The observation that best supports the idea that increased competition causes changes in a species is:
A population of deer show an increase in antler size over time, which allows them to obtain more mates during mating season.
This example illustrates how competition for mates can drive evolutionary changes, aligning with Darwin's concepts of natural selection and descent with modification. Increased antler size can provide a competitive advantage in securing mates, leading to reproductive success and potentially influencing the characteristics of future generations.