The Tail of the Peacock
If you give a female the choice between Zac Efron and Rowan Atkinson, the majority will choose Zac. This is because he is the more attractive of the two males.
A similar situation occurs among the peacock species. The female chooses the most attractive male to mate with.
This concept, of choosing a male based on physical features is known as sexual selection, a concept which Darwin first wrote about.
The sexual selection of the peacock has resulted in the evolution of the species, with the peacocks tails becoming more and more elaborate.
Contrary to the males' extravagant tail the female peahens have quite bland trains. The physical difference between the male and female peacocks is called sexual dimorphism.
- Natural Selection
- Sexual Selection
- Selection Pressure
- Darwin and Sexual Selection
- Peahen Preferences
- The Courtship Ritual
- Positive Feedback
- Genetic View of Sexual Selection
- Sexual Selection- Behavioral or Genetic?
- The Albino Peacock
- The Future of the Peacock
Natural selection is the differential survival and reproduction of individuals due to differences in their phenotypes. Darwin developed this theory as an explanation for adaptation and speciation; a mechanism for evolution. The amalgam of phenotypes make up an individual.
But when we are talking about the evolution of the peacocks bright, intricate and iridescent features, we need to be looking at a concept called sexual selection.
Sexual selection is a unique relative to natural selection that arises through preference by one sex for certain characteristics in individuals of the other sex. In other words it is a form of natural selection based on physical attributes. See 'Peahen Preferences'.
Sexual selection, while still evolving the organism, does not necessarily adapt them to the environment, rather, enhances the traits involved in mate acquisition. Natural selection does however produce individuals well adapted to the environment.
Sexual selection can produce individuals with elaborate ornaments that can be energetically costly to develop, maintain, or even lead to a direct survival cost for the individual. Therefore, sexual selection has the capacity to evolve maladaptive traits. (This is elaborated under the heading, 'The Future of the Peacock').
Sexual selection arises in response to either:
- Female Choice: Whereby the female chooses the male based upon the male's physical phenotypes.
- Male Competition: Intraspecific competition between the males for territory, access to females, or areas on mating grounds where displays take place. (See the 'Peacock Courtship video below)
So what is the selection pressure that drives the evolution of the peacock species?
Well, the peacocks aim is to attract a female to mate with and pass on their genes to the next generation. The selection pressure can arise from a lack of females in the species, meaning that there is intraspecific competition between the males. The pressure is on the males to be able to attract a mate, reproduce and pass on their genes.
Darwin and Sexual Selection
Darwin admitted (along with contemporary evolutionists) that mere survival of the fittest was insufficient in explaining the certain elaborate ornaments on certain organisms, like the peacock. Darwin developed a theory to explain this, Sexual Selection.
Darwin's famous example of this was the peacock's tail.
DARWIN ONCE WROTE:
'I remember well the time when the thought of the eye made me cold all over, but I have got over this stage of the complaint, and now small trifling particulars of structure often make me feel comfortable. The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!'
This video describes the controversy over sexual selection and also summarises the evolution of the peacock through sexual selection.
The peacocks' tail, through phyletic gradualism, has evolved due to the peahens pickiness. But what do the peahens looks for?
Well there are many attributes to which the peahen takes a fancy.
Before we start, it should be noted that peacocks do not see the same wavelength as humans on the electromagnetic spectrum. As humans we see, what we call, 'visible light" which corresponds to a wavelength range of 400-700 nanometers and a colour range of violet through to red. Peacocks however, do not see the beautiful array of greens and blues that humans do, instead they see ultraviolet.
As seen in the picture below, there is a clear difference between how humans see peacocks and how peacocks see other peacocks.
But how is this relevant you may be asking?
Well it shows that it is not necessary for the peacock to have phenotypes that display a magnificent array of colours. While that may be amazing for humans to see, it's not going to help the peacock find a mate. It is required however for them to have other qualities in their train, such as:
- Quantity of feathers- the more abundant the peacock's feathers are, the more 'attractive' they will appear to the peahen. A genetic mutation that creates a phenotype which allows the peacocks to grow more feathers would be advantageous in this situation as the female will choose that male to mate with. This therefore means that the genotype that allows more feathers will be passed on to the next generation. Overtime, this genotype will become the dominant trait.
- Size- This not only refers to the size of the feathers but also the size of the eyespots on the feather. A larger eyespot on the feather will appear more 'attractive' to the female. As talked about before, a genetic mutation which produces a large eyespot phenotype, would prove beneficial to the male peacock as it would make them more attractive to the peahen.
By clicking on the link further below (Under the heading 'Positive Feedback'), you will be able to see the change in size of the eyespot throughout time.
- distribution of colourful eyespots- The more equally spread and abundant the eye spots are the better the male looks to the female.
This can also been seen in the images above; the left peacock has unequally dispersed eyespots compared to the peacock on the right.
The Courtship Ritual
The peahens do not solely choose the male based on their tails, even though that is a major element of the selection process. They also choose based on the quality of the courtship dance. However, the more elaborate and beautiful the tail is, the more this will enhance the courtship dance, which can be seen below in the short clip.
The female chooses the male peacock based on the quality of the tail and the quality of the courtship dance. If the female chooses the male with larger eyespots than the other males, then there is a higher chance that the trait will be passed onto the next generation.
The peacocks will therefore evolve to have more elaborate and complex trains, with a high abundance of eye feathers and 'T' feathers, larger eyespots, more evenly distributed eyespots and better courtship dances; all these features being the favourable characteristics.
This is where things get interesting: since the tails become more and more impressive from one generation to the next, peacocks constantly need to do better and better to get the attention of a peahen. With each generation, the peahens prefer the peacocks with the most impressive tails, which leads to an increase in the average tail quality in the next generation, when the peahens will again select peacocks with the best tails, leading to an increase in quality, and so on.
This positive feedback, which is a hallmark of sexual selection via mate choice, leads to a runaway process where courtship signals become more and more extreme. This could eventually lead to the extreme trains becoming a disadvantage in the environment (See 'The Future of the Peacock')
Genetic View of the Sexual Selection
For natural selection to occur there must be a genetic variation within the population. The trait must then be passed on to the offspring.
Every detail in the peacock tail must be defined by genes in the genetic code of the peafowl. The complexity and detailed nature of the peafowl tail means there is a large amount of design information in the genetic code.
In relation to the peacock tail, there are an estimated 20 genes that are required to produce the tail. This amounts to a lot of genetic information.
Sexual Selection- Behavioral or Genetic?
At present, there is no conclusive evidence about the existence of a preference gene in the peahen. Further study needs to be done to investigate whether a preference gene really exists.
There are studies done regarding the peahen, which reveal that the female does generally select the male with more eyespots. However, it does not confirm the presence of a selection gene. So this still leaves the question of:
Is sexual selection behavioral or genetic?
The Albino Peacock
In the case of a white peacock, its unusual lack-of-color is due to a missing pigment. This missing pigment is dark and absorbs incident light, making diffracted and interference light visible (i.e. common peacocks).
The missing pigment in the peacock is a genetic mutation.
The peahen is attracted to colourful, bright peacocks, with an abundance of eyespots but as the white peacock lacks these features (as seen in the photo above), this mutation can be seen as a disadvantage to the peacock as the female peahen may not select this peacock to mate with.
This therefore means that the white peacock does not pass on its genetic mutation onto the next generation. Eventually, if the albino peacock is not chosen to mate with, the albino phenotype will no longer be present or occur in limited numbers among the peacock population.
However, the lack of colour leads the albino peacock to be better protected from predators. This means that the white phenotype gives the peacock a slight advantage over the average peacock.
The Future of the Peacock
The extravagant tail can also make the peacock vulnerable to predators as they are unable to run away and are easily seen by the predator.
This therefore means that the 'super males', i.e. the peacocks with maladaptive bright and colourful traits, are more prone to predator attack. Although the female may choose the 'super males' to mate with, the male may not be able to reproduce due to their maladaptive traits.
Once the peacock population reach a certain level of train quality, the negatives of having the most beautiful tail will outweigh the positives and the population is no longer stable.
Once the 'super males' die out, or become limited in number, it will make room for the ordinary males to be the selected peacock again. This is known as disruptive selection.
This is a step back in evolution for the peacocks as they are no longer increasing their tails' beauty by positive feedback.
The peacock is a historical example of how natural selection, or more specifically sexual selection, has resulted in the evolution of a species.