When Charles Darwin first presented his Theory of Natural Selection in 1858, there was scant understanding about the complexities of evolution. Darwin’s theory of evolution by natural selection defined itself as “the process by which a population’s heritable traits change over generations”. Basically, ‘natural selection’ is the means by which organisms with traits that help them survive and reproduce are more likely to pass those traits on to their offspring.
Darwin’s theory of evolution was a revolutionary idea in his age. One of the pillars of evolution is hybridisation. While not a direct synonym for natural selection, hybridisation shares a common purpose, and can be a factor in evolution and specialisation, which are both influenced by natural selection.
The world of flora and fauna is vast. According to the International Union for Conservation of Nature (IUCN), there are ~8.7 million extant species on Earth, of which only 1.2 million have been scientifically described and catalogued. Apart from these are those that have been hybridised. Typically, hybridisation is a natural process that occurs in both plants and animals. It is estimated that 25% of plant species and 10% of animal species hybridise with at least one other species. This is called ‘natural hybridisation’, which occurs by chance in nature, such as when species spawn in the same area due to habitat changes. Natural hybridisation can have positive and negative effects on genetic diversity, including increased allele (the alternative form or versions of a gene) diversity and speciation, or extinction and increased invasiveness. Some specimens of natural hybrids include: Bread and durum wheat (Hybrids of wild grass species), peppermint (water mint + garden spearmint), and rainbowfish (freshwater fish adapted to diverse climates).
Planting new species
Today, however, scientists have taken hybridisation miles ahead, developing a host of exotic new species in controlled environments by mixing genes. This brings us to the idea of artificial hybridisation, a deliberate process often used in agriculture – to boost yield, and increase productivity and income.
Scientists have experimented developing certain exotic crops. The first plant hybrid was produced by Thomas Fairchild in 1717. He created a plant hybrid by crossing two varieties of flowering plants – Sweet Willam (Dianthus barbatus) and Carnation (Dianthus caryophyllus) – resulting in a ‘Fairchild’s Mule’. Later, his research formed a base for other scientific studies on crop improvement in breeding programmes.
Plant hybrids are the offspring of two different varieties of plant species, and often have a greater genetic diversity than the parent plants. In short, plant hybridisation is a process through which new fruits and vegetables can be derived. These hybrids are often developed for disease resistance, to change the size of the parent plant, flower or fruit, to increase flowering, taste or even the colour of the fruits.
Plant hybridisation is where the pollen of one plant is transferred to flowers of a different variety. But before doing so, the breeder needs to decide which plant he wants to use as the female, and from which plant he wants to extract the pollen. Once this is decided, the pistil is pollinated manually, and it is also ensured that all the stamens are removed from the plant to prevent self-pollination.
Prominent plant hybrids
Hybrid lilies: Hybrid lilies or liliums are often termed Asiatic hybrids or Oriental hybrids. They can be easily grown in pots, with ample light and water
Triticale: Triticale is a hybrid crop combining the high yield of wheat with the hardiness and disease resistance of rye
Pluot: This is the offspring of plum and apricot, containing the shape of the plum and the smooth skin and juicy nature of the apricot
Tangor: Tangor is created by combining tangerine and orange. Slightly smaller than an orange, it is sweeter and more fragrant
Cutting-edge merge
By now, it’s clear that hybridisation is a process through which two organisms of different breeds, species or varieties are combined to develop a new entity that possesses the traits and characteristics of both these sources. It is not just limited to biology, and can be applied across various fields like chemistry, technology, culture and society.
There are studies suggesting that the concept of hybridisation dates back to Ancient Mesopotamia, over 5,000 years ago. This civilisation is believed to have introduced one of the first animal hybrids called ‘Kunga’. Archaeologists have termed this creature ‘the first beast of burden’, and DNA tests reveal that it may have been the earliest ‘man-made animal’.
Hybridisation in animals is a process where parent animals closely related to the same species are bred together to form a hybrid. For instance, the genes of lions and tigers which belong to the family ‘Felidae’ are bred to give birth to a ‘liger’ (lion + tigress) or ‘tigon’ (tiger + lioness). This is possible because lions and tigers are very closely related. However, this is not so easily possible among several other species, including Asian and African elephants, which separated biologically and geographically millions of years ago.
The process of hybridisation has biological significance, since it helps improve genetic variety, thus playing an influential role in evolution. These hybrids can produce offspring, and this happens when hybrids mate with other hybrids or with the same species as one of their parents. For instance, lions and tigers hybridise to produce ligers or tigons, and the offspring that turn out fertile, can mate with other ligers, lions, tigons or tigers. But these fertile hybrids can create complexities in natural science and a rift in the biological world. Since hybrids are a result of two distinct species, they exhibit unique abilities and characteristics apart from other animals. They may also show mutations and deficiencies, making them susceptible to often fatal health conditions.
Prominent animal hybrids
Ligers: They are the largest cats with a tawny coat and faint tiger-like stripes. Like tigers, ligers enjoy swimming, while possessing the social nature of lions. They are also docile, and usually sterile, being unable to reproduce. They also fail to display the predatory instincts of their parents (they cannot hunt for their food)
Tigons: Their characteristics are distinct from ligers. They are smaller, and often have a mix of tiger stripes and lion-like colouration. Tigons exhibit characteristics of both parent species, but are less aggressive and more active than ligers. They are also comparatively reserved
Zorse: An offspring of a zebra stallion and a horse mare, it is infertile. Zorses often resemble horses, with zebra-like stripes on their necks, legs or rumps. They are slightly smaller than a horse, but have a strong sturdy build, and are often temperamental and wild like zebras. Zorses inherit the stubborn nature of zebras, which makes it harder to train them
Zonkey: A cross between zebras and donkeys, the zonkey is a sterile animal that usually has a donkey-like body, with faint or pronounced zebra stripes on their legs. Zonkeys are smaller than horses, but larger than donkeys, and have a donkey’s resilience and a zebra’s strength, making them more independent, and thus difficult to be tamed
Wolfdog: A wolfdog is an offspring of a wolf and a domestic dog, and is often known to be independent, territorial and aggressive. The size of a wolfdog depends on the dog breed used in the cross. It is a highly intelligent canid, needing special care and training to handle its wild nature
Ethical questions
While hybridisation is seen as a great jump in biological research and genetic technology, it raises a bunch of ethical questions. Critics to animal hybridisation generally consider it unethical, since one is creating an animal that did not evolve in the natural world, has no place there, could trigger imbalance in nature, and has potentially unknown problems, such as a compromised immune system and a potential lack of shields against natural threats and afflictions. Some argue that there is little regulation, and the legal status of hybrid animals is uncertain, while others cite that most captive animal operators breed hybrid animals for commercial gains, rather than for scientific reasons. The best instance of this is the pet trade, where a lot of breeds of dogs, cats, guinea pigs, and ornamental fish, are a result of hybridisation. For instance, a Boxer is a hybrid ‘pedigree’ dog breed that descended from extinct bullenbeisser breeds, which were crossed with the English Bulldog and Mastiff in Germany in the 19th century. Today, more dogs are getting hybridised into new breeds, largely for ornamental reasons, such as the puggle (pug + beagle) and goldendoodle (golden retriever + standard poodle). Crossbreeding may open the animal to many genetic diseases that are common in purebred dogs. Besides, it could lead to mutations, affecting growth and lifespan.
More Hybrid Facts
Agricultural use
Hybridisation is a form of selective breeding that is used by agriculturists to create high-yielding crops
Animal hybridisation
This is used for medical research, commercial purposes and to test the potential limits of evolution
Rapid adaptation
Hybridisation can lead to the creation of novel genotypes that can adapt quickly to new environments
Hybrid zones
When previously isolated populations come together, a hybrid zone can form, containing the parental types
Genetic diversity
The diverse genotypes in a hybrid zone can help map genes that contribute to reproductive isolation