How did two unrelated species come to co-evolve? I am thinking about fruit trees and birds, specifically fig trees. The fig is a fruit that surrounds hundreds of small seeds-and the fruits is very high in sugar, and hence attractive to birds. In addition, the fruit has a pronounced laxative effect upon the birds who eat it-which means that the seeds have an excellent dispersal mechanism-plus, they reach the ground surrounded by fertilizer. How did such a symbiotic relationship come about? Was it simply a case of figs which bore better fruit were dispersed more efficiently?
Evolution continues to amaze!
A mutant fig tree started growing bizarre, sweet fruit. Birds were attracted to it. Its seeds were spread more widely and it outcompeted other figless fig trees.
If there were no birds, then the mutant fig tree would have died out.
I’m surprised you didn’t mention the fig wasp, which is necessary for the figs to bear fruit. But it’s the same issue.
The fig wasp is actually a much more appropriate example than the bird one in the OP. The bird example is opportunistic…the bird is not affected in its behavior by the fig’s evolution at all. It would do the same thing to any small, sweet fruit.
But the fig wasp’s evolution has been intimately entwined with the fig’s for ages. The cycle is complicated…the early fig fruits attract the fig wasp to enter and lay its eggs. The wasp loses its wings and antennae on the way into the fruit, so it’s going to die in there. It lays its eggs, fertilizing the (currently female) fruit/flowerhead as it does so. The fruit and the larvae develop together. The male wasps chew their way out, releasing the female wasps, who have pollen attached to them (as the fruit/flowerhead has developed male pollen-bearing organs in the development time) and go to another currently female fruit/flowerhead to start the cycle over. It’s confusing, but the chart I linked to helps you to understand it.
The fig/wasp pair has evolved a remarkable interdependence.
Strictly speaking, the “fruit” of a fig tree is actually an inflorescence (a cluster of flowers) that has grown inward. On that basis, it is very easy to see how cooperative evolution between ficus and wasps has occurred; the wasp species that originally pollinated the early ancestors of ficus trees found safe haven for nesting in the blooms, and especially those that were ingrown; as the blooms became more ingrown, the wasp species became adapted to spending more of their life their, until they finally evolved to lose flight once selecting and entering a fruit. Because of their peculiar breeding cycle (the female wasps are impregnated by their male siblings) it is very easy for rapid adaptation and speciation to occur.
In general, fruits are specifically evolved to attract scavengers who will inject the sugary fruit and excrete or otherwise distribute the seeds some distance away from the host, hence why fruits are typically bright, juicy, and readily accessible, while nuts and most pulses are protected by an inedible and often rigid shell. This symbiosis between organisms is called mutualism, and is a special case of coevolution.
Coevolution is not a special type of evolution; it is just a consideration of the intertwining of adaptive pressures. Species do not evolve in a vacuum; they adapt in response to environmental conditions, including those imposed by other species. One might as well speak of coyotes and rabbits having coevolved, each serving the needs of the others; the rabbit in providing a meal for the canine, the coyote in controlling the hare population so as to prevent overfeeding or disease.
Stranger
what about the role of animals dispering tree seeds (nuts). Have trees evolved large nut/seeds because of the attractiveness to squirrels? I have observed squireels burying acors-is that a big factor in the oak trees dispersal?
No, quite the opposite. Trees evolved nuts to try to stop squirrells finding them attractive. Plants evolved big seeds because big seeds can produce more vigorous seedlings by storing more food. Animals discovered that the food store in the seed could be eaten by them. Of course that killed the seeds so the trees evolved a thick coat that the animals couldn’t chew through the seed. Rodents then evolved sharp ever-growing teeth to chew through thick seeds. And so the evolutionary arms race went on and on. Some oak trees tried to stop squirrells taking to many of their seeds by germinating vary rapidly, so the squirrells evolved to kill the seed embryo before storing the seed.Of course just to keep things complicated some trees do get some advanatge from thier seeed predators by dispersion, including some oak trees.
But the general idea behind the evolution of nuts was to make the seed as unattractive as possible to animals. If a plant just wanted to make a seed attractive to dispersers then it would simply coat it with a soft fruit. Because it is just carbohydrate fruit can be made from air and water, which costs the tree virtually nothing. In contrast nuts are loaded with with proteins and oils, not to mention the seed itself, so when a nut is eaten it is a disaster for the tree. In addition fruits are favoured by birds and bears, which are far, far better dispersers than squirrels.
It’s probably a factor, but there are plenty of oaks that produce acorns that either squirells will only eat when they are starving or that squirrells always kill when they carry off. These species are more widely dispersed than oaks whose seeds are routinely buried intact. As such we can safely conclude that the benefit to oak trees in terms of dispersal is probably slight. The real benefit from squirells is probably that they ensure that a good supply of seeds aren’t consumed by the rats and other seed predators.
have dispersed just as far as