Lecture Outline Ch 23 Natural Selection
I.
II.
Charles Darwin’s Theory of Evolution
A.
Darwin publishes his observations and conclusion in On the Origin of Species in 1859
and challenges the theory of special creation.
B.
Darwin’s research produced evidence that species are not independent and do
change over time. (Box 23.1, Fig. 23.1)
C.
Researchers before Darwin noticed these same patterns in nature that he observed
(Box 23.2), but Darwin recognized that these patterns can be explained by the
process of descent with modification (natural selection).
D.
Researchers since Darwin have used a variety of approaches to gather evidence in
support of Darwin’s theories.
The Pattern of Evolution: Have Species Changed through Time?
A.
Evidence That Species Have Changed through Time
1. The fossil record was initially organized based on the relative age of the fossils.
a. A fossil is any trace (bones, branches, shells, tracks, etc.) of an organism
that lived in the past. (Fig. 23.2)
b. Fossils can be dated based on the age of the sedimentary rock in which
they are found.
2. As the fossil record has expanded, more evidence for evolution has emerged.
a. Extinction
(1) Many fossils do not resemble any known living plants and animals.
(2) As more examples were unearthed, biologists concluded that these
species had died out (i.e., become extinct).
(3) Darwin interpreted fossils as evidence that species changed over time.
b. Transitional forms
(1) Law of succession: Fossils found in a certain geographical region
frequently closely resemble the species living in that region. (Fig. 23.3)
(2) Darwin interpreted the law of succession as evidence that species
change over time, because extinct species seemed to be related to
existing species.
(3) The discovery of transitional form fossils that have traits intermediate
between those of older and younger species has added support to
Darwin’s interpretation. (Fig. 23.3b)
c. Environmental change
(1) Fossils of whales have been found high in the Himalayans, and
fossilized shells from extinct clams were found in the European Alps
and the desert of the Grand Canyon.
(2) Biologists infer that since Earth’s topography and environment have
changed over time, so have organisms.
3. Vestigial organs in living organisms support the idea that species change over
time.
a.
b.
B.
Darwin first described a variety of traits in animals that have either
reduced or no function. (Fig. 23.4)
Biologists interpret the existence of these traits as evidence that organisms
change over time.
Evidence That Species Are Related
1. Geographic proximity of similar species
a. Similar, but distinct, species are often found living close together in the
same geographic area. (Fig. 23.5a)
b. These similar species are part of a phylogeny (a family tree), and their
relationships can be diagrammed on a phylogenetic tree (a branching
diagram that indicates genealogy). (Fig. 23.5b)
2. Homologies
a. Structural—similarities in body parts of different organisms. (Fig. 23.6)
b. Developmental—similarities in the overall morphology of embryos and in
the fate of embryonic tissues. (Fig. 23.7)
c. Genetic—similarities in genes and protein sequences in different
organisms. (Fig. 23.8)
d. Distinguishing homology from analogy
(1) Analogous structures appear similar, but they are different in
structure, organization, or function. (Fig. 23.9a)
(2) Homologous structures appear similar, and they are similar in
structure, organization, and function. (Fig. 23.9b)
III. How Natural Selection Works
A.
Four postulates of natural selection, the outcome of which is evolution:
1. Individual organisms in a population vary in traits (size, shape, color, etc.).
2. Some variations in the population are heritable: can be passed on to offspring.
3. More offspring are produced than can survive; that is, only a subset will
survive to reproduce.
4. Individuals with traits conferring an advantage are more apt to survive to
reproduce; if favorable traits are heritable, the population evolves.
B.
Fitness
1. The ability of an individual to survive and reproduce = “Darwinian fitness.”
2. Fitness is estimated by measuring the number of offspring produced by one
individual vs. another in the population.
C.
Adaptation—a heritable trait that increases the fitness of the individual with the
trait relative to others that lack the trait.
IV. Evolution in Action: Recent Research on Natural Selection
A.
How did Mycobacterium tuberculosis (Fig. 23.10) become resistant to antibiotics?
1. Historical background
a. Prior to antibiotics, TB caused 25% of all deaths in New York City and 33%
of all deaths in Paris in the 1800s.
b. 1950 to 1990—Antibiotics successful at treating TB in developed nations.
c.
2.
3.
B.
Late 1980s—Resurgence in TB in developed countries due to strains
carrying antibiotic resistance. (Fig. 23.11)
Case study
a. HIV-positive patient admitted to hospital in Baltimore with active TB.
b. Lung cultures taken at week 1—TB bacteria are present.
c. Patient is given rifampin antibiotic for 40 weeks, released when lung
cultures show no bacteria.
d. Week 48—Patient is readmitted with TB symptoms; lung cultures again
show TB bacteria.
e. Patient is given further antibiotic treatment, but dies in 10 days.
Evidence that evolution of resistance in TB bacteria occurred in the patient
a. TB bacteria from lung cultures at week 1 are sensitive to rifampin, but TB
bacteria cultured at week 48 are resistant to rifampin.
b. DNA of TB bacteria from week 1 and week 48 are sequenced and
compared; they differ by one nucleotide in the gene coding for RNA
polymerase enzyme.
(1) Rifampin binds to the RNA polymerase enzyme of week-1 TB bacteria
and interferes with enzyme activity; prevents growth of the bacteria.
(2) Rifampin does not bind to the mutant form of RNA polymerase in the
bacteria from week 48, due to a single amino acid change in the
enzyme; thus the enzyme functions and the bacteria grow.
c. The development of antibiotic resistance in the TB patient exhibits the four
postulates of natural selection.
(1) The two populations of TB bacteria show variability in their
characteristics, and the variation is heritable—the difference is in a
gene (natural selection postulates 1 and 2).
(2) Nonrandom selection occurred—that is, only bacteria developing
resistance to rifampin survived to reproduce (natural selection
postulates 3 and 4).
Why Are Beak Size, Beak Shape, and Body Size Changing in Galapagos Finches?
1. Peter and Rosemary Grant approached this question without knowing the
alleles responsible for variation in these traits, nor the molecular mechanism for
adaptation.
2. The medium ground finch population of the island Daphne Major varies in
beak shape and body size, and these traits are heritable. (Fig. 23.12a)
3. The drought of 1977 provided a unique opportunity to study natural selection
in this finch population.
a. 84% of the medium finch population died of starvation during this year,
because most plants were not able to produce seeds during the drought.
b. The Grants observed some notable differences between surviving birds
and the dead birds.
(1) The survivors had much deeper beaks.
(2) The researchers hypothesized that the deeper and larger beaks
allowed those individuals to eat the only remaining seeds that were
tough and difficult to crack.
c.
d.
C.
V.
(3) In the year following the drought, the average beak size of the next
generation of medium finches in that population was considerably
larger than that of the population before the drought. (Fig. 23.12b)
Over the last 30 years, this population of finches has experienced changes
in beak shape and body size. (Fig. 23.13)
These observations support the hypothesis that natural selection can lead
to a change in a population over time.
Can Natural Selection Be Studied Experimentally?
1. Candace Galen observed alpine skypilot plants above and below timberline.
a. Plants growing in the treeless tundra above timberline have larger flowers
with longer stalks, are sweet smelling, and are pollinated by bumblebees.
b. Plants growing in the stunted forest at or below timberline have smaller
flowers with shorter stalks, are “skunky” smelling, and are primarily
pollinated by flies.
c. The observations demonstrate natural selection postulate 1: Variation
exists in the population of alpine skypilot plants.
2. Hypothesis: Natural selection produced the differences in the alpine skypilots
due to differences in pollinators; bees prefer large, sweet-smelling flowers.
a. Prediction 1: Individuals with larger flowers attract more bees.
b. Prediction 2: Individuals that attract more pollinators have higher fitness.
3. Experimental protocol (Fig. 23.14)
a. Allow bees to pollinate skypilot plants in the field, collect the seeds, and
plant them at random locations in the field.
b. Control: Hand-pollinate wild skypilot plants, collect the seeds, and plant
them at random locations in the field.
c. Observation: Bee-pollinated plants produced offspring with larger flowers
than those from hand-pollinated plants.
4. Conclusion: Bees can exert natural selection on the population of alpine
skypilot plants.
The Nature of Natural Selection and Adaptation
A.
Selection Acts on Individuals, but Evolutionary Change Occurs in Populations
1. Alpine skypilots did not get bigger and sweeter because bumblebees landed on
them, but because the individuals with big, sweet flowers were pollinated more
frequently and were more reproductively successful.
2. Individuals may change during their lifetime (acclimation), but only because
they inherited the ability to adapt (natural selection).
B.
Evolution Is Not Progressive
1. Evolution does not always result in more complex organisms.
2. Evolution simply results in organisms adapted to live in different
environments; no organism is “higher” than another. (Fig. 23.15)
C.
Not All Traits Are Adaptive
1. Many of the traits that any individual organism has do not necessarily increase
its fitness, and are therefore not adaptive.
a. Vestigial organs serve little to no physiological purpose.
b.
2.
Silent mutations create new alleles that are neutral and do not change the
fitness of the individual.
Even traits that are functional do not have unlimited adaptive potential.
a. Genetic constraints: Sometimes nonoptimal traits are propagated simply
because they are genetically linked with an optimal trait.
b. Historical constraints: Natural selection can act only on traits that exist in
the ancestral population.