The research, by Niwa et al, analyses sequences of DNA found in the cell compartments known as mitochondria from the Japanese sardine. Models show that analyses using the traditional model of gene divergence distribution produces misleading results for marine species with a high reproductive capacity. This is because of what's termed a 'reproductive skew' – that is, a large variance in number of offspring among different families in a population. The skew can produce an excess of recent mutations or the different versions of the same genes that mutation creates, called alleles.
Besides apparent excesses of rare alleles, extremely low ratios of effective population size to census size are often reported in abundant marine species. The genetically effective population size indicates how rapidly allele frequencies are likely to change and in this way lose genetic variation at a much higher rate than expected in the actual population.
Alternative models offered different accounts in the study. Under the Kingman model the genetic imprints in many marine species are generally interpreted as indicating a growth in population after the glacial period. This may, however, appear paradoxical, since, in large populations with offspring evenly distributed among families, allele frequencies change little from one generation to the next, and little genetic diversity is lost.
In sharp contrast, allele frequencies can change rapidly in the Beta model under the skewed offspring-number distribution. When excesses of rare alleles and small effective population sizes are prevalent, as is the case for most marine species, the this model better accounts for fluctuations in population abundance and large reproductive variance present characterized by the skew.
Marine populations with a high reproductive capacity are characterized by intermittent, extremely large recruitments. Within the context of current fisheries management of abundant species, recruitment is assumed to be influenced less by variance in individuals' reproductive success than by environmental variability. The results of this study, however, predict that fluctuations in population abundance are driven by dynamics an individual level. In years of extremely large recruitment, genetic variation can decline, because juveniles may represent offspring from only a few families. Over long periods, the evolution of reproductive traits and population dynamics of highly fertile marine species have probably been greatly influenced by reproductive skew.
This editor's choice paper was published alongside a Food for Thought piece, which presents the context and highlights the importance of the above study.
Dried fish (Engraulis japonicus); photo from digital collections at the Japan Fisheries Research and Education Agency's Library