Download Chapter 17 Microevoltion

Chapter 17 Microevolution
Designer Pets
• The many varieties or breeds have been
produced through artificial selection by
humans
• Some breeds would have been selected
against by nature had it not been for
human protection
The Great Chain of Being
• Many of the ancient Greeks, including
Aristotle, attempted to explain the natural
world by making direct observable
• By the 14th 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
Continue…
• The chain extended from lowest level
forms to spiritual beings
• Each being (species) had its fixed place in
the divine order—unchanged and
unchanging since creation
Biogeography
• Is the geographic distribution of life forms
on Earth.
Questions from Biogeography
• During the global voyages of the 16th
century scientist asked the following
questions:
– Where do all these species “fit” into the great
“Chain”?
– Why were certain species found in only some
parts of the world but not others?
– How did so many species get from the center
of creation to islands and isolated places?
Comparative Morphology
• Studies of the comparative morphology of
seemingly unrelated animals led to
questions of why certain structures should
be so similar
• Example: Pelvic Girdle in snakes and tail
bones in humans
Questions About Fossils
• Studies of sedimentary beds revealed that
deposits had been laid down slowly, one
above the other
– Fossils: layers held recognizable remains or
impression of an organisms
– The arrangement of the layers suggested that
different organisms had lived at different times
• De Buffon’s explanation: perhaps species
originated in more than one place, and
perhaps species became modified over
time -- EVOLUTION
New Evidence into Old Beliefs
• Georges Cuvier believed in an original
creation of all species
• Cuvier suggested that the abrupt changes
in the fossil record in different rock strata
reflected the concept of catastrophism
– After each catastrophe, fewer species
remained.
– The survivors were not new species, it was
just that their ancestors’ fossils had not been
found
Lamarck
• Theory of acquired characteristics is the
idea that simple forms had changed into
more complex ones by a built-in drive for
perfection up the Chain of Being.
– Example: Giraffe stretching its neck to reach
higher branches would result in offspring with
longer necks
• Experiments could not uphold his thoughts
on acquired traits
Larmarck’s and Darwin
&Wallace’s View
Charles Darwin
• Father of Evolution
Voyage on the Beagle
• Darwin traveled on the Beagle for five
years as naturalist.
– He collected and studied variety of plants and
animals
– While on the trip he read Lyell’s book
Principles of Geology, which proposed the
theory of uniformity – the notion of gradual,
lengthy molding of the earth’s geologic
structure
– Thus, the earth was millions of years old and
not thousands of years – evidence of
Evolution
Darwin’s theory takes form
• Darwin returned after five years at sea and
began pondering the “species problem”
what could explain the remarkable
diversity among organisms?
• 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?
Variation in Traits
• Thomas Malthus had suggested that as a
population outgrows its resources, its
members must compete for what is
available.
• 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.
Continue…
• 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 ha descended from common
ancestor
Darwin’s Finches
• 1) Finches on the Galápagos Islands resembled a
mainland finch but there were more types.
2) Galápagos finch species varied by nesting site,
beak size, and eating habits.
3) One unusual finch used a twig or thorn to pry out
insects, a job normally done by (missing)
woodpeckers (Darwin never witnessed this finch
behavior).
4) The variation in finches posed questions to Darwin:
did they descend from one mainland ancestor or did
islands allow isolated populations to evolve
independently, and could present-day species have
resulted from changes occurring in each isolated
population?
Example of Darwin’s Finches
Continue…
• Darwin reasoned that a population is
evolving when it heritable traits are
changing through successive generations.
• 1858, Darwin received a paper from Alfred
Wallace, who had developed much the
same theory on natural selection but
independently of Darwin
• Darwin and Wallace presented a joint
paper but Darwin published (alone) his
ideas in book form in 1859
Individuals Don’t Evolve–
Populations Do
• 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
behavior traits in common.
The Gene Pool
• Individuals of the same populations
generally have the same number and
kinds of genes
– All of the genes in the entire population
constitute the gene pool
– Each gene exists in two or more slightly
different molecular forms called alleles, which
offspring inherit and express as phenotype
• Each particular mix of alleles depend on
these five factors:
– Gene mutations create new alleles
– Crossing over and genetic recombination are
normal results of meiosis
– Independent assortment of chromosomes
occur in meiosis
– Fertilization between genetically varied
gametes produces “new” combinations of
genes
– Abnormal changes in chromosomes or number
can occur.
Stability and Change in Allele
Frequencies
• Allele frequencies are a measure of the
abundance of each kind of allele in the
entire in the entire population
• Evolution can be detected by a change in
allele frequencies from the genetic
equilibrium as established by the HardyWeinberg rule
Hardy Weinberg: five conditions necessary
for stable population
• No mutations occurring
• Population is very very large
• Population is isolated from other
populations of the same species
• All members survive, mate, and reproduce
(no selection)
• Mating is random
Continue…
• Because of these five conditions are rarely
fulfilled in natural populations, any
derivation from the reference point
established by the “rule” will indicate
evolution
• Microevolution is the change in allele
frequencies brought about by mutation,
genetic drift, gene flow, and natural
selection
Mutation 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 interactions
• Mutations are the only source of new
alleles – the genetic foundation for
biological diversity
Three types of mutations
• Lethal Mutations: is expression of a gene
that results in death
• Neutral Mutations: where or not they are
expressed in phenotype, have no effect on
survival and reproduction
• Beneficial Mutations: are those that
bestow survival advantages
Section 17-5
• Read Hardy-Weinberg on your own
Natural Selection Revisited
• Natural Selection probably accounts for
more changes in allele frequencies than
any other microevolutionary process
Major Points to Natural Selection
• Observation: All populations have the
reproductive capacity to increase in
numbers over generation
• 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
• Inferences: Because adaptive traits
promote survival and reproduction, they
must increase frequency over the
generation and less adaptive traits must
decrease in frequency or disappear
• 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
Natural Selection: Adaptation
• Adaptation: Evolutionary term that
describe an organisms better suited to
their environment.
• Organisms that are adapted to their
environment will have the ability to survive
and reproduce.
• Extinction occurs when an organisms is
not adapted to their environment and will
not be able to reproduce
Organisms have Variations
• Variations that make adaptation possible
are those that are passed on from
generation to generation.
• Darwin could not state the cause of
variations because genetics was not yet
established.
Organisms Struggle to Exist
• Malthus proposed that human populations
outgrow food supply and death and famine
were inevitable.
• Darwin applied this to all organisms;
resources were not sufficient for all
members to survive.
• Therefore, there is a constant struggle for
existence; only certain members survive
and reproduce.
Organisms Differ in Fitness
• 1. Organisms whose traits enable them to
reproduce to a greater degree have a greater
fitness.
2. Darwin noted that humans carry out artificial
selection.
a. Early humans likely selected wolf variants;
produced the varieties of domestic dogs.
b. Many crop plant varieties can be traced to a
single ancestor.
d. Evolution by artificial or natural selection
occurs when more fit organisms reproduce and
leave more offspring
On the Origin of Species
• Darwin’s publish book: On the Origin of
Species
• It waited 20 years to publish his book
• He used this time to complete experiments
to prove his theory of Natural Selection
What is Directional Selection?
• Allele Frequencies in a consistent direction
• Shifts may be in response to
environmental pressures or occur as a
new mutation appears and proved
adaptive
EXAMPLES
• Peppered Moths:
• Pesticide Resistance
• Antibiotic Resistance
Stabilizing Selection
• Favors the most common phenotype in the
population
• It counters the effects of mutations,
genetic drift, and gene flow
Disruptive Selection
• Favors forms at the extremes of the
phenotypic range of variation and selects
against the intermediate forms
Sexual Selection
• Most species have distinctively male and
female phenotypes – sexual dimorphisms
• Sexual selection is based on any trait that
gives the individual a competitive edge in
mating and producing offspring.
• Usually it is the female that are the agents
of selection when they pick their mates
Maintaining Two or More Alleles
• 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
• Population is in balanced polymorphism
when nonidentical alleles for a trait are
being maintained at frequencies greater
than 1%
Sickle Cell Anemia
• Humans that are homozygous for sicklecell anemia develop the disease and die at
early age
• BUT
– Individuals with alleles for both normal
hemoglobin and sickle-cell hemoglobin
(heterozygous) have the greatest chances of
surviving malaria
Gene Flow
• Genes move with individuals when they
move out of (emigration), or into
(immigration), a population
• The physical flow (a result of shuffling)
tends to minimize genetic variation
between population
Genetic Drift
• Is the random fluctuation in allele
frequencies over time, due to chance
occurrences alone
• Affects: Small Populations
Bottlenecks and Founder Effect
• Bottleneck: Some stressful situations
greatly reduces the size of a population
leaving a few individuals to reestablish the
population
• Founder Effect: A few individuals leave the
original population to establish a new one
Genetic Drift and Inbred
Populations
• Inbreeding refers to nonrandom mating
among closely related individuals
• It tends to increase the homozygous
condition, thus leading to lower fitness and
survival rates