Proving Evolution in the Age of Genetics

Well, now, the structure is the same in every single species we've ever encountered; a double helix comprised of four different complimentary nucleotide bases.It's the sequence of those bases that is the difference between species, and it's established beyond a doubt that the mechanisms of evolution can change those sequences, without limit as far as we know.I mean this was one of the first things to be tested when we discovered the structure of DNA and the mechanisms of inheritance, and it was fairly trivial to do so. If you care to search at pubmed.org you can see thousands of papers getting right into the specifics of how a given selection pressure on a population caused an effect in the DNA of the individuals of the population. Yes, much. Not least of which is the observation of new species arising. When we check we find distinct genetic differences between the new species and its parent species. Well, the smallest possible change to a DNA sequence would be the substitution of one base pair with another. That's pretty common. That's impossible to answer at this point; we've only sequenced the genomes of very few species, so we have no basis to determine which are the most similar out of all possible combinations.

I think he's just fast and loose with his terminology. Of course, natural selection doesn't actually influence the genes of an individual. Only mutation does that. Natural selection selects among individuals, which causes changes the the frequency of alleles (alternate genetic sequences) in a population, which is what evolution is - changing allele frequencies.In other words, mutation causes changes in the genetics of an individual; selection causes changes in the genetics of a population.

It does, and it can. Not all dogs can mate with all other dogs. For instance good luck getting a St. Bernard and a bichon frisse to mate - they're physically incompatible. If it weren't for the existence of intermediate varieties of dogs, there'd be no gene flow between large dogs and small dogs, and they would be different species of dog. All it takes to do that is a change in the sequence of the nucleotides, which is what mutation and selection do.

So are we. How did it get that way?By the cessation of gene flow between two populations. That's how new species are formed. All dogs might be the same species now, but if you eliminated the intermediate dogs, you'd wind up with two populations of dogs that could not interbreed physically; after a few generations they'd no longer be genetically compatible, either.

The alleles do change, though mutation. To clarify, BTW, alleles don't have genotypes. Individuals have a genotype. What I'm telling you is that there is no quantum leap; horses and donkeys are different species, yet they're not completely genetically incompatible. Only incompatible enough that their interspecific offspring are sterile.There is no quantum leap. There's a continum of speciation, which leads to certain ambiguous situations where we're not sure if we're looking at multiple species or simply subgroups of one species.

I don't think they'd recognize each other as mates. (That's "behavioral reproductive isolation.)

Except they can be crossed to form ligers (which fans of Napoleon Dynamite know are bred for their skills in magic), which can at least occasionally breed true; or tigons, one of which at least was able to mate with a tiger.They don't mate in the wild because they don't share many habitats, and generally don't recognize each other as mates.So, yeah. Gene flow is possible between them, but negligible in the wild. That, and morphology, is sufficient reason to consider them different species.

Oh, you're talking about chromosome number. CB141: Differing chromosome numbers

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Chromosome counts are poor indications of similarity; they can vary widely within a single genus or even a single species. The plant genus Clarkia, for example, has species with chromosome counts of n=5, 6, 7, 8, 9, 12, 14, 17, 18, and 26 [Lewis 1993]. Chromosome counts in the house mouse species (Mus domesticus) range from 2n=22 to 40 [Nachman et al. 1994].

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Chromosomes can split or join with little effect on the genes themselves. One human chromosome, for example, is very similar to two chimpanzee chromosomes laid end-to-end; it likely formed from the joining of two chromosomes [Yunis and Prakash 1982]. Because the genes can still align, a change in chromosome number does not prevent reproduction. Chromosome counts can also change through polyploidy, where the entire genome is duplicated. Polyploidy, in fact, is a common mechanism of speciation in plants.

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DNA doesn't decribe organs. It only describes proteins. It's a common misconception that DNA is a "blueprint", in other words, it contains a kind of encoded diagram or plan of the organism's body. This isn't apparently true. All DNA contains is descriptions of proteins. If we've learned anything from the human genome project, it's that the human genome doesn't contain all that many genes. A lot less than we used to think, in fact.

Er, wait now, more chromosomes doesn't mean more genes. And humans only have about 25,000 genes to the fruit fly's 13,000, so that's nowhere near the 3-4 times you suggested. Mutation and natural selection can do this. I'm pretty sure examples have been given, but there's literally thousands of examples in the literature.

Then I guess I don't understand what you're talking about. I have a suspicion you're trying to compare the "complexity" or "advancedness" of an organism and are trying to find a representation of that advancedness in that organisms DNA; in other words you'd like to say that humans are more complex than fruit flies, and so we must have more complexity in our DNA, and so how do you increase the complexity of DNA in order to turn a fruit fly into a human?Well, it doesn't seem to work like that. The DNA of all metazoans seems to be roughly of the same complexity, no matter how complex the organism might be. Yes there is, but we might as well say that a given species has a random number of genes. Number of genes has very little to do with speciation, and it actually changes over time as genes get knocked out or disabled through mutation, or new genes are activated. For instance I heard somewhere that humans are losing genes on the Y chromosome as it gradually shrinks throughout the population.By way of comparison, humans have 20k to 25k genes. C. elegans, a commonly researched worm, has 19.5k, and a flowering mustard plant of the family Arabidopsis has 27k. That's like asking "how fast do you have to run to win a race." The short answer is, it depends. It could be one. It could be many. Often just one new protein spells the difference beween life and death.

What you describe is often called "species idealism", the idea that all members of a species are variants of one essential "perfect" member of their species, and that they all share an innate essence that makes them (for instance) dogs and not cats, or whatever.It's an error of thinking that leads so naturally to the concept of animals being grouped into original "kinds".

Transition from what to what? In order for there to be a transition, there has to be a period of no transition.But species are always in transition. We speak often of events that separate populations and eliminate gene flow in between them; these speciation events are the first step to a new species, but they do not themselves create new species.It would be better to ask "what allows a population of variant individuals to interbreed?" The answer to that will help illuminate you in regards to how populations differentiate into different species.