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
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,
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).
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
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
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,
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
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).
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,
Among fish, hybridization is known between species of rabbitfishes (the
family Siganidae) (Kuriiwa, 2007). Hybridization between catfish species
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,
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)