According to evolutionary theory, behaviors that optimize survival and reproduction are favored by natural selection and therefore more likely to persist. How, then, can we explain the evolution of altruism, which is by definition costly to the self

Altruism could be helpful for the survival of relatives

Explanation:

Altruism behavior is produced when an organism decreases its fitness in order to increase the chances of survival in other individuals in the population. It is believed that altruistic behavior could have evolved as an adaptation associated with the survival of relatives from the same family. In consequence, if this were the case, altruism could ultimately be considered as a selfish behavior because it aims at passing on of genes through the survival of relatives which could benefit with this behavior.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.