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Is the argument from similarity valid?

In his book “The Greatest Show on Earth”, Richard Dakwins says that the universal code of DNA is the greatest evidence that we have a common ancestor. He refers to the figure that chimps share 98% of our DNA. The problem is that a universal code across species also supports the idea of a common designer. If there wasn’t a universal code, it would be greater evidence against a designer. This goes for anatomical similarities as well. It seems that the evidence of similarity is only as good as our pre-supposed worldview.

Posted: February 9th 2011

Mike the Infidel www

Materialist science begins from no presuppositions. It makes no claims about the essential nature of a physical process – whether caused by purely natural means or a supernatural being. People who believe in an intelligent designer, on the other hand, make a huge array of presuppositions about the nature of reality, the limits of natural processes, what existence means, and so on.

One of these works from the evidence. The other works from pure assertion. Which one is more justified?

Posted: February 12th 2011

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logicel

Evolution is a scientific theory—extremely potent in its explanatory and predicting aspects—and a FACT, not a presupposed worldview. Christian creationism is, however, a presupposed worldview.

Posted: February 11th 2011

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SmartLX www

The evidence for evolution isn’t simply the fact of similarity, it’s the extent and the type of similarity.

All the square wooden furniture at Ikea is made from the same kinds of components, and in fact many of the same exact components, from nails and cam bolts up to large boards and planks with holes in the same places. We know they’re all designed, and by the same team of designers, so no evidence for evolution there.

If you were to find instructions for a desk where nails are to be hammered into a single block of wood apparently for no reason, but you knew of an earlier desk where another block is attached at the same spot, you wouldn’t think that this was part of the design. You’d think it was a holdover from the previous form that no one had bothered to remove when it ceased to be useful.

Vestigial features, like the human appendix or the leg-bones of a whale, are the equivalent in living things, and they’re everywhere. They might still have some limited function (or not), but you can easily see how much more useful they would have been in their host creatures’ differently shaped ancestors, and/or how much more useful they are in distant cousins.

They demonstrate one of two things:
1. that modern animals are descended from animals who were so very different that they could not have belonged to the same species or even genus (or “kind”) or
2. if they were all designed as is, then the designer was incompetent.
The first option is entirely consistent with evolution by natural selection. The second option is INconsistent with designs by any sufficiently powerful and knowledgeable being.

Vestigial features are just one instance of the evidence for evolution from similarity. The DNA equivalent, “junk DNA”, is another. Suffice it to say that the rest goes way beyond “all animals are similar, so they’re related”.

Posted: February 10th 2011

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Eric_PK

I think you are conflating two concepts. The first is that all life in earth is DNA-based, and the second is that organisms share a common ancestor.

If one were going to design a biosphere, it would make sense to use the same building blocks across the whole area, because the animals have to be able to eat the plants to survive.

However, there’s no requirement that you keep similarity between animals. You wouldn’t, for example, design human embryoes to have tails that are then reabsorbed, nor would you design whales with tiny vestigial thigh bones, nor would you design humans with wisdom teeth that most often cause problems. None of these make sense if a designer was doing things, because they are mistakes.

Before the ability to sequence dna was available, biology had classified all organisms by how close they appeared to be related. When sequencing came along, it showed that if you look at a genetic measurement of how closely related two species are across a broad range of species, there is widespread agreement with previous classifications. Evolutionary theory predicted the measurements before they were made, and when they were made they agreed with the theory.

Posted: February 9th 2011

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George Locke

One of the predictions of modern evolutionary theory is that related organisms will share much of their DNA. The closer the relationship, the more similar the DNA. Given multiple lines of evidence and argument that point to a close evolutionary tie between humans and other apes, the fact that as a group we share much of our DNA may be counted as a confirmed prediction.

A designer is presumably constrained only by his/her creativity. If there were a designer, and that designer were God, it seems fair to suggest that the designer would have infinite creativity. Rather than expecting that living things would have much in common, with commonalities correlated to relatedness, we would expect no constraints on design. There is no a priori reason to expect similar species to have similar genotypes. In fact, the design hypothesis can’t explain why all life is based on DNA at all: why don’t plants or bacteria use some other molecule to store their genes? If God had infinite creativity, surely it could have found other molecules that would have done the job just as well, right? Under the design hypothesis, the ubiquity of DNA (and of ATP, the genetic code, etc) can only be explained by appealing to the caprice of the designer. Needless to say, this isn’t much of an explanation.

Posted: February 9th 2011

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