I.Individuals Don’t Evolve—Populations Do
A.Examples of Variation in Populations
1.Populations evolve, not individuals.
2.A population is a group of individuals belonging to the
same species, occupying the same given area.
3.A population exhibits variation among the individual
members, but they also hold certain morphological,
physiological, and behavioral traits in common.
B.The "Gene Pool"
1.Individuals of the same population generally have the
same number and kinds of genes.
a.All of the genes in the entire population constitute
the gene pool.
b.Each gene exists in two or more slightly different
molecular forms called alleles, which offspring
inherit and express as phenotype.
2.Each particular mix of alleles depends on these five
factors:
a.Gene mutations create new alleles.
b.Crossing over and genetic recombination are
normal results of meiosis.
c.Independent assortment of chromosomes occurs in
meiosis.
d.Fertilization between genetically varied gametes
produces "new" combinations of genes.
e.Abnormal changes in chromosome structure or
number can occur.
C.Stability and Change in Allele Frequencies
1.Allele frequencies are a measure of the abundance of each
kind of allele in the entire population.
2.Evolution can be detected by a change in allele
frequencies from the genetic equilibrium as established by
the Hardy-Weinberg rule.
3.These five conditions are necessary for a stable
population:
a.No mutations are occurring.
b.The population is very, very large.
c.The population is isolated from other populations of
the same species.
d.All members survive, mate, and reproduce (no
selection).
e.Mating is random.
4.Because these five conditions are rarely fulfilled in natural
populations, any deviation from the reference point
established by the "rule" will indicate evolution.
5.Microevolution is the change in allele frequencies brought
about by mutation, genetic drift, gene flow, and natural
selection.
D.Mutations Revisited
1.Mutations are heritable changes in DNA that can alter
gene expression.
2.Mutations are random and the phenotypic outcome may
be neutral, beneficial, harmful, or even lethal to the
individual depending on other interactions.
a.A lethal mutation is an expression of a gene that
results in death.
b.Neutral mutations, whether or not they are
expressed in phenotype, have no effect on survival
and reproduction.
c.Beneficial mutations are those that bestow survival
advantages.
3.Mutations are the only source of new alleles–the genetic
foundation for biological diversity.
II. Natural Selection Revisited
A.Natural selection probably accounts for more changes in allele
frequencies than any other microevolutionary process.
B.The major points of Darwin’s theory of natural selection are:
1.Observation: All populations have the reproductive
capacity to increase in numbers over generations.
2.Observation: No population is able to increase
indefinitely, for its individuals will run out of food, living
space, and other resources.
3.Inference: Because more individuals are produced than
can survive to reproductive age, the members of a
population must compete for the available resources.
4.Observation: All the individuals have the same genes,
which represent a pool of heritable information.
5.Observation: Most genes occur in different molecular
forms (alleles), which give rise to differences in phenotypic
details.
6.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
.
7.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.
III.Directional Change in the Range of Variation
A.Directional Selection
1.Directional selection shifts allele frequencies in a consistent
direction.
2.Such shifts may be in response to environmental pressures
or occur as a new mutation appears and proved adaptive.
B.The Case of the Peppered Moths
1.At first the light-gray form of the peppered moth enjoyed
a survivorship advantage on the light-gray tree trunks, but
when industrial pollution darkened the tree trunks, the
numbers of dark-gray moths increased because they
escaped notice by bird predators.
2.Mark-release-recapture methods showed that more dark
moths were recaptured in the polluted (dark tree trunks)
area.
C.Pesticide Resistance
1.When insecticides are first applied, susceptible insects
(most of the population) die, but the few that have the
adaptation that affords survival will live and pass the
heritable character on; eventually most of the population
will become resistant.
2.In addition to pest species, pesticides kill natural enemies
thus allowing pests to multiply even more abundantly–pest
resurgence.
D.Antibiotic Resistance
1.Antibiotics are wonderful drugs that have proven very
effective in treating bacterial-induced diseases.
2.However, overuse of antibiotics has led to the selection of
resistant strains that are no longer susceptible to the drug.
IV.Selection Against or in Favor of Extreme Phenotypes
A.Stabilizing Selection
1.Stabilizing selection favors the most common phenotype in
the population.
2.It counters the effects of mutation, genetic drift, and gene
flow.
B.Disruptive Selection
1.Disruptive selection favors forms at the extremes of the
phenotypic range of variation and selects against the
intermediate forms.
2.Thomas Smith discovered African finches in which the bill
size was either large or small—no in between.
V.Special Types of Selection
A.Sexual Selection
1.Most species have distinctively male and female
phenotypes—sexual dimorphism.
2.Sexual selection is based on any trait that gives the
individual a competitive edge in mating and producing
offspring.
3.Usually it is the females that are the agents of selection
when they pick their mates.
B.Balancing Selection
1.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.
2.A population is in balanced polymorphism when
nonidentical alleles for a trait are being maintained at
frequencies grater than one percent.
C.Sickle-Cell Anemia—Lesser of Two Evils?
1.Humans that are homozygous for sickle-cell anemia
(HbS/HbS) develop the disease and die at an early age.
2.However, individuals with alleles for both normal
hemoglobin (HbA) and sickle-cell hemoglobin () have the
greatest chances of surviving malaria.
VI.Gene Flow
A.Genes move with individuals when they move out of
(emigration), or into (immigration), a population.
B.The physical flow (and resultant shuffling) tends to minimize
genetic variation between populations.
VII.Genetic Drift
A.Genetic drift is the random fluctuation in allele frequencies over
time, due to chance occurrences alone.
1.It is more significant in small populations; sampling error
helps explain the difference.
2.Genetic drift increases the chance of any given allele
becoming more or less prevalent when the number of
individuals in a population is small.
B.Bottlenecks and the Founder Effect
1.In bottlenecks, some stressful situation greatly reduces the
size of a population leaving a few (typical or atypical?)
individuals to reestablish the population.
2.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.
C.Genetic Drift and Inbred Populations
1.Inbreeding refers to nonrandom mating among closely
related individuals.
2.It tends to increase the homozygous condition, thus
leading to lower fitness and survival rates.