Hybrids between two species can produce “swarms” that flourish
Our spectacular swarm has since begotten additional swarms. Among the most headline-grabbing are killer bees, so named for their heightened aggression. Scientists typically refer to them as Africanized bees because the swarm was initiated by Apis mellifera scutellata, an African subspecies of the European honeybee imported to Brazil from South Africa and Tanzania in 1956 to introduce their heat tolerance genes. Typical European honeybee subspecies had floundered in the steamy Brazilian climate, producing negligible amounts of honey.
The African bees proved far more adaptable than anticipated—they soon escaped cultivation and moved northward at a rapid pace. Along the way, they hybridized with other populations of feral honeybees. By 1990, the swarm had reached the southern border of the United States.
While in some ways the hybrid worker bees are not as fit as workers from European subspecies, they seem to be more resistant to the parasitic mite Varroa destructor—thought to be a contributor to colony collapse disorder affecting honey bees throughout North America due to its capacity to transmit disease. This advantage may have allowed the swarm to move even further north.
So, single, random factors can influence whether a swarm succeeds or fails. Had the mite not been introduced from Asia, European honey bee genes may have prevailed in the southern United States—and the African strain’s genes would not have been particularly advantageous. But, since it was, the African genes carried by the swarm gave the hybrids an edge and allowed them to advance further.
A new set of swarms
Among vertebrate animals, fish are some of the most prolific hybridizers due to their reproductive habits. Many are broadcast spawners, meaning that females lay eggs and males spray their sperm over them. So they don’t actually “have sex” in the way humans do. They disperse their gametes into the environment in close proximity in the hopes that they will pair.
“Compared to many mammals, for example, where internal sex occurs, it's harder for hybrids to occur,” Fant says. “But in plants and in fish, it's more common. You release eggs and sperm into the environment and then hope that it finds a match.”
If closely related species spawn in the same area—as is the case with the aforementioned herrings and whitefish—they may cross-fertilize. Some of the most recently identified swarms are fish due to the prevalence of these types of events.
The introduction of the red shiner (Cyprinella lutrensis) to American river systems where it is not native has led to the formation of a hybrid swarm due to crossing with the related blacktail shiner (Cyprinella venusta), for example. Intriguingly, many of the hybrids most closely resemble the red shiner—a phenomenon called cryptic introgression. Had genetic analysis not been done to determine the parentage of these fish, appearances would have suggested that the red shiner had just pushed its relatives out. Instead, genetically, those relatives were swallowed by the swarm.
While these swarms of tiny fish range over miles of river, some swarms are compact. One study found that miniature swarms comprising various subspecies of the common wall lizard (Podarcis muralis) had formed in German cities—these swarms seemed to extend for ranges of only hundreds of meters. Some of the subspecies were native and others had been accidentally introduced from France and Italy. The swarms themselves appeared to have interbred with each other, further exchanging genetic material.
Here again, we face the challenge of defining hybrid swarms. If these are simply variations of the same species, are they really hybrids in a meaningful sense?
These localized phenomena among very closely related organisms pale in comparison to some truly massive swarms that seem to have developed right under our noses—as a result of our own activities. As it turns out, much of the population of wild rice (Oryzias rufipogon) may actually be a hybrid swarm, carrying a mix of domestic genes that escaped agricultural strains. The domestic species is believed to have been developed in Asia some 9,000 years ago, possibly on multiple occasions in different regions.
Humans have since carried this species, O. sativa, across the globe. Nearly all of what we now consider to be wild rice appears to carry genes from domestic rice. While rice is largely self-pollinating, it can also be pollinated by wind and insects. These mechanisms may have facilitated the movement of domesticated genes into the wild, ultimately resulting in the formation of swarms.
Compellingly, the major varieties of domestic rice, indica and japonica, share genetic sequences with wild rice found in the regions where they are cultivated. These sequences code for non-shattering grains and upright growth habits. While they are not always expressed in wild populations—grains that do shatter protect the seeds from complete destruction by insects feeding on them and are thus advantageous to wild strains—the genes remain in the populations. What was once thought to be the wild-origin species of cultivated rice may thus be the result of rice that has gone wild, breeding its parent species out of existence—or at least fundamentally altering it.
