We use variation at a set of eight human Y chromosome microsatellite loci to investigate the demographic history of the Y chromosome. Instead of assuming a population of constant size, as in most of the previous work on the Y chromosome, we consider a model which permits a period of recent population growth. We show that for most of the populations in our sample this model fits the data far better than a model with no growth. We estimate the demographic parameters of this model for each population and also the time to the most recent common ancestor. Since there is some uncertainty about the details of the microsatellite mutation process, we consider several plausible mutation schemes and estimate the variance in mutation size simultaneously with the demographic parameters of interest. Our finding of a recent common ancestor (probably in the last 120,000 years), coupled with a strong signal of demographic expansion in all populations, suggests either a recent human expansion from a small ancestral population, or natural selection acting on the Y chromosome.

No abstract

No abstract

No abstract

No abstract

Mutations of alleles at microsatellite loci tend to result in alleles with repeat scores similar to those of the alleles from which they were derived. Therefore the difference in repeat score between alleles carries information about the amount of time that has passed since they shared a common ancestral allele. This information is ignored by genetic distances based on the infinite alleles model. Here we develop a genetic distance based on the stepwise mutation model that includes allelic repeat score. We adapt earlier treatments of the stepwise mutation model to show analytically that the expectation of this distance is a linear function of time. We then use computer simulations to evaluate the overall reliability of this distance and to compare it with allele sharing and Nei's distance. We find that no distance is uniformly superior for all purposes, but that for phylogenetic reconstruction of taxa that are sufficiently diverged, our new distance is preferable.

Polyacrylam~de gel electrophoresis revealed three genetic polymorphisms among central California populations of the supralittoral copepod Tigriopus californicus. Laboratory analysis of mated pairs and their progeny confirmed the allelic nature of esterase (EST), phosphoglucose isomerase (PGI), and leucine aminopeptidase (LAP) electromorphs. Polymorphism within each enzyme system was localized such that only a few of the nine or more sites sampled were polymorphic while the others were fixed on the same allele. While the Pescadero site (located near the middle of the 250 km transect studied) was polymorphic for PG1 with two alleles at 0.5 frequency, one of these forms never reached a frequency of 0.04 or higher in any of the other populations sampled, including a population located only 1.5 km to the south. Similar population differentiation was observed with respect to the EST locus, and to a lesser extent, the LAP locus. EST phenotype frequencies at Moss Beach exhibited both microgeographic and temporal variation. No obvious patterns, however, were observed among the phenotype frequencies, and estimated allele frequencies indicate that remarkable consistency existed among all the 'Moss Beach population samples. These data indicate that T. californicuspopulations located within habitat patches are genetically relatively homogeneous, while populations occupying habitats isolated by stretches of sandy beach can show strong genetic differentiation over short geographic distances.

The advantage of sexual reproduction remains a puzzle for evolutionary biologists. Everything else being equal, asexual populations are expected to have twice the number of offspring produced by similar sexual populations. Yet, asexual species are uncommon among higher eukaryotes. In models assuming small populations, high mutation rates, or frequent environmental changes, sexual reproduction seems to have at least a two‐fold advantage over asexuality. But the advantage of sex for large populations, low mutation rates, and rare or mild environmental changes remains a conundrum. Here we show that without recombination, rare advantageous mutations can result in increased accumulation of deleterious mutations (‘evolutionary traction’), which explains the long‐term advantage of sex under a wide parameter range.