Lecture on Introgressive Hybridization - BIOL 3030, Principles of Evolution and Systematics

We have talked about the process of hybridization in a number of connections during previous lectures. Generally, the hybrid individuals produced are sterile, have low viability and soon disappear. This is what happens as reinforcement is occurring. However, sometimes the hybrids are apparently normal in all ways, are fertile, and can interbreed with members of both parental species and with other hybrids. In this case, the hybrids may form a genetic bridge through which gene flow can occur between two species. Since gene flow is not expected between two distinct species, this phenomenon merits further attention. The process is called introgressive hybridization.

Introgressive hybridization (also just called introgression) - the transfer of genetic material between two distinct species by the production of fertile viable hybrids and subsequent matings of hybrids with members of the parental species. Let's look at some examples of introgressive hybridization so we can understand the process better.

Barking Treefrog
Green Treefrog

The barking treefrog (Hyla gratiosa) and the green treefrog (Hyla cinerea) often breed in the same pond at the same time throughout the southeastern United States. Under natural conditions, the barking treefrog males call from the water at the edge or floating in the pond next to a lily pad. Green treefrog males typically call from up in bushes or vegetation on the edge. So the males have calling site differences, a type of ecological isolating mechanism.

However, alteration of habitats by humans has produced a large number of pond types which lack woody vegetation and emergent species on the edges. These include farm ponds, fish ponds, golf course ponds, industrial ponds, sewage lagoons, etc. In ponds of this type, breeding male green treefrogs are forced to sit and call on the grass on the edge of the pond. As a consequence, green treefrog males clasp barking treefrog females who are headed for the calls of their own males in the pond.

Male treefrogs tend to clasp anything that hops near them. Pairs consisting of a barking treefrog female clasped by a green treefrog male enter the pond and eggs are laid and fertilized. Evidently, there are few well developed isolating mechanisms between these species except call site. At any rate, the tadpoles survive, transform into hybrid adults and themselves have some success in mating. At some highly artificialized pond sites in the Southeast, many of the specimens found are intermediate between the two parental species.

The details of this introgression have been well studied. It was first discovered by Dr. Jack Mecham at the experimental fisheries ponds of Auburn University in Lee County, Alabama. A student at Tuskegee University subsequently studied the frogs by using electrophoresis and examining bone morphology and verified the hybrid nature of many of the specimens. A student at the University of Georgia examined the mitochondrial DNA and found that in most of the hybrids it was barking treefrog mitochondrial DNA. Keep in mind that mitochondrial DNA is inherited from the female parent only. The results therefore mean that the mating mistakes involved female barking treefrogs and male green treefrogs, as was suspected from earlier studies.
Apparently, disturbance of the habitat destroys ecological isolating mechanisms, causing species formerly reproductively isolated to begin genetic exchange.

Iris fulva
Iris hexagona

In the Gulf Coast area from the Florida Panhandle west to Texas, several native Iris species are present. This example deals with two of these. Iris fulva is a species of bottomlands, inhabiting shaded sites, on heavy soils, in areas influenced only by fresh water. Iris hexagona is a species of marshes, growing in open sunlight, on sandy soils, often in sites influenced by brackish or salt water. Before disturbance by man, intermediates between the two species were unknown. However humans have disturbed Gulf Coast habitats in many ways including cutting forests, building canals, restricting flow by dams, and transporting soils. All in all, the coastal habitats themselves have been hybridized. As a consequence, the two Iris species began to interbreed when brought into proximity. Now there are places where almost all conceivable intermediates between them are present. Once again, habitat disturbance has broken down the ecological isolation. Introgression is occurring.

Bufo americanus
Bufo woodhousei

In the case of the American toad (Bufo americanus) and Fowler's toad (Bufo woodhousei fowleri), the situation is similar to that in the Iris species. In the eastern midwest, these two species are undergoing introgression as a result of habitat disturbance by man. Originally, the American toad bred in woodland pools in shaded deciduous forests and Fowler's toad bred in open areas in prairies and woodland openings. Removal of forests, planting of trees in prairies, and agricultural and urban-related changes have resulted in an anthropogenic (man-created) habitat in many areas. Now the toads may breed in the same site and hybridization occurs. Again, disruption of habitat conditions decreased the effectiveness of habitat isolation.

NOTE: A student who studied these two toads once claimed that they were not hybridizing but that the specimens with intermediate features were the result of both species beginning to adapt to the same newly created set of conditions, i.e., the habitats resulting from human disturbance. This explanation would have meant that parallel evolution was occurring. As it turned out, the student was wrong. He had based his assumptions of no hybridization on analysis of calls, coming up with only two call types. Faulty technique and analysis were to blame. There actually were intermediate calls in some specimens, reinforcing the idea that they were hybrids. But even so, the idea that species can become similar in some respects because both are adapting to a habitat created by man is interesting.

Helianthus annuus
Helianthus bolanderi

In California, introgressive hybridization was suspected between two species of sunflowers. One of these, Helianthus annuus is native to eastern and central North America. It is also the cultivated species and was brought to California and grown in fields prepared and tended by man. Helianthus bolanderi is a native California species which has specialized habitat requirements. It occurs only on areas of serpentine rock, a type of rock very high in certain metals.

Investigation showed that after habitats were altered by man in the central valley of California, a 'new' type of sunflower appeared in the ditches, vacant lots, and roadside habitats. Its features seemed to be intermediate between those of the cultivated sunflower and the serpentine species. The conclusion was that the two species had hybridized and that some of the hybrids were successful in disturbed habitats produced by human activities.

Later, these conclusions were questioned, because genetic work seemed to indicate that the sunflower type in the disturbed areas had no genes from the cultivated sunflower. Rather, it appeared to be variants of the serpentine species that had the ability to grow off of serpentine rock and could colonize disturbed habitats. In other words, some specimens of a species had come to resemble another species as a result of adapting to an anthropogenic habitat. This is what the student working with the toads in the midwest had had proposed. With toads, we assume the idea was wrong. Here, it seems to be correct, if the genetic work is a reliable way to determine the relationships of the types involved. In other words, this may not be introgression at all, but merely an example of evolution in one species.


1. When humans brought cattle (Bos taurus) up to higher elevations in the Himalayas, the cattle did not do well. But some escaped and bred with the native bovid, the yak (Bos grunniens). The hybrids between these two species had characteristics of both. They were more easily handled than wild yaks and could do better at higher altitudes than domestic cattle. These hybrids are called the domestic yak.

2. Although essentially all known cases of introgressive hybridization are the result of the breakdown of isolating mechanisms as humans disturbed habitats or transported species to areas where they were not native, it it possible that natural habitat alteration can result in introgression. In areas where gallery forests run out into the African savannas along streams, the extent to which they penetrate into the savannas may vary over time because rainfall differences may cause the stream to shrink or elongate. In this case, there would be an area that is gallery forest at one time and savanna at another, depending on the recent history of rainfall. Some ornithologists have claimed that it is in this area where certain bird species, one from the savanna and another from the forest, undergo introgression.

3. In the Great Basin, the cool desert area between the Rockies and the Sierras and Cascades (nearly all of Nevada, parts of California, Utah, Oregon, Washington, Idaho, etc), geological evidence shows that many streams and rivers were present in the past. Some ichthyologists have hypothesized that a diverse fish fauna was present, perhaps with many related species inhabiting different habitats in the streams and rivers. The drying trend that destroyed most of these drainages during the past 15,000 years or so may have pushed many of the fish species together into the few remaining habitats that were left after the drying episode. There, they may have undergone introgression, perhaps to the extent that some species completely merged.

Evolutions and Systematics: http://www.auburn.edu/~folkegw/evsys/index.html