First of all: this blog is really helpful, great work!
So now, while trying to write a report on ants --focusing on aspects of evolution-- I stumble upon many many questions. I'm hoping you can help me out!
I was thinking about invasive species that are nowadays found in several continents, like for instance the Argentine fire ant.. Do all of their populations still belong to the same species? (no subspecies)
If so, how come? And would a male and female both from a different continent still recognize each other as potential mating partners or not. (If not, what's the term for that?)
What are specific factors that trigger origination of (sub)species of ants, or what causes the absence of it.
1b. Macro-evolution.. Can we speak of macro-evolution within the ant family, since there are so many different varieties. If not, could the relation between wasps and ants be an example of macro-evolution or is macro-evolution really about even bigger events?
2. Sexual selection
In the mating of ants, is there any 'conscious' selection going on from either gender. Are there species where an individual for mating is picked over another, based on qualities perceived?
Thanks for your questions! You've gotten to some really awesome, fundamental evolutionary biology questions here, and you're asking me to make generalizations across a family of insects with more than 100 million years of evolution and more than 10,000 species, so I'm not sure I can do them justice in a blog post, but I'll try!
Your sub-question under heading 1a: "what are the factors that trigger origination of (sub)species of ants, or what causes the absence of it," is a question that can only be answered in a very generalizable way: speciation occurs when some factor causes populations of organisms to begin separate evolutionary trajectories. Often, as you suggest with respect to invasive species, this happens because of geographic isolation (allopatric speciation), but there are many theoretical and empirical studies that allude to the possibility of sympatric speciation, which is when speciation happens while populations of organisms are still within "cruising range" of each other. A previous blog post elaborates on speciation in the context of nest parasites.
Your first question about invasive species is great and very timely! There have been at least two studies in the past few years which demonstrated that Argentine ants (Linepithema humile) from different continents actually recognize each other as nestmates! So I can't imagine there would be any trouble with mating there.
The authors of both papers suggest that nestmate recognition is maintained across different continents because there is a steady stream of new arrivals, which prevents the populations from drifting apart. However, the question of when exactly a speciation event happens is very difficult to pin down. In a classic paper on the "Evolutionary species concept," the icthyologist EO Wiley states that "A species is a single lineage of ancestral descendant populations of organisms which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate."
By this definition, you would really have to be able to predict the future: how can you know whether a newly isolated population will come into contact with propagules from its ancestral range? The idea that newly isolated populations are incipient species is tempting, but in practice, species can only be delimited if there are morphological and/or molecular differences in them, or, if you subscribe to the biological species concept, things could be argued to be different species if they generally choose not to mate with each other (for more information, you might want to look up pre-zygotic isolation and assortative mating...). In the case of the Argentine ant, I would expect a sudden drop in propagule pressure would result in the actual isolation of the disparate populations, but I would only expect this to happen if humanity drastically changed or ceased its practice of global trade and travel.
Speaking of pre-zygotic isolation and assortive mating, I think it makes sense to talk about sexual selection in the context of micro-evolution, because that's potentially a pretty important driver of speciation and trait evolution in sexually-reproducing organisms. Stearns and Hoekstra, an often-recommended text in evolutionary biology, defines sexual selection as: "The component of natural selection that is associated with success in mating." While at first somewhat disappointing, I think this definition is useful because it underscores the fact that sexual selection is a component of natural selection. Many ants form mating swarms, or leks, especially in desert and temperate zones. In these cases, there is a certain amount of "scramble competition," and differential levels of mating success have been demonstrated to correlate with individual traits. However, I am not aware of anything approaching the level of mate choice and the resultant secondary sexual ornamentations that has been demonstrated in some butterflies, odonates (dragonflies and damselflies), or other animals.
For a variety of reasons, the exaggerated secondary sexual traits that seem to emerge in classical examples of sexual selection (i.e., the tail of male peacocks) seem to be less likely to develop in eusocial insects (for a more in-depth perspective on this, check here, here, and here). In lekking species, there is likely to be a very important trade-off between the relative sizes of the flight muscles and the testis and ovaries. Some species of ants do not lek, and either engage in within-nest mating (intranidal mating: for example, some Cardiocondyla exhibit this incestuous behavior), and others engage in "mate-calling," like some moths. For these species, ability to give off (for the females) and recognize mating cues (for the males) is likely to be selected upon, but, to the best of my knowledge, selection on particular traits in these species has not been selected upon. By Stearns and Hoekstra's definition, sexual selection is likely to occur in any sexually-reproducing species, regardless of escalating selection for mate-choice.
I'll defer to Stearns and Hoekstra again for their definition of macro-evolution: "The pattern of evolution at and above the species level, including most of fossil history and much of systematics." By this definition, macro-evolutionary patterns are evident in any taxon above the species level, for example, the fact that genera such as Pheidole, Strumigenys, and Camponotus have many species, while Paraponera, Tatuidris, and Rostromyrmex have very few species is a macro-evolutionary pattern. More ant genera than usual seem to have arisen when the earths terrestrial vegetation came to be dominated by flowering plants, which is another macro-evolutionary pattern.
Traits can also exhibit macro-evolutionary patterns: ants are all eusocial - we don't know of any solitary ants. Asexual reproduction has cropped up several times in a variety of ant lineages, but does not seem to have persisted beyond a speciation event. The ability to cultivate fungus has only occurred once in the ants. The processes that gave rise to these patterns are somewhat outside of the realm of macro-evolution, but the justification for using the term "macro-", instead of referring to these patterns as just plain old "evolution," is that they cannot be predicted by an understanding of intra-specific evolution alone. This is analogous to the anti-reductionist argument that cell biology cannot be usefully predicted by chemistry alone--simply understanding osmosis and organic chemistry would not allow us to predict the utility of sexual reproduction, which in some situations can give rise to heterogamy, which in turn drives the emergence of a fertilization envelope, uniparental organelle inheritance, and, in one case, the tail of the peacock. In another case, irreducible patterns and processes random-walked their way to the population of weird little wasps that would become the ancestors of all ants.
As I said at the outset, one would need quite a bit more time and space to fully answer your questions, but I hope I've at least given you some food for thought.
Jesse Czekanski-Moir & the AntAsk Team