Evidence for evolution

I accept evolution on authority, not because I really know that much
about it.I understand the theory (in general). I don't need to know about that. What I am interested in is knowing what the most solid, most convincing evidence is for this theory.The audience: somewhat educated but not specialized. In other words, you cannot expect them to know the esoteric jargon of evolutionary biologists. Give it to us in plain language. (This might be one of
the reasons for the creationist movement. They don't know what the real evidence is.) I don't, not really.

Thread moved here from the Proposed New Topics forum.

There are two things to consider:
1) The question of whether evolution occured.
2) The way in which it occured.For the first I'd say that the fossil record, the order of it and the relation between different parts of it is very convincing that evolution has occured.It tells us that there was a time when there were no "modern" animals on the planet. There were no reptiles, birds or mammals. At later times there were some of these but not all.The relationship between the nature of the animals and the order in which they appear shows us that the nearer to the top (present) the more like they are today.For the second, the correlation between the DNA of living things and the relationships shown in the fossil record is a very convincing reason to say that the theory of evolution is, in fact, the way in which the developments we see occured.

Great! Very clear. (the DNA reference is not clear, but I can ask about that later).How do we know for sure what's later and what's earlier as regards the fossil records?

(remembering that I'm not a geologist but we have some here that will pounce on me if I get it wrong).Apparently it is pretty clear in most cases then a series of rock layers are reasonably undisturbed. (Some more obvious than others, if you look at some parts of the rockies here they are twisted like a rucked up rug.)When you look at those layers and the composition of the rock in detail you find that there is a pattern of a very specific layer being always under other layers.It seems pretty obvious that the lower layers were laid down before the ones above. This was used a couple of centuries ago to work out the ordering. This still, of course, doesn't tell you how old the layers are just the relative ages.When the details are looked at the geologists of the time figured that the total time involved had to be pretty great because they could see remains of processes (like river sediments etc) that take a long time to lay down.This was nailed down tight as it can be over a century later when it became possible to date the rock layers in an absolute way and independently of the order.This absolute radiometric dating matched the centuries old determined order. I'm not creative enough at this point to figure out anyway that could have happened if the order wasn't a meaningful measure of relative age.The match suggests that the absolute age can be taken as meaningful as well. (though there are other reasons).As for the DNA reference: Message 95The concept here is that there was a set of relationships worked out between the fossils and the time they were laid down. Then many decades later it became possible to compare DNA.If evolution occured you would expect a correlation between the already worked out relationships and the differences in DNA.The msg above shows how good that match is. Very convincing to me.

Great stuff, Ned. First you have the layers and dating and then you have much later the DNA (this is still vague to me) to back it up. I'll study those sites you gave me before asking more questions, but you've made the basics very clear to me.Thanks.

The DNA stuff does get a little more complicated, to say the least.

Is it a matter of looking at an earlier fossil and a later fossil and determining that they are closely related according to the DNA? Something like that?

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Is it a matter of looking at an earlier fossil and a later fossil and determining that they are closely related according to the DNA? Something like that?

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This has been done successfully but only for very shallow depths of time i.e. the oldest reliable fossil DNA sequences are about 80,000 years old. So one cannot look at fossils millions of years old and do a direct comparison. But you can for example do comparisons of DNA sequences among living species that are thought to be closely related or distantly related. Evolution would predict, that for a locus under weak or no selection that the gene would be more simlar among the closley related species (since less time has passed since they diverged and less mutations would have had time to occur) and the same locus would be much more divergent among more distantly related species. Up to a point, the accumulation of mutations can be "clocklike" and calibrated to a fossil date or known divergence event to determine the time of the split. But even without this, one can trace the relationships among the organisms in the form of a tree by the sequence similartiy or dissimilartiy...it is not so much different from morphology only the characters are base changes at the DNA level rather than say tooth differences among mammals. At some point, whole genome sequences will be compared among most species which will be really interesting in determining among group relationships.

What about the very fact that some DNA sequences in the bacterial plasmid codes for the same thing in human? Wouldn't this be an indication of something?

Sure..but strongly conserved sequences are not particularly useful in all levels of phylogenetic or relatedness analysis. If I look at a sequence that is identical between the two groups I am interested in, I am stuck other than to say they both shared a common ancestor with that had the gene or it arrived by horizontal transfer recently. The conservation indicates common ancestry but the variation allows you to tell who is more closely related to who.

Oh, I see. You get the DNA samples mostly from living species. Does the DNA degrade or something when it gets real old?

hi r,
Nucleic acids fall apart pretty quickly. In fact, most of the damage occurs shortly after death and not because of the long time the sample has been buried. For example, there is no correlation between age of a bone and the quality of DNA you can retrieve from it except at the upper boundaries. What I mean by this is I have mammoths that are 4,000 years old that yield no DNA or really badly degraded DNA whereas I have samples that are 13-20,000 years old that yield lots of DNA and are easy to work with. In the literature, this seems to also hold true. The upper bound is that nobody has reproducible DNA sequences from samples over 100,000 years old so that is about the only age-degradation correlation that holds up.When an organism dies, most of the organelles break apart releasing digestive enzymes which destroys proteins and DNA. RNA and DNA both hydrolyze easily (it takes a lot of energy derived from food to maintain a cell). In addition, there are lots of bacteria and fungi that find dead things very tasty and so a corpse will be consumed meaning even more nucleic acid destruction...think of a road kill...the first day you drive by it, it looks like an animal...a week or so later it may just be a mass of unidentifiable bone and some meat attached... In the case of frozen mammoths, the temperature was unlikely stable i.e. they freeze thawed and are like freezer burned meat...yet again, extra damage to biomolecules.In testing modern DNA, you either have freshly killed samples or take blood, hair, swabs from the mouth,cell culture or even feces and immediately prepare the DNA or freeze the samples in liquid nitrogen stopping the degradation process for a while...so working with modern tissues is really much easier since you can get a lot of good quality DNA.Hope this helped.

Has the relatively recent emergence of DNA analytic techniques verified the older conclusions based on the dating methods--the layer the fossil is in, and the "radio-active dating" or whatever they use, to the point of great certainty?Would you say that evolutionary theory (not all the details of how, of course, but general macroevolution, including human evolution)is much more certain than, say, Big Bang Theory?

Well, not being a geologist, molecular biologist, or other of that ilk ( ), I find that the evidence from biogeography is the most compelling. It's the study of the distribution of both living and extinct organisms, and of related patterns of variation over the earth in the numbers and kinds of living things. Both Darwin and Wallace independently derived the theory of evolution via natural selection from examinations of biogeography. As one example, whereas a particular habitat type might exist in multiple places around the world (similar physical conditions, availability of resources, climate, etc), species are invariably more closely related to species in adjacent, albeit different, habitats than they are to species in identical habitats elsewhere. A related but even more compelling line of evidence revolves around endemism - an endemic species is one that is found in a restricted geographic area and nowhere else — regardless of whether the specific habitat type exists elsewhere. Patterns of endemism showing in-place evolution are some of the most persuasive examples of evolution and adaptation that exist.I'll be happy to provide specific examples if you'd like.