There are over 9000 species of birds! Birds are easy to observe and they have been the subject of nature studies by many of Europe's explorers and biologists.
Some groups of birds are a fine example of a phenomenon in evolution called a ring species. A ring species is actually a group of related species that have evolved away from each other due to a central barrier that restricts their mixing together thus forcing speciation events to occur in the shape of a ring. Perhaps an illustration will make that definition clear.

Some gulls present a ring pattern in their distribution and speciation.
In the United Kingdom we have two species of large gull (relevant to ring speciation) -
the Herring Gull and the Black-backed Gull.
The Herring Gull has a grey back and pink legs while the Black-backed Gull has a black back (well named isn't it?) and yellow legs.
North America has only the Herring Gull - the one with a grey back and pink legs.

Siberia is home to the Vega Gull - a species very similar to the Herring Gull and it has the same grey back as the Herring gull but its legs are yellow like the Black-backed Gull.

This map shows the distribution of these gulls as well as some others involved in the "ring". Biard's Gull and Heuglin's Gull have slightly different degrees of darkness on their backs and act as intermediates between the Vega and Black-backed Gulls. As you travel west from Siberia the gulls take on a darker back. That is, as you move from Siberia westward (clockwise) towards Europe the gulls start to look more like Black-backed Gulls. So we have an almost continuos gradation of gull types.

Although it is impossible to prove the sequence of events, a reasonable story is that these birds evolved from a common ancestor and the Arctic Circle acted as a ring barrier to produce these different types of gulls.

Consider the Herring Gull (Larus argentatus) here in the UK. Long ago it may have colonised North America. Recall that the North American and European Herring Gull are both the same species (Larus argentatus). The long distance caused the "American" Herring Gulls to be isolated from the "European" Herring Gulls. The two types of Herring Gull started to drift apart with respect to their appearance, but only very slightly. Nothing obvious. When the "American" Herring Gulls colonised Siberia they independently evolved yellow legs.

This new leg colour probably had nothing to do with the habitat. It was just a chance mutation that did well within the new Siberian colony. This colony gave rise to today's Vega Gulls. Note that Vega Gulls are still Herring Gulls (Larus argentatus) but are so different that we call them a subspecies of the Herring Gulls. (We'll come back to the idea of subspecies later.)
The descendants of Vega Gulls moved westward and as they did so new mutations started to accumulate causing the populations to diverge according to the colour (darkness) of their backs. By the time the population made its way "back around" to Europe the gulls had taken on a very dark back.
Here in the United Kingdom we see the "old" Herring Gull, with its pink legs and light-coloured back, along side the "new" Herring Gull, with its yellow legs and dark-coloured back. Side by side they look very different. Indeed, the "old" Herring Gull is now considered to be a separate species from the "new" Herring Gull - which is now called the Black-backed Gull (Larus fuscus).

Of course, scientists simply name the gulls, but what do the gulls "think" of this?
Well, here in the UK any Black-backed Gull (Larus fuscus) is free to mate with any Herring Gull (Larus argentatus) but they chose not to. Apparently, Black-backed Gulls do not find the pink legs and light-coloured back of the Herring Gulls "attractive" enough to consider them as potential mates. So they only mate with other Black-backed Gulls. And the Herrings Gulls limit themselves to other Herring Gulls. This is another form of reproductive isolation but, unlike the barrier that started the speciation, this isolation is based upon behaviour.

So scientists and gulls agree - the Herring and Black-backed Gulls are too different to be anything else but two separate species.
These gulls are an example of a ring species.
At most points in the ring there is only one "type" and they differ from their neighbours only slightly. However, where the two end points of the "speciation ring" meet (for example, here in the UK regarding these gulls) we find two types so different from each other they "earn" the title of separate species.

Isolation so important to speciation that is worth summarising it here using my example. At first a physically barrier, the Arctic Ocean, prevented the gulls from mating but by the time they had migrated around the barrier, behavioural mechanisms had taken over. The Arctic Ocean still acts as an effective physical barrier between, say, American Herring Gulls and Heuglin's Gull. But, here in the UK the Herring and Back-backed Gulls are isolated by their own behavioural preferences.
Throughout this course you will see that isolation and the divergence it allows is a major force in evolution.

Of course, we cannot travel back in time to see this speciation occurring but the "story" I have just told you explains how these two species (and the subspecies in between) may have evolved. I (we) can only work with the data at hand so we must create a "story" of ring speciation that is reasonable and useful. Certainly some kind of ring speciation has occurred, but maybe it didn't start in the UK and progress they way I have explained (above).

Here's the distribution map again. (Here's the data.) Have a good look at it and try to imagine another "ring species story" to explain the distribution and the speciation Once you have thought of another scenario that would cause this ring to form continue to the next page.