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PubMed2026

Signals of Natural Selection Across Regions of Low Recombination in Wild Populations of the Purple Sea Urchin, Strongylocentrotus purpuratus.

Structural variants (SVs) are increasingly recognized as important components of genetic architecture. Yet our understanding of the evolutionary forces maintaining SVs in natural populations is limited. Chromosomal inversions in particular can facilitate local adaptation in populations with high gene flow, including many marine species. The purple sea urchin (Strongylocentrotus purpuratus) is a powerful system to study these dynamics due to its high gene flow, lack of population structure, and broad latitudinal range. We analyzed whole genome sequence data from 137 individuals sampled across seven populations to identify regions of low recombination using scans for elevated linkage disequilibrium and genetic differentiation. Such regions may arise from structural variants, including chromosomal inversions. We identified nine regions showing signatures of reduced recombination, including three way genotype clustering, long range linkage, and hanging bridge patterns frequently associated with inversion polymorphisms. The regions were polymorphic within locations and along the species range with three loci showing concordant signatures of balancing and spatially heterogeneous selection based on enrichment of outliers and distinct patterns of allelic age. Additionally, these loci showed enrichment for genes associated with biomineralization and development. Our results provide the first evidence for regions of low recombination in the purple sea urchin genome, several of which display genomic signatures consistent with structural variants such as chromosomal inversions. These findings add to growing evidence that regions of reduced recombination constitute an important component of standing genetic variation in natural populations and may play a key role in adaptation to heterogeneous environments.

Strongylocentrotus purpuratus↗
PubMed1975

Natural selection and the evolution of reproductive effort.

Reproductive effort is defined as that proportion of the total energy budget of an organism that is devoted to reproductive processes. Reproductive effort at a given age within a species will be selected to maximize reproductive value at that age. Reproductive effort is not directly affected by changes in juvenile survivorship, nor necessarily reduced by an increase in adult survivorship. Selection for high levels of reproductive effort should occur when extrinsic adult mortality is high, in environments with constant juvenile survivorship, and in good years for juvenile survivorship in a variable environment, provided that the quality of the year is predictable by adults. Data necessary to measure reproductive effort and to understand how selection results in different levels of effort between individuals and species are discussed. We make several predictions about the effect of increased resource availability on reproductive effort. The empirical bases for testing these predictions are presently inadequate, and we consider data on energy budgets of organisms in nature to be essential for such test. We also conclude that variance in life table parameters must be known in detail to understand the selective bases of levels of reproductive effort.

Age Factors↗
PubMed1975

Population structure, gene flow and natural selection in populations of Euphydryas phaeton.

An examination of seven proteins, presumably encoded by seven structural gene loci, in three local populations of the supposedly sedentary and colonial butterfly, Euphydryas phaeton revealed that three (43 per cent) were polymorphic with three to five alleles each. In addition to this high level of heterozygosity, no statistically significant differences in allele frequencies were found at two of the three polymorphic loci. Since the effective breeding size in each population was estimated to range from as few as 20 to 200 individuals, it appears that some level of gene flow between populations must be invoked to explain the high levels of genetic variability maintained in local populations of this butterfly, despite its apparently colonial nature.

Alleles↗
PubMed1975

Natural selection on the dentition of an Arikara population.

The tooth crowns of most mammals do not change in size except by attrition. Therefore, by comparing age groups within a population and finding differences in size or variation, the effects of such agencies as directional or stabilizing selection can be demonstrated. Mesiodistal and buccolingual measurements were taken on the maxillary and mandibular permanent dentition of a protohistoric Arikara Indian population from South Dakota. The tooth sizes of juveniles ages 6 through 15 were compared to those of adults ages 16 through 20. The two age groups were also compared by the cross-sectional areas of the posterior teeth. When a difference between age groups was demonstrated in either size or variation, a selection intensity was estimated using published graphs. Results from the comparisons show that adults, in general, have larger and less variable teeth than juveniles. The data suggest the possibility that both directional and stabilizing selection were operating on the Arikara dentition. Furthermore, the selection intensities suggest that selection was operating not on the individual teeth but on functional complexes such as the posterior teeth. It appears that in populations like the Arikara where crown attrition is severe, selection will be in the direction of large teeth.

Adolescent↗
PubMed1975

Population genetics of a "colonising" lizard: natural selection for allozyme morphs in Anolis grahami.

Allozyme frequencies of Anolis grahami from Bermuda were compared to those from Jamaica. These lizards were introduced into Bermuda from Jamaica in 1905. The magnitude of the genetic changes were consistent with the hypothesis that these changes were produced by random genetic drift. The changes for the seperate alleles were, however, more heterogeneous than expected if caused by random drift alone. It is concluded that not all the alleles examined have been selectively neutral.

Animals↗
PubMed1978

Change of gene frequencies by natural selection under population number regulation.

By incorporating a population number regulating mechanism into the formulation of genic selection involving a pair of alleles (A1 and A2) with respective frequencies x and I-x, it is shown that the change of x in one generation is given by deltax = sx(1-x)/W, in which W is the mean absolute selective value (in Wright's sense). It is also shown that, in the process in which advantageous allele (say A1) increases from a low frequency to a high frequency, quasi-equilibrium is rapidly attained where deltaW approximately 0. In this state we have W approximately 1 + (s2/c)x(1-x) in the case of logarithmic population number regulation, and W approximately 1 + s2x(1-x)/(cN) in the case of logistic regulation. In these expressions, s is the selective advantage of A1 over A2, and c is a coefficient relating to the total population number regulation. It is pointed out that the approximation formula deltax = sx(1-x) is valid under wider circumstances than usually suggested by the conventional treatment of genic selection.

Alleles↗