Evolution of corn did not involve GMO technology but did include selection of desirable genes.

The earliest events in maize domestication likely involved small changes to single genes with dramatic effects. We know the events were early because there is little variation in these genes between maize varieties, suggesting that modern varieties are descended from a single ancestor. That the small changes had dramatic effects also explains the sudden appearance of maize in the archaeological record. These examples show us that evolution doesn't always involve gradual change over time.

https://learn.genetics.utah.edu/content/selection/corn/

>>3415177

One of the first scientists to fully appreciate the close relationship between teosinte and maize was George Beadle [Nobel Laureate]. In the 1930s, Beadle studied teosinte-maize hybrids and showed that their chromosomes are highly compatible. Later, he produced large numbers of teosinte-corn hybrids and observed the characteristics of their offspring. By applying basic laws of genetic inheritance, Beadle calculated that only about 5 genes were responsible for the most-notable differences between teosinte and a primitive strain of maize.

Using more-modern techniques, another group of scientists analyzed the DNA from teosinte-maize offspring. They too noticed that about 5 regions of the genome (which could be single genes or groups of genes) seemed to be controlling the most-significant differences between teosinte and maize.

In recent years, geneticists have used advanced molecular-biology tools to pinpoint the roles of some of the genes with large effects, as well as many other regions across the genome that have had subtle effects on maize domestication.

https://learn.genetics.utah.edu/content/selection/corn/

>>3415333

Bt (Bacillus thuringiensis) GMO corn is used in integrated pest managment strategies to cobtrol destructive plant-feeding insect larvae.

Figure 1

European corn borer: shotholes and tunnel in leaf midrib (a), damage and fungal infection in non-Bt maize (left) and Bt maize (b), stalk tunneling (c), and adult female (left) and male (d).

Figure 2

Scientist inspecting lodged maize from rootworm larvae (a), maize roots from non-Bt maize hybrid (right) and coleopteran Bt maize hybrid (b), rootworm larva (white arrow) feeding on maize root (c), and adult western corn rootworm.

Figure 3

Insect resistance management (IRM) high dose and refuge strategy assumes resistance is recessive. Many susceptible moths (SS) are produced in refuge maize that mate with rare resistant (RR) moths. Mating of resistant (RR) and susceptible (SS) moths produces heterozygous (RS) moths that die when they feed on high-dose Bt maize. This strategy dilutes resistance genes and delays or prevents the evolution of resistance to Bt maize.

https://www.nature.com/scitable/knowledge/library/use-and-impact-of-bt-maize-46975413