Evolution
According to Darwin’s theory of evolution by natural selection, the individuals best suited to their environment will have a better chance of surviving and multiplying. This is sometimes called “survival of the fittest”. Their characteristics will be passed on to the next generation. Thus if a particular species is well adapted to its environment then little change will occur between generations. But if the species is not well adapted to its environment, large changes may occur between generations. Some organisms seem to have changed little over hundreds of millions of years: for example, insects such as cockroaches and vertebrates such as tortoises and crocodiles. When bacteria become immune to drugs it is because they have evolved into a slightly different bacteria which the drugs are ineffective against. What is evolution in your own words:
US: 
