Final answer:
Polyploidy is a key mechanism by which additional chromosomes are added in plants, leading to genetic variation and potentially providing evolutionary advantages. It is much more common and well-tolerated in plants compared to animals, where it often results in non-viable offspring.
Step-by-step explanation:
Polyploidy is a better explanation for how chromosomes were added in plants, rather than in animals. The phenomenon of polyploidy, which is the condition of having more than two complete sets of chromosomes, is much more common in plants than in animals. Allopolyploidy, a form of polyploidy that occurs as a result of hybridization followed by chromosome doubling, has played a significant role in the evolution of plant species, resulting in organisms like the cultivated forms of wheat, cotton, and tobacco which are all allopolyploids. In contrast, animals typically do not tolerate the chromosomal aberrations related to polyploidy and are less likely to survive or produce normal offspring when such genetic changes occur.
This difference may be because polyploidy in plants often acts as an adaptive mechanism rather than an error, thus contributing to increased genetic variation and potentially providing evolutionary advantages such as novel traits or increased vigor. Over half of the plant species studied are related back to an ancestor that went through polyploidy, evidencing its prevalence and significance in plant evolution.