The code that is DNA

[Paleophyte]

Yes, each successive representative in a line could likely mate with the previous one.  This has already been addressed...repeatedly...and is not an objection to speciation, or the definition of species.

Do you have an objection to speciation, an event that we have observed in the lab and the field...or not?

See this is what I mean lol. You asked what the issue is, I told you, and then it doesn't matter unless the issue is specifically what you want it to be.

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To expand on my previous post, the idea of species seems strangely unnecessary. It almost seems a byproduct of creationism, which does have the idea of kinds to which individuals belong embedded in it. Either that or we're dealing with a psychological predisposition to group things together, projecting categories into nature, where nature has not placed them.

The idea of species seems incompatible with evolution, which is ironic since evolution is almost the front-runner of the idea. The whole "notion of phylogeny," as you say, starts off with species. But common descent and gradual change ought to blur all boundaries between organisms, such that speaking of this or that species no longer makes sense.

Specially since you're also fond of genetic similarity representing relatedness. If I share 5% of my genome with mushrooms, the idea that I'm a different species from mushrooms ought to be reduced by 5%.

Aside from allowing us to distinguish between you and a mushroom, the species concept is a rather handy one for evolution. It marks the point of no return, where two populations are no longer capable or reproduction and no further genetic transfer is possible between them. Species can evolve separately.

Your difficulty with species and time is resolved simply by the fact that nobody will be travelling to the past to mate with their ancestors from millennia ago. Other than some squabbling between splitters and groupers when species are in the process of splitting this is rarely an issue.

Consider once more the inspection tour of our species, but this time let's include the chimp brigade for comparison. As you march down the lines of our most distant ancestors they're indistinguishable. They are, in fact, the same. As you keep marching you'll start to notice some subtle differences between the ranks of Homo and the ranks of Pan. You may never be able to put your finger on the exact instant, though the emergence of our chromosome 21 may mark this particular distinction, but at some point in your inspection it's going to become inescapable that you're dealing with two populations that are reproductively incompatible. Now isolated by an inability to interbreed, these two populations diverge geneticallly. Time and continued evolution produces more differences and divergence, leaving you with what are undeniably two entirely different species in the present.

Without species and speciation there would be significantly less of a barrier to genetic transfer between organisms and evolution would be much messier. You'd be a much larger proportion mushroom, much of it through more recent inheritance.