A Question on One of Rabbi Natan Slifkin's Ten Questions

In the June 2, 2014 issue of the Jewish Press online, Rabbi Natan Slifkin wrote an article entitled Ten Questions on Evolution and Judaism.

His questions, and their answers, appeared to be aimed at two types of people.  Those who say, "Evolution!  Ptui!" and those who say "Evolution, duh."  Since those two positions are extremes, I'd like to raise a problem with his second question/answer from a position which lies between those extremes.  I hope R' Slifkin will do me the honor of replying to this question.

2) Why should schools accommodate evolution? Isn’t it just a theory, not a fact?

Here's my problem.  He refers to "common ancestry" as "the fact of evolution", as opposed to "the theory of evolution", which refers to the way in which evolution happens.  In other words, gradual mutations.

He states:

One is common ancestry, the concept that all animal life arose from a common ancestor – simple organisms gave rise to fish, fish to amphibians, amphibians to reptiles, reptiles to birds and mammals (without getting into how that could have happened). This is supported by a wealth of converging evidence along with testable predictions. Common ancestry is considered by all scientists (except certain deeply religious ones) to be as well-established as many other historical facts, and is thus often referred to as “the fact of evolution.” It is of immense benefit in understanding the natural world – for example, it tells us why whales and bats share anatomical similarities with mammals, despite their superficial resemblance to fish and birds.

This troubles me.  I'm aware that there are similarities between species, and that it can be useful to draw conclusions from those similarities.  But I don't understand how it is a "fact" that bats and whales and chimpanzees have a common ancestor.  I understand how that has explanatory power, but not how it is necessary.  In other words, it's similar to the difference between correlation and causation.  The idea of common ancestry correlates with what we see, but it isn't the only possible idea that correlates with it.

By way of analogy, let's consider the musical concept of "variations on a theme".  In variations on a theme, a composer may create numerous compositions which are all... well, variations on a single theme.  If one were to discover these variations at different times, one might conclude that they started with one version, and that each successive version was a modification, one further step away from the original each time.  Kabbalists as well were known to write variant forms of a given text for different purposes.  The most famous case is probably the hymn Yedid Nefesh, which is sung in many congregations on Erev Shabbat.  For centuries, the words of this hymn were known, though there were minor variants.  In the mid-20th century, what has been determined to be a manuscript of the hymn in the author's own hand was discovered, and since then, a small number of congregations have switched to the substantially different version.  It has been theorized, however, that the author deliberately created a version of the hymn with fewer explicit Kabbalistic images for general consumption.

Creating similar but different versions of a single thing, to serve different purposes, is a hallmark of creative action.  Why do the anatomical similarities between different mammals require a common ancestor?  Even assuming that one species can evolve into another (something which has been demonstrated in the lab), how can it be established as "fact" that this is indeed the process by which all species came to be?  The idea of extrapolating such a process back to a point source seems like an extraordinary claim.

Of course, if one begins from the premise that God cannot have had a hand in it, Occam's Razor would lead us to the extrapolation to a point source.  It is certainly the simplest way to explain things in the absence of any intelligent design.  But how does that qualify it as "fact"?

Given two books by the same author, I can draw many conclusions about one from the other, particularly because I know they share an author.  Similarly, it's useful to do the same with biology.  But in terms of logic, I don't see how "the fact of evolution" follows as a necessity from the physical evidence.

(I expect that those who find mere mention of evolution anathema will consider this article nearly as bad, and that those who consider belief in the theory of evolution b'chol dikdukeha u-prateha to be the sine qua non of civilized thinking people will see this as a sign that I'm a primitive ignoramus.  Everyone is entitled to their opinions.  However, I have moderation enabled here, and I won't be allowing gratuitously obnoxious comments through.)