What likely caused the variations in mammalian forelimbs that were present in their common ancestor?

The variations in mammalian forelimbs that are observed in different species are primarily due to changes in the genetic codes over time. These genetic variations lead to alterations in the development and structure of the forelimbs as species adapted to different environments and lifestyles. The process of evolution operates through genetic mutations, which can create new traits that may confer advantages in survival and reproduction.

Over generations, natural selection acts on these traits, leading to the diverse forms of forelimbs seen in mammals today, such as the flippers of whales, the wings of bats, and the arms of humans. These adaptations arise not just from surface-level changes but are fundamentally rooted in the genetic information that dictates morphology.

The other options focus on aspects that are influenced by genetic changes but do not serve as primary causes in themselves. Muscle structure changes and behaviors are responses to genetic material rather than drivers of variation. Vestigial structures also result from evolutionary processes, but they are remnants of past adaptations rather than sources of new variation.