Final answer:
Paired external fertilization is often preferable to group spawning due to a higher certainty of paternity, better timing with environmental conditions, potential for protective parental behaviour, reduced competition among sperm, and the possibility for selective mating which all contribute to a higher offspring survival rate.
Step-by-step explanation:
Paired external fertilization offers several advantages over group spawning, making it a preferential reproductive strategy for certain species. One of the primary reasons is that it enhances the likelihood of a specific male's sperm fertilizing the eggs, leading to a higher certainty of paternity. This assures that his genes will be transmitted to the next generation, offering evolutionary rewards for parental investment such as mate guarding or selective mate choice.
In comparison to group spawning, which tends to produce a very high number of offspring with lower survival rates due to predation and environmental factors, paired external fertilization typically results in fewer offspring. However, the offspring that do result from paired external fertilization often have a higher survival rate because the act of fertilization can be more closely timed with optimal environmental conditions, and parents can engage in behaviours that protect the eggs from predators, such as nest-building or egg-tending.
Another advantage of paired external fertilization is the reduction of competition among sperm for fertilizing eggs, which can lead to a higher fertilization success rate for the participating male. The strategic release of gametes in a paired scenario allows for a synchronized interaction between the male and female gametes, which can also improve genetic compatibility and result in a better quality of offspring due to selective mating.