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 observation suggests that increased competition for mates drives the evolution of larger antlers, as those individuals with larger antlers may be more successful in competing for mating opportunities. This aligns with Darwin's concept of natural selection, where traits that enhance reproductive success become more common in a population over time.