What's empirical about evolution through natural selection?

Darwinian evolution through natural selection is the process whereby heritable properties establish themselves in a population as a result of the higher propensity of entities possessing those properties to survive and reproduce within a particular environment, which itself may change in time.

When stated in this form, evolution through natural selection depends on two elements.

• the possibility of heritable variation, which may itself be subdivided into two subcomponents, variation and inheritance, although this further subdivision isn't relevant to this post, and
• that survival and reproduction is a function of the suitability of an entity to an environment.

The second element is tautological. The entities that survive and reproduce in an environment are by definition better suited to that environment than those that don't. Darwin called this phenomenon "natural selection" to contrast it with selection performed by animal breeders. Thus if one asks what aspects of evolution is empirical, the answer would have to be whether or not heritable changes occur in nature. The rest follows logically.

Of course Darwin was quite aware of the existence of heritable mutations. We (as farmers) have been breeding plants and animals for centuries. One might call the process of plant and animal breeding "evolution through human selection." What Darwin and Wallace noticed was that selection by nature may be understood on the same terms as selection by man — that by virtue of the fact that some plants are more likely to survive and reproduce than others, the environment itself "selects" for breeding some plants and animals over others. In other words, the environment acts like the farmer. The primary difference is that farmers select variations on the basis of their value to the farmer. The environment selects variations without intention, purpose, or reason: those that survive are selected.

It's easy to see why this model led Darwin and Wallace to think that small changes built up over a long period produce major evolutionary shifts. That's the way it happens on the farm. What Darwin and Wallace apparently didn't count on was the possibility of major changes in the environment, which would lead to major changes in what survives.

As we observed above, the mechanism of evolution by natural selection has two components: variation and selection. Variation, for the most part, involves relatively small changes. It is these small changes that Darwin and Wallace focus on. But selection, i.e., what actually fits the environment, depends on the environment. If the environment changes significantly — the temperature rises or falls significantly, the oceans cover or uncover land masses, the bio-environment itself changes as a result of activities of successful species, etc. — what will survive under the changed conditions is likely to be significantly different from what survived under the previous conditions. Major environmental changes are likely to effect many species at once, leading to a cascade of changes. It is this process that produces the punctuated equilibrium model we now understand as characterizing evolution.

Here's how Darwin put it in the conclusion to "On the origin of species."

Under domestication we see much variability …

Variability is not actually caused by man … But man can and does select the variations given to him by nature, and thus accumulates them in any desired manner. He thus adapts animals and plants for his own benefit or pleasure. He may do this methodically, or he may do it unconsciously by preserving the individuals most useful or pleasing to him without any intention of altering the breed. It is certain that he can largely influence the character of a breed by selecting, in each successive generation, individual differences so slight as to be inappreciable except by an educated eye. This unconscious process of selection has been the great agency in the formation of the most distinct and useful domestic breeds. That many breeds produced by man have to a large extent the character of natural species, is shown by the inextricable doubts whether many of them are varieties or aboriginally distinct species.

There is no reason why the principles which have acted so efficiently under domestication should not have acted under nature. …

If, then, animals and plants do vary, let it be ever so slightly or slowly, why should not variations or individual differences, which are in any way beneficial, be preserved and accumulated through natural selection, or the survival of the fittest? If man can by patience select variations useful to him, why, under changing and complex conditions of life, should not variations useful to nature's living products often arise, and be preserved or selected? … I can see no limit to this power, in slowly and beautifully adapting each form to the most complex relations of life. …

On the view that species are only strongly marked and permanent varieties, and that each species first existed as a variety, we can see why it is that no line of demarcation can be drawn between species, commonly supposed to have been produced by special acts of creation, and varieties which are acknowledged to have been produced by secondary laws. …

As natural selection acts solely by accumulating slight, successive, favourable variations, it can produce no great or sudden modifications; it can act only by short and slow steps. Hence, the canon of "Natura non facit saltum," [nature makes no leap] which every fresh addition to our knowledge tends to confirm, is on this theory intelligible.