Genetic load

Consider n genotypes ${\displaystyle \mathbf {A} _{1},\dots ,\mathbf {A} _{n}}$, which have the fitnesses ${\displaystyle w_{1},\dots ,w_{n}}$ and frequencies ${\displaystyle p_{1},\dots ,p_{n}}$, respectively. Ignoring frequency-dependent selection, the genetic load ${\displaystyle L}$ may be calculated as:

${\displaystyle L={{w_{\max }-{\bar {w}}} \over w_{\max }}}$

where ${\displaystyle w_{\max }}$ is either some theoretical optimum, or the maximum fitness observed in the population. In calculating the genetic load, ${\displaystyle w_{1}\dots w_{n}}$ must be actually found in at least a single copy in the population, and ${\displaystyle {\bar {w}}}$ is the average fitness calculated as the mean of all the fitnesses weighted by their corresponding frequencies:

${\displaystyle {\bar {w}}={\sum _{i=1}^{n}{p_{i}w_{i}}}}$

where the ${\displaystyle i^{\mathrm {th} }}$ genotype is ${\displaystyle \mathbf {A} _{i}}$ and has the fitness and frequency ${\displaystyle w_{i}}$ and ${\displaystyle p_{i}}$ respectively.

One problem with calculating genetic load is that it is difficult to evaluate either the theoretically optimal genotype, or the maximally fit genotype actually present in the population.[4] This is not a problem within mathematical models of genetic load, or for empirical studies that compare the relative value of genetic load in one setting to genetic load in another.

Migration load is the result of nonnative organisms that aren’t adapted to a particular environment coming into that environment. If they breed with individuals who are adapted to that environment, their offspring will not be as fit as they would have been if both of their parents had been adapted to that particular environment.[27][28][29] Migration load can also occur in asexually reproducing species, but in this case, purging of low fitness genotypes is more straightforward.