A lazy Mediterranean holiday may seem a perfect time to rest your brain. For Helen however, idle moments on the beach are perfect for scientific questioning. “Why are clouds white?” she once asked, staring upwards; “why do your ears pop as your plane ascends?” she wondered on another occasion.
Figure 1. Donkeys on the island of Hydra. Image credit: dronepicr (via Wikimedia)
Recently, it was the absence of motor traffic on the Aegean island of Hydra that aroused her curiosity; she noticed that mules, donkeys and horses were clearly doing all the heavy lifting. As a lifelong city dweller, the differences between these three equines had not grabbed Helen’s attention up till now. Eagar to check out this gap in her knowledge she buttonholed a hapless waiter over her souvlaki that evening. “What exactly is a mule” she asked. Keen to be consulted he explained in broken English that it was a cross between a donkey and a horse.
A quick search on the internet provided the basic information Helen sought: mules are indeed a cross between a horse and a donkey. On hearing in a later discussion that mules themselves are unable to produce offspring, Mary asked bluntly: “what’s the point of mules then?”. Wikipedia provided the answer: they are better suited than either parent to the role humans give them – hardier and more patient than horses and less obstinate and more intelligent than donkeys.
Later, in a discussion group, Sarah searched further on the internet looking for an explanation of their infertility. She discovered that donkeys and horses have different numbers of chromosomes, 62 and 64 respectively. The consequence is that when they mate, the mule offspring picks up half from each parent: 32 from the horse and 31 from the donkey, a total of 63, an odd number. This unexpected fact, raised the question of what chromosomes actually do and why they usually come in even numbers.
A chromosome is a gigantic structure, at the molecular level. It is a mixture of the very long, thin thread of DNA that carries the genetic information in every cell of an organism, and a number of proteins that hold it together. Short stretches of the long DNA molecule make up the genes that go on to determine which proteins get made in our bodies and, hence, what we are as an individual. But the DNA molecule is unwieldy. Its length in just one microscopic human cell would be nearly two metres if stretched out in a line.
To fit this into the tiny nucleus inside a cell, ready to be passed on safely to a next generation cell, it must be wound up into a compact bundle. It’s for a similar reason that a length of wool gets wound into a compact skein or cotton thread is stored on reels.
Figure 2. How DNA coils up. Image credit: Thomas Splettstoesser via Wikimedia
Figure 2 shows the double helix of DNA with its genes, wrapped around a group of spherical proteins (the nucleosome) and this structure itself being would up in a spiral to form the chromosome. As the total length of DNA in a cell is so enormous it is arranged in a number of shorter lengths to be manageable. Each of these shorter stretches is a chromosome. In a human cell, for example, the total length of DNA is shared across 46 separate chromosomes. This means some of your genes are on chromosome no 1, some on no. 2 and so on.
All of us that reproduce sexually – i.e. most plants and animals – inherit half our genes from each parent. Humans get 23 from mum and 23 from dad – that’s why we resemble each of them to some extent. All the genes we need to produce the proteins that make up our bodies are present in each of our parents (apart from the genes on the sex-linked Y chromosome which is only present in males).
We have only 23 utterly distinct types of chromosomes but two versions of each: one from mum and one from dad, as the microscope image in figure 3 shows. The exception is the sex-linked chromosome which comes in two very different versions as the last pair in the image shows.
Figure 3: 46 chromosomes in a human cell, laid out as 23 pairs. Image credit: US National Human Genome Research Institute via Wikimedia
It’s not difficult to see there would be a problem if all the chromosomes from a normal cell in both our mum and dad were mixed at the moment of fertilisation. Each parent would contribute 46 , burdening the offspring with a double load of 92 chromosomes. This is why special cells have to be made for sexual reproduction: egg cells and sperms cells. These are produced in glands called gonads by a process that splits up our 23 pairs of chromosomes into just 23 single chromosomes; that’s what special about sperm and egg cells – they only have a single copy of each chromosome rather than two. Thus, when fertilisation eventually takes place, a single chromosome from each parent enters the fertilised egg, restoring the usual complement of 46.
Back to our equine friends. There’s no problem for a stallion mating with a mare – their 64 chromosomes are organised as 32 pairs of similar ones. Similarly for male and female donkeys (known as Jacks and Jennies): their 62 chromosomes comprise 31 similar pairs. But when they cross-breed it’s the hybrid mule they produce that has a problem. The mule’s odd number of 63 chromosomes doesn’t allow for pairing up. This problem, together with structural differences between the horse and donkey chromosomes, means the process of forming egg and sperm cells in a mule is severely impaired. The mule is infertile. Bizarrely, very rare cases of pregnant mules have been reported, so there must be some exceptional situations in which fertilisation can happen.
Discussion of mating between horses and donkeys raises an even more profound question. “What is a species?” as Mary put it. “Are horses and donkeys different species?”. These are not straightforward questions. The generally accepted definition of a species is a population of organisms that can mate with one another and produce offspring that are also fertile. As one horse can mate with another to produce fertile foals, they are each members of a single species. It’s known as Equus ferus caballus. Similarly, donkeys are members of a distinct species: Equus africanus asinus. Mules, on the other hand, are not a species, as their offspring are not fertile.
Other types of hybrid animal exist, though they are relatively rare. Ligers and tigons (lion + tiger), wholphins (whale + dolphin) and grizzlars (grizzly + polar bears) have been found in the wild or successfully bred in captivity. The offspring are often infertile, but sometimes they are not. These exceptions make defining a species a controversial business.
Species can also be classified in ways other than ability to produce fertile offspring. In the past, physical features (morphology), such as length of tail or number of wings, were used to define a species. More recently the precise sequences of “letters” in the DNA code has been used to characterise species. The variety of classification methods means that definitions are sometimes in conflict.
The great step forward in taxonomy, taken by the Swedish botanist Carl Linnaeus, was the creation of a hierarchical classification system. The diagram in figure 4 uses the red fox as an example. The various species of fox (red fox, arctic fox etc.) form a group known as a genus; in this case, Vulpes
Figure 4: biological classification. Image credit: Humzah Rouf Phumboo (via Wikipedia)
Other animals that share many characteristics with foxes, such as dogs, wolves and coyotes, are grouped together in the next higher category: “family”, in this case the Canidae. Related families are themselves grouped together as an “order”, and so on up the hierarchy illustrated in figure 4.
At the upper end of the hierarchy, “Kingdom” separates organisms into the basic categories with which we are familiar: animal, plant, fungi …. Plus one or two other, less familiar, ones. Below this “Phylum” includes broad categories such as Chordata (animals with backbones) and Arthropods with jointed legs. “Class” breaks these down into more specific groupings such as mammals, birds and fish. “Order” is further sub-divided into groups such as carnivores and primates. These are themselves broken down into families.
Breeds and varieties
Mention of cats and dogs brought discussion in the group closer to home.“What exactly are breeds?” asked Patrick. “Are all kinds of dog really members of the same species? If they are how do we account for the extreme differences between a St Bernard and a Chihuahua?”
Figure 4: Greyhounds belonging to Louis XV. Picture by Jean-Baptiste Oudry
Definitions vary slightly, with Wikipedia referring to a breed as “a specific group of domestic animals having homogeneous appearance, behaviour, and/or other characteristics that distinguish it from others of the same species”. Other definitions refer to plants as well as animals and to common ancestors somewhere along the line.
“Breed” is not scientifically defined; it’s a term developed by agreement between breeders. Individual organisms with desired characteristics are selected by breeders to mate with one another. The expectation is that the relevant traits are passed on to their progeny and continue down the generations. Dogs of all breeds, however distinct their appearance may be, are members of the same species: Canis familiaris. Dogs of different breeds can, of course, mate and in so doing, produce fertile, cross-breed offspring. In plants the principles are much the same though the terminology differs. Breeds are known as cultivars (or, more commonly, varieties) and cross-breeds, as hybrids. Typically, genetic material is transferred from one plant with a desired characteristic to a genetically different plant. For example, a variety that is disease-resistant may be crossed with one that grows larger. This might be achieved by deliberately fertilising one with pollen from the other or by grafting – physically cutting the stem of one and placing in contact with the other.
In addition to deliberate breeding by humans, geographical isolation of populations, by a mountain chain, for example, can also lead to physical and behavioural differences. As individuals from separate groups will not usually come into contact, their offspring will reflect characteristics of their own group only. With time, mutations will gradually occur naturally and randomly. As a result, the separate populations will gradually change in different ways. If the separate environments favour different mutations, the characteristics of successive generations may begin to diverge gradually. Such differences may eventually become significant enough for the group to become recognised as a subspecies.
If breeds, cultivars and sub-species exist in plants and animals, does something of the kind occur for Homo sapiens? Are differences between sub-populations of human beings comparable to breeds? What is the meaning of ethnicity or race in biological terms?
Cultural differences, such as language and national histories, are commonly used in definitions of ethnicity, rather than anything biological. Wiktionary, for example, defines it as “The common characteristics of a group of people, especially regarding ancestry, culture, language or national experiences”. It is understood by sociologists today as a purely social concept. The concept of race, on the other hand was considered by some anthropologists, as recently as the early twentieth century, to be biologically-based. Measurements of skull size and shape and other physical features were made of people from different parts of the world and hierarchical classifications devised. Some of the false conclusions drawn acted as a pseudo-scientific basis for the eugenics movement and, subsequently, abhorrent racial policies.
With today’s understanding of genetics, however, scientists are able to use DNA from different populations to study differences biologically. Changes in particular environments can indeed lead to genetic mutations which cause changes in appearance. For example, as Homo sapiens gradually spread out from Africa some 20,000 to 50,000 years ago, a mutation resulting in lighter skin pigmentation developed. Dark skin pigmentation is essential in sunny places, to protect against excessive ultraviolet (UV) radiation from the Sun. A modicum of UV is needed , however, for our bodies to produce the essential vitamin D. Natural selection led to humans with lower amounts of the dark pigment, eumelanin, in the more northerly climes, where UV levels were lower.
Scientists today recognise that the concept of “race” is genetically meaningless. Key evidence for this come from studies of differences in the genes sampled from populations across the world. It is found that, within a sub-population, or so-called “race”, differences in genetic make-up are greater than any differences between sub-populations. This has been shown, in particular in the continent of Africa. It turns out that the greatest degree of genetic diversity occurs between different populations within the continent of Africa. A person of African descent is likely to be more different genetically from another African than from a person of European or Chinese descent.
This investigation of variation amongst biological organisms, launched by an enquiry about mules, has led us in many different directions. The role of chromosomes in enabling fertility, the hierarchical classification system for living things; the breeds and cultivars that feature in our domestic lives and the false theories of human differences that continue to blight our societies. The exploration throws up a host of further questions: how do genes determine traits like stubbornness? How were our farm animals and pets domesticated? When does a breed or cultivar become a new species? What about the evolution of bacteria and viruses? How do variants fit into the picture? No shortage of material for future blogs!
© Andrew Morris 2nd July 2022