frequencies, while nearly the other half of the loci have different alleles and in different frequencies. The loci with nearly identical allelic frequencies are different when different pairs of species are compared. The patterns of allelic variation within and between species are inconsistent with the hypothesis that the variation is adaptively neutral. Migration or mutation cannot explain the patterns of genetic variation, either. Balancing natural selection is the main process maintaining protein polymorphisms in natural populations.