The Definition of a Species

If you ask the average woman on the street how to define "species" you will get an answer something like "different types of animals."

If you ask one of my students you will (I hope) get something like "a group of individuals with similar physical and behavioral traits that are capable of breeding with each other and producing fertile offspring."

If you ask a biologist the answer becomes a bit more complicated.

There is no good answer.  The definition I use for class is the best working definition I have found.  But Nature does not color within the lines.  In fact when you ask Nature to color inside the lines Nature replies "but the lines make up a princess and I want to draw a tiger."

Let's look at Gulls as an example of why the definition of species is such a hard concept to define.

Here is a picture of a gull.

If you ask the average woman on the street what this is a picture of she will say "a seagull."

Ask an avid birder and he will say "a Herring Gull"

Ask a snobby birder (a small subset of birders) and he will answer "a mature adult Larus smithsonianus," or maybe "an adult HERG."

Let's try another picture.

Bird on the right - another Herring Gull, bird on the left is a Lesser Black Backed Gull (Larus fuscus).

Clearing distinct species, right?  I mean one has a light grey back and one has a dark grey back.

Well....  Yes, pretty much everyone agrees that the Lesser Black Backed Gull is a different species than the Herring Gull.  But look a the "latin" or scientific names and you see they both contain Larus.  This means the two species are somewhat related.

Ok.  Bear with me.  One more.

Average Woman on the Street:  "I know this one - it's a Herring Gull!"

Good birder goes "Herr... wait... somethings not quite right..."

Did you notice the legs?  In the Herring Gull they are pink, but this guy?  They're kind of yellow.  Huh.  A different species?

Not quite.  The consensus is that this is a hybrid between the Lesser Black Backed Gull and the Herring Gull.  These hybrids are fertile.

So are the Herring Gull and the Lesser Black Backed two different species or just one?

Oh, it is SO much more complicated that.  These are two (three?) examples of the "Larus gull complex;"  a group of different species that are capable of interbreeding and producing offspring that can then breed with other Larus gulls.  Originally thought to be a Ring Species they are now considered to be more complicated than that.  

These are birds with distinctive physical characteristics and different behaviors that are genetically close enough to hybridize and produce fertile offspring.  Multiple species?  A single species with a lot of diversity? 

We try to put these gulls into neat and tidy boxes marked species and Nature just laughs.