Final answer:
Tortoises in the Galapagos Islands have longer necks than tortoises in dry lowlands due to natural selection. Longer necks allow tortoises to reach leaves and access more food, which increases their chances of survival and reproductive success.
Step-by-step explanation:
Giant tortoises in the Galapagos Islands have longer necks compared to those in dry lowlands because of a process called natural selection. Darwin observed that tortoises with longer necks had a better chance of reaching leaves and accessing food during times of drought when fewer leaves were available. Since they were able to eat and survive, these long-necked tortoises were more likely to pass on the long-necked trait to their offspring. Over time, only long-necked tortoises would be present in the population.