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Coevolution occurs when two species influence each other's evolution due to each exerting selective pressure on other. Examples of coevolution include bees - flowers, wolves - rabbits and HIV - humans.
In evolution there are many trends that are seen whether it's divergent evolution where similar creatures become more different or convergent evolution when different species are becoming more similar. Different variant on this is something called coevolution and that's when two species influence each other's evolution. Now this is because each species exerts selective pressure on the other species. Some common examples of this include bees in flowers, wolves and rabbits, even HIV in humans.
If we take a look over here, here's a picture of a bee and you can see it on a flower. Now flowers are influenced by the bees because the flower that has the best combination of colors and the best nectar for feeding the bee, will attract more bees. and if they provide lots of nectar, then that bee will have lots of food, bring it back to the hive and wind up having lots of little baby bees which will mean in the next generation even more bees will be coming top this particular flower. now why would the flower want to attract bees? Well, that's because if you look at the hairs here you can see little yellow spots, those are pollen. These are the reproductive structures that this flower is using to fertilize other flowers of the same species and it's basically how flowers engage in sex. And so by bribing the bee, come here, come here, come here the bee comes along, picks up some pollen and then flies off and finds a similar flower, lands and when that pollen rubs off against the female part of this flower. That pollen pollinates the flower thus allowing sex between the flowers so that the plant can have lots and lots of seeds. so it's advantageous to the flower to be really good at attracting the bees. Now the bees are influenced because if you notice it has these hairs. Why does it have so many hairs and especially look at its legs are furry. Why do they have that? Well, that's what helps pick up the pollen. Now why would the bee want to bother picking up the pollen bee sides in its mouth? well, remember every time a bee pollinates another flower that makes seeds which makes more flowers which means more food for the bees. so the furrier the bee, the hairier the bee the better that hair is at picking up pollen, the better food supply in the future that bee will have. And so, the flower exerts pressure on the bees, the bes exert pressure on the flower.
Similarly, wolves exert pressure on rabbits and vice versa. Now not in this I'm going to give you food if you help me have sex kind of situation. But wolves catch rabbits. which ones do they catch most easily? the slow stupid ones. So that tends to make for faster smarter rabbits. But then, what happens to the wolves? Well, the lame wolves the wolves who get kind of easily confused by ooh, rabbit tree, what do i eat? Those will die making for smarter wolves and the slow ones die so the faster wolves. So they keep influencing each other in this evolutionary arms race.
Kind of a parallel thing is going on with us and HIV. HIV is evolving to get better and better at infecting humans and humans are people whoa re easily infected and die quickly, there genes are removed. There's already been discovered some individuals who have genetic predespositions that make them a little bit more resistant to HIV influences. And so if this continues, if we don't use our intellect and our technology to defeat HIV, then we may see eventually a slow evolutionary pressure on humans to become more resistant to HIV.