Microevolution: genetic changes within populations Chapter 16
16.1 Variation in Natural populations:
Phenotypic variation: differences in appearance or function thatif based on heritable
information are passed from generation to generation.
16.1a Evolutionary Biologists Describe and Quantify Phenotypic Variation:
Quantitative variation: individuals differ in small, incremental ways. [Data is usually
displayed on bar graphs or as curves the width of the curve is proportional to the amount
of variation and the mean describes the average vale]
Qualitative variation: exist in 2 or more discrete states, and intermediate forms are often
Polymorphism: the existence of discrete variants of a character [poly= many,
16.1b Phenotypic Variation Can Have Genetic and Environmental Causes:
Phenotypic variation may be caused by genetic differences between individuals by
differences in the environmental factors that individuals experience or by an interaction
between genetics and the environment. [As a result, genetic and phenotypic variations are
not directly correlated organisms with different genotypes can exhibit the same
phenotype and vice versa]
Only genetically based variation is subject to evolutionary change
Since environmental factors can influence the expression of genes, an organism’s
phenotype is frequently the product of an interaction between its genotype and
How can we determine whether phenotypic variation is caused by environmental
factors or by genetic differences?
We can test for an environmental cause experimentally by changing one environmental
variable and measuring the effects on genetically similar subjects.
Artificial selection: a population expressing quantitative traits that have a genetic basis
can be selectively bred to change the average value of the trait. [Unethical issues arise]
16.1c Several Processes Generate Genetic Variation:
Genetic variation, the raw material molded by micro evolutionary processes has 2
1. The production of new alleles the production of new alleles arise from small
scale mutations in DNA
2. The rearrangement or existing alleles the rearrangement of existing alleles can
result from larger scale changes in chromosome structure or number and from
several forms of genetic recombination.
16.1d Populations Often Contain Substantial Genetic Variation:
Gel Electrophoresis: a technique biologists began to use to separate 2 or more forms of a
given protein if they differ significantly in shape, mass, or net electrical charge.
[Underestimates genetic variation because it doesn’t detent different amino acid
substitutions if the proteins for which they code migrate at the same rate] Microevolution: genetic changes within populations Chapter 16
Some variations detected in the protein coding regions of DNA may not affect
phenotypes because they do not change the amino acid sequence for which the
16.2 Population Genetics
To predict how certain factors may influence genetic variation, population genetics:
1. Describe the genetic structure of a population
2. Create hypotheses in mathematical models, to describe how evolutionary
processes may change the genetic structure under specified conditions.
3. Test the predictions of these models to evaluate the ideas about evolution that are
embodied within them.
16.2a All Populations Have a Genetic Structure:
In diploid organisms, an individual’s genotype includes two alleles at very gene locus.
Gene pool: The sum of all alleles at all gene loci in all individuals.
Genotype frequencies: the percentages of individuals possessing each genotype [used to
describe the structure of a gene pool, scientists first identify the genotypes in a
Allele frequencies: the relative abundances of the different alleles [since scientists know
that each diploid organism has 2 alleles at the end of each gene locus, they can calculate
16.2b The HardyWeinberg Principle Is a Null Model That Defines How Evolution
Does Not Occur:
Null models: predict what would be seen if a particular factor had no effect. [Serve as
theoretical reference points. Used when the study is observational rather than
experimental and there is no control]
HardyWeinberg principle: Specifies the conditions under which a population of diploid
organisms achieves genetic equilibrium [genetic equilibrium is the point at which neither
allele frequencies nor genotype frequencies change in succeeding generations.
Genetic equilibrium is only possible if ALL of the following conditions are met:
1. No mutations are occurring
2. The population is closed to migration form other populations
3. The population is infinite in size
4. All genotypes in the population survive and reproduce equally well.
5. Individuals in the population mate randomly with respect to genotypes.
[If all of the above conditions 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]
16.3 The Agents of Microevolution:
a populations allele frequencies will change over time and not match the predictions of
the HardyWeinberg model is one or more of the conditions are violated. The processes
that foster this micro evolutionary change are listen in the table below Microevolution: genetic changes within populations Chapter 16
16.3a Mutations Create New Genetic Variations:
Mutation is a major source of heritable variation.
for most animals only mutations in the germ line are heritable, mutations in other
lineages have no effect on the next generation
in plans mutations may occur in the meristem cells, which eventually produce
lowers as well as nonreproductive structures mutations may be passed to the
next generation and ultimately influence the gene pool.
Neutral mutations: neither harmful nor helpful
Deleterious mutations: alter an individual’s structure, function, or behavior in harmful
Lethal mutations: cause the death of the organism carrying it [if a lethal allele is
dominant both homozygous and heterozygous carriers suffer from its effects; if
recessive it effects only homozygous individuals]
Advantageous mutation: confers some benefit on an individual that carries it.
16.3b Gene Flow Introduces Novel Genetic Variants into Populations:
Gene flow: organisms or their gametes sometimes move fro