When individuals change, but not into another species, it is
called microevolution. This type of evolution is unchallenged. These groups go
by such names as race, breed, morph, variety, and strain. The differences in
these variations can be imperceptible or dramatic.
- Individuals Don’t Evolve - Populations Do
- Examples of Variation in Populations
- Populations evolve not individuals.
- A population is a group of
individuals belonging to the same species, occupying the same given area.
- A population exhibits variation among the individual members, but they
also hold certain morphological, physiological, and behavioral traits in
common. (theoretical and subjective)
- The "Gene Pool"
- Individuals of the same population [generally] have the same number
and kinds of genes. All of the genes in the entire population constitute
the gene pool.
- Each particular mix of alleles
depends on these four factors:
- Mutations create new genetic information or organization
- Crossing over and genetic recombination are normal
results of meiosis.
- Independent assortment of chromosomes occurs in meiosis.
- Fertilization between genetically varied gametes produces
"new" combinations of genes.
- Stability and Change in Allele Frequencies
- Allele frequencies are a measure of the abundance of each
kind of allele in the entire population.
- Evolution can be detected by a change in allele frequencies from the
genetic equilibrium as established by the Hardy-Weinberg rule.
- These five conditions are necessary for a stable population:
- No mutations are occurring.
- The population is very, very large.
- The population is isolated from other populations of the same
- All members survive, mate, and reproduce (no selection).
- Mating is random.
- Because these five conditions are rarely fulfilled in natural
populations, any deviation from the reference point established by the
"rule" will indicate evolution.
- Microevolution is the change in allele frequencies brought
- Genetic drift
- Gene flow (migration)
- Natural selection
Natural Selection Revisited
- Mutations are heritable changes in DNA that can alter gene expression.
- Mutations are random and the phenotypic outcome may be neutral,
beneficial, harmful, or even lethal to the individual depending on other
- A lethal mutation is an expression
of a gene that results in death.
- Neutral mutations, whether or not they are expressed in
phenotype, have no effect on survival and reproduction.
- Beneficial mutations are those that bestow survival
- Mutations are the only source of new alleles - the genetic foundation
for biological diversity.
The many types of selection.
- Natural selection probably accounts for more changes in allele
frequencies than any other microevolutionary process.
- The major points of Darwin’s theory of natural selection are:
- Observation: All populations have the reproductive capacity to
increase in numbers over generations.
- Observation: No population is able to increase indefinitely, for
its individuals will run out of food, living space, and other resources.
- Inference: Because more individuals are produced than can survive
to reproductive age, the members of a population must compete for
the available resources.
- Observation: All the individuals have the same genes, which
represent a pool of heritable information.
- Observation: Most genes occur in different molecular forms
(alleles), which give rise to differences in phenotypic details.
- Inference: Because adaptive traits promote survival and
reproduction, they must increase in frequency over the generations, and
less adaptive traits must decrease in frequency or disappear.
- Conclusion A population can evolve by natural selection, that is,
the traits characterizing the population can change over time when its
individuals differ in one or more heritable traits that are responsible
for differences in survival and reproduction.
- Directional Selection shifts allele frequencies in a
- The Case of the Peppered Moths
- Pesticide Resistance
- Antibiotic Resistance
- Stabilizing Selection
- Stabilizing selection favors the most common phenotype in the
- It counters the effects of mutation, genetic drift, and gene flow.
Disruptive selection favors forms at the extremes of the phenotypic
range of variation and selects against the intermediate forms.
Thomas Smith discovered African finches in which the bill size was
either large or small—no in between.
Most species have distinctively male and female phenotypes—sexual
Sexual selection is based on any trait that gives the individual a
competitive edge in mating and producing offspring.
Usually it is the females that are the agents of selection when they
pick their mates (…limited by cryptic coloration)
This is a variation on the stabilizing theme in which two or more
forms of a trait are maintained in fairly stable proportions depending on
survival value in the environment. (HbS vs. HbA)
- Genes move with individuals when they move out of (emigration), or into
(immigration), a population.
- The movement of human populations throughout history is the ultimate
puzzle of gene flow.
- Refer to Dr. Cunningham's paper and the Scientific American Article.
VII Genetic Drift
- Genetic drift is the random fluctuation in allele frequencies over time,
due to chance occurrences alone (sampling error).
- Bottlenecks and the Founder Effect
- In bottlenecks, some stressful
situation greatly reduces the size of a population leaving a few
individuals to reestablish the population. (cheetahs)
- In the founder effect, a few
individuals (carrying genes that may/may not be typical of the whole
population) leave the original population to establish a new one.
- Genetic Drift and Inbred Populations
- Inbreeding refers to nonrandom mating among closely related
- It tends to increase the homozygous condition, thus leading to lower
fitness and survival rates.
(Industrial melanism in