This week’s post is a guest contribution by Tom Houslay, a doctoral student at the University of Stirling in Scotland who studies sexual selection. Tom blogs about insects, sex, and evolution at his personal website, which would be totally NSFW if you were a sage grouse. If you have an idea for a post, and you’d like to contribute to Nothing in Biology Makes Sense, e-mail Jeremy to inquire.
Whenever I read the Dobzhansky quote from which this blog takes its name, it puts me in mind of another famous phrase:
“The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!”
Such were the words of Charles Darwin in correspondence with botanist Asa Gray in 1860, a year after the publication of On the Origin of Species. Taken at face value, it is easy to see why Darwin felt such discomfort when gazing upon a peacock’s feathers—if animals adapt to their environment, surely a cumbersome train of colourful feathers would be counterproductive? Not only should it attract the attention of predators, but it would surely impede any attempts to evade them.
While peacocks are the posterboys for such traits, they are far from the only offenders. Time and again in the animal kingdom, we see exaggerated ornaments, vibrant colours, and fantastic acoustic and visual displays. Frogs and crickets advertise their whereabouts with loud calls, fireflies flash patterns with bioluminescence; greater sage grouse strut brazenly in open pastures. While they undoubtedly brighten up the world around us, these behaviours and morphologies can seem not only unnecessary, but downright detrimental to the survival of an individual. How, then, can their existence be resolved with our knowledge of evolution? Furthermore, why are these characters generally seen only in the males of a species?
Evolution through the process of natural selection requires that there be variation in heritable traits among individuals in a population, and that different trait combinations have different rates of survival and reproduction. It is the field of reproductive success to which we can ascribe the morphological differences between males and females—as Darwin set out thoroughly in his later volume, <i>Selection in Relation to Sex</i>. While males carry huge numbers of cheaply-produced sperm, females harbour relatively few eggs, which are packed with nutrients and protection for their future offspring. Evidently, this imbalance in resource investment creates conflicting desires for those wishing to maximise the potential of their genes spreading to future generations. Males incur little cost by mating indiscriminately in order to fertilise as many eggs as possible; females, meanwhile, should generally benefit by being a little more discerning when it comes to mate selection.
(As always, please note that this is evolutionary theory, rather than life advice – I’d rather not be blamed for anyone suffering multiple paternity suits after reading this post and then ‘mating indiscriminately’.)
When selecting a partner, females often exhibit preference on the basis of ‘direct benefits’—that is, goods or services that a male can provide which the female uses to increase her own fertility directly—such as food, territory, or parental care. In some species, the male ornament is indicative of such offerings. However, in many others, males provide nothing other than genes during mating, so why should females persist in choosing on the basis of exaggerated traits?
Well, if females are only receiving genetic material from their selected partners, perhaps this gives us a clue as to what ornaments might indicate. While the female may not benefit directly from choosing a genetically ‘superior’ mate, if her offspring have an advantage (in viability or attractiveness) then they may have greater reproductive success—meaning she will eventually reap the rewards of selecting for indirect genetic benefits.
However, there is a special problem faced by those with such mating systems. The species we associate with having the most exaggerated ornaments are often those in which males mate multiply—for example, consider grouse and other animals where males gather in leks to display their wares to watchful females. We noted earlier the need for genetic variation for selection—but if females strongly prefer ornamented males advertising heritable differences, and only mate with a subset of available sires, then the genetic variation between males that is associated with the ornamental trait will eventually be depleted. This in turn should undermine the benefits of choice, as these depend on genetic variation for the ornamental trait. We call this problem the ‘lek paradox’.
One proposed solution to this is that ornamental traits evolve a close association with an organism’s ability to acquire resources and convert them to usable forms. The ornaments themselves become ‘condition-dependent’—higher quality males should acquire more resources, and so invest more in the trait than lower quality rivals can afford to. Resource acquisition includes a male’s ability to forage, catch prey, fight disease, and metabolise nutrients effectively. To the extent that sexual traits are condition-dependent, they reflect the efficiency of the vast majority of an organism’s functions—and, by extension, the quality of virtually the entire genome seo companies.
This ‘genic capture’ model means that selection is no longer acting on the variation in a single trait, but on almost all alleles. The involvement of so many loci helps to maintain genetic variation in the population, even before considering how large a target this provides for new mutations. Not only that, but the effects of an allele may depend upon the environment in which it is expressed; a genotype that performs ‘best’ in one environment may not necessarily do so in another. Thanks to such ‘genotype-environment interactions’, favoured alleles change in space and time, affecting the ability of an organism to express its exaggerated traits.
These ornaments and displays are essentially summaries of current ‘performance’, enabling males to show females just how great they are. So the next time that you gaze, as Darwin did, upon a feather in a peacock’s tail, there’s no reason to feel nauseous. It’s yet another way that evolution has brightened up this world.
Rowe, L., & Houle, D. (1996). The lek paradox and the capture of genetic variance by condition dependent traits. Proc. Royal Soc. B, 263 (1375), 1415-1421 DOI: 10.1098/rspb.1996.0207