Welcome to Part 2.
Now, it’s important to note that because speciation is a population phenomenon, the phenomenon doesn’t usually appear as a binary “all or nothing” process, but instead, takes place over multiple generations, with increasing instances of interfertility failure between populations over time, until the process becomes complete. Let us hypothesise that we have to hand, a population of organisms, along with the ability to capture complete genetic audits of that population and its descendants. At which point, we introduce a barrier between two subsets of that population, and continue compiling our genetic audits.
When a lineage A diverges into reproductively separated lineages, let’s call these A1 and A2 respectively, then the moment at which A1 and A2 are considered separate species, in accordance with the biological species concept, is the point where all members of lineage A1 become, in some future generation, reproductively incompatible with all members of lineage A2, and vice versa. This arises naturally from the biological species concept, which defines a species as a population within which reproductive compatibility is maintained, but which is reproductively incompatible with other lineages.
Of course, since any population of living organisms is a dynamic entity, as expounded above, this is not an “all or nothing” affair in the space of a single generation. The potentiating mutations driving speciation will appear in one (or possibly more than one) individuals first, and slowly spread through the population, until eventually, all individuals in that population possess those mutations, and reproductive incompatibility with other lineages, including lineages with which the lineage in question shares a common ancestor, will arise.
How long it takes for this to happen, however, is not yet quantitatively predictable. Depending upon many factors, such as the fecundity of the individuals in the population, the generational turnaround, the mutation rates of the genes in question, and how quickly those mutations become fixed in the population, speciation could take place in as little as five years (as happened with Dobzhansky’s laboratory population of Drosophila pseudoobscura, as documented in his 1971 paper), or could take as long as ten million years. Species with fast generational turnarounds and rapid mutation rates in the relevant genes will exhibit speciation events more quickly under relevant circumstances, than species with slow generational turnarounds and slow mutation rates. Unfortunately, we are not yet in a position to use this information in a quantitatively predictive manner, but we can use this in a qualitative manner, to predict that a genetically isolated lineage will eventually exhibit reproductive incompatibility. We will only find out that this has happened, however, when appropriate tests upon the populations in question demonstrate interfertility failure between relevant populations, and of course, with large populations, it will be impractical to test every possible combination of individuals. If you have two populations, each comprising a million individuals, then exhaustive testing would require 1012 trial matings. Sadly, we’re not yet in a position to try this out. In this sense, there is no well-defined point at which speciation occurs, because we lack the ability to perform massive numbers of trial matings to find that point. But that point does exist, as I’ve just explained.
In addition, if there are selective forces at work in the population in question, such as those arising from niche migration and trophic specialisation, or those arising from sexual selection, these forces will act to accelerate the speciation process. See the Lake Victoria Superflock of Cichlid fishes for an example of rapid speciation arising from both mechanisms.
Note however that in some instances thereof, the process is incomplete, as some species in that superflock, despite being defined taxonomically as different species, on the basis of anatomical differences, are still capable of producing fertile hybrids with other members of the same superflock, as Ole Seehausen documented in at least one of his papers, and indeed, as many aquarists have discovered when keeping these fishes in an aquarium. Indeed, the problem of a phenomenon called ‘hyperdominance’, which results in males of extremely aggressive, territorially demanding species, taking over the aquarium and mating with females of other species as well as their own, is a well-known and well-documented problem in the world of Rift Lake Cichlid keeping.
Anyway, let’s move on to what happens to our laboratory population, after the barrier is introduced. We begin taking snapshots of each generation, starting with Snapshot 1 for the first generation after the introduction of the barrier, and continue doing so for however many generations are required to observe the requisite effect. For the first, say, 300 or so generations, no interfertility failure is observed (tested for by appropriate experimental cross matings), while the requisite potentiating mutations are being acquired in each of the two populations. Then, data of the following sort appears in the genetic audit:
Snapshot 751: 1 individual out of 3,726,435 possesses the genes coding for interfertility failure with related populations
Snapshot 752: 3 individuals out of 3,727,201 possess the genes coding for interfertility failure with related populations
Snapshot 753: 9 individuals out of 3,726,974 possess the genes coding for interfertility failure with related populations
Snapshot 754: 17 individuals out of 3,727,032 possess the genes coding for interfertility failure with related populations
Snapshot 755: 30 individuals out of 3,726,883 possess the genes coding for interfertility failure with related populations
Snapshot 756: 47 individuals out of 3,727,265 possess the genes coding for interfertility failure with related populations
Snapshot 1,032: 2,788,015 individuals out of 3,727,354 possess the genes coding for interfertility failure with related populations
Snapshot 1,033: 3,113,282 individuals out of 3,726,558 possess the genes coding for interfertility failure with related populations
Snapshot 1,034: 3,305,188 individuals out of 3,727,220 possess the genes coding for interfertility failure with related populations
Snapshot 1,035: 3,497,185 individuals out of 3,726,298 possess the genes coding for interfertility failure with related populations
Snapshot 1,036: 3,592,407 individuals out of 3,726,504 possess the genes coding for interfertility failure with related populations
Snapshot 1,037: 3,664,803 individuals out of 3,726,901 possess the genes coding for interfertility failure with related populations
Snapshot 1,038: 3,709,592 individuals out of 3,726,448 possess the genes coding for interfertility failure with related populations
Snapshot 1,039: 3,747,662 individuals out of 3,727,108 possess the genes coding for interfertility failure with related populations
Snapshot 1,040: 3,726,992 individuals out of 3,727,888 possess the genes coding for interfertility failure with related populations
Snapshot 1,041: 3,727,143 individuals out of 3,727,143 possess the genes coding for interfertility failure with related populations
Snapshot 1,041, if it were possible to take it, would constitute the moment at which the population was genetically separated from surrounding populations of related organisms, to the point of constituting a completely separate species. Of course, we would have, in this hypothetical setup, a series of 10,41 snapshots, constituting perhaps a full genetic audit, beginning from the moment of isolation of the population at Snapshot 1, all the way through to Snapshot 1,041, where the population achieved complete interfertility failure with surrounding populations, including populations that shared a common ancestry.
Of course, the hypothetical data I’ve presented above are highly simplified, and my exposition assumes for simplicity, that the spread of the genes in question would be strictly increasing, whilst of course there is no reason for real world data to conform to this simple model. The actual plot of the numbers could be a curve with both local maxima and local minima (as defined precisely in mathematics), prior to the point at which the entire population possessed the genes in question, and all individuals in that population were no longer able to produce viable offspring with individuals outside the population in question. But of course, those of us who paid attention in class, know that the shape of the curve leading to the end result observed in Snapshot 1,041 above doesn’t in the least change the larger picture.
The issue isn’t that a snapshot such as my hypothetical Snapshot 1,041 could not exist in principle, rather that such a snapshot would be formidably difficult to produce in practice in a real world population. Not that this is regarded as a research obstacle from the standpoint of documenting speciation events. Any snapshot yielding data equivalent to that of the hypothetical Snapshot 1,041 above would be sufficient to document a completed speciation event, even if the actual spread of the genes had taken place some time earlier. We would know, from a rigorous standpoint, that the speciation event ran to completion at some antecedent time to the snapshot being taken in a real world scenario, but we would be unable to know that antecedent time with precision, courtesy of the practical issues I’ve discussed elsewhere.
Indeed, one of my favourite papers documents the first attempt by scientists to produce documentation of a speciation event in the wild, with something akin to the genetic audit trail given above, namely the paper on Cynotilapia afra, and the observed divergence of the two populations at Thumbi Island, those populations having been founded at a known date. Indeed, the Cichlid fish literature alone contains numerous papers, documenting attempts to produce data sets equivalent to the snapshots in my hypothetical example above, some of those papers being:
Adaptive Evolution And Explosive Speciation: The Cichlid Fish Model by Thomas D. Kocher, Nature Reviews: Genetics, 5: 288-298 (April 2004)
African Cichlid Fish: A Model System In Adaptive Radiation Research by Ole Seehausen, Proceedings of the Royal Society of London Part B, 273: 1987-1998 (9th May 2006)
Case Studies And Mathematical Models Of Ecological Speciation. 1. Cichlids In A Crater Lake by Sergey Gavrilets, Aaron Vose, Marta Barluenga, Walter Salzburger and Axel Meyer, Molecular Ecology, 16: 2893-2909 (22nd January 2007)
Cichlid Species Flocks Of The Past And Present by Axel Meyer, Heredity 95: 419-420 (20 July 2005)
Colour Vision And Speciation In Lake Victoria Cichlids Of The Genus Pundamilla by Karen L. Carleton, Juliet W. L. Parry, James K. Bowmaker, David M. Hunt and Ole Seehausen, Molecular Ecology, 14: 4341-4353 (4th August 2005)
Divergent Selection During Speciation Of Lake Malawi Cichlid Fishes Inferred From Parallel Radiations In Nuptial Colouration by Charlotte J. Allender, Ole Seehausen, Mairi E. Knight, George F. Turner and Norman MacLean, Proceedings of the National Academy of Sciences of the USA, [b]100(24): 14704-14079 (25th November 2003)
Divergent Selection On Opsins Drives Incipient Speciation In Lake Victoria Cichlids by Yohey Terai, Ole Seehausen, Takeshi Sasaki, Kazuhiko Takahashi, Shinji Mizoiri, Tohru Sugawara, Tetsu Sato, Masakatsu Watanabe, Nellie Konijnendijk, Hillary D. J. Mrosso, Hidenori Tachida, Hiroo Imai,
Yoshinori Shichida and Norihiro Okada, Public Library of Science Biology, 4(12): e433 (December 2006)
Evolutionary Conservation Of Microsatellite Flanking Regions And Their Use In Resolving The Phylogeny Of Cichlid Fishes (Pisces: Perciformes) by Rafael Zardoya, Dana M. Vollmer, Clark Craddock, Jeffrey T. Streelman, Steve Karl and Axel Meyer, Proceedings of the Royal Society of London Part B, 263: 1589-1598 (1996)
Frequency Dependent Natural Selection In The Handedness Of Scale Eating Cichlid Fish by Michio Hori, Science, 260: 216-219 (9th April 1993)
Genetic And Developmental Basis Of Cichlid Trophic Diversity by R. C. Albertson and T. D. Kocher, Heredity, 97: 211-221 (12th July 2006)
How Many Species Of Cichlid Fishes Are There In African Lakes? by George F. Turner, Ole Seehausen, Mairi E. Knight, Charlotte J. Allender and Rosanna L. Robinson, Molecular Ecology, 10: 793-806 (2001)
Hybridisation And Contemporary Evolution In An Introduced Cichlid Fish From Lake Malawi National Park by J. Todd Streelman, S.L. Gymrek, M.R. Kidd, C. Kidd, R.L. Robinson, E. Hert, A.J. Ambali and T.D. Kocher, Molecular Ecology, 13: 2471-2479 (21 April 2004)
Major Histocompatibility Complex Variation In Two Species Of Cichlid Fishes From Lake Malawi by Hideki Ono, Colm O’hUigin, Herbert Tichy and Jan Klein, Molecular and Evolutionary Biology, 10(5): 1060-1072 (1993)
Male-Male Competition And Nuptial-Colour Displacement As A Diversifying Force In Lake Victoria Cichlid Fishes by Ole Seehausen and Dolph Schluter, Proceedings of the Royal Society of London Part B, 271: 1345-1353 (2004)
Mechanisms Of Rapid Sympatric Speciation By Sex Revesral And Sexual Selection In Cichlid Fishes by Russell Lande, Ole Seehausen and Jacques J. M. van Alphen, Genetica, 112-113: 435-443 (2001)
Mitochondrial Phylogeny Of The Endemic Mouthbrooding Lineages Of Cichlid Fishes From Lake Tanganyika In Eastern Africa by Christian Sturmbauer and Axel Meyer, Journal of Molecular and Biological Evolution, 10(4): 751-768 (1993)
Molecular Phylgeny And Evidence For An Adaptive Radiation Of Geophagine Cichlids From South America (Perciformes: Labroidei) by Hernán López-Fernández, Rodney L. Honeycutt and Kirk O. Winemiller, Molecular Phylogenetics and Evolution, 34: 227-244 (2005)
Multi Agent Simulations Of Evolution And Speciation In Cichlid Fish by Ross Clement, Proceedings of the 15th European Simulation Symposium, 2003
Multilocus Phylogeny Of Cichlid Fishes (Pisces: Perciformes) : Evolutionary Comparison Of Microsatellite And Single-Copy Nuclear Loci by J. Todd Streelman, Rafael Zardoya, Axel Meyer and Stephen A Karl, Journal of Molecular and Biological Evolution, 15(7): 798-808 (1998)
Origin Of The Superflock Of Cichlid Fishes From Lake Victoria, East Africa by Erik Verheyen, Walter Salzburger, Jos Snoeks and Axel Meyer, Science, 300: 325-329 (11 April 2003) #
Phylogeny Of African Cichlid Fishes As Revealed By Molecular Markers by Werner E. Mayer, Herbert Tichy and Jan Klein., Heredity, 80: 702-714 (1998)
Sensory Drive In Cichlid Speciation by Martine E. Maan, Kees D. Hofker, Jacques J. M. van Alphen and Ole Seehausen, The American Naturalist, 167(6): 947-954 (June 2006)
Speciation Through Sensory Drive In Cichlid Fish by Ole Seehausen, Yohey Terai, Isabel S. Magalhaes, Karen L. Carleton, Hillary D. J. Mrosso, Ryutaro Miyagi, Inke van der Sluijs, Maria V. Schneider, Martine E. Maan, Hidenori Tachida, Hiroo Imai & Norihiro Okada, Nature, 455: 620-627 (2nd October 2008)
The Effect Of Male Colouration On Female Mate Choice In Closely Related Lake Victoria Cichlids (Haplochromis nyererei Complex) by Ole Seehausen and Jacques J. M. van Alphen, Behavioural Ecology & Sociobiology, 42: 1-8 (1998) *
The Effect Of Selection Upon A A Long-Wavelength Sensitive (LWS) Opsin Gene Of Lake Victoria Cichlid Fishes by Yohey Terai, Werner E. Mayer, Jan Klein, Herbert Tichy, and Norihiro Okada, Proceedings of the National Academy of Sciences of the USA, 99: 15501-15506 (November 2002)
The Evolution Of The Pro-Domain Of Bone Morphogenetic Protein 4 (Bmp4) In An Explosively Speciated Lineage Of East African Cichlid Fishes by Yohey Terai, Naoko Morikawa and Norihiro Okada, Molecular & Biological Evolution, 19(9): 1628-1632 (2002)
The Species Flocks Of East African Cichlid Fishes: Recent Advances In Molecular Phylogenetics And Population Genetics by Walter Salzburger and Axel Mayer, Naturwissenschaft, 91: 277-290 (20 April 2004)
The Streelman et al paper above on Cynotilapia afra is a particular case in point.
Part 2 ends here. Part 3 follows shortly.