Units of Selection

1. Units of Selection

2. We think that the only way that adaptations can arise is through natural selection. The effects of such adaptation can be seen at many levels: gene frequencies, frequencies of types of organisms, even populations.

3. But…. where is the power of natural selection focused. On what units does selection work? Why is this an important question?

4. At what level does the benefit lie?

5. Segregation distortion is an example of an adaptation at the gene level that may actually be disadvantageous to the organism that carries it.

6. Sometimes, certain cell lines in a body are favored at the expense of the rest of the body.

7. There are many examples of adaptations that benefit organisms.

8. Is it possible for natural selection to operate at the level of the group? Altruism – a behavior pattern that increases the number of offspring produced by recipient while decreasing that of the donor.

10. If such behaviors have a genetic basis, then the elimination of altruists should cause the frequencies of altruistic genes to decrease. We would not expect alarm calls to be an evolutionarily stable strategy. So, how do these behaviors persist?

11. If altruists “hand out” altruism indiscriminately, then selfish individuals will be favored because they will receive the benefits without any cost. Altruism must first be dispensed preferentially to other altruists. This is the only way for the behavior to evolve, and only then if the benefit to the altruist outweighs the cost.

13. W.D. Hamilton suggested that we expand our ideas about fitness and selection. He defined inclusive fitness as the relative ability of an organism to get its genes, or copies of its genes, into the next generation. Hamilton suggested that it is irrelevant to the evolutionary process whether an individual passes on its own genes or enhances the transmission of copies of them in another individual.

14. So, who carries copies of an individual’s genes? Obviously, relatives are the most likely. Altruistic behavior often takes place between genetic relatives, where it is explained by the concept of kin selection. Actually, we can calculate a coefficient of relationship, r, which is the average proportion of genes shared by two individuals.

15. Coefficients of relationship

16. An altruistic allele can increase in frequency if it increases the inclusive fitness of the individual. According to Hamilton, we can quantify the probability that an allele will increase in frequency. It can increase if: Where k is b/c, the ratio of the benefit to the recipient to the cost to the altruist.

17. A test – Aphelocoma coerulescens

19. Benefit of helping is 35% - 7% or 28%. Since there were approximately 2 helpers at each nest, divide by 2 to get: b = 14% Cost is more difficult. Mumme concluded that the cost could be zero (assuming that the helper jays would not have bred anyway), or could be as high as the reproductive success of jays without helpers, in which case: c = 7%

20. In this case, the tradeoff is between helping in the production of siblings, or producing their own offspring. So we must evaluate whether: rsibc > roffc In this case, regardless which estimate of cost we use: (0.5)(14) > (0.5)(0) or (0.5)(14) > (0.5)(7) So, selection favors helping.

21. Genetically based Cooperation

22. Honeybees are haplodiploid. Females are diploid and males are haploid. Unfertilized eggs develop into males, while fertilized eggs develop into females. This skews the typical coefficients of relationship. Sisters are more closely related to each other than parents and offspring are. According to the theory of kin selection, a sister is more valuable than a daughter. This explains the ultimate altruism – forgoing one’s own reproduction to care for one’s sisters.

23. Ecologically based cooperation

24. The social unit of acorn woodpeckers is a territorial group of up to a dozen members that gather and store acorns in holes drilled in a tree (granary). Granary trees are developed, maintained, and defended by all members of the group.

26. The system has three unusual features: • Division of labor. Breeders and helpers. • Mate sharing. Only one active nest at any one time. Among breeders, any female may breed with any male. • Infanticide. Females kills the first eggs laid by other females. When birds enter a group to fill a vacancy, they kill all the young.

28. How do we explain this? The territory is saturated, with little opportunity for young to disperse. Groups resist immigration. Best fitness option for young is to remain with the group and contribute to its reproductive success. Mate sharing brings advantages to the individual breeder. Woodpeckers that share mates raise more young than do monogamous pairs. Infanticide provides a genetic advantage. Infanticidal females stop when they can no longer be sure that the infant she is killing is not her own.

29. Beyond the idea of kin selection, which is not truly altruistic, group selection is thought to be a weak force at best in evolution.