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Lecture Outline
Impacts, Issues: Rise of the Super Rats
A. Rats are one of the most notorious pests of all time.
1. The rodenticide warfarin was very effective when it was first introduced in the
1950s.
2. Within a few years, rats developed resistance—the chemical would no longer
kill.
B. The resistant rats happened to inherit a gene, which made the chemical ineffective.
1. The survivors passed on the gene to their offspring; soon resistant rats were the
normal population.
2. This is an example of evolution.
16.1
Early Beliefs, Confounding Discoveries
A. Questions from Biogeography
1. By the fourteenth century, the ancient view of gradual levels of organization
from lifeless matter to the most complex organisms had been formalized into the
great Chain of Being.
a. The chain extended from lowest forms to spiritual beings.
b. Each being (species) had its fixed place in the divine order—unchanged and
unchanging since creation.
2. When global voyages of the sixteenth century revealed unusual species not
known in Europe, the students of biogeography began to question, “Where do all
these species ‘fit’ in the great Chain?”
3. Furthermore, if all species had been created at the same time and place, “why
were certain species found in only some parts of the world but not others, and
how did so many species get from the center of creation to islands and isolated
places?”
B. Questions from Comparative Morphology
1. Studies of the comparative morphology of seemingly unrelated animals led to
questions of why certain structures should be so similar, (for example: pelvic
girdle bones in snakes).
2. One explanation: Some body parts were so perfect at the time of creation there
was no need for any variation. But what about bones still present but without
function (ankle bones in whales, tail bones in humans)?
C. Questions About Fossils
1. Studies of sedimentary beds revealed that deposits had been laid down slowly,
one above the other.
a. The layers held recognizable remains or impressions of organisms—fossils.
b. The arrangement of the layers suggested that different organisms had lived
at different times.
2. Perhaps species had originated in more than one place, and perhaps species had
become modified over time—evolution!
16.2
A Flurry of New Theories
A. Squeezing New Evidence into Old Beliefs
1. Georges Cuvier believed in an original creation of all species.
2. Cuvier further suggested that the abrupt changes in the fossil record in different
rock strata reflected the concept of catastrophism.
a. After each catastrophe, fewer species remained.
Copyright © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
is a trademark used herein under license.
b. The survivors were not new species; it was just that their ancestors’ fossils
had not been found.
3. Lamarck formulated a theory of inheritance of acquired characteristics—the idea
that simple forms had changed into more complex ones by a built-in drive for
perfection up the Chain of Being; for instance, a giraffe stretching its neck to
reach higher branches would result in offspring with longer necks.
B. Voyage of the Beagle
1. As a child (early 1800s), Charles Darwin was curious about nature, but in college,
he first pursued pre-medicine and finally received a degree in theology.
2. Botanist John Henslow arranged for Darwin (at age 22) to sail around the world
as a ship’s naturalist.
a. Throughout the trip, Darwin studied and collected a variety of plants and
animals.
b. He was also reading Lyell’s Principles of Geology, which proposed a theory of
uniformity—the notion of a gradual, lengthy molding of the earth’s geologic
structure.
c. Thus, the earth was not thousands, but possibly millions of years old—
enough time for evolution.
16.3
Darwin’s Theory Takes Form
A. Old Bones and Armadillos
1. Darwin returned after five years at sea and began pondering the “species
problem”: What could explain the remarkable diversity among organisms?
2. In Argentina, Darwin had observed extinct glyptodonts that bore suspicious
resemblance to living armadillos; Darwin wondered if the present species had
evolved from the extinct one.
B. A Key Insight—Variation in Traits
1. Thomas Malthus had suggested that as a population outgrows its resources, its
members must compete for what is available; some will not make it.
2. Darwin felt that if some normally variant members of a population bore traits
that increased their survival, then nature would select those same individuals to
survive, reproduce, and possibly change future populations’ traits.
a. On the Galapagos Islands, the dozen or so species of finches all varied from
one another to some extent but resembled the mainland finches to some
degree also; perhaps they had descended from common ancestors.
b. Darwin reasoned that a population is evolving when its heritable traits are
changing through successive generations.
Copyright © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
is a trademark used herein under license.
C.
Natural Selection Defined
1. The major points of Darwin’s theory of natural selection as published in On the
Origin of Species in 1859 are:
a. Observation: All populations have the reproductive capacity to increase in
numbers over generations.
b. Observation: No population is able to increase indefinitely, for its individuals
will run out of food, living space, and other resources.
c. Inference: Sooner or later, individuals will end up competing for dwindling
resources.
d. Observation: All the individuals have the same genes, which represent a pool
of heritable information.
e. Observation: Most genes occur in different molecular forms (alleles), which
give rise to differences in phenotypic details.
f. Inferences: 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.
g. Conclusions: 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.
2. The theory has been debated, but more evidence is gathered every day.
16.4
The Nature of Adaptation
A. Adaptation is defined variously.
1. Short-term adaptations, such as an individual plant’s stunted growth on a windy
plain, last only as long as the individual does.
2. Long-term adaptations have some heritable aspect that improves the odds for
surviving and reproducing.
B. Salt-Tolerant Tomatoes
1. Tomatoes originated in South American soils with a high salt content; they were
adapted to these conditions.
2. Commercial tomatoes in today’s markets will not tolerate salt.
3. However, gene transfers can yield a salt-tolerant tomato that will grow in
irrigated plots.
C. No Polar Bears in the Desert
1. You can safely assume that a polar bear is well adapted to the Arctic
environment and would not do well in the desert.
2. Detailed knowledge of its anatomy and physiology might make you view any
animal with respect to its abilities to survive.
D. Adaptation to What?
1. The environment in which a trait evolved may be very different from the one
prevailing now.
2. For example, llamas are well adapted to high altitudes because of their
hemoglobin; however, their close relative the camel has the same capability and
lives in low altitude deserts.
16.5
Individuals Don’t Evolve, Populations Do
A. Variation in Populations
1. Populations evolve, not individuals.
Copyright © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
is a trademark used herein under license.
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.
a. Qualitative variation is called polymorphism.
b. Quantitative differences result in a continuous range of differences.
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 a phenotype.
2. Each particular mix of alleles depends on five factors.
a. Gene mutations create new alleles.
b. Crossing over at meiosis I results in novel recombinations of alleles in
chromosomes.
c. Independent assortment of chromosomes in meiosis mixes maternal and
paternal chromosomes in gametes.
d. Fertilization between genetically varied parents produces “new”
combinations of genes.
e. Changes in chromosome structure or number leads to loss, duplication, or
repositioning of genes.
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.
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,
natural selection, gene flow, and genetic drift.
16.6
Mutations Revisited
A. Mutations are heritable changes in DNA that can alter gene expression.
1. Each gene has a mutation rate, the probability of its mutating during or between
DNA replications.
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.
B. Mutations are the only source of new alleles—the genetic foundation for biological
diversity.
Copyright © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
is a trademark used herein under license.
16.7
Directional Selection
A. Directional selection shifts allele frequencies in a consistent direction, which may be
in response to environmental pressures or occur as a new mutation appears and is
proved adaptive.
B. 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.
C. 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.
D. Coat Color in Desert Mice
1. The larger population of rock pocket mice in the Arizona desert survives well
because of its genetically-determined lighter fur color that blends in with the
granite; predator birds cannot spot them easily.
2. A smaller population of mice has darker coats, which allow them to blend in
with the dark basalt (from lava) and avoid being seen by predators.
16.8
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.
3. Human birth weight that averages around 7 pounds is favored.
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.
16.9
Maintaining Variation in a Population
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. 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 sicklecell hemoglobin (HbS) have the greatest chances of surviving malaria.
Copyright © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
is a trademark used herein under license.
16.10
16.11
Genetic Drift—The Chance Changes
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.
3. Fixation means that one kind of allele remains at a specified locus in a
population.
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.
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.
Copyright © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
is a trademark used herein under license.