Different selection pressures might favor a group or an individual and lead to group or individual adaptations. Group selection refers to pressures that lead to group adaptations and more altruistic traits. Individual selection refers to pressures that lead to individual adaptations and more selfish traits.
An example of group selection
Cotton-top tamarins are an example of a species in which group selection has led to altruistic traits. Like other tamarin species, cotton-top tamarins live in altruistic groups of up to 15 individuals in which there is only one mated pair; the rest of the group consists of offspring from previous years and sometimes even unrelated adults (Zahed, Kurian, & Snowdon, 2010). While the mother nurses, the father and older siblings do much of the carrying of young. Genetically related individuals will benefit from altruism in an environment that favors group selection.
However, Hauser, Chen, and Chuang (2003) found that cotton-top tamarins will share food with unrelated individuals - especially if that individual has shared food with them before. In these groupings, altruism begets more altruism. Another reason why group adaptations in tamarins like this are beneficial is that tamarins are very tiny monkeys with a great variety of predators. In their groups, one member always acts as the sentry, alerting the others of predators and keeping the entire group safe (Kostan & Snowdon, 2002).
An example of individual selection
Cuttlefish are a fascinating example of individual selection leading to several different individual adaptations. In particular, the males participate in sperm competition for reproductive success. Males compete for access to females and cuttlefish have two methods they use to increase their chances for success: 1) sperm removal when they detect that a different male mated with their partner before they did; and 2) adjusting their amount of ejaculation depending on whether another male mated with their partner before them and depending on how much sperm they were able to remove in step 1 (Wada, Takegaki, Mori, & Natsukari, 2010). Initially, it appears that the larger males have the advantage when it comes to competing for access to females. However, there is an additional, even more incredible adaptation that smaller males employ to increase their success: they mimic females! While cuttlefish are known for their amazing camouflage abilities that help them hide from predators and sneak up on prey, smaller males also use this adaptation to disguise themselves as female cuttlefish and sneak past the larger males for access to a true female (Hanlon, Naud, Shaw, & Havenhand, 2005).
When examining the mating behaviors of cuttlefish, it is clear that individual selection has led to a very interesting variety of adaptations!
Hanlon, R. T., Naud, M., Shaw, P. W., & Havenhand, J. N. (2005). Behavioural ecology: Transient sexual mimicry leads to fertilization. Nature, 433(7023), 212.
Hauser, M.D., Chen, M.K., Chen, F., & Chuang, E. (2003). Give unto others: genetically unrelated cotton-top tamarin monkeys preferentially give food to those who altruistically give food back. Proceedings of the Royal Society B-Biological Sciences, 270 (1531), 2363-2370.
Kostan, K.M., & Snowdon, C.T. (2002). Attachment and social preferences in cooperatively-reared cotton-top tamarins. American Journal of Primatology, 57 (3), 131-139.
Zahed, S.R., Kurian, A.V., & Snowdon, C.T. (2010). Social dynamics and individual plasticity of infant care behavior in cooperatively breeding cotton-top tamarins. American Journal of Primatology, 72 (4), 296-306.
Wada, T., Takegaki, T., Mori, T., & Natsukari, Y. (2010). Sperm removal, ejaculation and their behavioural interaction in male cuttlefish in response to female mating history. Animal Behaviour, 79(3), 613-619.
Cotton-top tamarin image from http://www.pbase.com/image/20137906