Selection Pressures: Skills (DP IB Biology)

Modelling Sexual & Natural Selection

Selection in guppies

• Guppies are a species of fish found in the mountain forest streams of Trinidad and Tobago

• They exhibit variation in colour and pattern, as well as sexual dimorphism

  • Males tend to be brightly coloured while females are dull in colour

• The colouration (specifically the spots) of guppies provides them with camouflage from predators

  • Their spots mimic the gravel of the streambeds in their native streams

  • Some streambeds are finer and sandier while others are coarser and more pebble-like

  • Certain streams provide more natural protection through hiding spots from predators

John Endler's experiments with guppies

• In the 1970s a scientist named John Endler observed guppies in Trinidad and noticed that their colour patterning changed with predation pressure

• He conducted lab and field research on guppy populations to investigate the effects of natural and sexual selection on the evolution of the guppies

• Endler came up with several hypotheses:

  • When predators are present, the substrate type of the streambed impacts survival, causing spot brightness to change

  • When predation rate is low spot brightness on male guppies increases due to sexual selection

  • As predation increases the brightness of the spots decreases

Laboratory experiment

• Endler used a greenhouse to recreate a tropical environment

• Prior to the experiment the guppies were living in large tanks and breeding freely for six months with no predation

  • These guppies exhibited a wide range of spot size and number

• Endler counted and measured the spots on all of the guppies

• Guppies were then randomly assigned to go into ten ponds inside the greenhouse

  • Five ponds had coarse gravel

  • Five ponds had fine gravel

• The ponds were exposed to three different levels of predation:

  • Two ponds had no predation

    • One with fine and one with coarse gravel

  • Four ponds contained a dangerous predator, e.g. pike cichlid 

    • Two with fine and two with coarse gravel

  • Four ponds contained a weak predator, e.g. the killifish

    • Two with fine and two with coarse gravel

• Once the predators were introduced the experiment was allowed to run for five months

• Endler then counted and measured the spots on all of the guppies

• The experiment was allowed to run for a further nine months, after which more data was collected

Results

• Endler predicted that:

  • A high predation rate would cause the populations to diverge from each other

    • So the guppies in ponds with coarse gravel should have different patterns/colouration to those in ponds with fine gravel

  •  A low predation rate would allow the male guppies to become more conspicuous

    • The brightness of their spots would help them to attract females

• Endler found that when predators were introduced there were significant changes in the guppies:

  • In ponds with a high predation rate the mean number of spots decreased

  • In ponds with a low predation rate the number of spots continued to increase

  • In ponds with coarse gravel guppies tended to have larger spots, whereas in the ponds with finer gravel guppies tended to have smaller spots

    • This was true of ponds with a low and high predation rate

    • This can be interpreted as spot size mimicking gravel size

  • In ponds with no predation the opposite was observed; fine gravel ponds favoured large spots on male guppies while coarse gravel ponds favoured small spots

    • It was thought that not matching their background make males more conspicuous, which can help to attract females

Field experiment

• Endler transferred a number of dull male guppies from an area of high predation to an area of low predation

• He left them there for 15 guppy generations (two years) before returning to observe any changes

• When he returned he noticed that the male guppies had more colourful patterning

  • This was likely due to sexual selection and the absence of strong predation

Interpreting results

• The lab and field experiments that Endler carried out demonstrated that the evolution in the guppy populations isn't clear cut; it is a dynamic process of natural and sexual selection

• Natural selection occurs in guppies due to competition for food and avoidance of predators

  • Predators can spot brightly coloured males more easily, reducing their survival chances

  • This leads to selection for less highly coloured/spotted individuals within the population

• Sexual selection occurs as some traits (e.g. bright colouration) provide a reproductive advantage

  • Males with brighter and bigger spots are more likely to obtain a mate, reproduce and pass on their alleles that code for these specific characteristics

• A trade-off between these two types of selection occurs

  • Although in areas with high predation rates, brightly coloured males are less likely to survive, regardless of their reproductive advantage

Endler's results graph

Endler's experimental data from the lab and field are presented in several graphs. The colour of the guppies was evaluated by counting the colour, brightness and number of spots.