Hybrid sharks

Australian scientists have found the first evidence that shark species can hybridise. The Australian black-tip shark (Carcharhinus tilstoni) and the common black-tip shark (C. limbatus) are found along much of the northern Australian coastline and have extensively overlapping distributions. The larger common black-tip can tolerate cooler waters and it is therefore found in more southerly locations.

The find is also interesting because the hybrid sharks are morphologically indistinguishable from the Australian black-tip shark, but they are found further south like the common black tip. The scientists involved in the study believe that the hybridisation could allow the ability to tolerate cold water to spread into the Australian black-tip, which would facilitate a southerly range expansion. 

Common black-tip shark Carcharhinus limbatus

If the hybrid sharks mate with the pure-bred Australian black-tips and the genes for cold-tolerance spread in the population it would be an example of 'evolution in action'. Most people familiar with the theory of evolution by natural selection would know that the stuff that natural selection works on is variation in heritable traits. The best known way in which heritable variation can arise is through genetic mutation, but there are other ways it can occur, which include hybridisation.

Hybrids are rarely found in the wild. Partly this is because often they or their offspring are less fit and therefore do not persist for long when hybridisation occurs. The scientists involved in finding the hybrid sharks are now attempting to measure the fitness of the hybrids. I wish them luck because measurements of fitness in the wild are more rare than in the laboratory, but more informative about the process of natural selection. What's more, measuring fitness in wild populations is difficult, especially for a large animals that can move long distances.