Speculative evolution
Abstract
The present paper discusses the possibility that rare animal interspecific hybrids may evolve into entirely new species and occupy the highest trophic level. Such hybrid speciation may have occurred in historical times and represents an evolutionary mechanism. The subject of the current study, however, is typical cases of anthropologic organisms, whose evolution was facilitated or even caused by human activity. These are typical examples of the way that human civilization changed ecological systems and affected biodiversity.
Summary of interspecific hybridization
Previously, it was assumed that biological species are mostly reproductively isolated, or in other words, unable to crossbreed. Currently, this view is not so restrictive, especially when it was shown that even in the human genome there is strong evidence of historic hybridization with other species of the Homo genus. Much expectedly, hybridization is more common in taxonomically closer species. Therefore, interspecific (between different species of the same genus) hybrids are more common than intergeneric (between species of different genera) ones. In plants, hybridization tends to occur more frequently than in animals and some species are a result of such, including important crop plants like wheat. For a hybrid to evolve into a new species, it must undergo hybrid speciation. Both male and female hybrids must be fertile and should be capable to outcompete the parent species in the particular ecological region.
Hybrid infertility is explained largely by the different chromosome numbers in different species, or, even if this matches, the same gene may be on different chromosomes. This means that their sex cells – sperm and eggs, may lack a particular gene or even a whole chromosome. Plants usually overcome this by polyploidy. For example wheat contains chromosome pairs of three different genomes, coming from the three species that formed it. In animals, however, polyploidy is rare and most commonly, lethal. Some animal hybrids like the pizzly bear may be fully fertile, while others possess a different degree of sterility: 1) only females are fertile like in ligers and tigons; 2) both sexes are fully sterile. This is a major obstacle for animal hybrid speciation.
Close hybridization with unexpected consequences
Unlike hybridization between not closely related animals, hybrids between different breeds and subspecies are much more probable to have viable and fertile offspring. This is, of course, widely used in selective breeding in efforts to acquire new breeds with excelling characteristics. In some cases, however, crossbreeding might lead to unexpected results. The best-known contemporary example is the Africanized or killer bees. It is a hybrid of Western honeybee (Apis mellifera) and its subspecies (Apis mellifera scutellata), the African honey bee. The purpose was to improve honey production, but the outcome was an extremely aggressive and highly defensive hybrid, killing several people yearly. The killer bee crossbreeds with common honeybees readily and spread from Brazil to the USA in less than 50 years.
Similarly, the crossbreeding of different killer whales (Orcinus orca) ecotypes, occurring in captivity hides the potential risk of creating a behaviorally different population. Killer whales are considered a single species, inhabiting nearly every sea in the world and standing at 5-8 meters in length and 4-6 tones in weight, they are the unchallenged apex predator of the open ocean. Their family grouping in pods, cooperative hunting, and sophisticated vocalization make them even deadlier. The current killer whale population is divided into several ecotypes, differing in behavior, vocalization, and food specialization – some eat fish, while others prefer sea mammals and are even capable of dragging seals out of the shore. These ecotypes tend not to crossbreed, even if living in the same habitat, but this is not happening in captivity. So, if we remember the shark-becoming-smart horror film Deep Blue Sea [1], killer whales do already have everything that the sharks acquired through genetic engineering in it. In this respect, a hybrid, captive population may change its food preference and learn how to hunt on humans. If released in the wild, or escaping in some way, these orcas will become a danger, by attacking small boats, scuba divers, and even dragging tourists from the beach.
Nature found its way. The chromosome stabilization repair system.
The biggest obstacle in front of hybrid speciation is the chromosome number and mapping of the parent species. Even small differences in the gene distribution may result in non-viable sex cells during the homologous recombination. Most commonly, the heterogametic sex, being the male in mammals, is sterile in interspecific hybrids, which is called Haldane’s rule. Thus, a female hybrid may reproduce, but with a male of one of the parent species, which will prevent true hybrid speciation. The situation could be even more dramatic if the parent species have different chromosome numbers like in humans (23 pairs) and great apes (24 pairs). Thus, a resulting hypothetical hybrid will have an odd number of chromosomes – 47. The human chromosome 2 resulted from the fusion of two ape chromosomes and during sex cell division in the hybrid a viable egg or sperm should contain either the human chromosome or both of the ape. In any other case, there will be either gene loss or gene duplication, which is usually harmful. These mechanistic problems, however, are not the sole explanation of Haldane’s rule, and overcoming them might not be sufficient for male fertility and hybrid speciation.
The male sterility of hybrids seemed to be unavoidable, but soon this concept proved wrong. First, this effect diminished with further crossbreeding between the female hybrid and a male parent species. Thus, a third-generation male hybrid occurred to be fertile and capable to start a small population of hybrid animals, further leading to hybrid speciation, although it does not share an equal amount of genetic material from the parent species. In other words, the liger hybrid species is about ¾ lion and ¼ tiger, while the tigon hybrid species is the opposite.
Another important factor in this event was the so-called chromosome stabilization repair system. There is no conclusive research, indicating the evolutionary origin of this system. It is found only in mammals and represents a complex of proteins, involved in the mismatch reconstruction of chromosomes during the metaphase I of meiosis. During this phase, the chromosome reconstruction mechanism leads to extensive chromosome fusion in a way that the parent species‘ chromosome number would match each other. For example mules, hybrids of a male donkey (62 chromosomes) and a female horse (64 chromosomes) have a total of 63 chromosomes (31 donkeys and 32 horses). During the formation of sex cells, the chromosome reconstruction mechanism would rearrange the horse chromosomes and reduced them to 31, thus producing sperms and eggs with 31 chromosomes.
False hybrids
For the purpose of this article, a true hybrid is considered an animal, resulting from natural or laboratory merging of sperm and egg cells of the parent species. As such, true hybrids should contain half of the genetic information of both species. It is possible, however, to artificially introduce one or many genes of one species into gamete of another and produce incomplete hybrids. The humanized chimps, created by BioGen Research in Michael Crichton’s Next [2] are most probably false hybrids. False hybrids were also created intentionally in scientific efforts to recreate several extinct species thanks to preserved DNA fragments. Two notable examples exist, the mammoth-elephant and saber-toothed-lion hybrids. During their creation, the preserved DNA sequences were artificially complemented with DNA from extant relatives. The preserved genes of Smilodon fatalis and Mammuthus primigenius were used to replace homologous genes in the African lion (Panthera leo) and the Asian elephant (Elephas maximus) respectively. The resulting hybrids were called Panthera leo x smilodonis and Elephas maximus x mammuthus and contain between 36 and 42 percent of the extinct species DNA. They were successfully bred in captivity, but later escaped in the wild and established their own populations.
The new hybrid megafauna of North America
After the Great War, when large parts of North America were heavily depopulated, completely new megafauna evolved from previously captive animals. It consists of both feral populations of livestock and hybrid species. The hybrid speciation megafauna consists primarily of the following species: 1) beefalo, a cattle/American bison hybrid with an approximate DNA ratio of 65/35 percent. These hybrids were fully fertile and established a population of several million individuals; 2) feral mules, a horse/donkey hybrids with approximate DNA ratio of 25/75 percent, that developed fertility after several generations of hybrid with parent species mating; 3) American elephant, a mammoth/Asian elephant false hybrids. The three species form mixed herds and perform annual migrations in search of food.
At least three big cats’ hybrids also evolved and replaced the existing cougar populations. First, the saber-toothed lion false hybrids formed a stable population of several thousand individuals, mainly in Nebraska and Dakota. The two other major hybrids evolved from the continuous crossbreeding between thousands of tigers and lions held captive in numerous private zoos and safari parks. The larger ligers are predominantly social cats, living in family groups of several tens of individuals in open areas. They initiated like hybrids of male lions and female tigers and gained fertility after successional crossbreeding between female ligers and other male lions, thus gaining a DNA ratio of 78-to-22 percent lion-to-tiger. The smaller tigons are solitary, mountain predators, initiated by crossbreeding between male tigers and female lions and having the opposite parent DNA ratio. All three hybrid species are apex predators and often co-exist in the same area, but tend to avoid each other.
Figure 1. Male true (liger, standing) and false (saber-tooth tiger, sitting) hybrid. The sex chromosomes (X and Y) are shown, with the lion DNA in red and the foreign DNA in black. While in the liger the X chromosome is entirely of tiger origin and the Y chromosome is entirely of lion origin, in the saber-tooth the Smilodon and lion DNA is randomly distributed.
The last hybrid carnivore that evolved naturally due to habitat loss in the far North is the pizzly bear, a polar and grizzly bear hybrid (Ursus arctos x Ursus maritimus). They occupy mainly Alaska and Canada and are large, short-tempered predators, seen hunting either on the ground or in large water basins. They occurred mainly because of the Arctic ice cap melting, which forced the Polar bears toward Southern regions in search of potential prey. There, they started to overlap their habitat with grizzly bears and freely interbred with them.
Hybrid reptiles of the Everglades
The Everglades of Florida, much similar to other parts of North America, were heavily altered by anthropogenic factors and raised at least two hybrid species of great carnivorous potential. First, the large and overly aggressive caiman/alligator hybrid (Melanosuchus nigers x Alligator mississipiensis) emerged when black caimans, released in the wild crossbred with native American alligators. The resulting reptile easily reaches 6 m in length. Concerns of such possibility were never raised, although crocodilian hybrids like American/Cuban and Siamese/Saltwater crocodile hybrids were known to occur frequently.
The other reptilian hybrid is a cross between two introduced species, the Burmese (Python bivitattus) and the African rock pythons (Python sebae). Both of them were popular pets and both of them made it to the wild, where despite their origin on different continents, they successfully bred between each other. The resulting hybrid easily outcompeted the native Boa constrictor in both size and aggressiveness.
References
- Renny Harlin. 1999. Deep Blue Sea. Village Roadshow Pictures.
- Michael Crichton. 2006. Next. HarperCollins.