A type of natural selection, called “stabilizing selection”, is a process which helps to maintain the status quo within a species by decreasing genetic diversity – an idea based on the observation that most traits of a particular organism do not seem to change drastically over time. Historically, however, finding direct evidence for this type of selection has been elusive.
Genetic drift, on the other hand, refers to changes in the frequency of a particular gene variant (allele), which typically occurs in relatively small groups, decimated by a population bottleneck, due to the unpredictability of reproduction and other random occurrences.
This process falls under “weak selection” whereby several different phenotypes are selected in the same population because they confer a similar degree of fitness. Species exhibiting this characteristic are called polymorphic.
A classic example of genetic drift is Cepaea nemoralis, a type of polymorphic land snail, with varying shell colours, which are distributed unequally across populations, and had probably arisen due to random events.
In a new study, Hokkaido University researchers studied two ant species in Japan and Korea that are indiscriminate on the molecular and phylogenetic levels, meaning they could be considered one species, and found evidence for different selection forces at play on different parts of the body.
First, they observed that a number of tiny spurs on the front and back part of the leg are used for different purposes – the former was frequently used to clean the antennae, a major sensory organ in ants, and the latter was used only occasional to scratch the back surface of the abdomen where no important organs are situated.
The researchers studied ten populations of the same species, finding significant genetic differences between them, implying limited inter-population gene flow, and only limited evidence for inbreeding. This means that any observed morphological differences within and between the populations would reflect how strongly natural selection acted on each character.
Results showed a high degree of variation in the lengths of the less important hind spurs, which corresponds to the random genetic drift (different characteristics being retained because they have little to no effect on fitness), and a much smaller degree of variation in the length of the front spur.
“Our results suggest that stabilizing selection is at play, maintaining these functionally important characteristics in each population, while less important characteristics have diversified over time,” said Eisuke Hasegawa, an author on the paper published in Scientific Reports. “Further investigations should help us understand the evolutionary forces that generate diversity and similarity among populations.”