SPECIATION



EVOLUTIONARY MODEL

If the evolutionary model is true, then populations of one species can evolve into separate species. If this occurs gradually, it is expected that one could find examples of populations which are classified in the same species but which seem to be in the process of speciation. It is also expected that many populations which are classified as separate species would still be able to hybridize, to some degree. If evolution is correct, then hybridization between two species might produce a new race whose novel features could ultimately spur the formation of a new species.

CREATIONISM MODEL

If creationism is true, each created "kind" is completely unrelated and new "kinds" cannot evolve from others. In the past, creationists have typically argued that each species represented a separate "kind" and that no new species could ever evolve. Today, many to most creationists accept that new species can evolve from others, although they disagree on the nature of "kinds" and the mechanisms involved. Given that the Bible depicts crossing different kinds of life in a negative light (such as Levitcus 19:19 and Deuteronomy 22:9), it is not expected that such hybridization would be a positive source of variation and a mechanism to increase the diversity of life on earth.

INTELLIGENT DESIGN

If intelligent design is correct, then complex aspects of life cannot evolve gradually over time. Speciation is arguably complex in that it would involve mechanisms of reproductive isolation and adaptation to a local environment. As a result, it could be argued that intermediate steps in the process of speciation would be the opposite of what would be predicted in the intelligent design model. If the intelligent design model is correct, it is unexpected that a complex process such as speciation could make use of changes which had originally been adaptive for a separate region. For something to be "irreducibly complex", intermediate steps should have no adaptive values. Hybridization as a mechanism of speciation would rely on the adaptations of each of the two separate lineages to their local conditions.

If species could not evolve into new species, one would expect that it would be relatively easy to identify the species which are alive today. If species evolve, then one would expect that the situation would be much more complicated. Evolving species would produce a number of transitional states. Speciation could begin with two populations of the same species which begin to develop differences between them. After being isolated from each other, two such populations could become semi-species-populations which can interbreed but which rarely do. Eventually, newly evolved species could form part of a species complex in which the incipient species are almost entirely reproductively isolated but which can often hybridize with other species under certain conditions.The study of natural species has provided abundant examples of speciation in progress. Many modern populations have diverged to the point where they are close to being separate species.

Speciation is occurring in the passion-vine butterflies Heliconius himera and H. erato. Although these species can produce viable hybrids without any evidence of decreased viability or fertility, hybrids account for only about 10% of the offspring where the species overlap. Mate choice is the primary cause of the reproductive isolation of these incipient species (McMillan, 1997).


The populations of the extreme geographic limits of a single species may not be able to reproduce successfully. For example, males of U.S. populations of Drosophila pseudoobscura produce sterile male hybrids when crossed with females produces infertile male hybrids when males from the USA are mated to Drosophila pseudoobscura females from Colombia (Mallet, 2006). The species D. pseudoobscura and D. persimilis are thought to have diverged about 550 000 years ago which is about the same estimated date as that for the sibling species D. simulans, D. mauritiana and D. sechellia (Mallet, 2006).

Within the past million years, a number of incipient species of house mouse have differentiated from ancestral populations, of which the two best studied Mus musculus musculus and M. m. domesticus. These supspecies/incipient species do not overlap over most of their geographic range but hybridization can occur in the overlapping portions. A number of areas of the genome are accumulating subspecies-specific changes and some chromosomal inversions are known (Harr, 2006).
About 34 subspeces are known of the Western song sparrow (Tobin, 2001). The genes which cause hybrid sterility in the recently formed species pair of Drosophila mojavensis- Drosophila arizonae are not yet fixed in populations of Drosophila mojavensis (Reed, 2004). Chimps, gorillas, and orangutans all possess greater intraspecific genetic variations than humans do in spite of their small geographic distributions. One significant contributing factor is the existence of subspecies with little interbreeding between them (Wall, 2006).


Populations which seem distinct at the extremes are linked by intermediates, such as the Eastern milk snake (Lampropeltis triangulum triangulum), scarlet kingsnake (Lampropeltis triangulum elapsoides), and Red milk snake (Lampropeltis triangulum syspila) (Raven, 2002).


Leopard frogs (Rana pipiens complex) were once classified as a single species. Now, they are classified as four species (Rana pipiens, Ranablairi, Rana utricularia, and Rana berlandieri) which rarely if ever hybridize. (Hybrids which survive embryonic development are often sterile or suffer decreased viability.) (Raven, 2002).


SPECIATION GENES
A variety of genes have been identified which contribute to the reproductive isolation and speciation of related populations. Genetic analysis indicates that there are often multiple genes capable of killing hybrids or rendering them infertile. (For example there are thought to be 190 such genes acting on D. simulans × melanogaster crosses.) It may be that the reproductive isolation of populations involves a greater number of genes as more time passes (Mallet, 2006). Some, but not all, of the genes which cause male infertility in hybrids affect the motility of sperm (Reed, 2004). Often, hybrid sterility is caused by loci on the X chromosome interacting with dominant alleles on the autosomes. When only one gender of hybrids is sterile, it is typically male (known as Haldane's rule) (Chang, 2007).

Hybrid sterility and female species preferences help to isolate populations and are both polygenic. Drosophila melanogaster and the species of the Drosophila simulans group provide evidence of this (Noor, 2001). There are at least 3 autosomal loci which produce sterile hybrids of D. persimilis and D. pseudoobscura (Chang, 2007). Hybrid sterility is caused by at least 3 loci in Drosophila mauritiana and at least 2 loci in Drosophila sechellia (Cabot, 1994).
One of the sites which controls hybrid sterility in crosses between Drosophila mauritiana into D. simulans is a locus named Odysseus which has undergone a very rapid rate of positive selection in recent history. Different alleles of this gene are nearly fixed in the two species and crosses between the two result in sterile males as a result. The 15 amino acid changes in the OdsH homeobox gene between these two species is a greater difference than that which separates comparable regions of nematode worms and mice (Ting, 1998).


Genes known to cause hybrid sterility include MYB-like Hmr (Hybrid male rescue), nuclear pore complex protein Nup98, and Zhr (Zygotic hybrid rescue) in D. simulans × melanogaster crosses (Mallet, 2006). Variations in the genes Lethal hybrid rescue (in Drosophila simulans) result in lethality to hybrid males resulting from interspecific crosses (Brideau, 2006; Barbash, 2003).


The M and S populations of Anopheles gambiae are in the process of becoming separate species. Although they can interbreed, hybridization account for only 1% of the matings. These two forms are adapting to different types of habitats. There are no known fixed chromosomal inversions which are accumulating genes involved in their reproductive isolation, but there is an area near the centromere of the X chromosome which seems to be accomplishing a similar result, perhaps because of the reduced amount of recombination which occurs in the centromere (Stump, 2005). M and S are reproductively isolated forming incipient sympatric species. There are 3 separate areas of the genome which seem to be responsible for this reproductive isolation (Turner, 2007).
A gene has been identified which is a factor in the reproductive isolation of two species of mice, Mus musculus and Mus spretus (Harr, 2006).

 

INTERSPECIFIC HYBRIDIZATION
Even after populations have diverged sufficiently to be considered as separate species, hybridization between them may still be successful.
Among fungi, natural hybrids between species of the fungus genus Heterobasidion (Garbelotto, 2004). A new species of European fungal pathogen on alder trees has evolved recently through hybridization of existing species. It is likely that disruption of environments favors the production of new fungal pathogens (Brasier, 1999).
Many trees can hybridize, such as staghorn sumac and smooth sumac.
Among invertebrates, interspecific hybrids are known between moth species Heliothis virescens and H. subflexa (Teal, 1995). Hybridization can occur between the oysters Crassostrea angulata and Crassostrea gigas (Leitao, 2007).

Among fish, hybridization is known between species of rabbitfishes (the family Siganidae) (Kuriiwa, 2007). Hybridization between catfish species (Dunham, 2007).
Jefferson Salamanders can interbreed with Blue-spotted salamanders and this hybridization has resulted in two species composed solely of females: the silvery and Tremblay's salamanders. Males can still mate with these all-female species but their sperm doesn't contribute to the genetic material of the offspring, it merely stimulates the development of the eggs. Spotted turtles can hybridize with bog turtles.


Some species of piculets, such as Picumnus cirratus can hybridize with other species and can be divided into several races (Winkler 1995; Short, 1982). Hybridization is known to occur between 20 species pairs in 8 genera of woodpeckers, sometimes forming a "superspecies group" (Short, 1982). Genetic evidence indicates that the largest woodpecker genus, Picoides, consists of several groups which are more closely related to other woodpecker genera than they are to each other (Weibel, 2002). Red bellied woodpeckers are known to hybridize with four others (Melamerpes uropygialis, M. hoffmannii, M. superciliaris, and M. aurifrons) (Winkler 1995; Short, 1982).Pileated woodpeckers form a superspecies with the lineaged and black bodied woodpeckers with which they can hybridize.

A number of interspecific hybridizations are known to occur within raptors, including peregrine and prairie falcons, peregrine and lanner falcons, merlins and Eurasian kestrels, black and red kites, rough-legged hawks and common buzzards, buzzards and goshawks (although infertile), and a red-backed and Swainson's hawk (Weidensaul, 2000).Among Darwin's finches, interspecific hybridization can occur if a male imitates the song of the males of a related species (Grant, 1997).
Hummingbirds do not mate for life and males even attempt to mate with other species (and a number of hybrids between species have been produced)(Greenewalt, 1960). A number of warbler species can hybridize, such as the blue winged and golden winged warblers, Nashville warbler and American redstart, blue winged and Kentucky warblers, and Northern parula and yellow-throated warblers (Garrett, 1997).


Hybridization was successful between female llamas and male dromedary camels (Wolfrom, 2003). A wholphin is a rare hybrid between a bottlenose dolphin and false killer whale which is known in captivity and reported in nature as well. Zebra/horse hybrids are known as zorse or hebra (depending on whether the male was a zebra or horse, respectively (Wikepedia, 2007). Before the equid known as the quagga became extinct, crosses between horses and quaggas were known. Although mules are typically sterile, some horse-donkey crosses produce fertile offspring. One hybrid is known to have been born between African and Asian elephants, although it died after 10 days (Shoshani, p. 52). Hybridization among lemurs (and even the production of fertile hybrids) is possible in interspecific matings. (Horvath, 2007). Some whale species are closely related enough to hybridize, including the genera Grompus and Tursiops (Gaskin, 1982).


Liger and tigons are represent hybrids between lions and tigers (which vary in whether the male parent was a lion or tiger, respectively). Ligers can weigh more than a thousand pounds (450 kg) and, when standing on a prop stretch 12 feet. Their meals can reach 50 pounds of meat per meal. They are fast runners and can reach 50 mph. Not only are they the largest form of modern cat, they are also larger than any known fossil cat. Tigons are usually smaller than either of the two parent species but a tigon of the Shalambala Preserve during the 1970s-1980s raised a tigon which measured 13 feet from nose to the tip of the tail.
A number of names have been created for the crosses between cats of the genus Pantera. For example, a male lion can produce offspring known as ligers, liguars, and liards with female tigers, jaguars, and leopards, respectively. A male tiger can produce tigons, tiguars, and tigards by female lions, jaguars, and leopards. A male jaguar can produce jaglions and jagupards with lions and leopards. A male leopard can produce leopons, doglas, and leguars with lions, tigers, and jaguars.
Some of these hybrids have been fertile (and have even more interesting names such as li-liger, ti-liger, tig-liger, ti-tigon). Although oral tradition from India records that doglas can be produced from leopard and tiger crosses, modern attempts have resulted in stillbirths of spontaneously aborted fetuses (Wikepedia, 2007).

 

HYBRIDIZATION AS SPECIATION

The hybridization between species can not only produce fertile offspring, it can also be a mechanism of speciation. One proposed mechanism for speciation is that the hybrids between two existing species would have a variety of traits which would allow novel adaptations. Genetic analyses of goats produce different relationships among species depending upon whether nuclear or mitochondrial genes are used. One explanation for this discrepancy is that the Pliocene ancestor of modern goat species was a hybrid between two parental species. It appears that the mitochondria from a species adapted high altitude enabled novel adaptations in goats (Ropiquet, 2006).


Butterflies of the genus Lycaeides living in alpine environments seem to be a species which resulted from the hybridization of the parent species L. melissa and L. idas (Gompert, 2006). There are about 11,500 identified species of ants (in 288 genera) which represent about 1% of modern insect species. The ant genus Pheidole is the most speciose genus known in the modern world with more than 1100 species (almost 10% known species) (Moreau, 2008).


A new species of fungus evolved from a hybrid between species Phytophthora cactorum and P. nicotianae. Another species of this genus, Phytophthora alni, also seems to have resulted from the hybridization of P. fragariae and P. cambivora. The hybrid species is tetraploid (Ioos, 2006). A new species of European fungal pathogen on alder trees has evolved recently through hybridization of existing species. It is likely that disruption of environments favors the production of new fungal pathogens (Brasier, 1999).


There are about twenty wild species of wheat, genus Triticum. Humans had domesticated one species, Triticum monococcum (with 14 chromosomes) by 11,000 years ago. Later, a polyploidy accident in Tritcum monococcum resulted in the species Triticum turgidum (the summer wheat used for noodles) with 28 chromosomes. A fertilization error involving Triticum turgidum (28 chromsomes) and the wild species Triticum tauschii (14 chromosomes) produced the species of wheat used for bread, Triticum aestivum (42 chromsomes) (Campbell, 2003).