What groups of people did manifest destiny affect?what symbols can represent one of the key events form the period of Manifest Destiny (in a political cartoon)?
Can't draw you a political cartoon, but I can give you some ideas for one. The main groups of people who were affected by Manifest Destiny would include Native Americans, who had their tribal lands overrun by settlers, profiteers and the US Army. Their way of life would change forever because of Americans' ovewhelming desire to move west.
European immigrants were affected by the large amount of cheap or free farmland that attracted them to America in the first place. Robber barons were further enriched by the resource wealth and railroad opportunities in the west. Lastly, it affected the soldiers sent west to protect the territorial gains of the US.
As for symbols on a political cartoon you might consider images of wagons, soldiers and miners heading west. You could include Natives, an image of California in the distance. If you want to specify it towards one event from that time period, consider the War with Mexico and California on the other side of the Mexican Army from the Americans. You could use a caption such as "Just a little farther now, boys" said by President James Polk.
It would seem that any political cartoon about Manifest Destiny should include some representation of Native Americans. One format that I have seen work very well is the idea of an "Uncle Sam" depiction that is larger than life, and overrunning the Native Americans, who are depicted proportionally smaller. In this the depiction shows that Manifest Destiny empowered the American government over all else, including the Native Americans. The fact that Manifest Destiny helped to bolster the ability of the American Government to take land that did not belong to it lends itself to being depicted in a cartoonish manner where theft and intimidation is validated by the government.