Relaxed selection and loss of non-beneficial traits

In the latest issue of Trends in Ecology & Evolution, there is a review and metaanalysis of the fascinating phenomen of trait loss after the disappearance of selection pressures maintaining the traits. Classical cases is the loss of vision among cave-dwelling fish or loss of flight ability or antipredator adaptations among birds and insects invading oceanic islands with few predators. This study is briefly reviewed at Science Daily, and one of the co-authors is Andrew P. Hendry, the external opponent of Fabrice Eroukhmanoff's Ph.D.-thesis on November 20 2009.

Fascinating questions to adress here is why do some traits disappear fast, while others take much longer time to decay? According to the results it seems as if two factors might be important in determining the speed by which traits are lost when no longer maintained by selection:

1/Traits that are energetically or nutrient-wise costly are more likely to disappear fast. Examples of such traits are the armour-plate reductions in marine sticklebacks, which disappear fast when these sticklebacks invade freshwater environments, where the minerals that are needed to produce these plates are scarce.

2/Traits that have a relatively simple genetic basis, and which are governed by one or a few loci are lost faster than traits governed by many traits. Examples of such traits include the loss of vision among cave-dwelling animals. Although many genes might influence vision, it might be sufficient with mutations in one or a few genes to cause blindness.

These interesting questions also apply to some of the study systems we are working in our lab, e. g. the Podarcis-lizards that Anna studies on the islets in Greece, where predators are few or the isopods that Fabrice have studied in Lake Krankesjön and Lake Tåkern which have invaded a new limnetic habitat (stonewort), where the isopoods seem to have evolved a suite of different anti-predator adaptations, perhaps as a response to a changed predator regime (invertebrates vs. fish) or perhaps even relaxed overall predation.