In relation to evolution, biological fitness describes an organism’s ability to pass on genetic material to its offspring

• Consequently, fitness does not simply refer to an organism’s capacity to survive – but its subsequent ability to reproduce
  

• Fitness is measured by reproductive success – an organism that fails to reproduce has low fitness, regardless of how long it lives for 
  

Certain species have evolved characteristics that favour successful mating over individual survival (sexual selection)

• Male birds of paradise have bright and elaborate plumages, along with intricate courtship behaviours, to help attract mates
  

• Certain species of frog have distinctive mating calls, with louder calls being selected for (easier to locate the mate)
  

• Female deer select males with larger antlers, despite being energetically costly to grow and maintain
  

The use of sexual competition as a selection pressure has led to the development of pronounced sexual dimorphism in certain animal species

Sexual Selection

Endler Experiment

Guppies are a species of fish that possess characteristics as a consequence of both natural selection and sexual selection

• Guppies that live in proximity to predators exhibit dull and drab colouration due to natural selection (better camouflaged for survival)
  

• However, guppies that are not exposed to dangerous predators tend to have brighter colours due to sexual selection (preferred by females)
  

John Endler was a Canadian biologist who undertook experiments on guppies to model sexual and natural selection

• He transferred drab guppies from a region with predators to a region free of predators and then monitored them over many generations to see how they evolved
  

• He transferred drab guppies from a region with predators to a region free of predators and then monitored them over many generations to see how they evolved
  

• He noted that the population had rapidly evolved to favour brightly coloured male fish, demonstrating the significance of the predatory fish as a selection pressure
  

Guppy Data