Chapter 16 Textbook 01/26/2014
all species differ in some way, like there length, size and mass.
Phenotypic variation is difference in functions or appearance between individual organisms.
Darwin’s theory of evolution recognized the importance of heritable phenotypic variation.
Quantitative Variation; individuals differ in small, incremental ways.
Mendel studied exhibited the qualitative variation; they exist in two or more discrete states, and
intermediate forms are often absent. Ex. Snow geese either have blue or white feathers.
the existence of discrete variations of a character is called a polymorphism (poly=many, morphos=form).
We describe phenotypic polymorphisms quantitatively by calculating the percentage or frequency of each
Phenotypic variation within populations may be caused by genetic difference between individuals, by
differences in the environmental factors that individuals experience, or by an interaction between genetics
and the environment.
Knowing whether phenotypic variation is caused by genetic differences, environmental factors or an
interaction of the two is important because only genetically based variation is subject to
How can we determine whether phenotypic variation Is caused by environmental factors or by genetic
we can test for an environmental cause experimentally by changing one environmental variation and
measuring the effects on genetically.
Artificial Selection (Mendel’s Suggestion)selective breading of animals or plants to ensure that certain
desirable traits appear at higher frequency in successive generations.
Genetic variation, the raw material moulded by microevolutionary processes, has two potential sources:
1.the production of new alleles
2. the rearrangement of existing alleles.
Most new alleles arise from smallscale mutations in DNA. The shuffling of existing alleles into new combinations can produce an new number of genotypes and
phenotypes in the next generation.
How much genetic variations actually exist within populations?
in the 1960’s, evolutionary biologist began to use gel electrophoresis to identify biochemical polymorphisms
in diverse organisms.
Studies of chromosomal and mitochondrial DNA suggest that every locus exhibits some variability in its
Summary of Chapter 16.1
Phenotypic traits exhibit either quantitative or qualitative variation within populations of all organisms. These
discrete differences are called polymorphisms.
Genetic variation, environmental factors, or an interaction between the two cause phenotypic variation
within populations. Only genetically based phenotypic variation is heritable and subject to evolutionary
Genetic variation arises within population largely through mutation and genetic recombination. Artificial
selection experiments and analyses of protein and DNA sequences reveal the most populations include
significant genetic variation.
to predict how certain factors may influence genetic variation, population geneticists describe the genetic
structure of a population.
The HardyWeinberg principle of genetic equilibrium is a null model.
The sum of all alleles at all gene loci in all individuals is called the population’s gene pool.
To describe the structure of a gene pool, first identify the genotypes in a representative sample and
calculate genotype frequencies, the percentages of individuals possessing each genotype.
Allele frequencies are the relative abundances of the different alleles.
For a locus with two alleles, scientists use the symbolp to identify the frequency if one allele, aq to
identify the frequency of the other allele.
*look at example page 369*
A null model predicts what would scientists see in an experiment if a particular factor had no effect. Null
models serve as a theoretical reference points against which observation can be evaluated. The HardyWeinberg Principle specifies the conditions under which a population of diploid organism
achieves genetic equilibrium, the point at which neither allele frequencies nor genotype frequencies change
in succeeding generations.
The HardyWeinberg principle is a mathematical model that describes how genotype frequencies are
established in sexually reproducing organisms.
This model, genetic equilibrium is possible only if all of the following conditions are met:
No mutations are occurring
The population is closed to migration from other populations.
The population is infinite in size.
All genotypes in the population survive and reproduce equally well.
Individuals in the population mate randomly with respect to genotypes.
If the conditions of the model are met, the allele frequencies of the population for an identified gene locus
will never change, and the genotype frequencies will stop changing after one generation.
If they are not met, microevolution may be occurring.
a populations allele frequencies will change over time and not match the predications of the Hardy
Weinberg model of one more of the conditions of the model are violated.
A mutation is a heritable change in DNA can introduce new genetic variation into a population.
A mutation is usually rare and between 1 gamete in 100000 and 1 in million.
Mutation is a major source of heritable variation .
In most animals mutations occur in the germ line that are heritable and usually have no effect on the next
In most plants mutations occur in the meristem cells, which usually have an effect on the next generation.
Neutral Mutation are neither harmful nor helpful, they may change an organism’s phenotype without
influencing its survival and reproduction.
Deleterious mutations alter an individuals structure, function, or behavior in harmful ways. Lethal allele
is dominant form its effects; if recessive it affects only homozygous recessive individuals. Sometimes a change in DNA produces an advantageous mutation, which confers some benefit on an
individual that carries it.
Organisms or their gametes sometimes move from one population to another. If the immigrants reproduce
they may introduce novel alleles into the population they have joined.
Gene flow violates the HardyWeinberg condition that populations must be closed to migration.
the evolutionary importance of gene flow depends on the degree of genetic differentiation between
populations and the rate of gene flow between them.
Chance events sometimes cause allele frequencies in a population to change unpredictably. This
phenomenon known as genetic drift.
Genetic drift has dramatic effects on small populations which clearly violate the HardyWeinberg condition
of infinite populations size.
Chance deviations from expected resultswhich cause genetic driftsoccur whenever organisms engage in
sexual reproduction, simply because their population sizes are not infinitely large.
Genetic drift generally leads to the loss of alleles and reduced genetic variability. Population bottlenecks
and founder effects often foster genetic drift.
Population bottlenecks Population bottlenecks occur when a population’s size is reduced for at least one