Bees provide a great example of co-evolution in action and they're incredibly important pollinators. They consume nectar and pollen, collecting both while they forage. Flowers have evolved into specific color, fragrance and shape combinations that make them attractive and accessible to bees (and often unappealing or inaccessible to their competitors). Bees have paid back such flowers by evolving specific body parts that make them more efficient at collecting -- and inadvertently passing on certain portions of -- pollen as they make their rounds.
The Transporters of Pollen
Pollen can be carried by wind, rafted by water or shuttled around by any manner of creatures, be they bees, beetles, birds or bats, and deposited on the female reproductive part of another flower. That might sound pretty hit or miss, and it is, which is why plants -- particularly gymnosperms -- produce lots of pollen.
In order for plants to successfully spread their pollen, many coevolved with other creatures to get the job done more frequently and more efficiently. This happened a number of ways. With flowering plants, for example, those with the tastiest pollen were more likely to attract pollinators, so they were the ones that had the best chance of propagating their species. Flowering plants also leverage shape, color and scent to bring in more customers, sometimes in ways that might seem surprising. Many beetle species are attracted to flowers that produce scents we would consider highly unappealing. Some of these plants, among them the common household philodendron, attract beetles by heating up through a chemical reaction. It causes them to produce an odor reminiscent of decomposing organic matter, which the beetles are naturally drawn to. One Sumatran plant, known as the devil's tongue, smells so foul it has reportedly made people pass out. It's pollinator? A species of carrion beetle.
Bright colorful flowers are most likely to attract diurnal creatures, while white or light yellow ones are most likely to be spotted by nocturnal animals. There's also the production of nectar. Many proficient pollinators, such as bees, bats and hummingbirds thrive on nectar, so having nectar cups suited for the pollinator's mouthparts was another important specialization to develop. Lastly, the positioning of plants' sexual parts evolved, too. Those specimens whose arrangement best catered to a potential pollinator's feeding habits were most successful. So stamen that were most likely to be brushed against by a pollinator -- and therefore more likely to be brushed off and carried away -- were the most ideally positioned for evolutionary perseverance.