Why does a mango have one big seed, while a papaya has many small ones?
Different plant families have different characteristics. Mangoes are in the cashew family Anicardiaceae and papaya are in the Caricaceae family. They’re not closely related at all.
What are the reasons behind evolving with opposite seed dispersal strategies then? Perhaps papaya seeds are designed to be eaten by animals, which might prove difficult with a mango seed.
Depends on what animal. Avocados were adapted to be spread via giant ground sloths, but now rely entirely on humans to spread an fertilize them.
The real difficulty is in pooping out mango pits. :eek:
At least coconuts float, which can be useful in dispersal.
Evolution has no goal or direction, it is simply the result of what random mutations happen, and if they are useful at that time. Small seeded mangos and large seeded papayas would likely work, too, it just didn’t happen to happen that way.
I was thinking of what animals might be able to feed on mangoes and disperse their seeds, but could only think of primates. South Asia, where it looks as though the mango first originated, does have a lot of monkeys. I hadn’t considered extinct animals, so the giant ground sloth distributing avocado seeds is interesting.
I would not be surprised to find that mangoes were eaten and the huge seed dispersed by huge critters that are no longer with us. Kinda what it is likely that took place with the avocado:
To be more accurate than one of the comments: I just imagined the giant armadillos and other mega fauna firing seeds at each other.
Quick–buy this.
Sure, evolution isn’t goal-oriented, but what are you responding to here? Surely not just that the OP framed the question with “why”? That’s obviously shorthand for “how did it come about that”, and it’s shorthand that evolutionary biologists (and all scientists) use all the time.
The OP is quite reasonably asking how it came about that two superficially similar plants came to evolve rather different reproductive strategies. And it’s a really interesting question. Optimizing reproductive strategy is something that will obviously be subject to strong natural selection, and seed size and number is certainly something that can respond quickly to selective pressure, so it’s not something that we should shrug off with “either one is fine, it just happened that way”.
I’m completely happy with that answer, because that is the correct answer.
Could’ve been a primate! Gigantopithecus was certainly big enough, ate fruit and lived in southeast Asia.
It is not the correct answer, even in a pedantic technical sense. Mutation is a random process, but natural selection is not. Some traits are completely attributable to chance (genetic drift), but reproductive strategy is certainly not one of them.
But the problem is that we usually don’t know. Look at the opposite end of evolution, extinction. The honey bees are dying right now and are being watched closely and we still can’t agree on what is causing it. Same goes for many other ongoing evolutionary and extinction events. Sure, we can say mangoes got big seeds to be dispursed by mastadons and avocados small seeds to be dispersed by monkeys–but small, tough seeds that need digestive juices to germinate would also work well in mastadon mangoes. And large seeds would also work well for monkey avocados, who could carry the fruit off, gnaw around the seed, and toss the seed away. Both strategies would have worked in both cases. And never mind that similar fruit that would have had similar dispersers came up with different seed strategies–for instance, apple-like seeds and peach-like seeds. Apples could have just as easily had pits and peaches could just as easily had lots of small seeds. It still comes back to “just because.”
Having done a bit of googling, it looks as though monkeys might indeed carry mangoes and so disperse seeds that way. Elephants, also found in South Asia, apparently eat mangoes and would likely be able to pass the seed through their digestive system and so disperse the seeds in their faeces.
I found this article, which discusses the large size of mango seeds in more detail. It also discusses the avocado, and suggests that the seeds of both these fruits were once dispersed by now-extinct megafauna, namely the giant sloth and the Gomphothere. The latter of these were once found in Eurasia and became extinct relatively recently, only a few thousands of years ago. Hominids might have been responsible for continuing mango seed dispersal after the Gomphotheres’ demise.
As stated in the article:
It describes an example of evolutionary anachronism from the animal kingdom: the pronghorn antelope of North America. This even-toed ungulate is the second fastest animal in the world after the African cheetah, and much faster than any extant predators. The reason for its speed is that it was once hunted by a species of cheetah that previously existed in North America but is now extinct.
A couple of other things I was ruminating and wildly speculating. Perhaps the mangoes cultivated today have grown in size compared with the original wild mangoes they derive from, which originally had smaller seeds. Also, if mango seeds float, perhaps the annual monsoon rains that South Asia experiences might be able to carry seeds to other locations.
“We don’t know” and “it just happened by chance” are very different statements. These different reproductive strategies certainly did not evolve by chance, and it’s certainly worthwhile to discuss possible reasons behind the one-big-seed and many-small-seeds strategies, even if we don’t know the exact distribution vectors involved for these specific plants.
And this isn’t evolutionary psychology, where hard evidence is almost impossible to come by. Often field scientists do spend time analyzing what’s really happening in specific cases. Finch beaks in the Galagos?
Surely having a big seed confers a big headstart to offspring, akin to placental mammals versus marsupials and monotremes. The megafauna facilitated this advantage.
And perhaps, if seed dispersal is being achieved primarily through megafauna, a big seed has a better chance of surviving mastication and passing through the digestive tract of a large animal?
Yes, it give a big headstart to the new plant. But it also takes many times the energy to produce, and if that one seed doesn’t end up in the right place the whole effort is wasted.
There are a lot more plants that produce hundreds of small seeds than ones that produce one big seed. It’s such a common pattern that it really stands out when we see a plant that produces really big seeds like mangoes or coconuts.
The point is, there is no one single strategy that is the right one–the mango “chose” one large seed, but couldn’t two smaller but still large seeds also work? Couldn’t a dozen small seeds with a thick, woody shell that is very bitter if bitten into work? You are looking at the one strategy that was chosen and working backwards to a “just so” story that this one strategy was inevitable.
And a lot of this is contingency and exaltation quality fly modifying a strategy that already exists. Maybe the only reason mangos have one large seed instead of a dozen rock-hardd, bitter ones is because the mango ancestor already had a single, slightly biggish seed “left over” from an earlier dispersal strategy. (Why do birds have feathers and not membrane wings, which worked fine for bats and pterosaurs? Because their ancestor had feathers for other purposes, and they proved useful to make flight worthy.)