Learning the secrets of survival
A new study of the demographic structure of contemporary killer whale populations reveals complex interactions between social evolution, adaptation and genetic diversification in these marine mammals.
Orcas (Orcinus orca) are the largest members of the dolphin family and are found in virtually all of the world’s oceans - from the Arctic to the waters surrounding Antarctica. They are generally referred to as killer whales, but that name tends to obscure the fact that these mammals live in complex communities and display a rich repertoire of social behavior. An international team of biologists led by Professor Jochen Wolf, who was recently appointed to the Chair of Evolutionary Biology at LMU, has now taken a closer look at the population genetics of the orca. The species has evolved into several distinct ecotypes, which differ from each other – often quite significantly – in morphology, behavior and feeding habits, and exploit specific ecological niches. The primary aim of the study was to gain insights into ecotype evolution and assess the roles of genomic, environmental and sociobiological factors in this process. The findings have just appeared in the online journal “Nature Communications”.
In the study, Wolf (who currently also holds a professorship at the University of Uppsala in Sweden) and his collaborators determined the complete genomic sequences of 50 orcas representing several different ecotypes, and compared them with one another. Detailed analysis of these data enabled the investigators to reconstruct the recent genetic history of the species. The results indicate that the adaptive radiation which gave rise to the different ecotypes began a little over 200,000 years ago. Moreover, the genetic data reveal that the evolution of new variants was preceded by a relatively sudden reduction in the range of genetic variation present in the population. This so-called bottleneck effect implies a contraction in the size of a breeding population due to the emergence of physical, genetic or behavioral barriers between previously interbreeding groups. This process promotes genetic diversification and –in the case of persistent reproductive isolation owing to social cohesion, for instance – the evolution of specific adaptations to the local environment. Indeed, the analysis of genomic evolution in orcas confirms that the different ecotypes adapted rapidly to their new environments and the food sources they offered. Thus, the researchers detected characteristic genetic differences between orcas from different habitats, including variations in genes involved in the development of adipose tissue, skin regeneration and amino acid metabolism.
These findings reveal that the various ecotypes reacted in a flexible manner to changing environmental conditions and were able to exploit the available ecological niches. The animals developed sophisticated collective hunting strategies, and the formation of stable kinship groups also favored social learning. However, the very fact that each ecotype became better adapted to its particular habitat inevitably altered the prevailing selection conditions. According to the authors of the study, orcas reveal “at the genomic level” how the factors population history, ecological adaptation, evolution and cultural development interact in shaping a species. – In this case, of course, the term culture is used in its broad evolutionary sense to mean the intergenerational transmission of adaptive behavioral strategies by means of social learning.
Nature Communications 2016