I hope you weren't thinking this is true! How much genetic differentiation happens within dimorphic or polymorphic species? Think about the difference between males and females in sexually dimorphic organisms. Consider the difference between the juvenile forms such as the catepillar, or tadpole and the adult.
Below are abstracts of recent papers on polymorphism and dimorphism. Back to Finding Order in Chaos
The genetic basis at Alzheimer's disease is becoming clear; it appears to fit the postulates of the two main theories of the evolution of senescence, with both polymorphic alleles and rare mutations involved.
We present a survey of passerine birds designed to investigate the frequency with which sexual dimorphism in coloration or color pattern has evolved from monomorphism (or the converse). Based on the number of genera that have both a monomorphic and a dimorphic species, and the minimum number of changes inferred to have occurred between genera, the transition between dimorphism and monomorphism has occurred at least 150 times. Using the Sibley/Ahlquist phylogeny, we obtain maximum likelihood estimates of the probability that one state will be in the other after one million years of 0.01 to 0.02 (monomorphism to dimorphism) and 0.03 to 0.04 (dimorphism to monomorphism). The rate of transition from dimorphism to monomorphism appears to be higher than the converse, and there are more monomorphic than dimorphic species. We conclude that the transition between alternative states is not difficult, and that the evolution of sexual dimorphism, given appropriate selection pressures, is unlikely to be constrained.
Individual plants of several Amelanchier taxa contain many polymorphic nucleotide sites in the internal transcribed spacers (ITS) of nuclear ribosomal DNA (nrDNA). This polymorphism is unusual because it is not recent in origin and thus has resisted homogenization by concerted evolution. Amelanchier ITS sequence polymorphism is hypothesized to be the result of gene flow between two major North American clades resolved by phylogenetic analysis of ITS sequences. Western North American species plus A, humilis and A. sanguinea of eastern North America form one clade (A), and the remaining eastern North American Amelanchier make up clade B. Five eastern North American taxa are polymorphic at many of the nucleotide sites where clades A and B have diverged and are thought to be of hybrid origin, with A. humilis or A. sanguinea as one parent and various members of clade B as the other parent. Morphological evidence suggests that A. humilis is one of the parents of one of the polymorphic taxa, a microspecies that we refer to informally as A. ''erecta.'' Sequences of 21 cloned copies of the ITS1-5.8S gene-ITS2 region from one A. ''erecta'' individual are identical to A. humilis sequence or to the clade B consensus sequence, or they are apparent recombinants of A. humilis and clade B ITS repeats. Amelanchier ''erecta'' and another polymorphic taxon are suspected to be relatively old because both grow several hundred kilometers beyond the range of one of their parents. ITS sequence polymorphisms have apparently persisted in these two taxa perhaps because of polyploidy and/or agamospermy (asexual seed production), which are prevalent in the genus.
Sex differences are intrinsically interesting, particularly in the brain. When sexually dimorphic structures mediate learning, and when such learning ability is necessary to compete for mates, then such differences are best understood within the framework of sexual selection. By categorizing recent studies of sex differences in the brain by their role in mate competition, theories of sexual selection can be used to predict and characterize the occurrence of dimorphisms among species with different mating systems.
We examined Variation on the nonrecombining portion of the human Y chromosome to investigate human evolution during the last 200,000 years. The Y-specific polymorphic sites included the Y Alu insertional polymorphism or ''YAP'' element (DYS287), the poly(A) tail associated with the YAP element, three point mutations in close association with the YAP insertion site, an A-G polymorphic transition (DYS271), and a tetranucleotide microsatellite (DYS19). Global variation at the five bi-allelic sites (DYS271, DYS287 and the three point mutations) gave rise to five ''YAP haplotypes'' in 60 populations from Africa, Europe, Asia, Australasia, and the New World (n = 1500). Combining the multi-allelic variation at the microsatellite loci (poly(A) tail and DYS19) with the YAP haplotypes resulted in a total of 27 ''combination haplotypes''. All five of the YAP haplotypes and 21 of the 27 combination haplotypes were found in African populations, which had greater haplotype diversity than did populations from other geographical locations. Only subsets of the five YAP haplotypes were found outside of Africa. Patterns of observed variation were compatible with a variety of hypotheses, including multiple human migrations and range expansions.
The evolution of distyly from tristyly has occurred repeatedly, especially in the Lythraceae. However, the evolutionary forces involved remain unclear since species exhibiting transitional stages between tristyly and distyly have rarely been studied. The self-compatible, wetland perennial Decodon verticillatus (Lythraceae) may provide this transitional variation since populations commonly lack style morphs, particularly the mid-styled (M) morph. In dimorphic populations lacking the M morph, anthers positioned at the mid level in both the long- (L) and short-styled (S) morphs have lost their target stigma, setting the stage for either evolutionary repositioning of mid-level anthers to increase pollen export to L and S stigmas, or increased variability in mid-level anther position resulting from relaxed selection. We examined these two hypotheses by comparing floral morphology in eight dimorphic and ten trimorphic populations from throughout the species' range. We found no evidence that loss of the M morph has led to evolutionary modification of mid-level stamens. While mid-level stamens of the S morph were 11.0 +/- 4.0% (mean +/- 1 SE) longer than those of the L morph in dimorphic populations, divergence in stamen length between morphs occurred to the same extent (10.4 +/- 2.0%) in trimorphic populations and cannot be attributed to the absence of the M morph. Analyses of variability using median ratio tests revealed no difference in the variability of mid-level stamen length between dimorphic and trimorphic populations. Mid-level stamens were not more variable than long- and short-level stamens within dimorphic populations. The consistent divergence in midlevel stamens between the L and S morphs may reflect morph-specific differences in the optimal position of mid-level anthers for maximizing cross-pollination and avoiding self-fertilization.
At least two adaptive processes can lead to the evolution of sexual dimorphism: sexual selection (e.g. male-male combat) or natural selection (e.g. dietary divergence). We investigated the adaptive significance of a distinctive pattern of sexual dimorphism in a south-eastern Australian frog, Adelotus brevis. Male Adelotus grow larger than female conspecifics, have larger heads relative to body size, and have large paired projections ('tusks') in the lower jaw. All of these traits are rare among anurans. We quantified the degree of dimorphism in Adelotus, and gathered data on diets and mating systems of this species to evaluate tile possible roles of sexual selection and dietary divergence in favoring the evolution of these sexually dimorphic traits. Analysis of prey items in alimentary tracts revealed significant sex differences in prey types. For example, females ate proportionally more arthropods and fewer molluscs than did males. However, this difference is likely to be a secondary consequence of habitat differences between the sexes (due in turn to their different reproductive roles) rather than a selective force for the evolution of sexual dimorphism. Calling males spend their time in moist habitats where pondsnails are abundant, whereas females are more often encountered in the drier arthropod-rich woodlands. A three-year behavioural ecology study on a field population revealed that reproductive males engage in agonistic interactions, with the sexually dimorphic tusks used to attack rivals. Larger body size contributed to male reproductive success. Small males were excluded from calling sites and, among the calling males, larger animals had higher reproductive success (numbers of matings) than did smaller individuals. Hence, the atypical pattern of sexual dimorphism in Adelotus brevis seems to have resulted from sexual selection for larger body size and tusk size in males. in the context of male-male agonistic behaviour, rather than natural selection for ecological divergence between the sexes. (C) 1997 The Linnean Society of London.
Marine populations of the threespine stickleback (Gasterosteus aculeatus) have mostly been described as monomorphic for the complete lateral plate morph, especially in northern waters. An examination of stickleback populations from 27 marine and estuarine sites along the coast of Norway showed that polymorphism is more common than expected; in fact, only 15% of the samples were monomorphic. Populations from estuarine habitats (low salinity) were more polymorphic than populations from marine habitats. A study of morphometric characters shows that the plate morph variation reflects true polymorphism, not just immigration of freshwater sticklebacks or hybridization. Plate number variation within each plate morph was also recorded, but no pattern of variation could be detected, either geographically, or between estuarine and marine habitats.
This review summarises current understanding of the evolution of self-incompatibility inferred from DNA sequence analysis. Self-incompatibility in many plant families is controlled by a single, highly polymorphic S-locus which, in the Solanaceae, encodes an allelic series of stylar ribonucleases known as the S-RNases. PCR approaches are a convenient way to examine the diversity of S-RNase sequences within and between wild populations of a self-incompatible species and provide a unique view into the species' current and historic population structure. Similar molecular appoaches have also been used to show that S-RNases are involved in self-incompatibility in families other than the Solanaceae. A model for the evolution of ribonuclease-based self-incompatibility systems is discussed.
Sexual size dimorphism (SSD) is often attributed to sexual selection, particularly when males are the larger sex. However, sexual selection favoring large males is common even in taxa where females are the larger sex, and is therefore not a sufficient explanation of patterns of SSD. As part of a more extensive study of the evolution of SSD in water striders (Heteroptera, Gerridae), we examine patterns of sexual selection and SSD in 12 populations of Aquarius remigis. We calculate univariate and multivariate selection gradients from samples of mating and single males, for two sexually dimorphic traits (total length and profemoral width) and two sexually monomorphic traits (mesofemoral length and wing form). The multivariate analyses reveal strong selection favoring larger males, in spite of the female-biased SSD for this trait, and weaker selection favoring aptery and reduced mesofemoral length. Selection is weakest on the most dimorphic trait, profemoral width, and is stabilizing rather than directional. The pattern of sexual selection on morphological traits is therefore not concordant with the pattern of SSD. The univariate selection gradients reveal little net selection (direct + indirect) on any of the traits, and suggest that evolution away from the plesiomorphic pattern of SSD is constrained by antagonistic patterns of selection acting on this suite of positively correlated morphological traits. We hypothesize that SSD in A. remigis is not in equilibrium, a hypothesis that is consistent with both theoretical models of the evolution of SSD and our previous studies of allometry for SSD. A negative interpopulation correlation between the intensity of sexual selection and the operational sex ratio supports the hypothesis that, as in several other water strider species, sexual selection in A. remigis occurs through generalized female reluctance rather than active female choice. The implications of this for patterns of sexual selection are discussed.
Molecular methods allow an extremely fine strain typing that can be used to establish the population structure of bacterial species. This methodology has been used to characterize a collection of 74 uropathogenic Escherichia coil obtained from three hospitals located in geographically distant towns in Spain, some representatives of the ECOR collection and other reference strains. Genomic DNA was analyzed by RAPD (Random Amplified Polymorphic DNA) that can characterize a bacterial strain to the level of defining individual clones. The 16S rDNA-23S rDNA spacers were amplified by PCR and submitted to restriction analysis. Finally, the presence or absence of G adhesins in Escherichia coli as well as the type of adhesin (three types are known) have been shown by PCR amplification followed by digestion with restriction enzymes. As expected a wide diversity was shown by RAPD and identical patterns were only found in the case of strains isolated from the same individual, an obvious case of relapse. Analysis of the spacers' restriction patterns showed the presence of two markedly differentiated clusters that we have named alpha and beta. Both RAPD and spacer restriction patterns originated similar clusters of strains showing a consistency in the evolution of the global genome with the sequence variation of the ribosomal spacers. Furthermore, most of the strains having G-adhesin, with only a few exceptions, corresponded to the alpha rRNA spacer group. The two spacer types detected were also consistent with some phenotypic markers such as sucrose and raffinose utilization. The alpha and beta clusters could be intraspecific groups produced by partial sexual isolation or other barriers that are originating a divergent evolution.
Optimality arguments and modifier theory are reviewed as paradigms for the study of the evolution of recombination. Optimality criteria (such as maximization of mean fitness) may agree with results from models developed in terms of the evolution of recombination at modifier loci. Modifier models demonstrate, however, that equilibrium mean fitness can decrease during the evolution of recombination rates and is not always maximized. Therefore, optimality arguments do not successfully predict the conditions under which increased or decreased recombination will evolve. The results from modifier models indicate that decreased recombination rates are usually favored when the population is initially near a polymorphic equilibrium with linkage disequilibrium. When the population is subject to directional selection or to deleterious mutations, increased recombination may be favored under certain conditions, provided that there is negative epistasis among alleles.
In a recent review of the distribution and evolution of sexual dichromatism in the Old World viperid snakes of the genus Vipera, Shine and Madsen proposed that males in sexually dichromatic taxa had evolved bright contrasting colour patterns because these function to confuse visually guided predators by inducing a ''flicker-fusion'' illusion and thus enhance survival of males that move about rapidly in search of mates. Here we provide support for the flicker-fusion hypothesis obtained from a capture-recapture study of a colour polymorphic population of the adder, V. berus, that consists of a sexually dichromatic zigzag colour morph and a monochromatic melanistic colour morph. We used mark - recapture data to estimate and compare annual capture and survival probabilities of melanistic and zigzag male and female individuals. Our results revealed that in any given year zigzag males survived significantly better than did melanistic males, whereas melanistic females survived better than zigzag females. This is consistent with the flicker-fusion hypothesis and suggests that selection from visually guided predators acts on the interaction between colour pattern and behaviour.