Isolating Barriers Among Closely Related Species

Isolating barriers :

Isolating barriers prevent interbreeding between species :

Why is it that closely related species, living in the same area, do not breed together? The answer is that this is prevented by isolating barriers. An isolating barrier is any evolved character of the two species that stops them from interbreeding? The definition specifies "evolved characters" to exclude non-interbreeding due to simple geographic separation. Interbreeding between two geographically separate populations of a species is impossible, but the geographic separation is not an isolating barrier in the strict sense. Geographic separation alone does not have to be an evolved character, and is unlikely to be an evolved character when it is between two populations of a species. One subpopulation can colonize a new area without any genetic change, or the populations may have been separated by a geographic accident, such as the formation of a new river. Courtship, however, is an example of an isolating barrier. If two species do not inter-breed because their courtship differs, then the courtship behavior of at least one of those species must have undergone evolutionary change.

The most important distinction is between prezygotic

and postzygotic isolation. Prezygotic isolation means that zygotes are never formed, for instance because members of the two species are adapted to different habitats and never meet, or have different courtships and do not recognize each other as potential mates. 

Alternatively, the members of two species may meet, mate, and form zygotes, but if the hybrid offspring are inviable or sterile then the two species have postzygotic

isolation.

Sperm or pollen competition can produce subtle prezygotic isolation :

Over evolutionary time, differences accumulate between species and the result is that they become fully isolated by both prezygotic and postzygotic isolating barriers. They will evolve different appearances, different courtships, different ecological adaptations, and different and incompatible genetic systems. However, closely related and recently

evolved species may be only partly isolated, and then research can reveal which isolating barriers are at work. One factor that has been investigated recently in several species is "gametic isolation". The simplest kind of gametic isolation occurs when the sperm and eggs of two species do not fertilize each other. But a process called "sperm competition" can cause a subtler kind of gametic isolation. Two species may not interbreed because the sperm, or pollen, of species 1 outcompetes that of species 2 when fertilizing the eggs of species 1, but the sperm, or pollen, of species 2 outcompetes that of species 1

when fertilizing the eggs of species 2. Wade et al. (1993), for instance, studied reproductive isolation between two beetles, Tribolium castaneum and T.freemani. T. castaneum is a worldwide pest of stored flour called the flour beetle, and T.freemani is a closely related species that lives in Kashmir. The two species are not isolated at the premating stage: males of both species copulate indiscriminately with females of both species.  Quote a remark about the mating propensities of male flour beetles, who "will attempt copulation with other males, dead beetles of both sexes, or with any object, such as a lump of flour or frass, which looks like a beetle."

Wade et al. (1993) did an experiment in which they put female T.freeman in one of three situations: 

(i) with two successive males of T.freemani; 

(ii) with two successive males of T. castaneume; or 

(iii) with one male T.freemani and then one male T. castaneum. The female beetles laid a similar number of eggs in all three cases, and a similar percentage of the eggs hatched and grew up (though the interspecies hybrid offspring are sterile). This shows that the sperm of male T. castaneum are capable of fertilizing T.freemani eggs. When female T.freemani were put with males of both species (condition (iii)), less than 3% of the offspring were hybrids -over 97% of the eggs had been fertilized by the T.freemani male's sperm. The reason is that when two males inseminate the same female, their sperm compete inside the female to fertilize her eggs. In this case,

when no T.freemani sperm are present, T. castaneum sperm can fertilize the eggs, but when T.freemani sperm are present they outcompete the T. castaneum sperm Sperm competition is causing reproductive isolation.  It is a form of male competition, and its out

come may well be influenced by female choice. In this case, the "choice" would be effected by the female's internal reproductive physiology. The experiment matters not only for revealing the nature of reproductive isolation in this pair of beetles, but also shows what needs to be done in research on prezygotic isolation. An experiment in which males of one species are simply crossed with females of another species is inadequate to measure prezygotic isolation. When male T. aastaneum are put with female T.freemani they produce hybrid offspring. We might falsely conclude that these two species are not prezygotically isolated. But if the females are put

with male T.freemani and T. castaneum, hardly any hybrid offspring are produced and the prezygotic isolation is revealed. Isolation by sperm, or pollen, competition has recently been found in many species.

Closely related African cichlid fish species are prezygotically isolated by their color patterns, but are not postzygotically isolated :

Cichlid fish are found globally in warm freshwater environments, but they are famous for the huge numbers of species that have evolved in the East African lakes. They are also famous as a conservation disaster, as a large but unknown number of species have been lost following the introduction of a predatory fish, the Nile perch, into the lakes, together with increasing lake eutrophication. Here we concentrate on the reproductive isolation between two cichlid species that live in Lake Victoria.

Cichlids often have beautiful color patterns, and Pundamilia nyererei and P. punda-milia are related species that differ in color. For simplicity, we can refer to P. nyererei as red and to P. pundamilia as blue, but the color illustrations show that the words red and blue hardly describe the gorgeous colors of the two species. Seehausen & van Alphen (1998) performed a laboratory experiment on the mating preferences of the two species. They first tested the preferences of females of both species for males of one species or the other, in normal light. The result was that the females of both species preferred conspecific males (Figure 13.4 scroll down). The two species show prezygotic isolation by mating behavior. Seehausen and van Alphen then repeated the experiment, but in monochromatic light, in which the color difference

between the two species was invisible (Plate 6). Now the females of both species showno preference between red and blue males. The experiment shows that the prezygotic isolation is due to the color patterns of the two fish species.

Seehausen's lab has also measured postzygotic isolation. The two species will interbreed in the lab and produce hybrids. The hybrids are fertile, and by 2001 five generations of hybrids had been successfully bred: the two species are not postzygotically isolated. In conclusion, P. nyererei and P. pundamilia are isolated prezygotically by color pattern but not postzygotically.

The main point of Seehausen's experiment here is to show how isolating barriers can be investigated, but the results have two other interests. One is in relation to conservation. The color differences between the two species become less visible in cloudy, eutrophic waters. Pollution in Lake Victoria is making it more likely that the two species hybridize. Pollution is leading to a loss of biodiversity, not by the normal mechanism of extinction but by removing the isolating barrier between closely related species. The other interest is in relation to speciation, and illustrates a similar point to the study of flour beetles. Mate preference, like sperm competition, is a form of sexual selection. Sexual selection is thought to drive speciation, particularly sympatric speciation. The African lake cichlids provide some of the strongest

evidence for sympatric speciation. Seehausen's experiments, which show that mating preferences are the first kind of isolation to evolve in these fish,

fits in with the broad idea that sexual selection has contributed to the spectacular radi-

ation of cichlids in East Africa.

In conclusion, over evolutionary time the amount of isolation between two species will increase and the species will eventually be isolated by most of the barriers.  Experiments can be done to reveal what the particular isolating barriers are between closely related species. These experiments can reveal what isolating barriers are at work in the early stages of speciation. 

Figure 13.4
Mating preferences (a form of prezygotic isolation) in two cichlid
species from Lake Victoria, Africa. The two species are referred to as the "red" and "blue" species. Individual females of each species were given a choice of two males, one from each species. A preference for males of the red species was arbitrarily defined as a positive preference; a negative preference indicates a preference for males of the blue species. Females preferred conspecific males in normal white light, but the preference disappeared in monochromatic light, where the two species were visually indistinguishable.