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Realized Heritability
Selection can act on any phenotypic variation, but can only cause evolutionary change if the
variation is genetic.
Population biologists often use an index called realized heritability, h2, to quantify the degree to
which a trait in a population can be pushed by selection. To calculate this index, we find the
response to selection by subtracting the average of the second generation from that of the entire
first generation. We also find the selection differential by subtracting the average of the selected
parents from that of the entire first generation. Realized heritability is the response divided by
the differential.
realized heritability =
realized heritability =
response to selection
selection differential
avg1st gen − avg 2 nd gen
avg1st gen − avg selected parents
A low h2 (less than .01) occurs when the offspring of the selected parents differ little from the
original population, in spite of a big difference between the population as a whole and the
selected parents.
A high h2 (greater than .6) occurs when the offspring of the selected parents differ from the
original population almost as much as the selected parents do.
To quantify any increase in number of hairs made by selecting and intermating the hairy portion of a population,
students would first want to record the number of hairs on each plant in the experimental population (Generation 0) of size = n. Then calculate the average number of hairs on a
representative plant = x. The standard deviation is a calculation that described the average amount
that individuals vary from a population average and is a useful statistic in helping to understand
how the hairy phenotype is inherited.
If students select the 10 percent hairiest plants from the population and calculate the average or
mean number of hairs from this subpopulation then the difference between the population and the
selected sub-population mean is the selection differential. Seed is produced on the selected plants.
Then the progeny from the selected subpopulations are counted for hairs and averaged. The difference between average number of hairs from the original population, Generation 0 and the average of
Generation 1 is known as the response to selection. The inherited change in the population due to
the 10 percent selection for hairiness is known as the realized heritability, h 2 , and is the proportion
of the response to selection to the selection differential.
Estimates of realized heritability can range from low, < 0.1 to high, > 0.6 and are useful in predicting
the rates at which a population can change through selection.
selection differential
h 2 = response to selection
IV. HERITABILITY
One of the most useful results of performing this kind of analysis is that
we can determine how much of the phenotypic variation is due to each
component. Selection, remember, can act on any kind of phenotypic
variation, BUT ONLY CAUSES EVOLUTIONARY CHANGE IF THE
VARIATION IS HERITABLE
We can express the amount of variation that is genetic in origin as the BROADSENSE HERITABILITY, hB^2 = VG/VP
or the percentage of phenotypic variation that is genetic
this could include dominance and epistatic variation, and since
resemblance across generations is due almost exclusively to additive
variation, we will be more interested in the NARROW-SENSE
HERITABILITY, h^2 (usually just "heritability"), which is the
proportion of phenotypic variance that is due to additive genetic causes:
VA/VP
Why Care about Heritability? Because we can predict the response to
a given amount of selection. As we learned, selection acts on phenotypic
variation (VP), but changes are transmitted through genetic variation. In
fact, through additive genetic variation (VA).
Since heritability is the proportion of additive genetic variation, we can
predict the response to selection, R, with h^2 and s, the selection
differential:
R = h^2s
Obviously, when h^2 = 0, there will be no response, and when there is
positive h^2, there will be some response to selection.
heritability
A measure of the degree to which the variance in the distribution of a phenotype is due to
genetic causes. In the broad sense it is measured by the total genetic variance divided by
the total phenotypic variance. In the narrow sense it is measured by the genetic variance
due to additive genes divided by the total phenotypic variance.
heritability in the broad sense
See broad heritability.
heritability in the narrow sense
The proportion of phenotypic variance that can be attributed to additive genetic variance.
realized heritability
Heritability measured by a response to selection. The ratio of the single-generation
progress of selection to the selection differential of the parents.
selection differential
The difference between the mean of a population and the mean of the individuals selected
to be parents of the next generation.
selection progress
The difference between the mean of a population and the mean of the offspring in the
next generation born to selected parents.
heritability (in the narrow sense)A measure of the extent to which genes inherited from parents
determine phenotypes; defined as the ratio of the additive genetic variance to total phenotypic
variance; also called realized heritability. (taken as definition of realized heritability in this
glossary)