Download Darwin and Natural Selection

Evolution
Unit 12
p. 6-14, 417-428, 499-515
Darwin and Natural Selection
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Beagle Expedition: found
unique species that looked
similar to species from
other parts of the world.
 Noticed changes in form
among members of the
same and closely related
species.
Voyage of the HMS Beagle
The Origin of the Species
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On The Origin of Species
by Means of Natural
Selection Nov. 1859
Charles Darwin
He avoided the term
"evolution”
“light will be thrown on
the origin of man and his
history".
"endless forms most
beautiful and most
wonderful have been,
and are being, evolved."
Natural Selection
 Natural
selection: mechanism for change
in populations that occurs when organisms
with favorable variations for a particular
environment survive, reproduce, and pass
these variations on to the next generation
(those with less favorable traits are less
likely to survive).
 Referred to by Darwin as “decent with
modification”
Evolution of Humans

The Descent of Man,
and Selection in
Relation to Sex
Charles Darwin 1871
 Darwin outlined
sexual selection to
explain the evolution
of humans
Darwin’s 4 Facts
 Exponential
Growth of Populations
 Factors will prevent Exponential
Growth(Steady State)
 Variability among species
 Traits are inheritable (NOT AQUIRED)
Darwin’s 2 Inferences
 Competition
for resources
 Individuals most fit for their environment
will survive.
4 Tenets of Natural
Selection
 All
populations have genetic variation.
 The environment presents challenges to
successful reproduction.
 Individuals tend to produce more offspring
than the environment can support.
 Individuals that are better fit their
environment tend to live longer, reproduce
more, and spread their genes throughout the
population.
Natural Selection
 Natural
Selection is accepted as the driving
force for evolution.
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What is a the difference between a scientific
hypothesis, law, and theory?
Hypothesis, Theory, Law

Hypothesis- explanations that are testable through
experimentation or observation.
 Theory- comprehensive explanation of facts, laws, and
reasoning that is supported by many observations and
multiple lines of evidence. A working model of a
natural phenomena. Theories are accepted by the
scientific community.
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ex. Modern Atomic Theory, Cell Theory, Theory of Evolution
Law- short, usually mathematical, explanation of a
scientific relationship.

ex. Law of Conservation of Matter/Energy, Gas Laws, Law of
Gravity, Thermodynamics, Newton’s, Kepler’s
Evolution Terms
 Species-
group of organism capable of
breeding to produce fertile offspring.
 Population- a particular species in a given area
that is interbreeding.
 Changes
happen to individuals, populations of
species evolve.
Evolution Terms
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Reproductive Fitness- Advantage of a particular
genotype to survive and reproduce at a greater rate.
Fitness is specific to a particular environment.
(Consider both the biotic and abiotic environment).
As the environment changes, so do the fitness values
of the genotypes.
Fitness is a property of a genotype, not of an
individual or a population.
Individuals with the same genotype share the same
fitness within the same environment.
Fitness is measured over one generation or more.
The concepts of fitness and adaptation are relevant
ONLY in a particular ecological context. There is no
such thing as fitness in an absolute sense.
Evolution Terms
 Adaptation-
a genetically determined
characteristic that improves an organism’s
ability to survive and reproduce in a
particular environment.
 Adapt- the evolutionary process by which
organisms become better suited to their
environments
Evolution Terms

Structural Adaptations:
 Can change the size and/or shape of a body part
 For example: thorns, wings, mimicry (copy the
appearance of another species), camouflage (blend
in with environment)
 Change within a population over time (anywhere
from 100 years to millions of years) depending on
type of adaptation, rate of reproduction and
environmental factors
 Physiological Adaptations:
 Help populations overcome chemicals they
encounter (antibiotics, pesticides, herbicides, etc)
 Develop rapidly (example: bacterial resistance to
penicillin)
Evolution Terms
 Genetic
variation- small differences among
individuals within a population
 Genetic Variation must be present for
evolution to occur.
 Polymorphic- the presence of two or more
alleles (variations) of a gene in a gene
pool.
Sources of Genetic Variation
 Mutation-
change in DNA sequence that can
slowly introduce new alleles to a population
(can be harmful, beneficial or have no
significance)
 Meiosis
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Random alignment of chromosomes.
Crossing Over- exchange of homologous
chromosome segments.
 Sexual
Reproduction (fertilization)- combination
of alleles of two individuals
 Immigration- incoming alleles to a gene pool
Mutation = a heritable change in the
nucleotide sequence of DNA, resulting in an
alteration in the products coded for by the
gene. (about 0.5% of DNA bases are variable)
Consider how the amount of genetic
divergence (change) forms a
continuum:
Microevolution
small changes
Macroevolution
large changes
Microevolution = adaptation
Macroevolution = speciation
Microevolution vs.
Macroevolution
 Microevolution-
changes within a species
that can occur over dozens or hundreds of
generations.
 Macroevolution- evolution over long
periods of time that involve origin of new
species.
Microevolution
 Population
genetics- change in the genetic
composition of populations.
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Uses Mendel’s Law of inheritance,
Biochemical analysis of genes and proteins,
and Mathematical models to understand
microevolution.
Gene Pool- all the genes of a population of
organisms. (refers to a single population, not
the entire species)
Mechanisms for Evolutionary
Change
 Natural
Selection
 Artificial Selection
 Gene Flow
 Genetic Drift
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Founder Effect
Population (Genetic) Bottleneck
Microevolution in Large
Populations
 Natural
selection is the main factor that
changes the gene pool of large populations.
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Peppered Moth (p.423)
Malaria/ Sickle Cell Anemia (p.423-424)
• http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.
html
 Gene
Flow
 Mutation
Gene flow = introduction or loss of new
alleles into the population through
immigration or emigration.
Microevolution in Small
Populations
 Genetic
Drift
Founder Effect
 Genetic Bottleneck (Population
Bottleneck)

Inbreeding
Inbreeding Depression
Genetic drift = random shifts in allele
frequencies in small populations
Founder’s Effect
 Small
number of individuals of a large
populations migrate and start a new population.
 Gene pools of new population can change
greatly from the original population if the new
population is small.
 Causes a loss of genetic variation.
 Can cause inbreeding- the rise of homozygosity
within a population.
Genetic Bottleneck
 The
number of organisms in a population is
drastically reduced (hunting, disease, loss of
habitat, etc.)
 Large populations can become inbred
 Inbreeding can lead to the increase of the
frequency of harmful recessive alleles in the
gene pool.
 Inbreeding Depression- increased
homozygosity of a population leads to reduction
os fertility and survival.
Speciation = Origin of a new
Species
 Species
= group of organisms that look
alike and have the ability to interbreed and
produce fertile offspring in nature
 Speciation occurs when
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Interbreeding is prevented
Production of fertile offspring is prevented
Speciation
 Species
organisms that can reproduce
successfully.
 Genetic Isolation causes speciationpopulations gene pools become so different,
they can no longer interbreed.
 Prezygotic Isolation mechanisms keep
members of different populations from fertilizing
each other.
 Postzygotic isolation mechanisms keep
populations from producing viable or fertile
offspring.
Prezygotic Isolation
 Geographic
Isolation- barrier (mountains,
canyons, oceanns, etc.) keep members from
meeting each other.
 Ecological Isolation- populations do not
crossbreed because their habitats differ
adaptive radiation.
 Behavioral/ Reproductive isolation- differences
in courtship and/or mating season
 Chemical isolation: sperm may not be attracted
to an egg or may not penetrate the egg when
they find it.
Adaptive Radiation
 Adaptive
radiation is rapid evolutionary change
characterized by an increase in the
morphological and ecological diversity of a
single, rapidly diversifying lineage. Phenotypes
adapt in response to the environment, with new
and useful traits arising. This is an evolutionary
process driven by natural selection.
Natural Selection leads to Adaptive Radiation
and Speciation
Geographic
Isolation
Postzygotic Isolation
 Zygotes
may not develop properly.
 Odd chromosome numbers in hybrids (mules,
ligers, zonies, etc.) complicate meiosis.
 Plants can become polyploid, making mating
difficult with plants having the parental # of
chromosomes.
Hardy-Weinberg
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Idealized mathematical model of allele frequencies in
gene pools.
 Hardy Weinberg uses allele frequency to calculate the
genotype frequency.
 Assumptions
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Organism is diploid
Generations do not overlap
Population size is large
Mutation is negligible
Random Sexual Reproduction
Migration is negligible
Natural Selection does not operate
Under these assumptions, allele frequencies remain
stable over time.
Hardy-Weinberg
 p-
frequency of one allele
 q- frequency of another allele
 p2- frequency of homozygous p genotype
 2pq- frequency of heterozygous genotype
 q2- frequency of homozygous q genotype
p
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+q=1
p2 + 2pq + q2 = 1
Pace of Evolution
 Gradualism-
speciation can occur at a slow but
constant accumulation of changes.
 Punctuated Equilibrium- species experience
long periods of no change between short bursts
of change.
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Has been proven by evolution of E coli bacteria in
nutrient-poor growth conditions.
Explains gaps in the fossil record between related
species, short transition leads to little chance for
fossils.
Types of Natural Selection
 Stabilizing
Selection - favors average traits
(average sized spiders fair better than
both large and small)
 Directional Selection - favors one extreme
of a trait (the longer the beak the better)
 Disruptive Selection - favors both
extremes of a trait (intermediate at
disadvantage - often eliminated)
Types of Evolution
 Divergence
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(ex: adaptive radiation);
Single populations splits to adapt to different
environments
 Convergence
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Unrelated species become similar as they
adapt to a similar environment.
Analogous structures
• Same use, different form
• Not evidence of a common evolutionary
ancestor
Evidence for Evolution
 Fossil
Record
 Anatomical relationships
 Embryological development
 Genetic comparisons
Fossil Record
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Fossils can help provide a record of the history
of life and shows evolutionary change.
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Shows physical record or extinct organisms.
Shows organisms before and after divergence of
species.
Shows the rate of evolutionary change.
Radiometric dating- use of the ratio of
radioactive isotopes, can be used to put a
relative date on a fossil.
 The record is incomplete - there are large gaps therefore each and every step of evolution can
not be followed.
 Paleontologists look for intermediate species to
provide a step by step sequence of evolution.
The Fossil Record
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The succession of forms in the fossil record
clearly suggests that organisms change
through time, and have descended from a
common ancestor.
 Different groups appear in the fossil record at
different times, with a general trend toward the
simplest organisms appearing the earliest..this
is at odds with the view that they were all
created at the same time.
 Many forms have gone extinct, another
observation that is at odds with the view that
each species was specially created for a
purpose.
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In some cases, a direct line of descent, and
change through time, can be observed in
fossils. Foraminifera, small oceanic
protozoans, leave a continuous fossil record
in oceanic sediments. It is possible to trace
their gradual evolution over millions of years.
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Since Darwin’s day, our knowledge fossil
record has improved tremendously, we can
trace the evolution of many different groups
through fossils: horses, for instance, have a
superb fossil record, showing many instances
of speciation and many intervals of
evolutionary change.
Key Facts from the Fossil Record
 Animal
and Plant forms have changed
over time.
 The timespan of evolution is immense.
 Extinction is the fate of most species that
have ever existed.
 Environments in every locale have
changed, often drastically.
Anatomical Comparisons
 Homologous
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Same form, different use
Indication of common evolutionary ancestor
 Vestigial
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structures
structures
Evolutionary remnants of structures that were
important in a past ancestor
Homologous Structures
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Closely related species frequently have
homologous structures: structures that are similar
in their fundamental layout and construction,
although they may serve very different purposes.
 For example, the forelimbs of mammals are
constructed from the same skeletal elements:
The wings of a bat, a whale, a human, a dog,
etc. all contain the same bones, despite their
different uses.
 This suggests that common ancestry, rather than
design, plays a role in the construction of species.
Vestigial Structures
Many species retain structures that only make sense in
light of their ancestry.
These structures are typically reduced and nonfunctional,
but they are inherited from ancestors, in whom they were
important to survival or reproduction
Example-Whales have
an excellent fossil record-showing
transitional forms
Comparative Development and Embryology
 If
members of a taxonomic unit share a
common ancestry, it is reflected in their
development:
 Two of the many examples:
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limb bud development in whales
extraembryonic membranes of the amniote egg
Embryological Development
 There
are noted similarities between the
embryological development of all
vertebrates (fish, chicken, rabbit, human,
etc)
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Presence of gills and tails in all supports
evidence that gill-breathing vertebrates
preceded air-breathing, terrestrial species
Snake Chicken
Possum
Cat
Bat
Human
Embryology

Closely related species go through similar stages
of development, although the adults may not
resemble each other very closely.
 For instance, all vertebrate embryos develop gill
pouches at some stage, even though in many
species, they are lost later. This is suggestive of a
common origin for vertebrates.
 Embryological development is often suggestive of
evolution: birds have many developmental features
in common with reptilian ancestors, land vertebrate
embryos have many features suggestive of an
aquatic existence (gill pouches, a notochord,
blocks of segmented muscle).
Genetic Comparisons
 All
living things use DNA.
 All living things use the same genetic code.
 Can show relationships between species
(human and chimpanzee DNA are 99%
identical and are closer than chimpanzees
are to apes)
 Can help reveal when species diverged
from their ancestral types
Modern Evidence
 Since
Darwin’s time, there have been
hundreds of studies of evolution.
 Natural selection has been measured in
many organisms in the field, and in
laboratory populations.
 An understanding of evolution has also
become important to combating disease.
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The synthesis of Darwin’s theory with Mendelian
genetics led to our modern understanding of
Evolution.
Several early twentieth century evolutionary
biologists are widely credited with developing our
modern understanding:
R.A. Fisher
J.B.S. Haldane
Sewall Wright
Theodosius Dobzhanski
Thomas Hunt Morgan