Corn cross pollination

​Zea mays is a species that has sufficient morphological distance between the male and female flowers that most pollen does not fertilize its mother plant.  This character is unique from most other crop plants in which the stamens and pistils are parts of the same flower.  Although other species may have other strategies to reduce self pollination, like timing, the distance between the ear shoot and the tassel plus abundance of pollen that easily floats in air encourages pollination of other corn plants rather than self pollination.
 
This characteristic of corn promoted significant genetic diversity ultimately resulting in ease of adaptation to a wide range of environments as humans selected the genetics suited to their needs and environments.  This is diversity to us as we selected genes for less starch in sweet corn, endosperm explosiveness in popcorn, soft starch in floury corn and hard endosperm of flint corn.  Corn varieties are adapted to short seasons of temperate zones and to longer seasons near the equator.  Even as pathogens evolve adaptation of some corn genetically controlled resistance, genetic variability within the species is eventually identified to produce the metabolic products needed to resist the pathogen.
 
Open pollinated varieties maintain genetic heterozygosity as meiosis after fusion of male gamete with that of the egg cell, creates new mixes of the genes.  Consequently, each seed in an open pollenated corn pollinated plant when at least one of the parents is heterozygous will have some genetic variation from adjacent seeds.  Corn breeders have used this principle to get new, desired genetic characters.
 
Genetic variability is advantageous when identifying useful characters, but not when attempting to get optimum grain production.  The advantage of crossing two homozygous plants of two distinct genetic backgrounds is producing predictable and consistent hybrid plants. Controlled selfed pollination for several generation ultimately results in homozygosity, with essentially each seed having identical genetics.  This selfing process also creates homozygosity of some genes that have negative affects on the plants, including characters affecting seed size and volume.  Identifying homozygous inbreds that essentially overcome these negative genes of another hybrid parent is the plant breeders job for selecting potential high performing hybrids.
 
Cross pollination characteristic of corn has allowed genetic variability and human effort to select and self pollinate to create inbreds and then crosses to make specific hybrids, allowing this species to be a major international crop.