2) They must be together for awhile (geographic isolation --> different populations).
I have two problems with this: (1) is that, as presented the bb population just got there and had not had time to interbreed -- therefore "for awhile" does not readily apply, they weren't there long enough, and (2) the bb population could just as easily be a migratory variation to a non-migratory aa population -- they could pass through every year on their migratory route, but still never interbreed because they aren't there during breeding season, whether they could "
potentially interbreed
and give fertile offspring" or not.
There are behavior barriers to mating that prevent interbreeding even if it is possible. The best example of this is the "ring species" the asian greenish warbler:
Greenish warblers
quote:
Greenish warblers (Phylloscopus trochiloides) inhabit forests across much of northern and central Asia. In central Siberia, two distinct forms of greenish warbler coexist without interbreeding, and therefore these forms can be considered distinct species. The two forms are connected by a long chain of populations encircling the Tibetan Plateau to the south, and traits change gradually through this ring of populations. There is no place where there is an obvious species boundary along the southern side of the ring. Hence the two distinct 'species' in Siberia are apparently connected by gene flow. By studying geographic variation in the ring of populations, we can study how speciation has occurred. This unusual situation has been termed a 'circular overlap' or 'ring species'.
Each variation breeds with the neighboring one except at the northern overlap in Siberia where the
viridanus and
plumbeitarsus varieties ignore each other.
quote:
It is relatively easy to hear the song differences in the field, and playback experiments have shown that the birds distinguish between types of songs. A male greenish warbler will aggressively respond to songs that it recognizes as belonging to its own species, intending to chase an intruding male out of its territory, but it usually will not respond to song from a distantly related population. Females probably also use the song differences to distinguish among potential mates.
Different behavior affecting breeding.
The alleles in the area of overlap between
viridanus and
plumbeitarsus are in different proportions than in the rest of the habitat for each subspecies , but they are not interbreeding so it doesn't matter.
Forget the storm - It was just an obvious diversion for those students who didn't do their homework.
Agreed. BUT if one variety had survived more than the other then it would have been natural selection changing the frequency of alleles in the combined population (if we agree that they now make one population, otherwise it is just survival preference for one population over another).
BTW: How would you define a population that "breeds" asexually.
A very good question.
What makes a sexually breeding population 'special' is the population dynamics of gene flow between individuals within a species. This can act to hold a species in stasis where it is adequately adapted to its habitat to survive, as sexual pressure can tend to direct the population as a whole towards an average.
In asexual populations the gene flow is only in one direction, and as each individual organism picks up mutations those will only be passed to their offspring. It very well could be that what we see as single cell "species" are the remains when interconnecting variations have been removed by natural selection.
Genetics can measure the DNA and see how closely related populations are, behavioral and ecological studies can show how they have same reactions to chemicals and habitat, but this may only be an artifact of removal of intermediates to other single cell species.
Certainly such populations are isolated from other similar ones -- in many cases (but not all).
I did a google and found:
(1)
http://members.aol.com/darwinpage/mayrspecies.htm
quote:
There are a number of suggestions of how species taxa in asexual organisms can be delimited and defined, but this is outside the present discussion. However, I find that any endeavor to propose a species definition that is equally applicable to both sexually reproducing and asexual populations misses the basic characteristics of the biological species definition (the protection of harmonious gene pools).
He also talks about other definitions of species that are more historical, but which would have been in effect when asexual species were first type cast.
(2)
Observed Instances of Speciation
quote:
4.2 Cases Where the Biological Species Concept Does Not Apply
There is no unambiguous criterion for determining that a speciation event has occurred in those cases where the BSC does not apply. This is especially true for obligately asexual organisms. Usually phenetic (e.g. phenotypic and genetic) differences between populations are used to justify a claim of speciation. A few caveats are germane to this. It is not obvious how much change is necessary to claim that a population has speciated. In my humble opinion, the difference between the "new species" and its "ancestor" should be at least as great as the differences among recognized species in the group (i.e. genus, family) involved. The investigator should show that the change is persistent. Finally, many organisms have life cycles/life histories that involve alternative morphologies and/or an ability to adjust their phenotypes in response to short term changes in ecological conditions. The investigator should be sure to rule these things out before claiming that a phenetic change constitutes a speciation event.
That seems fairly reasonable. This would be the same kind of difference that is used in paleontology to seperate fossils into different species over time when there is no way to see if {organism 1 million years ago} can breed with {organism 5 million years ago}.
(3)
http://www.msu.edu/course/plb/418/speciesconcepts.htm
quote:
Diagnosability Species Concept
The smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals.
Genealogical Species Concept
Species are defined by basal exclusivity; individuals of a species are more closely related to one another than to any individuals outside the group.
Reasons there is little agreement on species concepts- Variation is complex
- Breeding systems are variable
- Levels of gene flow is variable
- Speciation is ongoing so all stage are present in the environment
- Relative to the number of species in the environment we know too little to generalize
This is the only one they list for asexual (they also list BSC - biological species concept - and some others)
(4)
Birky Lab Home
quote:
Theory and some experimental evidence suggests that the loss of sexual reproduction should reduce the effectiveness natural selection. Asexual lineages should accumulate detrimental mutations, leading to extinction. They should also have difficulty retaining and fixing advantageous mutations, which would make it difficult to adapt to new environments and speciate. In fact the definition of species in asexual organisms is controversial, since the "biological" species definition cannot be applied. We are studying the long-term consequences of the loss of asexual reproduction, and the preliminary results are exciting! The bdelloid rotifers are a widespread group of freshwater invertebrates which have been reproducing asexually for at least 40 million years and undergone substantial differentiation into species differing in morphology, habitat, and behavior.
we used basic population genetic theory to show that asexual organisms, like sexual organisms, should fall into clusters representing independently evolving lineages. We devised a new species concept, the Evolutionary Genetic Species Concept, for asexuals which describes clusters that are comparable to biological species in sexual organisms. We also devised a species criterion, the 4-time rule, that can be used to identify such asexual species from DNA sequences of small samples.
Asexual organisms are also not constrained by the population gene sharing of sexual species so they can have more variation within a population and the only thing holding that in check is survival of those individuals.
From these it looks like asexual species are populations of organisms with similar characteristics that have become isolated by natural selection and time.
This is more of an "amoeba" in time description that what remains after selection over time is a species...
That's the best I've got at this time.
Enjoy.
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