Final answer:
Reproductive success is quantified by the number of offspring that survive to reproduce, known as fitness. For external fertilization, adaptations may include releasing large amounts of sperm, while internal fertilization may involve traits that ensure mating success.
Step-by-step explanation:
The reproductive success of an organism is best measured by its number of offspring that survive to reproduce. This concept, often termed as fitness in evolutionary biology, reflects the relative ability of an organism to survive and produce fertile offspring. Regarding adaptations for reproductive success, in species that utilize external fertilization, males may evolve adaptations such as the release of large quantities of sperm to increase the likelihood of egg fertilization.
In contrast, for species with internal fertilization, males might develop traits that ensure successful mating and sperm transfer to the female. These could include physical adaptations, mating behaviors, or strategies that outcompete other males. The concept of reproductive success is deeply interwoven with darwinian fitness and natural selection, where the variations of traits among individuals and their ability to compete for resources significantly influence which individuals will have more offspring.