Guppy Fish-Amazingly Fast Evolution

 
In this article we learn about the evolution of guppy fish and how they can evolve amazingly fast- in a little over one year!

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.

Trinidad and Tobago shown on satellite image of South America

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….

pair of guppies female larger than male

….and are often more brightly colored than the females.

Guppy coppia gialla showing more brightly coloured male

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 John Endler, evolutionary biologistrivers 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.

Tobago river

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…..

examples of male adult guppiesImage credit: openi.nlm.nih.gov

…..while in another nearby stream they would be less brightly colored with only a few small splodges of color

male guppy not brightly colouredImage credit: nas.er.usgs.gov

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.

  • So male guppies, some of which lived in the same mountain stream, had evolved different colors, patterns and markings. Why was this?

To answer this question we first of all need to understand ‘sexual selection’.

Evolutionary pressures-sexual selection at work

On the Origin of the Species through Natural Selection-first edition‘Sexual selection’ is a type of ‘natural selection’ in which some ‘organisms’ produce more offspring that other ‘organisms’ of the same species because they are better at securing mates.

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 lion roaring showing his impressive mane

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…

Raggiana Bird of Paradise, the national bird of Papua New Guinea, sitting on a tree displaying impressive tail

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.

peacock courting a peahen

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.

brightly coloured male guppy (Poecilia reticulata ) swimming next to a drab female

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.

  • So why do female guppies choose males with bright colors?

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.

Usain Bolt 2012 Olympics standing on the athletics track with his muscular physique in evidence

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)

  • So if evolutionary pressures pushed male guppies to evolve brighter and brighter skin colors to attract a mate, why was it that in some mountain streams in Trinidad and Tobago, Dr Endler found guppies that had not evolved bright colors but had a ‘drab’ appearance?

male guppy not brightly coloured with a few yellow splodges and white skin

Dr Endler found that this question could be answered with reference to the guppy predators.

Evolutionary pressures- struggle for survival

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.

Crenicichla punctata

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!

pair of guppies female larger than male

Science experiments- reproducing findings in a laboratory

Coarse Gravel used to line the bottom of Dr Endler's fish tanks in famous guppy experiment

coarse gravel

fine gravel which was used to line the bottom of Dr Endler's fish tanks

fine gravel

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.

Experiment 1

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.

  • You can see the results of the experiment below; what do they tell you?

guppies in tanks with predators-some in gravely lined ponds and others in fine sand lined pondsImage Credit:evolution.berkeley.edu/

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.

Experiment 2

No guppy predators were placed into the fish ponds and the guppies were left to breed freely

  • How do you explain the results in the diagram below?

guppy experiment no predators present sexual selection means that guppies with large spots will predominateImage Credit:evolution.berkeley.edu/

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.

Guppy art

  • Using this template of guppies of different shapes and sizes, create ponds/rivers with predators, no predators, coarse gravel and fine gravel etc

Evolution of guppies-further reading

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