Re: Poisonous fruits and edible seeds

Your question is not easy to answer because plants are so diverse in their seed 
dispersal methods. There are also several methods for dispersing seeds, and 
many plants use more than one method. Some plants use wind, water, gravity, 
attach their hooked fruits or sticky seeds to animals to hitch a ride, or 
launch them. So it would not be necessary for such fruits/seeds to be edible 
although some are, such as the water-dispersed coconut. It costs a plant energy 
to synthesize poisonous or bad-tasting compounds, termed secondary compounds, 
to make fruits or seeds poisonous or bad-tasting. Therefore, it may give the 
plant an advantage not to have to synthesize so many secondary compounds. In 
evolutionary terms, if a fruit was very tasty and seeds easily digested, and 
there is a large animal population, all the fruit and seeds might be eaten and 
the plant would not be able to reproduce by seed. In that case, any mutation 
that made fruit or seeds inedible could be very beneficial. However, animals 
would probably not eat all the seeds every year. So even if all the seeds are 
eaten some years, it still could be successful seed dispersal if the plant was 
a long-lived tree, and if once every few years, some seeds survived and 
sprouted. That may be enough to successfully propagate and spread a long-lived 
species.

In the other extreme, if a fruit and seed was very poisonous, you might expect 
no animals would disperse its seeds by eating them. That means a mutation to 
make fruit edible could be an advantage. However, there are always some animals 
that may mistakenly eat even poisonous fruit/seed. Alternately, the species 
could rely on seed disperal methods that did not require animals to eat the 
fruit. They might also rely mainly on vegetative propagation, which is very 
important in the plant kingdom.

The plants you mention, rice, wheat, walnut, carrot and potato, have been 
selected for hundreds or thousands of years for larger and more tasty fruits or 
edible parts. Wild carrots, for example, have much smaller storage roots that 
are not nearly as tasty as cultivated carrots. In many cases, selecting for 
better taste means selecting fruits, seeds or edible plants parts with fewer 
secondary compounds. One thing to remember is that fruits or seeds that are 
poisonous or bad tasting to people are not all poisonous or bad tasting to 
animals. For example, the "hot" capsaicin in wild pepper does not deter birds 
from eating them even though mammals have adverse reactions to capsaicin. 

Seeds are dispersed in a great variety of ways, so it is often better to 
consider specific examples. Walnuts in the Eastern USA rely largely on 
squirrels who carry away the walnuts and bury them. The squirrels either forget 
where some are buried or let them buried too long, and they sprout. Blue jays 
are considered very important dispersers of oak acorns over long distances 
(over 1 km). It works in these cases even though the seeds are edible because 
so many seeds are produced that the animals either can't eat all the stored 
seeds before they sprout or forget where they stored some of them. 

Many plants have coevolved with animals which disperse their fruits or seeds. 
For example, seeds of several USA wildflowers have an elaiosome, an external 
area high in proteins or fats, that serve as food for ants, which disperse the 
seeds.

References


Plant Secondary Compounds


Seed Dispersal by blue jays


Seed dispersal of wild chiltepin peppers


More seed dispersal methods: Ants


Ant Seed Dispersal


Drift Seeds And Drift Fruits: Seeds That Ride The Ocean Currents


Blowing In The Wind: Seeds & Fruits Dispersed By Wind



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