Final answer:
Vertebrates generally have the most introns, which are noncoding sequences within genes. These introns are spliced out of pre-mRNA during RNA splicing, and while bacteria generally lack introns, vertebrate genes can contain numerous such sequences.
Step-by-step explanation:
The species that generally have the most introns are vertebrates. Introns are noncoding sequences found within genes that are transcribed into pre-mRNA but are then removed during the process known as RNA splicing. Bacteria typically do not have introns in their genes; their gene coding regions are continuous. In stark contrast, the genes of higher eukaryotes, including vertebrates, often contain multiple introns. These introns can sometimes number in the tens or even over seventy in a single gene. The role of introns in gene expression and evolution is complex and multifaceted, and they may contribute to regulatory sequences, but the exact biological significance of having numerous or lengthy introns is not fully understood. It is a matter of probability that introns are more likely to be involved in crossing over because they are typically longer than exons.