A guppy (Poecilia reticulata) is a species of small fish which originates in the freshwater mountain streams of north eastern South America and the islands of Trinidad and Tobago.
Guppies are ‘sexually dimorphic’,meaning that the males and females of the species look quite different from each other.
The males are smaller than the females….
….and are often more brightly colored than the females.
In the 1970’s an American evolutionary biologist by the name of Dr Endler studied wild populations of guppies in their natural habitats among the mountain rivers of Trinidad and Tobago and Venezuela. Dr Endler took photos of thousands of guppies and took measured them, their colors and their markings in great detail.
Through his research Dr Endler noticed how male guppies living in different mountain streams had very different colors and markings. In one stream male guppies would have bright and colorful rainbow markings…..
…..while in another nearby stream they would be less brightly colored with only a few small splodges of color
Not only did Dr Endler noticed striking differences in the coloration of guppies in different streams, but he also noticed striking differences in the coloration of guppies in different parts of the same mountain stream.
To answer this question we first of all need to understand ‘sexual selection’.
In his 1859 book ‘On the Origin of the Species through Natural Selection’, Charles Darwin explained that sexual selection:
“.. depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring.”
In other words, males of the same species have to compete with each other to secure a mate so they can reproduce and pass on their genes to the next generation.
Lions find a mate by making themselves more attractive to lionesses; they grow bigger manes than their lion competitors….
Male birds of paradise find a mate by making themselves more attractive to female birds of paradise; they grow longer and more colorful tail feathers than their male competitors…
Peacocks find a mate by making themselves more attractive to peahens; they grow longer and more colorful tail plumages than their male competitors. Such magnificent tail plumages are a sign to peahens that a peacock is healthy and could be a good choice of mate.
Through his research Dr Endler concluded that male guppies find a mate by making themselves more attractive to female guppies; they develop brighter and more colorful markings than their competitors, giving themselves a better chance of being chosen as a mate by a female guppy.
So the brighter and more colorful a male guppy is, the more likely a female guppy will choose him as a mate, giving him a better chance of passing his genes onto the next generation.
Bright colors could indicate good genes, in the same way that the muscular physique of a human athlete may indicate his/her health and vitality.
This is guppy ‘sexual selection’ at work and it is the evolutionary force that pushes guppy coloration toward conspicuousness. (ie a coloration that attracts notice or attention)
Dr Endler found that this question could be answered with reference to the guppy predators.
While the evolutionary pressure on male guppies in some circumstances is to evolve brighter colorings and markings, in certain situations the evolutionary pressure to survive is greater than the evolutionary pressure to reproduce.
In some rivers in Trinidad and Tobago there are large populations of guppy predators in the form of pike cichlida, which hunt and eat guppies. Predatory fish like pike cichlidas prefer to eat colorful male guppies because they are a source of food that is easy to hunt down and catch.
In those rivers, or parts or rivers, populated by pike cichlidas (crenicichlas) the evolutionary pressure is for male guppies to camouflage themselves so they can avoid becoming a tasty meal for the pike cichlidas.
Male guppies’ first priority is to avoid becoming breakfast for pike cichlidas. Having evolved camouflaged skins which give them the best chance of survival, only then do they consider strategies which will allow them to reproduce. Staying alive takes priority over reproducing!
In those rivers with a high risk of predation from pike cichlidas, female guppies just have to be less fussy about who they choose for a mate. They can’t choose bright colorful males to mate with because they have all been eaten! They just have to choose drab, boring colored males!
After completing his study of wild guppies in the mountain streams of Trinidad and Tobago the evolutionary biologist Dr Endler wanted to find out if he could replicate (or reproduce) the findings of his field work in a laboratory.
He built a number of fish ponds in a greenhouse. Male and female guppies were placed randomly in the ponds; some ponds had coarse gravel placed on the bottom while others had fine gravel placed on the bottom. No predators were introduced to any of the ponds and all guppies were allowed to breed freely.
After five months of breeding freely the main experiments began.
A guppy predator, the pike cichlida, was placed into each of two ponds which had coarse gravel lining the bottom; other pike cichladas were placed into each of two other ponds, this time with fine gravel lining the bottom.
The experiment was allowed to continue for a further 14 months, during which 15 generations of guppies were born.
a) Those male guppies with coarse gravel lining the bottom of the pond evolved large spots to help them avoid being eaten by the resident Pike Cichlada. Guppies had an evolutionary advantage if they were born with larger spots since they were better camouflaged and the pike cichlada was less likely to eat the guppies with larger spots and more likely to eat the guppies with smaller spots.
Over time the evolutionary pressure was for these guppies to evolve larger spots
Those male guppies with fine gravel lining the bottom of the pond evolved smaller spots to help them evade being eaten by the resident pike cichlada. Guppies had an evolutionary advantage if they were born with smaller spots since they were better camouflaged; the pike cichlada was less likely to eat the guppies with smaller spots and more likely to eat the guppies with larger spots.
Over time the evolutionary pressure was for these guppies to evolve smaller spots.
The results of Experiment 1 show that the evolutionary pressure to survive was more important than the evolutionary pressure to breed.
No guppy predators were placed into the fish ponds and the guppies were left to breed freely
a) Male guppies in fish ponds with coarse gravel lining the bottom evolved smaller spots. These smaller spots helped the male guppies stand out against the coarse gravel lining the bottom of the pond. Male guppies which were more visible stood a better chance of finding a female guppy to mate with.
Male guppies with smaller spots therefore stood a better chance of breeding and passing their ‘small spot genes’ onto the next generation.
b) Male guppies in fish ponds with fine gravel lining the bottom evolved larger spots. These larger spots helped the male guppies stand out against the fine gravel lining the bottom of the pond. Male guppies which were more visible stood a better chance of finding a female guppy to mate with.
Male guppies with larger spots therefore stood a better chance of breeding and passing their ‘large spot genes’ onto the next generation.
The results of Experiment 2 show that, in the absence of any threat of predation, the male guppies did not need to evolve bodies that were camouflaged; instead they evolved to ‘stand out’ and become more visible to female guppies.