1. Species Are Not Immutable: given enough relative change, groups with common ancestry will not be able to or will choose not to mate.

This is just common sense. If two animals are different enough, they will not, or even will not be able to breed. And the difference between 'will not choose to' and 'will not be able to' may only be a matter of time, given natural selection.

2. Evolution Is Gradual: Spontaneous, large scale change is not possible. Between two largely differing points in the ancestry, it will be possible to find an intermediate stage of development.

Insofar as the fossil record is complete, this assertion has not been disconfirmed. We do not, for example, find humans before we find mammals. As the years have gone by since 1859 and the fossil evidence has mounted, the age of each new piece of fossil evidence has placed it within this series just as predicted. This is also supported by genetic evidence. Random DNA mutations correspond to relatively small scale variations in physicality. The fact that you can catch the common cold each year and not become immune to it is an indication of this kind of variation.

3. Evolution Is Driven By Natural Selection: populations are variable; some variations give a differential advantage in numbers of offspring; some of those variations are genetic and will be passed on.

Natural selection is extremely well-confirmed, empirically. It has been observed in the wild and in the laboratory numerous times. Given what we now know about DNA, it makes perfect sense. It also explains why antibiotic-resistant strains of bacteria have been bred by the improper use of antibiotics, and why we can breed animals for traits by practicing artificial selection.

4. Evolution Leads to Speciation: Granting Assertion 3 leads to a mechanism by which the truth of Assertion 1 is manifested.

To grant Assertion 3, which is very well-confirmed empirically, and not to grant this assertion is like admitting that I can walk at a rate of 1 foot every hour, but arguing that I could never go a mile at that rate. If you admit that natural selection is a mechanism for change over time (however slow), given enough time, there will be big changes.

As for hard evidence, there is plenty. The Talk Origins Archive at lists four new species that have arisen in historical times, fully cited in the scientific literature.

Also, consider what are known as 'ring species'. As species spread around some very large obstacle, such as an inhospitable climate or geography, they adapt to their new locations. But when the species meet again on the other side of the obstacle, they no longer interbreed, even though around the other side of the ring, they still do. Several such species have been identified.

As Don Lindsay describes it:

There are several ring species, but the most famous example is the herring gull. In Britain, these 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

The mechanism for Speciation has been demonstrated in the laboratory, as well. John Rennie (2002) cites the following experiment:

William R. Rice of the University of New Mexico and George W. Salt of the University of California at Davis demonstrated that if they sorted a group of fruit flies by their preference for certain environments and bred those flies separately over 35 generations, the resulting flies would refuse to breed with those from a very different environment.

Of course, artificial selection by humans has produced the same result. Dogs do not breed with wolves, nor do pigs breed with wild boar.

5. All Organisms Share a Single, Common Ancestry: This is an conclusion which follows from all of the previous assertions.

It would be hard to argue that the other assertions are true and this one is false. It would be like admitting that all of the stars and galaxies are heading away from each other, but denying the Big Bang. It is of course possible to deny the Big Bang; but to make a credible scientific argument, you should come up with a different naturalistic explanation for why the stars and galaxies move as they do.

Please return to Empirical Doubts.