The Origin of Species

The Biological Species Concept.

Linnaeus, the founder of modern taxonomy described species in terms of their physical form or morphology. The morphospecies concept is still the most common method used for describing species. This concept was, and is useful in the field, since its basis is observable and measurable anatomy. However it does not address the discontinuity that exists between species from standpoint of evolutionary theory. Of course Linnaeus was not looking to describe the evolutionary relationships between species.

In 1942, Ernst Mayr proposed the Biological Species concept, as an alternative to the morphospecies. A biological species is defined as...

The biological species concept has as its central theme the notion of gene flow, or rather the lack of gene flow between different species. Therefore if two individuals are of the same species they can exchange genetic material, which means they can successfully breed and produce viable offspring. If two individuals cannot interbreed and produce viable offspring they are, by this definition different species.

This biological species concept cannot be applied to all organisms. For example it cannot be applied to species that reproduce only by asexual means. In these organisms there is no sexual mating and no exchange of genetic material. This biological species concept cannot be applied to extinct organisms represented only by fossils since we have no evidence of whether or not they could interbreed. Finally organisms that are separated by geographical barriers might be the same species but are prevented from interbreeding by such separations.

In some cases the determination of a species is not possible. For example there are four phenotypically distinct populations of the deer mouse Peromyscus maniculatus found in the Rocky Mountains. These populations are geographically isolated and are refereed to as subspecies. At certain points the populations do overlap, and at these locations there is some interbreeding, which produces some gene flow between the populations. Two of these populations are the exception; P. m. artemisiae and P. m. nebrascensis do not interbreed in the area where they overlap. However they are not entirely reproductively isolated as genes from each of these populations can flow through each of the other two subspecies populations. For example populations A and C cannot interbreed, but both can breed with population B. Therefore genes from population A can reach population C through population B. There are many other examples where there is a blurry distinction between populations with limited gene flow and a full biological species with isolated gene pools. It is unlikely that we will see a single unambiguous definition of a species, which will apply in all cases. However this biological species concept does allow us to examine some of the mechanisms which keep two populations genetically isolated.

Reproductive barriers that separate species.

A reproductive barrier is any factor that prevents two species from producing fertile hybrids, thus contributing to reproductive isolation. There are several reproductive barriers and most species are separated by a combination of more than one of these barriers.

The various reproductive barriers which isolate species can be classified as either Prezygotic or Postzygotic. Prezygotic barriers are mechanisms which prevent the fusion of the egg and the sperm so that no zygote can form. Postzygotic barriers are mechanisms that prevent a zygote from developing into a fertile adult offspring.

Prezygotic Barriers.

Habitat Isolation: When two closely related species live in entirely different habitats they will seldom encounter each other and therefore there will be few opportunities to mate. For example there are two closely related species of garter snake but one lives primarily in the water while the other lives on land. They very seldom ever cross paths so they never breed in the wild.

Temporal Isolation: If two species breed at different times of the day, or in different seasons they cannot mix their gametes, because when one species is ready to breed the other is not. For example, brown trout and rainbow trout live in the same streams, but brown trout breed in the fall and rainbow trout breed in the spring.

Behavioural Isolation: Each species may use different signals to attract a mate. If one species does not recognize the signals the two species will not mate. Many of these signals can be sights, smells or sounds. For example male fireflies of different species signal to the females of the same species by blinking their lights in a characteristic pattern. The females will discriminate among the different signals and respond only to flashes of their own species by flashing back and attracting the males. Many animals use characteristic scents (chemical signals called pheromones) to attract a mate. Only members of the same species will recognize these smells. Many birds recognize members of the same species by their songs.

The mating ritual behaviour between these two Blue Footed Boobies ensures they recognize each other as been of the same species.

Mechanical Isolation: Many species cannot mate because their reproductive anatomies are incompatible. For example male dragonflies use a pair of special appendages to clasp females during copulation. When a male tries to mount a female of a different species he cannot grasp her because his clasping appendages don't fit her body shape.

Gametic Isolation: This mechanism is due to the incompatibly of sperm and eggs from different species. The gametes do not fuse because they do not recognize each other. Gametic recognition may be based on the presence of specific molecules on the coats around the eggs that adhere only to complementary molecules on the sperm cells of the same species. Such molecular recognition mechanisms are what enables a flower to discriminate between pollen of the same species and pollen from a different species.

Postzygotic Barriers.

Reduced Hybrid Viability: Genetic incompatibility between the two species may abort the development of the embryo at an early stage. The genes from each species that control the development of the embryo may not be compatible and conflicting instructions may cause the embryo to develop abnormally. For example several species of frogs of the genus Rana can live in the same ponds. Some times there are hybrid zygotes formed but these fail to develop.

 

Reduced Hybrid Fertility: If two species can produce hybrid offspring that are viable, reproductive isolation is intact if these offspring are sterile and cannot breed. Therefore the hybrid generation can not pass the genes on any subsequent generations, and genes cannot flow between each of the original populations. One cause of such hybrid sterility could be that the number or structures of the chromosomes of the parent species are so different that meiosis cannot occur and gametes are not produced. One example of this form of reproductive isolation occurs when horses are breed with donkeys to produce mules. The mules are viable but are sterile.

In this photo you can see the horse on the left , the donkey in the middle and their offspring a mule on the right.

Hybrid Breakdown: When some species cross mate the first generation of offspring are viable and fertile. However when these hybrids mate with each other or with either of the parent species offspring of the next generation are feeble or sterile. In cotton plants some species can produce fertile hybrids, but the offspring of these hybrids don't produce viable seeds.

Introgression

Despite these mechanisms that prevent gene flow between different species, sometimes alleles do seep through these barriers. The transplantation of alleles between species is called Introgression. For example corn (Zea mays) contains some alleles traceable to the closely related wild grass teosinte (Zea mexicana). The occasional hybridization does not erase the boundary between corn and teosinte.

Geographical isolation and the origin of species.

Reproductive barriers form boundaries around species, and the evolution of these barriers is the key biological event in the origin of a new species. One essential event in the origin of a species occurs when the gene pool of a population is separated from other populations of the parent species. This genetically isolated group can now follow its own evolutionary path as changes in allele frequencies caused by selection, genetic drift, and mutations occurs undiluted by gene flow from the parent population. Speciation episodes can be classified into two modes based on the geographical relationship of the new species and its ancestral species.

Allopatric speciation: This speciation occurs when the initial block to gene flow is a geographical barrier that physically isolates the splinter population from the parent population. Such geological events as the formation of a river or a deep canyon, formation of mountain ranges or the separation of whole continents. For example two squirrel species exist on opposite side of the Grand Canyon. The canyon prevents gene flow between the two species. Small populations may become isolated from their parent populations when they travel to a new location. For example the small birds Darwin found on the Galapagos Islands were transported by storms to the islands from the mainland of South America. The extent of development of a geographical barrier necessary to isolate two populations depends on the ability of the organisms to disperse. For example, the Grand Canyon is a barrier to the squirrels, but not to birds.

For many centuries two populations of squirrels have been isolated by the Grand Canyon, one population on the north rim and one on the south. They have now evolved into two different species.

Sympatric speciation: A new species can originate in the geographical midst of the parent species. This is called Sympatric speciation. In this case reproductive isolation evolves without geographical isolation. The type of speciation can occur very quickly even in one generation if the genetic change results in a reproductive barrier between the mutants and the parent population. Many plants have originated from improper cell division that results in extra sets of chromosomes, a mutant condition known as polyploidy. Depending on the origin of the extra sets of chromosomes, polyploids are classified in two forms, Autopolyploids and Allopolyploids.

Autopolyploid: An organism that has more than two sets of chromosomes, but all sets are derived from the same species. For example, if a nondisjunction in the production of gametes by either mitosis or meiosis, results in diploid gametes. If these plants self-fertilize the offspring would have four sets of chromosomes, they would be tetraploids. These tetraploids would be able to mate with other teraploids but if they mated with either of the diploid parent species the offspring would be triploid, and would be sterile due to impaired meiosis of unpaired chromosomes. Such as instantaneous special genetic event would thus produce a postzygotic barrier, which would isolate the gene pools of the mutant tetraploids from the diploid parent population.

Allopolyploid: This is the production of a polyploid species from the contributions by two different species. This type of sympartric event is more common that the above autoployploidy. The evolution of an allopolyploid begins when two different species interbreed to produce a hybrid. These hybrids are usually sterile, because the two haploid sets of chromosomes don't match up during meiosis. If the species can reproduce asexually as many plants do, then the sterile hybrid population can maintain itself until the next event occurs.

There are at least two mechanisms that can transform these sterile allopolyploid hybrids into fertile polyploids.

1. The cells in the reproductive organs (antheridia and archegonia) produce the cells that undergo meiosis to produce the eggs and sperm. If these antheridial or archegonial cells undergo a mitotic nondisjuction that creates a tetraploid germ cell, the gametes produced from this tetraploid germ cell will be diploid. If two diploid gametes were to fuse the resulting offspring would be tetraploid with two matching sets of homologous chromosomes and would therefore be fertile.

2. If there was a nondisjunction, that produced a diploid gamete by the one of the parent species, and that unreduced gamete was to fuse with a normal haploid gamete it would produce a sterile triploid hybrid. If there was another nondisjunction in the sterile triploid allopoyploid hybrid, which produced an unreduced triploid gamete, and that gamete was to fuse with a normal haploid gamete from either of the two original parent species, then the offspring would have an even number of homologous chromosomes and be fertile polyploids.

Speciation of polyploids, especially allopolyploids, has been very important in plant evolution. Some alloployploids are very vigorous because they contain the best qualities of both parent species. The accidents required to produce these new plant species (interspecific hybridization coupled with nondisjunction) have occurred often enough that between 25% and 50% of all plant species are polyploids.