February 2012

Last month, biologists announced the discovery of hybrid sharks in Australian waters. The new sharks may not warrant a marine park attraction — they look much like their closely-related parent species — but do represent an unexpected twist of biology and evolution. This is the first time that scientists have found evidence of shark hybridization — an event that was thought to be rare because, unlike the many fish that simply release eggs and sperm into the water, sharks mate. Clearly, though, the widely-distributed common blacktip shark and the Australian blacktip shark (which is restricted to northern and eastern Australia) have few qualms about each other: 57 apparently healthy hybrid individuals were discovered in the first investigation of these animals. What does this mean for the future evolution of blacktip sharks?

Where's the evolution?

One of the roles that hybridization plays in evolution is to spread genetic variation — the raw material of evolution — from one species to another. These new gene versions might turn out to be advantageous, disadvantageous, or completely neutral, depending on the situation. In the case of the Australian blacktip shark (Carcharhinus tilstoni), introducing DNA from the larger, common species (C. limbatus) results in individuals that have larger bodies than purebred Australian blacktips. While the Australian blacktip is usually restricted to tropical waters, hybrid blacktips have been found further south in cooler water. The researchers hypothesize that these larger-bodied sharks may be able to outcompete purebreds in the southern part of the range, and may even extend the species’ range further south. If these C. limbatus genes are truly advantageous, then through the process of natural selection, we may observe them becoming more and more common in the population in future generations.

Unfortunately, some articles in the popular press have misrepresented this aspect of hybridization, and of evolution in general. Several reports on this research have suggested that the Australian blacktips are adapting to cope with climate change and make it sound as though the species began hybridizing intentionally. Of course, this is not the case. Sharks are not accepting mates of a different species because they’ve noticed that water temperatures are on the rise, and evolution is not bringing these two species together in order to ensure their survival in our rapidly changing oceans. Natural selection has no foresight. It simply selects among individuals in a population, favoring traits that enable individuals to survive and reproduce and yielding more copies of those individuals’ genes in the next generation. This may mean that hybrid individuals and their genes are favored in certain situations (e.g., when oceans are warming) — but it does not mean that the hybridization occurred because it would be advantageous for the species.

Of course, since these sharks were discovered so recently, we don’t know for sure whether hybrids have an advantage over one purebred species, both, or neither. Hybridization can affect evolutionary trajectories in several different ways. If hybrids have low fitness (i.e., poor reproductive and survival abilities) in comparison to purebreds, hybridization can actually reinforce barriers between species. Individuals carrying genes that cause them to accept mates from the other species (i.e., “less-choosy” genes) will have offspring with low fitness, and hence, the less-choosy genes are likely to be weeded out of the population. On the other hand, as described above, if hybrids have a fitness advantage, genes from the other species (e.g., genes resulting in larger body size) may spread throughout the population. And in some instances, if hybrids are fit, fertile, and isolated from the parent species, a third species may form as a direct result of the hybridization. This has happened in some plants, like sunflowers. Through hybridization, it is also possible for the two parent species to fully merge, resulting in a single daughter species. This seems to be occurring with some species of cichlid fish in Lake Victoria and with stickleback fish in the lakes of British Columbia.

In the case of the blacktip sharks, scientists are not even sure if hybridization is a recent phenomenon or an ancient one that we’ve only recently discovered. It could be that the two species have been hybridizing for many generations and that, because of improved technology and additional attention to ocean ecology, we’ve just discovered it. There are many examples of long-standing “hybrid zones” — regions where two species come into contact with one another and hybridize. For example, in Central Europe, the carrion crow (which lives in Western Europe) and the hooded crow (which lives in Eastern Europe) form a narrow hybrid zone, currently running from Italy up to Denmark, which may have been in existence for the last 10,000 years! Perhaps hybrid sharks are new to science, but not to nature. Alternatively, it is possible that the hybridization of common and Australian blacktip sharks is recent and has been brought about by environmental factors such as human fishing practices or climate change. Further studies should help clarify whether or not hybridization is recent and how this is likely to shape the future course of the sharks’ evolution. Whatever the outcome, it will have come about because of the mechanistic processes of evolution, not because of any intention on the on the part of the sharks.