Why Can't Different Species Mate With Each Other?

The tree of life is huge but is seems like every animal has it's designated spot. How do scientists draw the lines between species?

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The latest draft of the tree of life, published in the journal Proceedings of the National Academy of Sciences, charts out the over 2.3 million species of animals, plants, fungi, and bacteria living on Earth. Previous versions of the tree only charted about 100,000 species, but this new, comprehensive tree charts out every living species currently known to man. There's an interactive version online which you can check out here. This massive tree is truly a sight to behold, but begs the question: how do scientists tell different species apart?

The old-fashioned way of defining a species was through a process known as morphology--examining them visually. But species can also be defined biologically. The Biological Species Concept (BSC), which was introduced by biologist Ernst Mayr in the mid-20th century, basically says that any two animals that can create fertile offspring are of the same species. There are problems with both ways of identifying species, however. Two animals of the same species can look wildly different from each other (just think of all the different kinds of dogs that exist). Conversely, two different kinds of animals can evolve to adapt to similar environments and end up looking similar (fossas, a descendant of the Mongoose, look a lot like cats).

A study recently published in the journal Biology Letters described what they thought to be six different species of Rock-Wallabys capable of mating. Using genetic analysis, the researchers found gene flow between the species, suggesting something else was going on, forcing them to rethink their theory of evolution of that species. The study suggests that these animals were mating, and giving birth to fertile offspring, meaning they were more like a single species than scientists previously believed.

Molecular genetic sequencing is changing how biologists define species. By comparing the DNA of different animals, scientists can see exactly how closely related they are. This is a huge advance in the field of taxonomy and helped scientists understand how different animals have evolved. For example, a study published in the Journal of Biogeography found that although we share physical features with orangutans, (like beards on men and similar shoulder blades), we only share 97 percent of our DNA with them. By contrast, we share 99 percent of our DNA with chimps and bonobos, meaning we're more closely related to them.

Learn More:

Rock-wallaby interbreeding causes rethink on evolution (phys.org)
"It was previously thought that mating between different rock-wallaby species could not result in fertile offspring. This is because of the differences in the way their genetic material is packaged into chromosomes."

Humans Related to Orangutans, Not Chimps, Says New Pitt, Buffalo Museum of Science Study of Human Origin (University of Pittsburgh)
"Researchers propose new grouping for humans, orangutans, and common ancestors and lay out a scenario of the migration and evolution of 'dental hominoids' in the 'Journal of Biogeography.'"