Module 3.1: Fitness- a Mechanism for Change
In a biological setting
fitness simply refers to reproductive success and reflects how well an organism
is adapted to its environment. It is a key concept rooted at the core of the
theory of natural selection. There are several ways to measure fitness; for example,
absolute fitness (wabs) which measures the ratio of a given genotype
before and after selection while, relative fitness (w) measures differential
reproductive success which is, the proportion of the next generation’s gene
pool that is inclined from a particular genotype compared with competing
genotypes. Absolute fitness is calculated as the product of the proportion survival
and the average reproductive rate. Relative fitness of each genotype is calculated
by dividing each genotype’s survival and/reproductive rate by the highest
survival and/or reproductive rate among the given genotypes. The overall survival
rate is the percentage of individuals born that survive to reproductive age. Reproductive
rate for any given genotype or phenotype is the average number of offspring born
per individual.
A beetle is polymorphic
for color, it comes in black, brown, and yellow color morphs. Birds and lizards’
prey upon them, so due to differences in survivorship, the fitness of the color
morphs differ. AA – black morphs have 67% chance of survival to adulthood, Aa –
brown morphs have 93% chance of survival to adulthood whereas, aa – yellow morphs
have 11% chance of survival to adulthood. Assuming they have an equal number of
offspring, this example will show the relative fitness of each phenotype.
wAA = 93/93 = 1
wAa =67/93= 0.7204
waa = 11/93 = 0.1183
The relative fitness of the black, brown, and yellow morphs is 1,
0.7204, and 0.1183, respectively. Because more brown morphs will survive to reproduce,
the next generation will probably contain a higher frequency of A alleles. This
example also indicates that reproductive success does not always depend on traits
such as strength and speed; reproductive success can also be accomplished by colorful
displays.
Sources
Evolutionary 'selection of the fittest' measured for the
first time | EurekAlert! Science News
Fitness is an inherently complicated concept, and scientists STILL don't have a "standard" way of measuring fitness "correctly". Fitness involves both survival and reproduction, as you said, and I'd be interested in hearing how you would combine those two things into a single measure of fitness (though there's no right or wrong answer). Measuring fitness as survival is problematic because then long-lived species like elephants would have inherently higher fitness than something like a beetle, even though the beetle produces many thousands of offspring per generation and elephants only a few. And at the same time, measuring fitness just as reproductive capacity is also problematic for the same reasons. So what would you do to fix that?
ReplyDeleteThank you for taking time to read my post! Your reply helped me understand how it is quite a challenge to come up with a standard way of measuring fitness. Maybe instead of considering number of offspring one could focus on the proportion of subsequent generations that carry an organism's genes (reproductive success)?
ReplyDeleteYES! Ideally we'd be able to track the number of offspring over many generations, but this is VERY difficult. So that's why we use these easier-to-measure proxies for fitness like body size or age.
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