What is fitness- (as it relates to evolution)?
I am largely motivated by a Wilkins post over at Evolving Thoughts, where he attempts to define fitness as part of the “basic concepts” series, and largely (I think) misses the point. (Although the more philosophical points are excellent)
John’s Definition: Fitness is the rate of increase of X in a population P.
This is wrong- fitness is not a rate, but instead a probability or propensity. Fitness if the propensity of a given allele to find itself propagated in the next generation. Following this, alleles conveying fitness benefits are statistically over-represented in the next generation as compared to alleles with lower fitness. The opposite it true as well, alleles that have negative fitness consequences are under-represented…
Rate however is important- and I suspect that this is what Wilkins was trying to emphasize. Take 2 alleles, identical in their fitness profiles, except for the rate in which the find themselves in the next generation is greater in the 1st, here it is easier to see that the faster of the 2 has higher fitness. Rate is important, but saying that rate alone defines fitness is myopic.
My Definition: The concept of fitness relates to the propensity of a given allele to found in future generations. The rate at which this occurs is not the definition of fitness, but instead a key parameter that modifies the fitness of any given allele.
I will point readers to Elliot Sober (1984) and John Endlers “Natural Selection in the wild” for superb discussions of fitness.
Topics to be covered in later posts:
- How do we measure fitness?
- What are fitness landscapes?
- Inclusive Fitness
4 responses so far ↓
1 John Wilkins // Jan 21, 2007 at 4:28 pm
First of all, it’s Wilkins, not Wilkes.
Second, I give a reason why I don’t think the propensity interpretation works. Perhaps you missed that.
Third, the rate of increase gives you the representation in the subsequent generations.
But I appreciate your comments. Perhaps you can elaborate.
2 Matt // Jan 21, 2007 at 6:13 pm
Whoa John, nice to know one can have a friendly disagreement- You take difference of opinion like a champ…
Anyway, Sorry about the name- mine is often misspelled as well- so I understand..
I read your propensity arguement- but don’t agree, I will elaborate, but need more time than I have right now to develop my thoughts..
3 Rich Lawler // Jan 24, 2007 at 5:53 pm
Alleles don’t have fitnesses, just marginal fitnesses; this is because we assign fitness values to genotypes, not alleles. Marginal fitness is the probability that an allele will pair up with another allele to produce a genotype that has some particular fitness value.
I tend to think that fitness is best thought of as the rate of growth of a genotype or strategy (e.g., invasibility). We want to consider absolute rates of growth, rather than per-generation rates (as your definition seems to imply), since we need to account for differences in generation times, and only the per-unit-time rate of increase can do this. Only rate-insensitive (i.e., not dependent on generation times) measures of fitness can predict changes in genotype frequency.
All that said, McGraw and Caswell (1996 in Am. Nat) used a propensity framework of fitness to measure the *rate* of growth of individuals in a population. In essence, they combine and operationalize the propensity of individuals to survive, grow, reproduce… and use these data to calcluate the “growth rate of the individual,” which allows individual fitness to be properly analyzed in a rate-insensitive demographic framework.
4 Matt // Jan 24, 2007 at 6:57 pm
Rich,
We do assign fitness to genotypes- say DRB- but it is the specific rare allele that confers high fitness. The genotype is comprised of many genes- each of which has X nummber of alleles.
Im not sure about needing to acct for difference in generation times- as we are always taking about populations. As generation size is determined by the whole cohort- it is equal (and does not need to be accounted for) for all individuals being compared. Right???
I don’t know the paper you cite- but will be the end of the evening…
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