Variety: the spice of life sciences

Pat Monaghan and Tim Birkhead call for variation to be the theme once more in university biology

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Bees ( June 2013)

The absence of natural, individual variation, both genetically and environmentally induced, in model organisms is hardly representative of the real world

A study published in the British Medical Journal last year commented that only one in four young men has “optimal” sperm quality: the rest produce ejaculates with lots of duds. Is poor sperm quality the fault of tight underpants or a hot laptop resting on the testes? Does it jeopardise our future?

Before we start worrying about what medical science can do to rectify this apparent deficiency, we should think about it from another perspective, look at sperm quality in other species and ask whether it varies according to their various mating systems. Such comparisons can tell us what we might expect human ejaculate quality to look like.

It turns out that in those species with promiscuous females, males need high-quality ejaculates so that their sperm have a chance of outcompeting those of their rivals. In contrast, in species with monogamous females, males can manage with low-quality ejaculates. Chimpanzees are famously promiscuous and have superb sperm; gorillas on the other hand are like us in that female promiscuity is low and males get by with ejaculates that contain relatively high numbers of duds. So, poor-quality sperm is just the way we are, but this becomes apparent only after comparing ourselves with other species and doing so within a conceptual framework that allows us to interpret the comparisons.

This sperm example illustrates the importance of knowing how and why closely related animal species can differ from each other. Currently there are two very different research and training approaches to such biological variation in the life sciences.

In the first (bio)medical approach, research and training tend to treat variation as a nuisance, and the focus is on commonality. Biomedical degrees rarely provide students with any sense of the diversity of life or the evolutionary relationships between different types of organism. Being different is considered equivalent to being deviant, and the benchmark for deciding whether something is worth studying or not is how relevant it is to humans. Biomedical teaching and research on non-human animals typically focuses on a handful of “model species” – viewed as surrogates for humans, or at best as representative of the taxon to which they belong.

Remarkably, model organisms in practice have not been chosen with either of these qualities in mind. Instead, they have been selected usually because of their rapid development, short lifespan, insensitivity to their environment and the ease with which they can be reared in the laboratory. On top of this, many model organisms have been “standardised” by intense selective breeding specifically aimed at reducing variation among individuals. This uniformity is important in that it helps with the replication of experiments and reduces the number of animals that need to be studied. But the absence of natural, individual variation, both genetically and environmentally induced, is hardly representative of the real world.

The use of model organisms – mainly Drosophila flies, mice, rats, chickens, zebra fish, yeast and the nematode worm C. elegans – has yielded great benefits and as a result we now know much more about many fundamental and highly conserved areas of cell biology and biochemistry. But this understanding has come at a price: knowledge has been canalised. Minimising environmental effects means that there is an overemphasis on genomic effects and an almost complete lack of appreciation of how variation is generated by environmental factors, why this is important and what it can tell us.

The other approach to biology is one that is best represented by the discipline of zoology. Indeed, zoology is perhaps now the only discipline in which students are taught about the nature, causes and consequences of variation among and within species. It considers animals in the context of their environment. Zoologists are excited by variation, and want to know how and why animals differ at the cellular, individual, population and species level. Natural selection is the conceptual framework that links variation in biological traits with variation in the environment.

All species have an ancestral legacy that shapes their current body form, physiology and behaviour, but zoologists also recognise that both within and among species, animals differ in many important ways. Consequently, to a scientist with a zoological background, the idea that the fly Drosophila melanogaster is representative of the biology of all arthropods or that a small-bodied, short-lived mammal such as the mouse is a good surrogate for a large-bodied, long-lived mammal such as the human seems ridiculous.

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