If the change is slow they have time to adapt. An example of a fast change is human deforestation. With their habitat suddenly removed, the organisms living there will have to find new shelter and food fast. It's likely that they'll not even be able to find anything to survive off of and die, because organisms tend to have very specific needs. For example, pandas would go extinct if bamboo was eradicated.
Freshwater fish would not be able to adapt if it was suddenly dropped into salt water. They would die. An example of a slow change would be the slow addition of salt to freshwater. Fish that are better able to survive within higher levels of salinity will be more likely to reproduce. Slowly through reproduction a new type of fish suited to salt water may be developed over several generations.
A peptide bond is a chemical bond formed between two molecules when the carboxyl group of one molecule reacts with the amino group of the other molecule, releasing a molecule of water.
