Final answer:
Autotetraploids are formed from a single species' mitotic failure leading to doubled chromosomes and potential sympatric speciation, whereas amphidiploids (allopolyploids) result from hybridization of two different species followed by chromosome doubling, allowing previously sterile hybrids to reproduce.
Step-by-step explanation:
Autotetraploids are polyploid organisms with multiple sets of chromosomes originating from a single species, due to a failure in cytokinesis after mitosis. This results in the doubling of the species' original chromosome number (e.g., from 2n to 4n). If such a plant, for instance, with a diploid number of 6, produces autopolyploid gametes that are also diploid instead of haploid, these gametes will have twice as many chromosomes (2n = 6 instead of n = 3) and will not be compatible with normal gametes. These autopolyploid individuals can then either self-pollinate or cross with others like them, potentially leading to sympatric speciation.
In contrast, amphidiploids (also known as allopolyploids) are the result of hybridization between two different species, with a subsequent doubling of chromosomes. Here, the first-generation hybrid is usually sterile due to the inability to undergo normal meiosis. However, a chromosome duplication restores the ability to undergo meiosis and produce fertile offspring, resulting in a new organism with both sets of parental chromosomes (e.g., AABB).
Overall, both autotetraploids and amphidiploids represent mechanisms of polyploidy and speciation, indicating their significance in the evolution and adaptation of plant species.