Speciation: What is it and How does it Happen?

So how do new species develop?

First, we have to remember that ‘species’ is a poorly defined concept and there is no line that automatically denotes one species from another.  So, there’s a lot of issues with the concept of classification, much less microevolution vs. macroevolution.  Because the definition of macroevolution almost depends on how we define species. 

If we reclassify a new species into a new genus, then macroevolution happened.  That’s a cheesy way to do it, but that’s technically all it would take.

We wil probably never see much in the way of macroevolution.  Humans see intermediates for organisms.  See this discussion for a longer description of this.

Here, we will discuss speciation.  How it happens and what it means.

First, the types of speciation. Let’s say we have a cline of populations.

A – B – C- D – E – F

So what happens if C and D are split apart by a mountain range? That’s called allopatric speciation.  Now keep in mind that this isn’t a fence being put up.  It’s a major change in the environment.  Massive forest fire, mountain range, volcano, etc.  There will be seperation of the these closely related species for hundreds if not thousands or millions of years.

This can be seen in the Kaibab squirrels of the North Rim of the Grand Canyon.  These have recently been reclassified as a sub-species of Abert’s squirrel, which lives on the south rim of the Grand Canyon.  Presumably, 17 million years ago, these were the same species.  At some point during the development of the canyon, the squirrels could no longer cross the river and were stuck.  Over time, they changed.  (This isn’t the best example of this, but the Abert squirrel is a wicked cool looking animal.  The best examples remain the Galapagos tortoises and finches.)

Peripatric speciation is when a small group becomes different from the larger population of a species.  This can occur due to the genetic drift or founder effect.  Basically, a small group within a population changes and becomes (either instantly or over time) reproductively isolated from the parent population. 

In the founder effect, you would have a single individual that has a particular trait that is a parent of a large number of offspring and those offspring all have that trait.  Over time, that trait can become fixed in the smaller offshoot population but may not even exist within the larger population.  Even something as simple as a thicker coat of fur can result in peripatric speciation, if the trait causes the new population to be reproductively isolated from the parent population. 

A great example of peripatric speciation is the London Underground mosquitoes

Parapatric speciation occurs when a group is prevented from sharing genes with an otherwise connected population.  Ring species are an example of parapatric speciation.  Individuals from each population may meet one another from time to time, but there are reproductive differences that prevent mating (time of season, behavior to induce reproduction, etc).

Sympatric speciation occurs within a population.  It basically occurs when otherwise minor differences are amplified to the point of speciation.  The daughter species both occupy the same location and freely intermingle, but there is something that causes reproductive isolation.

The African lake chiclids are examples of sympatric speciation.  In these cases, sexual selection resulted in minor differences being increased and resulting in coloration and mating behavior differences between populations.  In captivity, these wildy different species will sometimes interbreed.  The speciation in these chiclids has occured within the last 5000 years without intervention by humans.

There is recent evidence that other species have undergone or are undergoing sympatric speciation.

Polyploidy is a common method of speciation in plants.  It occurs when the offspring has some multiple of the orginal chromosomes.  For example, if a particular plants has 18 chromosomes and the gametes have 9 chromosomes, then there may be offspring with 27, 36, or more chromosomes. 

These polyploid plants may or may not be able to reproduce with the parent species (or at all), but recent research indicates that a large percentage of modern plants are the result of polyploidy.  Technically, this is a subset of sympatric speciation, but its prevelence encourages separate mention.

One last concept, then we can get into what this all means.

The Wallace Effect is a process by which natural selection increases reproductive isolation.  If changes have occured to two separated populations of species, then when they are able to intermix again, there may be incomplete speciation.  This would result in offspring that are hybrids, which are generally not fertile.  Even if the hybrid is fertile, it may contain traits from both subspecies that don’t work well together or reduce the fitness of the individual (at least compared to the two parents).

For example, I have a hybrid cat.  One parent was obviously a domestic cat and one was obviously a bobcat.  At 22 pounds, he’s a monster, but friendly to a fault.  This particular cat has most of the body of a bobcat.  Giant chest, heavy muscles, giant paws and claws.  However, he has the hips and hind legs of a house cat.  He, almost literally, cannot support his own weight on his back legs.  He has horrible hip problems.  He’s getting to be old and I suspect we’ll have to put him down soon.  In the wild, he probably wouldn’t have lasted as long as he did with me.

So those are the types of speciation.

What does it all mean?  How does it apply to macro evolution?

Remember, none of this can be taken in isolation and that we’re talking about millions of years for the evolution of our modern species.
So we have a cline: A – B- C- D- E- F

We’re already getting into potential speciation between A and F.  But what happens if a mountain or canyon develops between C and D?  Well, then over the years, the two groups A,B,C and D,E,F will further change.  Now we have definately have two species (at a minimum).  We can observe other changes to these populations over a few million years.

A’s environment gets a little wierd resulting in parapatric speciation.  F wanders of from D and E and results in peripatric speciation.  D and E intermingle to the point that the genes have mixed and results in one species where there might have been two.  Now we have

A1 – A – B – C  ///  DE  // F

You can play this little game for a few dozen million years and get some radically different organisms.

Taking the example of the skink that it is switching from laying eggs to live births, we see that some changes are not nearly as difficult to accmplish as we thought.  We know there are carnivores that eat plants (heck, look at the label on your cat or dog food).  We know there are herbivores that will eat meat.  We know there are land animals that spend the majority of their lives in the water.  There are marine animals that breed on land, there are land animals that breed in water.  We know there are birds that swim, some flying birds swim very, very well.

Little changes, easily add up to big ones.

Finally, I’ll give some instances of speciation that humans have observed.  Remember that the only reason these are only species changes is because that’s what we call it.  One of the defining characters of E. coli is that it cannot use citrate as an energy source… so what do we call E. coli that can?

This one is really important and shows how arbitrary classification is.  But it is a whole new genus.  Muntzig, A, Triticale Results and Problems, Parey, Berlin, 1979. Describes whole new *genus* of plants, Triticosecale, of several species, formed by artificial selection.

Speciation in Insects
1. G Kilias, SN Alahiotis, and M Pelecanos. A multifactorial genetic investigation of speciation theory using drosophila melanogaster Evolution 34:730-737, 1980. Got new species of fruit flies in the lab after 5 years on different diets and temperatures. Also confirmation of natural selection in the process. Lots of references to other studies that saw speciation.
2. JM Thoday, Disruptive selection. Proc. Royal Soc. London B. 182: 109-143, 1972.
Lots of references in this one to other speciation.
3. KF Koopman, Natural selection for reproductive isolation between Drosophila pseudobscura and Drosophila persimilis. Evolution 4: 135-148, 1950. Using artificial mixed poulations of D. pseudoobscura and D. persimilis, it has been possible to show,over a period of several generations, a very rapid increase in the amount of reproductive isolation between the species as a result of natural selection.
4. LE Hurd and RM Eisenberg, Divergent selection for geotactic response and evolution of reproductive isolation in sympatric and allopatric populations of houseflies. American Naturalist 109: 353-358, 1975.
5. Coyne, Jerry A. Orr, H. Allen. Patterns of speciation in Drosophila. Evolution. V43. P362(20) March, 1989.
6. Dobzhansky and Pavlovsky, 1957 An incipient species of Drosophila, Nature 23: 289- 292.
7. Ahearn, J. N. 1980. Evolution of behavioral reproductive isolation in a laboratory stock of Drosophila silvestris. Experientia. 36:63-64.
8. 10. Breeuwer, J. A. J. and J. H. Werren. 1990. Microorganisms associated with chromosome destruction and reproductive isolation between two insect species. Nature. 346:558-560.
9. Powell, J. R. 1978. The founder-flush speciation theory: an experimental approach. Evolution. 32:465-474.
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14. de Oliveira, A. K. and A. R. Cordeiro. 1980. Adaptation of Drosophila willistoni experimental populations to extreme pH medium. II. Development of incipient reproductive isolation. Heredity. 44:123-130.15. 29. Rice, W. R. and G. W. Salt. 1988. Speciation via disruptive selection on habitat preference: experimental evidence. The American Naturalist. 131:911-917.
30. Rice, W. R. and G. W. Salt. 1990. The evolution of reproductive isolation as a correlated character under sympatric conditions: experimental evidence. Evolution. 44:1140-1152.
31. del Solar, E. 1966. Sexual isolation caused by selection for positive and negative phototaxis and geotaxis in Drosophila pseudoobscura. Proceedings of the National Academy of Sciences (US). 56:484-487.
32. Weinberg, J. R., V. R. Starczak and P. Jora. 1992. Evidence for rapid speciation following a founder event in the laboratory. Evolution. 46:1214-1220.
33. V Morell, Earth’s unbounded beetlemania explained. Science 281:501-503, July 24, 1998. Evolution explains the 330,000 odd beetlespecies. Exploitation of newly evolved flowering plants.
34. B Wuethrich, Speciation: Mexican pairs show geography’s role. Science 285: 1190, Aug. 20, 1999. Discusses allopatric speciation. Debate with ecological speciation on which is most prevalent.

Speciation in Plants
1. Speciation in action Science 72:700-701, 1996 A great laboratory study of the evolution of a hybrid plant species. Scientists did it in the lab, but the genetic data says it happened the same way in nature.
2. Hybrid speciation in peonies http://www.pnas.org/…/061288698v1#B1
3. http://www.holysmoke…new-species.htm new species of groundsel by hybridization
4. Butters, F. K. 1941. Hybrid Woodsias in Minnesota. Amer. Fern. J. 31:15-21.
5. Butters, F. K. and R. M. Tryon, jr. 1948. A fertile mutant of a Woodsia hybrid. American Journal of Botany. 35:138.
6. Toxic Tailings and Tolerant Grass by RE Cook in Natural History, 90(3): 28-38, 1981 discusses selection pressure of grasses growing on mine tailings that are rich in toxic heavy metals. “When wind borne pollen carrying nontolerant genes crosses the border [between prairie and tailings] and fertilizes the gametes of tolerant females, the resultant offspring show a range of tolerances. The movement of genes from the pasture to the mine would, therefore, tend to dilute the tolerance level of seedlings. Only fully tolerant individuals survive to reproduce, however. This selective mortality, which eliminates variants, counteracts the dilution and molds a toatally tolerant population. The pasture and mine populations evolve distinctive adaptations because selective factors are dominant over the homogenizing influence of foreign genes.”
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Speciation in microorganisms
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7. Boraas, M. E. The speciation of algal clusters by flagellate predation. EOS. Transactions of the American Geophysical Union. 64:1102.
8. Castenholz, R. W. 1992. Speciation, usage, concept, and evolution in the cyanobacteria (blue-green algae). Journal of Phycology 28:737-745.
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New Genus
1. Muntzig, A, Triticale Results and Problems, Parey, Berlin, 1979. Describes whole new *genus* of plants, Triticosecale, of several species, formed by artificial selection. These plants are important in agriculture.

Invertebrate not insect
1. ME Heliberg, DP Balch, K Roy, Climate-driven range expansion and morphological evolution in a marine gastropod. Science 292: 1707-1710, June1, 2001. Documents mrorphological change due to disruptive selection over time. Northerna and southern populations of A spirata off California from Pleistocene to present.
2. Weinberg, J. R., V. R. Starczak and P. Jora. 1992. Evidence for rapid speciation following a founder event with a polychaete worm. . Evolution. 46:1214-1220.

Vertebrate Speciation
1. N Barton Ecology: the rapid origin of reproductive isolation Science 290:462-463, Oct. 20, 2000. http://www.sciencemag.org/cgi/content/full/290/5491/462 Natural selection of reproductive isolation observed in two cases. Full papers are: AP Hendry, JK Wenburg, P Bentzen, EC Volk, TP Quinn, Rapid evolution of reproductive isolation in the wild: evidence from introduced salmon. Science 290: 516-519, Oct. 20, 2000. and M Higgie, S Chenoweth, MWBlows, Natural selection and the reinforcement of mate recognition. Science290: 519-521, Oct. 20, 2000
2. G Vogel, African elephant species splits in two. Science 293: 1414, Aug. 24, 2001. http://www.sciencemag.org/cgi/content/full/293/5534/1414
3. C Vila` , P Savolainen, JE. Maldonado, IR. Amorim, JE. Rice, RL. Honeycutt, KA. Crandall, JLundeberg, RK. Wayne, Multiple and Ancient Origins of the Domestic Dog Science 276: 1687-1689, 13 JUNE 1997. Dogs no longer one species but 4 according to the genetics. http://www.idir.net/…2dog/wayne1.htm
4. Barrowclough, George F.. Speciation and Geographic Variation in Black-tailed Gnatcatchers. (book reviews) The Condor. V94. P555(2) May, 1992
5. Kluger, Jeffrey. Go fish. Rapid fish speciation in African lakes. Discover. V13. P18(1) March, 1992.
Formation of five new species of cichlid fishes which formed since they were isolated from the parent stock, Lake Nagubago. (These fish have complex mating rituals and different coloration.) See also Mayr, E., 1970. _Populations, Species, and Evolution_, Massachusetts, Harvard University Press. p. 348
6. Genus _Rattus_ currently consists of 137 species [1,2] and is known to have
originally developed in Indonesia and Malaysia during and prior to the Middle
Ages[3].
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[2] D. Morris. The Mammals. Hodder and Stoughton, London, 1965.
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7. Stanley, S., 1979. _Macroevolution: Pattern and Process_, San Francisco,
W.H. Freeman and Company. p. 41
Rapid speciation of the Faeroe Island house mouse, which occurred in less than 250 years after man brought the creature to the island.

Speciation in the Fossil Record
1. Paleontological documentation of speciation in cenozoic molluscs from Turkana basin. Williamson, PG, Nature 293:437-443, 1981. Excellent study of “gradual” evolution in an extremely find fossil record.
2. A trilobite odyssey. Niles Eldredge and Michelle J. Eldredge. Natural History 81:53-59, 1972. A discussion of “gradual” evolution of trilobites in one small area and then migration and replacement over a wide area. Is lay discussion of punctuated equilibria, and does not overthrow Darwinian gradual change of form. Describes transitionals

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