Depends on where they colonize from/what they bring.
I imagine that most native species of tortoises and iguanas will go extinct very quickly-with predation from humans and rats definitely (and dogs and pigs possibly). From an ecological perspective, Polynesians on the Galapagos would be a massive disaster, wiping out much of the island's diversity (though Darwin's finches might yet survive and inspire future scientists...)
IIRC the islands are pretty arid. According to galapagospark.org, San Cristobal island saw the cultivation of sugarcane, so the Polynesian crop package could work there. The other islands may see crop cultivation take a backseat to fishing, sealing, and gathering seabird eggs as happened IOTL with the Moriori and many of the Maori.
The aridity means that the Galapagos Polynesians would be limited in their ability to grow/harvest timber for boats, which would limit contact with the mainland. However, there are some uber-good trading opportunities, so even with limited resources I expect there to be trading voyages. Sugarcane would be an excellent crop to trade, and sea lion pelts could also be an item that could interest mainlanders. In exchange, wood products, tools and jewelry could be traded.
I expect a degree of agricultural interchange to occur as well. Manioc would be an interesting crop for people who live on an arid island, as would sweet peppers. Guinea pigs might interest Polynesians as well.
The Polynesian crops would definitely interest the Native Americans, particularly sugarcane which is basically like crack in carbohydrate form and bananas which create a lot of calories for very little labor. Depending on which animals the Polynesians brought, you might see anything from a widespread adoption of domestic chickens* and pigs to widespread problems with disease and food spoilage caused by invasive Polynesian rats.
RE: the ASB thing, that refers to evolutionary changes in the distant past that would normally butterfly humans. Evolutionary changes after humans arrive on the scene, or in isolated areas which wouldn't affect human evolution don't count.
*Chickens might have been introduced by Polynesians to South America IOTL, but only one site has been found with chicken bones dating to earlier than Columbus. The lack of redundancy in the record requires skepticism IMHO.