Nature’s hybrids

Polar Bear

This week, I recorded my first interview for the Irish science podcast, Scibernia. I interviewed Prof. Dan Bradley of Trinity College Dublin. He was part of a team that discovered that modern polar bears descended from a common female ancestor, the Irish brown bear. This research has received national and international news coverage so I was delighted to have the opportunity to discuss it with Prof. Bradley.

The team showed that over 20,000 years ago hybridisation occurred between Irish brown bears and polar bears. From this, I wondered what causes modern hybrids and can we learn from species that interbred in the past. Hybridisation is caused when different species of plants or animals interbreed. Hybrids are often considered unnatural as people are usually intentionally or accidentally responsible for their creation. Well-known animal hybrids that are purposely bred by people include ligers (male lion/female tiger), zedonks (male zebra/female donkey) and mules (male donkey/female horse).

There are two main reasons that people have influenced hybrids in nature: habitat change and introduction of species. Reduction of sea ice in the Arctic Circle is forcing animals to move to new areas where they can mix with closely related species. Arctic hybrids such as grizzly-polar bear and beluga-narwhal have been recorded over the past few years. Closer to home, a proportion of introduced sika and native red deer populations in north and east Ireland are thought to be sika-red deer hybrids.

Conservation groups are concerned that interbreeding could result in driving some animals to extinction. Grizzly-polar bear hybrids in captivity have been shown to have a hybrid advantage i.e. perform better than their original species. However, this can be lost in future generations, leaving the hybrid weaker than either the polar bear or grizzly bear. Beluga-narwhal hybrids lack the narwhal’s tusk which is important for successful breeding. It has a similar role in attracting a mate as the feathers of male peacocks and the manes of male lions.

During my interview, Prof. Bradley mentioned that “to some extent hybridisation between closely related species is part of evolutionary history”. The hybridisation of polar bears and Irish brown bears occurred with no human influence showing that hybrids have occurred naturally in the past. It is an example of how hybrids can influence the evolution of a species and perhaps even enhance its survival.

By U.S. law, hybrid animals can be removed from the wild to enhance the survival of the protected species. Conserving hybrid species is only recommended as a last resort. These measures could prevent species adapting and evolving through hybridisation. An ice-free summer is predicted for the Arctic by 2030, so unfortunately we will discover the fate of the polar bear and its potential future hybrids very soon!

Click here to listen to my interview with Prof. Dan Bradley. Edwards CJ, Suchard MA, Lemey P, Welch JJ, Barnes I, Fulton TL, Barnett R, O’Connell TC, Coxon P, Monaghan N, Valdiosera CE, Lorenzen ED, Willerslev E, Baryshnikov GF, Rambaut A, Thomas MG, Bradley DG, & Shapiro B (2011). Ancient hybridization and an Irish origin for the modern polar bear matriline. Current biology : CB, 21 (15), 1251-8 PMID: 21737280

Kelly BP, Whiteley A, & Tallmon D (2010). The Arctic melting pot. Nature, 468 (7326) PMID: 21164461

Polar Bear Photo: Ansgar Walk / Wikimedia Commons