Narrowsense heritability VAVP - PowerPoint PPT Presentation

Title: Narrowsense heritability VAVP


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Narrow-sense heritability VA/VP This is the
heritability in the breeder's equation r h2
s Narrow-sense heritability is viewed as the
single most important descriptive statistic about
the quantitative genetics of a given trait in a
given population. It indicates the evolutionary
potential of the trait. How do we estimate
narrow-sense heritability? Resemblance of
relatives ...
Started here 15 Feb. 2007
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Least-squares linear regression slope
narrow-sense heritability
N 50
If have no maternal effects or common-family
environ-mental effects
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Least-squares linear regression slope
narrow-sense heritability
N 50
Low repeatability would add a lot of "jitter" to
the data points and reduce the slope
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In principle, can use any relatives, just need to
know the expected causes of resemblance. For
example, in an organism that had no paternal
care, might measure offspring and only the
fathers. Double the regression slope to estimate
narrow-sense heritability.
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Another common "breeding design" is to mate each
father (sire) with multiple mothers (dams) and
measure trait of interest in the offspring
only. This half-sib, full-sib design allows
estimation of narrow-sense heritability. In
particular, the among-sire component of variance
is proportional to additive genetic effects (if
have no non-genetic paternal effects). Can also
estimate realized narrow-sense heritability from
a selective breeding experiment because r/s
h2
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This equation describes the response to
directional selection of a single phenotypic
trait, given its narrow-sense heritability and
the intensity of selection r h2 s However,
organisms comprise many phenotypic traits, and
they may be correlated. Therefore, selection that
affects one may affect another.
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Imagine two traits that are positively correlated
in a population before selection.
Trait B
Trait A
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Imagine that selection eliminates all individuals
with values lt 1.0 for Trait A.
Die Survive
Trait B
These survivors will also have values for trait B
that tend to be greater than the mean for the
whole population before selection.
Trait A
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In addition, traits may be genetically
correlated, i.e., tend to run together in families
Therefore, to understand phenotypic evolution, we
need to consider both phenotypic and additive
genetic correlations between traits.
Trait B in Offspring
Trait A in Parents
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Genetic correlations can either slow (constrain)
or accelerate (facilitate) phenotypic evolution,
depending on whether they are in the same
direction as selection. Genetic correlations are
caused by pleiotropy (one gene affects gt 1 trait)
and/or by linkage disequilibrium (non-random
associations among alleles at different loci
during the gametic phase), one cause of which is
physical linkage on chromosomes. Pleiotropy is
generally thought to be the more important
cause. Genetic correlations can themselves evolve
in response to selection because alleles with
different pleiotropic effects may be
favored. Thus, a genetic correlation between two
traits might indicate the action of past
selection.
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Multivariate Selection Theory
DZ G P-1 s
multivariate response to selection a vector
indicating the cross-generational change of
character means or traits Z1, Z2, Z3, etc.
additive genetic variance covariance matrix
inverse of phenotypic variance covariance matrix
vector of directional selection differentials
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b can be calculated as the vector of partial
regression coef ficients in a multiple regression
to predict fitness.
Fitness bo b1Z1 b2Z2 b3Z3 residuals
If all relevant traits have been included in the
analysis, then these describe selection that is
acting directly on the individual traits.
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Hypothetical Example
Imagine a population of birds in which we measure
clutch size at fledging, which is a major
component of Darwinian fitness, for 50 nests. In
this species, males help to take care of the
young, both feeding them and protecting the
nest. So, for each nest, we also measure male
body mass and take a blood sample to determine
his circulating testosterone level.
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Clutch Size
Body Mass (grams)
Clutch Size 0.824 0.107 Body Mass, r2
0.187 Standardized Regression Coefficient 0.433
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Clutch Size
Testosterone (ng/ml)
Clutch Size 8.594 - 3.458 Testosterone, r2
0.304 Standardized Regression Coefficient -0.551
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Testosterone (ng/ml)
Body Mass (grams)
r 0.263, 2-tailed P 0.065
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Clutch Size 0.824 0.107 Body Mass, r2
0.187 Standardized Regression Coefficient
0.433
Clutch Size 8.594 - 3.458 Testosterone, r2
0.304 Standardized Regression Coefficient
-0.551
Multiple Regression Clutch Size 3.429 0.154
Body Mass -
4.482 Testosterone
Multiple r2 0.662 Standardized Partial
Regression Coefficients Body Mass
0.621 Testosterone -0.714
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Lecture 13 Phenotypic Plasticity
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How do complex traits evolve?
Complex traits are At relatively high levels
of biological organization Comprised of many
subordinate traits Affected by many genes and
environmental factors http//complextrait.org/
regarding human diseases and disorders
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Classic complex traits
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The ultimate complex trait
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Selection acts hierarchically
In animals, selection generally acts more
directly on behavior than on the subordinate
traits that determine performance abilities
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At any level of organization ...
Phenotypes may be affected by environmental
factors, i.e., their expression may be "plastic"
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Phenotypic Plasticity
The ability of an individual organism to alter
its phenotype in response to changes in
environmental conditions. or The modification of
developmental events by the environment. or The
ability of one genotype to produce more than one
phenotype when exposed to different environments.
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The ability of one genotype to produce more than
one phenotype when exposed to different
environments.
Highly Variable Plasticity, strong
Genotype-by-Environment Interaction
No Plasticity
Plasticity
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Features of "Phenotypic Plasticity"
Something in the internal and/or external
environment changes (usually) Organism senses
that change Organism alters gene
expression Usually, the altered gene expression
yields additional observable phenotypes
Includes "acclimation" and "acclimatization" as
well as learning and memory.
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Features of "Phenotypic Plasticity"
Something in the internal and/or external
environment changes (usually)
Changes in ambient temperature, humidity
oroxygen concentration would constitute external
environmental factors, and many organisms respond
to these with phenotypic plasticity that involves
multiple organ systems and multiple levels of
biological organization. Mechanical overload of
the heart is an example of an environmental
change that occurs within an organism, and it
leads mainly to organ-specific changes that
necessarily involve fewer levels of biological
organization.
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Features of "Phenotypic Plasticity"
Organism senses that change
Some changes may occur without any formal sensing
by the organism, e.g., as a result of direct (and
possibly differential) effects of temperature on
the rates of ongoing biochemical and
physiological processes.
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Features of "Phenotypic Plasticity"
Organism alters gene expression
Some plastic responses need not involve changes
in gene expression (transcription) but instead
could occur via phosphorylation of existing
proteins, changes in protein levels caused by
variation in protein ubiquitination, or
stimulation of existing microRNAs.
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Features of "Phenotypic Plasticity"
Usually, the altered gene expression yields
additional observable phenotypes
In principle, lower-level traits might change in
offsetting ways, such that a higher-level trait
could show little or no apparent change. For
example, it would be theoretically
possible (though perhaps unlikely) for exercise
training to cause an increase in maximal heart
rate but a reduction in stroke volume, such that
cardiac output was unchanged.
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Hierarchical masking effects
Compensatory plasticity at lower levels could
lead to reduced plasticity at higher levels
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Features of "Phenotypic Plasticity"
The changes may or may not be reversible. The
changes may or may not be adaptive in the sense
of increasing the organism's reproductive success
(Darwinian fitness).
The idea that environmentally induced
modifications are adaptive in the sense that they
improve organismal function and/or enhance
Darwinian fitness has been termed the "beneficial
acclimation hypothesis." In general, non-adaptive
plasticity might be expected to occur any time
that an organism is exposed to environmental
conditions with which it is "unfamiliar" in terms
of its evolutionary history.
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Features of "Phenotypic Plasticity"
In some cases, behavioral plasticity can shield
lower level traits from selection. For example,
gravid lizards may become more wary.
At the population level, phenotypic plasticity in
behavior and other traits can facilitate
invasions of new habitats.
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Classic Cases of Phenotypic Plasticity
Poorly fed and well-fed sibling echinopluteus
larvae of the sea urchin Lytechinus variegatus on
day 4 of development. Note greater investment in
ciliated band and internal skeleton under low
food conditions. Photo by J. S. McAlister.
http//www.unc.edu/podolsky/plasticity.htm
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Classic Cases of Phenotypic Plasticity
Carotenoid coloration is phenotypically plastic,
and diets lacking carotenoids result in very
little color in normally pigmented species, such
as the house finch (Carpodacus mexicanus).
Population differences in carotenoid have been
related to the presence of specific food plants.
Price, T. D. 2006. Phenotypic plasticity, sexual
selection and the evolution of colour patterns.
J. Exp. Biology 2092368-2376.
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Taylor, C. R., and E. R. Weibel. 1981. Design of
the mammalian respiratory system. I.
Problem and strategy. Respiration
Physiology 441-10. Passage from page 3
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Many such examples do seem to be adaptive, i.e.,
to confer higher Darwinian fitness (or at least
they increase organismal performance at some
task), so we can proceed to ask ...
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To be or not to be
when shouldplasticity evolve?
Stopped here 15 Feb. 2007