Now let's consider this matter of fitness a bit more thoroughly. Darwin talked about the survival of the fittest, a comparative term which suggests that some individuals are fittER than others. The concept of genetic fitness is usually defined as the CONTRIBUTION TO THE NEXT GENERATION OF INDIVIDUALS HAVING ONE GENOTYPE IN COMPARISON TO THE CONTRIBUTION OF INDIVIDUALS HAVING OTHER GENOTYPES.
A very interesting thing about fitness is that an individual's total fitness is something which he can be conceived of as sharing in part with his relatives. Many of you will already know and the rest of you will be able to figure out from your text the reasons why you share 1/2 your genes in common with each of your parents, 1/2 with each of your full siblings, 1/2 with each of your own offspring, 1/4 with each of your half siblings, grandparents, grandchildren, or first cousins, aunts and uncles, 1/8 with each of your great-grandparents, great-grandchildren, second cousins, great aunts or great uncles.
Now, as we have said, an individual's total or INCLUSIVE FITNESS consists of that individuals total capacity to get his or her genotype into the next generation. One component of inclusive fitness is called PERSONAL FITNESS and it involves one's own capacity to reproduce. However, since genetic material is shared with relatives, another component of fitness (called SHARED FITNESS) involves fitness shared with relatives. Since you share a proportion of your total genetic material with relatives, individuals can also increase the representation of their own genes in future generations by assisting the reproduction of relatives. These facts can be summed up in the following verbal equation:
INCLUSIVE FITNESS = PERSONAL FITNESS + SHARED FITNESS
Now this idea has some very interesting behavioural ramifications. It leads us into a new and different way of looking at some old concepts. Those concepts are ALTRUISM, SELFISHNESS, and SPITE.
1. When an animal behaves in a way which reduces that animal's personal fitness while increasing the personal fitness of another animal, the behaviour of the animal whose personal fitness is being decreased is said to be ALTRUISTIC. For example, one animal might protect another in some dangerous situation. By exposing himself to danger, the protector would be lowering his or her own personal fitness. By protecting the other animal and thus removing a danger to its life and potential reproductive success, the protector would be raising the personal fitness of the animal protected.
2. If an animal is not going to be altruistic, it may be selfish. Defined in a theoretically interesting way, SELFISHNESS consists of behaving is such a way as to increase one's own personal fitness at the expense of the personal fitness of another organism. In other words, selfish acts (by this definition) are acts which enhance one's own reproductive success at the expense of the reproductive success of another organism. NOT ALL ACTS WHICH PROMOTE ONE'S OWN REPRODUCTIVE SUCCESS ARE SELFISH.
3. Finally, a third possible type of behaviour is SPITE. Spite is defined here as an act which lowers one's own personal fitness while simultaneously lowering the personal fitness of another. (E.g. risking one's life in an effort to harm another individual)
A very interesting set of questions concerns circumstances under which it is biologically adaptive for animals to behave in altruistic, selfish, and spiteful ways. When will natural selection favour altruism? When will it favour selfishness? When (if ever) will it favour spite?
To answer these questions, we are going to have to write some simple mathematical equations. We'll consider altruism first.
LET:
B = Benefit to a recipient of an altruistic act
C = Cost to the altruist
r = the proportion of genes held in common by the altruist and recipient
(coefficient of relatedness)
Altruism will be favoured by natural selection whenever:
or
(The two equations are equivalent!!!)
These equations make it possible to visualise situations in which altruism would be adaptive (i.e. selected for by natural selection). For example, since full siblings share 1/2 their genes in common, it would be biologically adaptive for one full sibling to sacrifice his or her life to save 3 or more full siblings or other relatives with whom r=1/2.
To understand selfishness,
LET:
G = Gain to selfish individual
H = Harm done to recipient of act
Then, selfishness will be adaptive whenever:
Altruism can be increased by either increasing B or reducing C. Some individuals
(young more than old) can be helped more than others (increasing B). Other
individuals (homosexuals, monks, nuns, post-menapausal women) are functionally
sterile and thus have little or no personal fitness to lose. These people
should be more altruistic than individuals with high personal fitness (e.g.
adolescents and young adults).