How did Endler's guppy observations separate sexual from natural selection?

Endler's guppy work illustrates how an easily noticed male colour pattern can improve mating success while increasing exposure to predators. Being noticed can help a male attract a mate but also make it easier to catch. These effects can favour different appearances.

Sexual selection acts through differences in gaining mates or fertilisations. If females prefer a particular male pattern, males with that pattern may mate more often. Viability selection acts through differences in survival. A conspicuous pattern, one that stands out against the background, may make a male easier for a predator to spot.

The simple prediction is therefore conditional:

Endler's 1980 study examined how colour patterns evolved with different predators and backgrounds. By changing these conditions experimentally, researchers could test predictions about colour change, not just compare attractive and unattractive fish. "Shallow water" alone does not always explain low survival. You also need to know which predators are present, whether they eat adults or juveniles and how visible the fish are.

An invented calculation illustrates why both stages count. Suppose 100 conspicuous males have a survival probability to breeding of 0.40 and survivors average three offspring. Their expected contribution is $100\times0.40\times3=120$. If 100 less conspicuous males survive with probability 0.75 and survivors average two offspring, their expected contribution is $150$. The larger mating output per survivor does not compensate for the survival cost in this example. These are explanatory numbers, not Endler's results.

To separate the processes in a study, measure mating outcomes and survival separately rather than guessing both from the final colour frequencies. Controlled observations of mate choice test preference. Predator treatments or survival comparisons test effects on survival. Repeating the study and using similar starting populations helps separate selection from chance differences.

Sexual selection is usually considered a component of natural selection broadly defined, though teaching comparisons often contrast it with survival selection. Neither process means guppies choose their inherited colour to suit an environment. Population patterns emerge because inherited variants leave different numbers of descendants.

Put the two selection pressures side by side in the guppy question in the Biology SL mock exam. Explain your choice using both mating success and survival, while keeping the role of predators and the visual background in mind.