@parados,
The biological species concept remains in effect.
A species is a population whose members are able to interbreedfreely under natural conditions.
Not all bilogists agree with the biological definition because its full of exceptions and difficulties (eg
are Ursus horribilis and Ursus maritimus merely subspecies of each other? . The majority of evolutionary biologists accept the biological definition . They understand the concepts of hybridization in species at geographic contact points or in captivity. Or, hybridization in our gardens by artificial diversity expansion via propogation (Both asexual and sexual). The existence pf hybrids proves only that members of the hybridizing species are closely related to each other. When ranges are proximally forced by environmental factors and niches are compressed , (as is occuring in the High Arctic), some hybridization may occur especially when additional environmental niches occur via some kind of external intervention (like Garbage dumps in the CHrurchill area of Hudson Bay-here there be polar bears, AND barren ground grizzlies). These two animals, while initially having different habitats, are now both becoming m ore opportunistic and niches (and chances for hybridization) overlap.
Studies on polars and grizzlies show that the females produce chemically different pheromones , while brown bears produce pheromones of very similar nature Hence weve seen some hybridization between these two.
Using SNP and STR alleles using " genetic knock- out species" has shown that we can retrograde speciation via altering an SNP to a previous nucleotide letter (for example, genetic knock out chickens and mice can be induced to recapture morphological fetures that had been lost since the Jurassic (eg, chickens with teeth buds) and the Cretaceous (Insectivores with marsupial traits).
While speciation is a favorite topic to beat about the head and neck, the fact is that the biological definition "works fairly well" and, after all. Hardy- Weinberg predicts that a large amount of genetic variation occurs during every mating congress. The "hopeful monsters" of the Hardy Weinberg endmembers may just be sitting there waiting for extinction, OR, the environment may change , giving a hopeful monster a slight advantage.
The two competing definitions of species in use by evolutionary biologists (not so much competing as they are indpendently focused) are the biological definition on which I expanded a bit, and a "genetic definition" which is more loaded with statistical inference and population stats (gene frequencies) The first looks at the phenotypic variation, and the latter looks at the genotypic.
I just stand around the genetics labs and wait for film at 11. Paleo species are easy. We just measure a significant change in a fossil, measure its mass and metrics, and then carefully try to fit it within a geologic period (plus or minus a chron or three). Geologists /paleontologists. IMHO, have revoked their hold on evolutionary evidence, since most of us use fossils more for applied science rather than the theoretical. In fact, genetics is showing that the clades of heretofore assumed-to-be "related" chronospecies are more easily assigned genetic relationships among totally different species rather than some that only appear morphologically related.
For example, weve had several species of "sabre toothed" placental mammals , marsupials , and even a few from among members of one order of multituberculates (An order that was wiped out in the Cretaceous extinction). It wasnt only "cats" that produced sabre toothedness it was at least 10 separate chronospecies of different orders. The thing in common that defined sabretoothedness was a single SNP(ic) allele.
Anomie claimed that all the evidence for evolution is "merely" circumstantial. So is the evidence of the theory ofUniversal magnetism and the ATomic theory. Being merely circumstantial relates to one , maybe two components of the scientific underpinnings. WHEN ALL the "circumstantial evidence" supports the theory, it becomes compelling to all but those whose worldview requires stubborn adherence to ignorance of the theory .
The theory works fine, is testable, is the basis of predictions, is the basis of much of recent pharma advances, and, unlike any competing hypothesis of how life developed on the planet NOTHING REFUTES IT.