The modern theory of Evolution started, of course, with Darwin and Wallace. Darwin published 'On the Origin of Secies' after receiving a letter from Wallace with exactly the same idea in it.

The theory of evolution is based on a few observations and inferences. They are:

Darwin collected some Finchs from the Galapagos islands. There are many birds very similar, but with differences in their eating habits. These differences are aided by their beak shape and size. He wanted to know about how these differences came about.

Darwin lived in a time when the only real alternative to evolution (which was already around at the time, as seen by the works of Lamarck and others) was the current ideas of theology which said that each species was immutable. It should be pointed out that some naturalists did see that species were changeable at this time, but they were a minority. Darwin, Wallace (and Lamarck of course) did not think that the creationists of their day could be right. They saw that species change forms all the time, and it was how different species arise that they were trying to answer. This is what we need to concentrate on firstly. Not the evolution of all life forms over hundreds of millions of years, but whether new species can form which are recognised as species. The 19th century creationists said they could not, that each species was fixed by the Creator. The first step is to show that these ideas held about the imuutability of species are wrong. This may seem trivial, but it is an important point. We need to know if species can form over time from other species.

It has been shown that speciation events occur, as shown by the experiments in labs with Drosophila were ones where specialisation of the mating ritual have caused mates to choose discriminately.

Isolation is the way that new species form. The most obvious example is a physical barrier such as a mountain range or an ocean, but behavioural differences can also cause seperation. Harrison did some work on the Sawfly Pontanica Salicis whose larvae produce galls on willows. There are a number of different races, such as one that produce galls on Salix andersoniana. Harrison toook some of these and isolated them so that they could only produce galls on a different willow S. rubra. In the first generation, the majority of the larvae died. After about four generations the survival rate was back to normal, after another three generations, the flies were allowed to choose species of willow again, they stayed with the S. rubra. The mating also took place between these and other flies that populated the same Willow. This shows isolation occurs due to behavioural differences also, and it shows how different populations can occur even when a species lives in the same geographic region. This seperation can eventually, over time, lead to the formation of new species

Take a look at the number of sub species of the big cats there are, 7 sub species of lion (Panthera leo) 8 sub species of tiger (P. tigris) and 7 sub species of Leopard (P. Paruds). This is a great example of different varieties of species arising from what was obviously a much more general form of each.

In Britain, the small woodland birds the Chiff Chaff Phylloscopus collybita and the Willow Warbler P. trochilis are so alike that even when seen by experts it is virtually impossible to tell them apart. Only by their call can they be shown to be different. Each has become more choosy in its partner choosing, in the same way that Drosophila have shown to be in labs. There are many different examples of this, which is called adaptive radiation.

What it does show, though is that the 19th century naturalists were right. Species do change. New species do originate.

Another reason Darwin thought species are related is the fact that hybrids can be produced. Darwin reasoned that hybrids could only form if, relatively recently, the two species were the same. If they were the same species, then their offspring would be fertile, but as they are not, too much time must have passed since the two seperated. For example, horses (Equus cabalus) and donkeys (E. asinus) can mate and produce hybrids. This is evidence that some time ago, they were close enough to be considered the same species. There is also the zebra (Equus burchellii) which is also very closely related to the horse that it would share a common ancestor with it and the donkey.

Lions and tigers also produce hybrids. Some time ago, lions and tigers had a common ancestor, and some process seperated them into the two species. This must have happened before the subspecies formed. Some time ago there would have been one species of tiger and one species of lion. These would have a common ancestor that would be a more general form of both.

These examples are important to the point I am making. One point to realise is that the number of examples is immense, because of the very variability in species that we acknowledge is so difficult to put into nicely labelled boxes. There is one more example, I would like to consider. Another one from Britain, the Herring gull, (Larus argentatus) and the Lesser black backed gull (L. fuscus). These do not interbreed, they are seperate species. They are related, though.

"In Britain, these [The Herring gull] are white. They breed with the herring gulls of eastern America, which are also white. American herring gulls breed with those of Alaska, and Alaskan ones breed with those of Siberia. But as you go to Alaska and Siberia, you find that herring gulls are getting smaller, and picking up some black markings. And when you get all the way back to Britain, they have become Lesser Black-Backed Gulls. "

Ring species Accessed 2/7/04

The other point that I would like to make is to drive home the fact that there is a continuum from one species to many. We have examples of ring species, where there are two species connected by a very roundabout route, of sub species, where distinct varieties exist, but are not distant enough to be real species, of species that are very distinct but which occasionally interbreed, to hybridisation, where two species are more distant, to full blown speciation seen in labs with drosophila and with examples of the warblers in Britain.

This quote sums it up better than I can: "In isolation now, the new population can become so differentiated that if contact is reestablished with the parent population there will be no effective interbreeding. This can range from formation of hybrids with reduced fertility through hybrids with reduced viability, to reduction in fertilisation, reduction in mating, to, finally, no mating attempts whatsoever - to mention a few of the many possible blocks to efficient sexual reproduction. In these instances where reproductive efforts are wasted, selection will favour those individuals who mate within their own populations forming productive unions, and the process of isolation will be pushed rapidly to completion."

What we can see is a continuum from a single species that can become isolated and that this isolation can form different races and then sub species and then species.

If your only objection to the above is that they are all the same 'Kind' i.e. Cat kind, warbler 'Kind', finch 'Kind' etc then you are on the right track to understanding evolution. The fact that they are all very similar to each other is a fundamental point about evolution. The large order changes can be discussed later.