Encyclopedia of Evolution.pdf - Online Reading Center

mutations can occur in buds. A branch that grows from
a bud that has a somatic mutation is genetically different
from the other branches of the tree. Each branch of a tree
may differ in its ability to resist herbivores (see coevolution).
Insect herbivores, which have several generations in
a single year, can evolve much faster than trees. It has been
suggested that the generation of somatic mutations allows
trees to keep up with the evolutionary pace of the insects
that eat their leaves. If this branch produces seeds, the
genetic differences can be passed on to the next generation.
Does a tree function as a genetic and even as an evolutionary
population?
Measuring Genetic Variability in Populations
It has often proved very difficult to measure variability for
any given gene, or overall genetic variability, in populations.
Several approaches have been used:
Phenotypic traits. One approach is to estimate variability
of a measurable phenotypic trait (e.g., area of a leaf, or
length of a bird’s beak). This variability can be expressed as
a variance, as a standard deviation, as a standard error, or as
a coefficient of variation. This variability among individuals,
however, is caused partly by genetic differences among the
individuals (the heritability) and partly by differences among
the individuals in the environmental conditions they have
experienced. Variance, as originally defined by Fisher, is the
average squared deviation from the average value; because
of the square component, variance is symbolized as s2 . The
heritability component of the variance is symbolized as h2 .
Genetic and environmental variability is also symbolized by
the letter V. Thus, total phenotypic variability (VP) consists
of genetic variability (VG) and environmental variability (VE). Genetic variability can itself be divided into two components:
the variability that results from diversity of alleles (additive
genetic variability, VA) and the variability that results from
one allele being dominant over another (dominance variability,
VD). Therefore VG = VA + VD. Heritability can be
expressed as either broad-sense or narrow-sense heritability:
• Broad-sense heritability is VG/VP = VG/(VG+VE), the proportion
of phenotypic variation that is genetic.
• Narrow-sense heritability quantifies only the additive
genetic variation and is therefore VA/VP = VA/(VA+VD+VE). Narrow-sense heritability can also be quantified as the
proportion of total variability in offspring that can be
explained by the variability in the parents in a statistical
analysis.
Heritability calculations assume that parental and offspring
environments are uncorrelated; if they are correlated,
environmental effect can be mistaken for genetic inheritance.
Researchers can sometimes avoid this problem experimentally.
They can, for example, switch eggs among bird nests,
with the result that the offspring of each parent experiences
each of the nest environments. This cannot be done in all
experimental systems.
Heritability is a measure of genetic variation among
members of a population; it is not the same as inheritance. If
population genetics
there is only one allele for a locus in the entire population, and
that allele has a strong effect on a trait, then the trait is very
strongly inherited but has a heritability of zero. Heritability is
not a fixed number for a population, since it depends upon the
amount of plasticity. Consider a genetic disease that is caused
by a single allele and that is invariably fatal; the heritability
would be 100 percent. But as soon as a medicine is discovered
that cures the disease, the heritability drops to zero percent.
Proteins. Another approach is to measure the variability
of proteins. Differences among proteins can be very important,
as enzyme proteins control all the chemical reactions in
an organism. Different forms of the same enzyme, determined
by different alleles at the same locus, are called allozymes.
An individual that is homozygous for one of the alleles of the
gene that encodes this protein will produce just one allozyme;
an individual heterozygous for this gene can produce two
allozymes; the individuals in the population can collectively
produce more than two allozymes. However, two allozymes,
while they differ in structure, may not differ in function. That
is, they may have exactly the same effect on the phenotype
of the organism and on its ability to survive and reproduce.
Therefore the study of allozymes may reveal greater variability
than the study of phenotypic traits.
Proteins can be separated from one another by electrophoresis.
A mixture of proteins is placed at one end of a
gelatin-like sheet (usually composed of the complex sugar
agarose). An electric current creates positive and negative
poles on this sheet. Proteins, which have a slight negative
charge, diffuse through the molecules of the gel toward the
positive pole. Because they must diffuse through an obstacle
course of agarose molecules, the smaller proteins move faster
than the larger proteins. Therefore electrophoresis separates
proteins on the basis of size as well as of charge. If a type of
protein from an individual is placed on a gel, the protein will
show up as a band, after the appropriate staining is used to
make it visible. If the individual is homozygous for that allozyme,
one band shows up; an individual heterozygous (producing
two allozymes) will have two bands. Electrophoresis
was first used to study the protein variability of populations
by Richard Lewontin and John Hubby (see Lewontin, Richard)
in the 1960s.
DNA. Another approach is to measure the variability of
the DNA itself. Measuring the overall variability of the DNA
itself would seem to be an ideal quantification of genetic variability.
However, since most of the DNA in a eukaryotic cell
is noncoding DNA, a measure of overall DNA variability
may be meaningless as an indicator of how much evolution
may be possible in that population. Even measuring the
variability in the DNA of a specified gene may not be very
meaningful, as much of this variability may not produce differences
in proteins. However, differences between populations,
or differences between species, in their DNA (whether
it codes for proteins or not) is routinely used as a measure
of how much evolutionary divergence has occurred between
them (see DNA [evidence for evolution]).
Measurements of genetic variability can be used to reconstruct
population and species history. For example, biologists