Evolution of fruit trees and selective pressures on fruit size?

Question

My understanding is that fruit trees make fruit because animals will eat it along with the seeds and then disperse the seeds in their f***s, creating a selective pressure in favor of sugary fruit.

I'm just wondering what the selective pressure is for fruits to be so large (besides agriculture)...it seems that there would be a strong pressure to become smaller since as long as the animal eats the fruit the seeds will be dispersed, and smaller fruit means the tree can save more energy.

I guess it could just be agriculture/selective breeding that made fruit as large as it is today. I'm wondering if there's anything else.

Answer 1

Well, when you consider that tens of thousands of flowering plants package their seeds in what we refer to as "fruit" in the common sense, and you realize that there is considerable variation in fruit size *in nature* among species, then the answers above (i.e., to the effect that it's all due to human breeding) fail to capture the whole picture. Indeed, jeremy c's claim that "friut [sic] size isn't a chosen attribute unless humans look for this trait" is just false.

There are many animals (esp. birds and mammals) that are especially attracted to large fruits. Selection, therefore, will favor large fruits in species that rely on such animals for seed dispersal.

Answer 2

friut size isn't a chosen attribute unless humans look for this trait. most of the time the trees that have bigger fruit have only a large animal that will dispurse the seeds. such as elephants and durian fruit.  

Answer 3

Plants have a number of biological constraints on fruit size. Many large fruit vary in size depending on the number of fertilized seeds they bear. This is true for apples. They can have a maximum of 10 or 12 fertilized seeds depending on variety. More seeds fertilized then the fruit is larger. So here pollination impacts the fruit's potential size.

Plants have more flowers than they can fruit. They drop excess fruit early in growth to provide more space. This fruit drop or abscission is from competition between the developing fruit. Eliminating more immature fruit yields larger mature fruit.

http://jxb.oxfordjournals.org/cgi/conten...

Carbohydrate production effects the size. Local leaves are the major contributors to fruit on a branch so herbivore load has it impact also. Plants able to defend against herbivores can produce larger fruit to attract dispersal. Fruit may have greater potential size than they can ever reach in the wild.

http://aob.oxfordjournals.org/cgi/conten...

Birds can swallow larger oval or elongated fruit than they can globular fruit because they can only open their beaks a certain distance. Oval fruited plants would have a  greater chance of attracting birds by offering greater rewards. So shape & size in fruit is often linked with the bird's beak size.

http://www.springerlink.com/content/m72k...

A large number of plants setting larger fruit come from polyploid plants. Plants have the ability to survive a doubling of their chromosomal count in each cell. Since they lack highly specialized organ function common to animals a doubled genetic expression is sometimes beneficial. This leads to larger flowers with stronger abilities to attract pollinators and larger fruit to attract dispersers. Flower petals can be thicker and flowers can be longer lasting in polyploid plants so offer longer opportunities for pollinators to find them. Fruit from tetraploid (4n) apples can be twice as large as diploid fruit but may be watery compared to diploids. The disadvantage is crossing with diploids (2n) produces triploids (3n) that are not fertile with the odd chromosomal count. If the tetraploid plants can become established so they can breed together and there are resources to support their increased demands they will persist. These are often associated with mammalian seed dispersal rather than just bird.

http://www.ces.ncsu.edu/fletcher/program...

There are multiple genes in plants that control potential size in mature fruit. However they may not be expressed in combination very often. Wild fruit usually has a large variation of alleles per gene so on average few plants would receive the exacts set that combine to give maximal size. Small and round will be more common in offering the best packaging shape with good seed to pulp ratio  for a range of birds and animals to disperse. Small fruit are easier to remove, carry and disperse. Small fruit offer less attractive shelters/food options to egg laying insects seeking to hide their young from predators and provide food. Neither bacteria nor insects will disperse seeds so must be defended against stealing the fruit while keeping the fruit attractive to dispersers. So a plant wants to protect its fruit from everybody until the seeds are ready.

http://www.ingentaconnect.com/content/be...

http://209.85.173.104/search?q=cache:cev...

http://www.evcforum.net/cgi-bin/dm.cgi?a...

Answer 4

many large fruits were artificially bred, actually.