I went to America a few weeks back and met someone who held an opinion I had never seriously considered before - they didn't believe the theory of evolution. To me, this was bizarre, like saying you didn't believe in tables or something. Since then I've started noticing nodes like The Theory Of Evolution Is Not A Valid Theory. This is my small attempt to stand up for sanity.

The five virtues of a valid hypothesis are laid out in 'The Web Of Belief' by W.V. Quine and J.S. Ullian (one of the foremost works of ontology). Those virtues are conservatism, modesty, simplicity, generality and refutability. So how does the theory of evolution hold up?

  • Conservatism
    Defined as: "The less rejection of prior beliefs required, the more plausible the hypothesis - all other things being equal." It seems evolution doesn't do very well here, as it turned most previously held ideas on their heads. But note the rider that Quine and Ullian added: "all other things being equal". Before Darwin, the origin of the species was an area populated by many beliefs but few theories. Evolution does not affect any other proven facts, only theistic beliefs and human vanity.

  • Modesty
    Evolution is entirely self-contained. There is no corollary which we must hold to be true before we can accept evolution as being true. Compare with other theories, which require a belief in a second hypothesis (for instance, the existence of an omnipotent god). Evolution stands or falls on its own.

  • Simplicity
    The cast of Seinfeld are descended from amino acids? Preposterous! (well, except for George) Actually, while the implications of the theory are enormous, the fundamental idea of natural selection is extremely simple. We know that offspring are slightly different from their parents. We also know that organisms that are suited to their environment will live longer than unsuited organisms. Occam would be happy.

  • Generality
    Evolution is not a special case for humans. It applies to all organisms everywhere in the universe. This makes the theory not only valid, but extremely useful. It can even be applied to non-biological entities, such as E2. Other theories about the origin of humanity suffer here, as they treat humanity as a special case and fail to account for the emergence of the billions of other species to have lived on this planet.

  • Refutability
    The theory of evolution does not contain loopholes or unverifiable opinions. It does not, like most other theories which deal with mankind's origins, assume some momentous event of which no records or evidence exists. It can be refuted quite simply - produce a fossil which clearly doesn't fit into any evolutionary tree. If even one appears that can't be explained, then the entire theory is bunk and we have to go looking for a new one.
Of course, none of this actually proves the theory of evolution. But it is, without a doubt a valid theory.
Notes:
1. This is not intended to be a discussion of biology, but logic. As I said, it was inspired by The Theory Of Evolution is not a valid theory, which attempts to use rules of basic logic to disprove the theory. Oh, and by the way, predictiveness is not an essential virtue of a good hypothesis.

2. Religion is mentioned quite a bit here. It's not that I have a problem with faith, it's just that creationism is pretty much the only alternative to evolution. If anyone knows of a decent scientific alternative to evolution, please let me know. I'd love to hear it.



BTW - I, personally, don't like back-and-forth arguing between writeups, so I'm only going to answer questions by /msg. Any errors in this writeup which are brought to my attention will gladly be amended.
There are many biological reasons, also, why the Theory of Evolution is valid. I will attempt to cover the current evidence biologists use to support the theory, adding my own attempt to stand up for sanity. As for bol's mention of religion; many religions have come to accept that creation is not a valid theory.

Biologically, there are 6 major areas that support Evolution: Fossil Record, Embryology, Comparitive Anatomy, Study of Intermediate Forms, Biogeography and Molecular Biology. These are discussed briefly below.
  1. Fossil record

  2. Embryology
    • During the embryonic development organisms often develop structures that resemble those seen in ancestral adults.
      e.g. Human embryos with gills, gills are lost later in development

  3. Comparitive Anatomy
    • Organisms may share basic structural forms that suggests common ancestry (Homologolous structure).
      e.g. Forelimb structures

  4. Study of Intermediate Forms

  5. Biogeography
    (the study of the distribution of organisms)
    • Evidence of modern animals being decended from ancestors in the same area.

  6. Molecular Biology

As with bol's write-up, these facts do not prove the theory of evolution without doubt, but they do show it to be a very valid theory.
This is NOT a cut-and-paste write-up.
Evidence for Evolution

Biogeography presents the most easily noticeable evidence for evolution. Species in different parts of the world are different, with many areas having their own indigenous ones. On the other hand, species close to another geographically are usually much more similar. Islands are examples of this--commonly groups of islands have closely related, but still different, species on them. This shows the migration of a group of individuals from one species to another location, which then forms a separate species. Another case, the fact that species inhabiting very similar environments located in different regions of the world are not similar, shows that the randomness of evolution produces different results even under similar circumstances. This is contrary to many previous beliefs (held by people as renowned as Christopher Columbus) that life in similar environments is similar, but which do not hold up to actual observations.

The fossil record also provides a large amount of evidence for evolution. The ages of fossils conform to the ideas of evolution. Prokaryotes, declared to have evolved first, are the oldest of fossils, followed by eukaryotes. Vertebrates come towards the very end, with humans one of the youngest of species. The fossil record also provides many examples of transitional fossils, which show a progression of intermediate species between one species and another. The differences in fossils of similar species over time show modifications amassing into much more visible changes.

Similarities between related species also supply evidence for evolution. Species often share the same underlying structure, for example the forelimbs of mammals are very similar, despite having very different functions. These homologous structures are a result of the two species resulting from a common ancestor. Likewise, vestigial organs (such as the appendix in humans) are left over from ancestral species, in which they served some important function, but this function became useless in subsequent descendants, and so the organ became less and less function.

Possibly the most important evidence for evolution comes from molecular biology. Common genes between species, even those as different as humans and bacteria, are often similar in function, and those among even more closely related species can be nearly identical. Proteins that do common tasks are also often very similar between species. This is a result of organisms sharing a common ancestor. Genes that are absolutely necessary for life are, as a result, prevalent in all life, and remain unchanged because any changes must be fatal almost all of the time.

The theory of evolution is a valid theory because it is in fact testable despite what The Theory Of Evolution is not a valid theory states. The test for the theory was stated by Charles Darwin in On The Origin Of Species.
The test is whether there exists a biological structure which cannot have come into existence by a series of smaller evolutionary steps.
Individuals who believe that there do indeed exist these structures call their theory Irreducible Complexity. They examine complex structures such as the eye, and brain but also many other structures to check whether it seems possible for it to have come from the process of evolution.
That makes it a hypothesis according to science, which defines a hypothesis broadly as any idea which there exists a test/tests for which would falsify that idea.
A theory is normally a collection of multiple hypothesises to explain something, and which has been tested over and over and repeatedly failed to be falsified. That does not make proven however, The problem of proof says that nothing in science is ever scientifically proven no matter how many commercials state otherwise. However it does mean the idea is well supported by testable evidence, that the tests done are repeatable and the same or very similar results will be found if those tests are repeated. As such theories are assumed true until new evidence comes along which challenges that belief.
Evolution meets all the conditions for a theory as defined by science. It is therefore a theory.

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