New discovery: Molecular variation in one gene affects the growth of natural populations

For the first time, ecologists have been able to show that molecular variation in one gene may affect the growth of a population in its natural habitat. Research Professor Ilkka Hanski, University of Helsinki, and Dr Ilik Saccheri, University of Liverpool, UK, discovered that the population growth of the Glanville fritillary butterfly (Melitaea cinxia) is affected by the allelic composition of the phosphoglucose isomerase (Pgi) enzyme. The result challenges previous views according to which allelic variation in populations, and possible consequent differences in individual performance, would be of no significance for population growth.

It has been observed in previous studies on the Glanville fritillary and Colias butterflies that the individuals’ Pgi genotype affects their flight metabolic rate and reproductive performance. The present result proves that these differences between individuals also have repercussions at the population level. The role of Pgi is emphasised by the fact that variation in the other genes studied showed no connection to population growth.

How strongly the differences in Pgi alleles affect population growth depends on ecological factors. The size of the habitat patch and its connectivity to existing populations affect migration and gene flow between populations. The study shows that genetic factors and the structure of the habitat together influence variation in population growth. The results also show that the structure of the habitat and the dynamics of the species in a fragmented habitat maintain variation in the gene encoding for the Pgi enzyme

According to previously held views, populations dynamics in natural populations is mainly influenced by environmental factors and demography, that is birth rate, death rate and migration. Genetic variation and natural selection have been thought to have no direct effect on population growth or a weak effect at most.

Scientists analysed hundreds of well-characterized local populations of the Glanville fritillary in the Åland Islands in Finland, where the butterfly has a large metapopulation that has been studied for 15 years.