What is fitness?

By Branden Holmes (Surroundx) on November 28, 2010

When evolutionary biologists speak of the "fitness" of an individual, what they usually mean these days is what is called reproductive success (RS): the extent to which some of their genes will still survive in the population x number of generations later. Because it is not enough just to only consider the contribution of offspring into the next generation. It is good that an individual may have many offspring, even twice as many as the average. But if none of those offspring are likewise endowed with fecundity, it doesn't matter how many offspring that individual has because they will not be disproportianally represented in terms of genes many generations later. How durable a species or gene is (the probability that that species or gene will still be extant after many generations) is what really counts in evolution. This is simply a temporal extension of reproductive success, where the period concerned is many, many generations later, and the entity concerned is the population as opposed to the individual, because individuals do not live for many generations. So, inheritability of fecundity is essential, otherwise the gene pool will simply be diluted and directional selection of fecundity will be impossible. But a number of other definitions for fitness are in wide use, somewhat confounding the problem of multiple definitions, which also partially overlap in meaning:

1. Relative fitness: the number of offspring produced by an individual, compared with other individuals in that population.

2. Absolute fitness: whether the genes passed on from one individual to its offspring, increase in frequency within a single generation.

3. Darwinian fitness: the capacity for a particular individual to be better (positive) or worse (negative) at carrying out a particular action, or actions, relative to other individuals in the population i.e. speed, evasiveness etc.

4. Allelic fitness: the definition used by population geneticists. The reproductive/fecundity benefit of a given gene at a single locus on one of the chromosomes, to an individual which possesses that particular allele. In other words, whether the allele has a positive, neutral or negative effect on the reproductive success of an individual which possesses that allele.

5. Inclusive fitness: an individuals reproductive success, plus the reproductive success of the individuals immediate family (mother, father, brother/s and sister/s), weighted according to their relatedness ('coefficient of relatedness').

6. Neighbour-modulated fitness: an individual may produce more offspring than they can adequately care for. These extra offspring may be cared for by relatives, thus counting as their own offspring in the equation, even though they do not take care of them themselves.

7. Adapted fitness: the degree to which an individual is suited to its environment, relative to other individuals within that same population.

All of these various definitions are warranted because each describes a particular aspect of organisms which varies sufficiently that some individuals are better equipped to survive and thrive than others. Generally it is individuals at one end of the trait value spectrum which are at an advantage ("directional selection"). However, for some traits individuals with an average trait value survive best ("stabilizing selection"). We label this advantage "fitness", regardless of the nature of the trait, whether physical, behavioural or genetic. Scientists of different disciplines are interested in different aspects of organisms, so we would expect as many different definitions as there are ways of looking at the subject. It is important to acknowledge all of these different definitions of "fitness" because they are all valid.

Related articles:
Evolution, fitness, natural selection

About Branden Holmes
I am an amateur evolutionist interested in the theoretical side of the subject.

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