Your "story" may be different from mine but here I have assumed that the Vega Gull started it all. I imagine that the Vega Gull hopped over to North America creating a colony of Herring Gulls which developed the mutation that causes their legs to be pink. These American Herring Gulls colonised all of North America and eventually made the big leap across the Atlantic to colonise the UK. Meanwhile, the Vega Gulls from Siberia had been moving westward across Russia and developing dark backs as they colonised the new lands, giving rise to Birula's Gulls and Heuglin's Gulls. By the time they colonised Europe their backs were so dark that they had become the Back-backed Gulls. In the UK the Black-backed Gulls met the Herring Gulls and we have our two species.

Which "story" is correct? Well, with this data it is hard to say. It might help to take careful measurements of all the different gulls involved in the ring and try to figure out the most likely story. Certainly modern techniques in biochemistry and genetics would provide useful data. Regardless of the actual events, ring speciation has occurred with these gulls.

We see ring species throughout the world with other animals and even among plants. The ring may be formed by any physical barrier that inhibits individuals from getting together for mating. The ring acts as a physical barrier to their reproduction. These barriers may be oceans (as in the example), mountains, deserts or even lakes. Once the two ends of the ring met the two forms are so different that they are different species and their reproductive isolation continues due to non-physical barriers such as behavioural differences.

I hope the examples in this lesson have convinced you that we can observe evolution on a small scale.

• You've seen examples of how artificial selection has been used over a long period of time to domesticate dogs, crops, etc. in a purposeful way.
• You've seen evidence of selection quickly influencing the evolution of colouring in Peppered Moths and pesticide resistance in insects all by accident.
• I've described carefully controlled experiments of evolution in the laboratory in which rats were artificially selected for different degrees of tooth decay.

All of these show how selection can influence the types of organisms in subsequent generations and thus give rise to different populations.

I've also shown you some examples of true speciation occurring.

• Polyploidy in plants is a fine example of how laboratory work in evolution can be used to better understand speciation in the wild.
• The formation of ring species illustrates how one species can diverge into two species, given enough time and isolation.

You may be wondering if these processes can operate over a longer scale and give rise to greater diversity. We answer that in our next lesson as I show you more evidence for evolution by Natural Selection.