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Evolutionary Fitness: How to Measure It

    Natural Selection is a process of sorting by the reproductive success that occurs in populations of replicating units. Where units can be defined as molecules, cells, organisms, or larger units.  There are four conditions for natural selection to occur: (1) the units expressing the trait must vary in their reproductive success; (2) the trait must vary among the units in the population; (3) the correlation between the trait and reproductive success must be nonzero (i.e. positive correlation with reproductive success will yield an increase in frequency and vice versa); and lastly (4) the trait must be heritable. 

    Fitness is a measure of relative reproductive performance that can be used to predict long-term dynamics. In other words, fitness is an organism's ability to survive and reproduce to contribute genetically to the next generation. Heritable traits that have greater fitness will increase in a population subject to natural selection. While heritable traits that have less fitness will decrease.  

    Absolute Fitness is the number of progeny of an organism and Relative Fitness is the reproductive rate is relative to the maximum reproductive rate in a population. To measure this, I will use cats with different eye colors as a model to demonstrate fitness. 

Figure 1: Three generations of a black cat with brown eyes


Figure 2: Three generations of a black cat with blue eyes 


Table 1: Shows the absolute and relative fitness of the cats with different eye colors

    The cat with the brown eye color has the highest absolute fitness and maximum relative fitness compared to the population, therefore, has the highest fitness. There will be more black cats with brown eyes in the population than blue eyes.

This example was adapted by an example found on yt. 

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  1. Ashley PanagakisFebruary 24, 2021 at 10:35 AM

    Hi Carmen,
    I really liked your definitions of natural selection, fitness, and how you connected these two. I think your example of measuring fitness was also interesting to see how the cat's eye color affected fitness.

    ReplyDelete

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