Final answer:
The mallard duck and the moose both have metatarsal bones due to a common ancestor but have evolved different foot structures - webbed and hooved, respectively - to suit their different habitats and lifestyles.
Step-by-step explanation:
The presence of metatarsal bones in both the mallard duck and the moose can be explained by the concept of homology, which indicates they share a common ancestor that had metatarsal bones in its feet. However, the differing shapes of their feet - webbed feet in ducks for swimming and hooved feet in moose for walking on hard ground - are an example of adaptive evolution. This adaptation is a result of natural selection acting on the specific environments and lifestyles of these species, leading to the different foot structures that optimize their ability to move efficiently in their respective habitats.
Mallard ducks have webbed feet, which provide a large surface area that is beneficial for propulsion in water. In contrast, moose have feet with hooves that are better suited for supporting their heavy body weight on solid ground while also allowing them to move quickly to escape predators. This differentiation is an illustration of how the same skeletal structure, the metatarsal bones, can undergo evolutionary changes to fit the specific needs of an organism based on its ecological niche.