1001
Principle ANTH Figures in the History of Evolutionary Thought
Karl vonLinne (1707-1778)
o Swedish botanist
o Devised the system of organism classification
o pseudo-Latinized organisms (even own name Carolus Linnaeus)
o Principle of Fixity of Species
All species were created by God, and each species was fixed in its
characteristics and was unchangeable
Regarded variation within a species inconsequential
Jean Baptiste Lamarck (1744-1829)
o One of the first people to propose that species do change over time (there is
evolution of species)
o He was an invertebrate zoologist
o Theory of Inheritance of Acquired Characteristics (Theory of Use and Disuse)
Four components/tenets to theory
All organisms have needs in order to survive (food, avoid predators
and other conditions)
Met by modification of organs/tissue or the establishment of new
organs to satisfy these needs
The continued use of these organs leads to increase in organisms
complexity and functional capacity, disuse of it would led to its
degeneration and loss
Changes in the organs that occur within ones lifetime become
inheritable and can be passed on to offspring
George Cuvier (1769-1832)
o Contemporary of Lamarck
o The renowned comparative anatomist of his day
o Disagreed with Lamarck that changes within ones lifetime would be anything
but disadvantageous
o Theory of Catastrophism
In the past, there were a series of sudden violent cataclysmic events
that caused the extinction of animals in the affected areas
After the events subsided, the area was relatively depopulated.
Animals from nearby areas migrated in and repopulated (these animals
were similar to the extinct ones, but not identical)
Able to demonstrate that any one species does not change over time,
they are fixed by God by certain events. The immigrant animals that
repopulated the area satisfied the evidence of a series of similar
looking animals across time
Charles Lyell (1797-1875)
o Impressed that sun, wind, rain could change physical geography by slow,
progressive cumulative events
o Rejected Cuviers Theory of Catastrophism
o Theory of Uniformitarianism
Forces existing today toe shape the earth are the same that were
present in the past (ex. the formation of a mountain or the change in
the direction of flow of a river)
Should not rely on explaining past events by forces that we do not see
today (must explain the past by what we see today)
Greatly expanded the theory on the age of the earth (had to have been
millions of years old for the earth to be made into the shape it is
gradually
Important to Darwins theory of Evolution
Thomas Malthus (1766- 1834)
o Cleric who published important essay called Essay on the Principle of
Population (1798)
o Argued that over time, human population size has the potential to increase
exponentially, however the food supply can not increase this way and will
serve as population control (starvation, war over food, etc)
o This was important for Darwins research on evolution
o Limitation: didnt take into consideration that the slope of the food supply line
will change over time due to increase technology
Charles Darwin (1809-1882)
o Accepted job as naturalist on the Beagle as is circumnavigate the globe
o His ideas about the concept of a species and whether or not it could change
over time were altered during this trip
o Made many of important observations on the Galapagos Islands (600 miles of
northwest coast of Ecuador)
13 different species of finches on this island that were different than
the species on the mainland South America
Artificial Selection (managed breeding in domesticated species)
Could radically change nature of a group by choosing which
domestic animals (ex. dogs) could breed
Alfred Russell Wallace (1823-1913)
o Colleague of Darwin
o Vocation was to collect specimens and sell them (traveled to south America,
East Asia, etc)
o While under a fit of malaria, had an idea and wrote a three page paper
Darwin was alarmed this was the same idea that he had been
working on for 20 years and Wallace would get credit
Darwin wrote a short paper on the same ideas, and both papers
were read
o In 1859 (know date), was the publication of Darwins On the Origin of Species
by means of Natural Selection
Created uproar in Victorian England the inference that humans had
predecessors (monkeys) was never stated, but inferred
He never championed idea in public, had surrogate to do this (Thomas
vanHucksley was his bulldog)
Never talks about how a new species comes about in his publication
o Tenets of Darwins (and Wallaces) Theory of Evolution by Natural Selection
Reproductive potential of a population or species is great or manifold
Resources in the environment are in limited or short supply (there will
be competition among species for food)
Biological variation is a normal feature within a species
Any hereditary variation that allows any organisms a slight advantage
in the competition for resources will be perpetuated.
Organisms with favorable traits will survive and produce more
offspring (over time and across generations, there will be
increase in frequency of these with the favorable traits)
History of Heredity
o Gregor Mendel (1822-1884)
Austrian Monk
How were traits inherited?
Used pea plants and choose 7 characteristics (with 2
expressions and no intermediate type, 2 contrasting traits) to
test
Published experiment results in 1866 (know)
Presented results in statistical manner that confused everyone
(no one presented work in this way)
o It was not appreciated until independently rediscovered
later
Results of Experiment
self fertilization plants for years until had absolutely pure strains
(only produced one expression seed texture round or
wrinkled)
cross pollinated these two plants so the resulting generation
would be one or the other
o 1ST generation: all the plants had round seeds (100%
round, round x round)
o then self-pollinated these plants and the 2nd generation
75% of seeds were round and 25% were winkled
o trait for wrinkled seed texture was there hidden, and
expressed in 2nd generation
cross pollinated these results for 3rd generation
o results of round x round were all seeds were round
(100%)
75% round
25% wrinkled
o results of wrinkled x wrinkled were 100%
Parental
First Generation
Second Generation
Third Generation
Round (50%)
100% round seeds
(round x round)
75% seeds were round
(round x round)
100% round and in other
times 75% round and
25% wrinkled
Wrinkled (50%)
25% were wrinkled
(wrinkled x wrinkled)
Uniformly wrinkled seeds
(100%)
R = round and w = wrinkled
Principles derived from Mendels experiment
o Principle of Segregation
Genes occur in pairs in an individual organism.
In the production of sex cells, that pair of genes is separated and each
sex cell gets one of the genes
Use punnett square to predict the outcome of the experiment (means
for expressing all possible outcomes)
Use symbols to represent the characteristics (R= round,
w=wrinkled)
o RR = round texture
o Lowercase ww=wrinkled texture
= 4 Rw (100% Round)
= 1 RR, 1 ww, and 2 Rw (3 Round and 1 wrinkled)
(100% round)
(100% wrinkled)
(75% round and 25% wrinkled)
Definitions
o Gene segment of DNA that codes for the production of a protein
o Allele alternative form (variants) of a gene
o Genotype set of genes that an individual has
Having Rw gene
o Phenotype physical manifestation (expression) of the genotype
Having round seed texture
o Homozygous situation of an individual having two copies of the same allele
o Heterozygous situation of having one copy each of different alleles
o Dominant allele expressed phenotypically in both homozygous and
heterozygous situations
o Recessive allele - allele that is expressed phenotypically only in a
homozygous situation
Alleles are considered dominant because they are producing more
protein products than what is classified as recessive allele, or that the
recessive allele is not producing any protein product
Ex. O allele in blood type does not produce anything
Principle of Independent Assortment
o The segregation of one pair of genes does not influence the segregation of
another pairs of genes
Note: Segregation separation of pair so that sex cells get only one of
each pair
o Seed texture: Stem Length
R,w
T,s
Genotypes
(1) RRTT
(2) RRTs
(2) RwTT
(4) RwTs
(1) RRss
(2) Rwss
(1) wwTT
(2) wwTs
(1) wwss
Phenotypes
Round, Tall (9)
Round, short
(3)
Wrinkled, Tall
(3)
Wrinkled, short
(1)
Seed texture 12 Round, 4 wrinkled (3:1)
Stem Length 12 Tall, 4 short (3:1)
(Ts x Ts)
Results: 3 tall, 1 short
Sorted in the next generation independently from one another
o More genotypic variability in a population (species consists of many
populations) then there is phenotypic variability
Within a species, despite the small phenotypic variability, there is a
wide pool of variability (this is fundamental for the success of a
species)
Without genetic variability, there would be no natural selection
Mechanisms of Cellular Reproduction
o Aim: How variability is produced
o Mitosis
Somatic cells (i.e. not sex cells) undergo mitosis
Ex. Liver, muscle, skin cells
Functions are two fold 1) for the growth of an organization (increase in
cell number) and 2) maintenance of organs and tissue
Cells die before organs and tissue do, so must be replaced
Cells have two parts of its life span only near the end of its life time
that it undergoes mitosis
Most cells life happens during interphase (before it gets into
mitotic phases)
Interphase
Genes are active (cant distinguish chromosomes)
Near the end of this phase, the chromosomes replicate (syn:
duplicate)
Two chromatids are formed, and are bound together
(temporarily) by a centromere
Then we enter Mitosis
Prophase (first stage of Mitosis)
Nothing much is going on that is different from the lat stage of
interphase (chromatids still held together by centromere)
Metaphase
Chromosomes align in the middle of the cell independent of
each other
Spindle fibers attach to the centromeres
Anaphase
Centromeres divide (this defines anaphase)
The two chromatids can now migrate to different ends of cell
Telephase
Cell divides (end up with two cells)
Results of Mitosis
1) Two daughter cells are produced, each with the diploid
chromosome number
2) both daughter cells are genetically identical with each other
and with parent cell (this is the ultimate function of mitosis)
o Definitions:
Homologous chromosomes chromosomes with the same
sequence of genes (you receive one of each homologous
chromosomes from your mother and one from father)
Ex. At the same location on each chromosome are the genes
for the same thing (ex. seed texture, stem length, etc)
Chromatids identical strands of DNA (homozygous for each position
on each strand)
Centromere constricted portion of a chromosome that holds two
chromatids together
Centromeres are late replicating DNA
Diploid full compliment of chromosomes (human # = 46)
o Meiosis (reduction division)
Sex cells or gametes undergo meiosis
Function: to produce sex cells to transmit chromosomes to the
offspring
Interphase
Genes are active in producing proteins
Near terminal part, chromosomes replicate (duplicate)
o Two identical chromatids
Prophase I
Homologous chromosomes will align, or pair, with each other
(called synapsis)
One strain on each chromatid crosses over (recombination physical exchange of genetic material) centromeres still
present
Metaphase I
Chromosomes align in center of cell
Homologous chromosomes remain paired with one another
(travel together to the center of cell)
Non-homologous pairs of chromosomes align independently
from one another
Anaphase I
Unlike mitosis, centromeres do not divide pulled as unit to the
sides of the cell
Telephase I
Cell divides and two cells are produced
Have reduced the number of chromosomes in each cell
Interphase II - Nothing is happening different from the end of
Telephase I
Prophase II nothing new is happening
Metaphase II
Chromosomes align in the middle part of the cell
Alignment of non-homologous chromosomes is independent of
one another
Spindle fibers form and attach to centromere
Anaphase II
Centromeres divide releasing two strains of a chromosomes
from one another so they can be pulled to opposite sides of cell
Telephase II
Each cell divides so there are now four cells total
Definition
Synapsis pairing of homologous chromosomes
Crossing over (recombination) physical exchange of DNA
between homologous chromosomes only
o Only occurs in meiosis
Haploid one half of chromosomal compliment (number)
human = 23
o Parent cells have diploid number (humans=46 )
o Result of Meiosis
Four cells are produced, each has the haploid chromosome number
Each daughter cell is genetically different from the parent cell and from
one another
Ultimate function of meiosis the production of genetic variable sex
cells
Comparing Mitosis and Meiosis
o Number of daughter cells produced
Mitosis: 2
Meiosis: 4
o Chromosome number in daughter cells
Mitosis: diploid
Meiosis: haploid
o Genetic structure of the daughter cells compared to the parent cell and each
other
Mitosis: daughter cells genetically identical to each other and parent
Meiosis: genetically different from parent and each other
o Functional result
Mitosis: exact duplication of parent cell to daughter cell
Meiosis: virtually infinite variability among daughter cells
o To illustrate meiosis, you could put prophase, metaphase I, metaphase II, and
Telophase II together and be able to explain the complete process
In Metaphase I, the chromosomes can be set up in any order and the
result will be the same. This could result in even more variation of
daughter cells in Telophase II
In metaphase II, there are 2 chromosomes and 2 orientations for each
of them. There are two orientations for each chromosome in
metaphase II. This is 4 times 4, 16 variations of daughter cells
As humans, we have 23 pairs of chromosomes, 46 total. We
have approximately 70,000,000,000,000 variations of daughter
cells
o Mechanical differences between Mitosis and Meiosis
Prophase
Mitosis nothing happens
Meiosis synapsis of homologous chromosomes and crossing
over between those
Metaphase
Mitosis chromosomes align independently from one another
Meiosis metaphase I homologous chromosomes remain
paired and non-homologous chromosomes align independently
Anaphase
Mitosis centromeres divide
Meiosis anaphase I centromeres do not divide
o Metaphase II chromosomes align independently of one
another
o Anaphase II centromeres divide
o Activities of a cell during Interphase
Replication of chromosomes
Occurs in nucleus of the cell where the chromosomes are
present
Replication occurs during interphase
Two chromatids are synthesized (formed) and held together
temporarily by centromere
As a chromosome is made up of DNA, replication of a
chromosome is identical to replication of DNA
o DNA double helical molecules, two strains twisted upon
themselves
o DNA is made up of nucleotides (consists of phosphoric
acid, a sugar deoxyribose, and a nitrogen containing
base)
o Four bases in DNA: Adenine (A), Cytosine (C), Guanine
(G), Thymine (T)
o Bonding pairs of bases in DNA are Adenine-Thymine,
and Guanine-Cytosine
Gene activity
January 30, 2007
Interphase
Replication of a Chromosome
o Replication is semi conservative, in that each DNA strand acts/serves as a
template (a master string for copying) for the formation of another strand. As
a result of this, 2 chromatids are produced, and each chromatid is comprised
of 1 parental strand of DNA and one newly synthesized daughter strand.
The two chromatids are temporarily held together by a centramere.
DNA
DNA has 4 nucleotides.
o Nucleotides consist of phosphoric acid, sugar, and base.
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Bonding Pairs:
o A-T
o G-C
Gene
Comments
o Definition: Gene: A sequence of nucleotides (segment of DNA) that codes
for the production of a polypeptide (i.e., protein)
Protein: macromolecule consisting of amino acids that is functional in
the body. A protein may consist of 2 or more polypeptides.
Amino Acid: molecules of a protein. There are 20 amino acids; 10 of
them we can produce in our bodies and the other 10 we get from our
diet.
o Correspondence between gene and protein
A gene consists of a series of nucleotides
A protein consists of a series of amino acids
As a gene codes for a protein, it is the series or sequence of
nucleotides which determines the series or sequence of the amino
acids. The gene codes the protein because the nucleotides determine
the amino acids.
Function of a Gene
o Proteins are synthesized at ribosomes, which are in the cytoplasm. Genes
are in the nucleus; therefore, you have to get the genetic code from the
nucleus to the cytoplasm.
Transcription is the formation of messenger RNA (mRNA). This is
transcribing /copying of the genetic code from DNA to mRNA.
In RNA, uracil replaces Thymine. Bonding pairs are A-U and
G-C.
mRNA and DNA codons
o Definition: codon: triplet sequence of nucleotides that
specifies a particular amino acid
o There are 64 codons and 20 amino acids
TA
A
TA
A
AU
T
AT
U
CG
G
CG
G
AU
T
AT
U
TA
AT
CG
AT
GC
AT
TA
TA
AT
GC
AU
TA
TA
AU
mRNA
single
strand
C
T
A
A
T
GC
AT
TA
TA
AT
DNA
C
U
A
A
U
mRNA
Translation: formation of a protein; transfer RNA (tRNA) carries an
amino acid to the ribosome.
tRNA has an anticodon
o anticodon: triplet sequence of nucleotides that serves to
bond a transfer RNA to a codon of mRNA
Structural genes determine the structural proteins of the body.
Regulatory genes determine the time and rate of activity of
structural genes.
(For a test know which nucleotide bonds with what, 64 codons, 24 amino acids, etc.)
Two classes of genes:
1) Structural genes code for the structures in the body
2) Regulator genes control the activity of structural genes ; one regulator gene
controls a multitude of structural genes
Mutation
Mutations are errors in replication, and there are absolutely fundamental to the
variation and evolution of species. Errors in biology are influential in evolutionary
success.
Evolution: a change in allele frequencies over time; it occurs over generations, so
individuals DO NOT evolve, their species does
Mutation is the only way of introducing new alleles into a species. Most mutations
are harmful or neutral (i.e., neutral with respect to natural selection), though the
natural selective advantage, disadvantage, or neutrality is relative to the
environment. The spray of a mutation in a population is by natural selection.
o Mutation is a random event. They dont necessarily occur at advantageous
times; rather, they act randomly.
2/1/07
Mutation
Classification of mutations
o Point mutation change in a single nucleotide that results from an error in
the replication of DNA
Results in a new protein
Consequences of point mutation
No change in the amino acid sequences of a protein. (Due to
the redundancy of the genetic code, multiple condons may
specify the same amino acid). This is a neutral mutation
(neutral with respect to natural selection)
Change in the amino acid sequence of a protein
o Change may not by functionally important. The mutation
is neutral with respect to natural selection
o May be functionally important, and may either be
advantageous or disadvantageous with respect to
selection
A single point mutation can have very significant effects on an
individual (sickle cell anemia results from a single point
mutation)
T-A
A-T
C-G
A-T
G-C
A-T
T-A
T-A
A-T
Replication
T-A
A-T
C-G
A-T
G-C
A-T
T-A
T-A
A-T
T-A
A-T
C-G
A-T
A-C
A-T
T-A
T-A
A-T
Transcription
T -A
Translation 6yu
A-U
C-G
Methioinine
A-U
Phenylalanine (serine)
A-U
asparagines
A-U
AUG
UUU
AAU
T-A
T-A
A-U
o Mutation rate is estimated to be 1x10-5 (1/100,000) per gene per cell
generation
75% of us are carrying mutations with respect to our parents (most
have little or no effect on us)
The mutation rate for a gene itself is to low to be the principle force in
evolution
Provide variation on which natural selection can act upon
(produce different proteins it can act on)
Mutation persists because of natural selection
Become widespread within a population or species due to
natural selection
o Chromosomal mutation
The position of a gene on a chromosome may be important in its
expression
If the change the position of a gene, you may change its phenotypic
expression
Ex. 2 chromosomes (series of genes on both)
Gene A1 - A4 on chromosome strand #8
Gene b1-b5 on chromosome #46
Ga1
Gb1
Ga2
Gb2
Ga3
Gb3
Ga4
Gb4
Gb5
The point of the breakage of the genes becomes sticky and
could result in
Ga1
Ga2 Gb3 Gb4 Gb5
Gb1
Gb2 Ga3 Ga4
An imbalance in chromosome number
Chromosome number 1-22 non-sex chromosomal
o #1 longest of the non-sex chromosomes (and will have
the most genes)
#23 sex chromosome (is the shortest chromosome)
o Has the fewest genes
o Failure of centromere to divide in Anaphase II, both
chromosomes are pulled to the same side and one will
have 24 chromosomes
If this is the one fertilized, then the person will
have 47 chromosomes (down syndrome 47
chromosomes with three copies of #21 instead of
two)
#21 is the 2nd shortest of the chromosomes (has
second genes)
fewest There is no instant when a person has 45 chromosomes all
fatal
Genome is tightly integrated, and so a random change at the
nucleotide level or above, affects the phenotypical outcome of
the individual
o Most random changes are disadvantageous
X chromosome is a lot longer than the y (XX Xy)
o How is this imbalance rectified in the genome?
In every female sex cell, one gene is turned off
(dense lump in DNA)
No gene is turned off in the male sex cells
Extra chromosomes are turned off, and only one is
active in each cell
Genotype and Phenotype at the level of species
o Genotype set of genes an individual possesses
o Phenotype physical expression of those genes
Can subdivide traits based on phenotypic distributions
Two broad categories of phenotypes
o Traits that show discontinuous variation a few easily distinguishable
phenotypes (usually)
Usually results from inheritance involving one gene
Ex. 1: Mendelian traits
Ex.2: ABO blood group shows this discontinuous variation (three
alleles and the resulting phenotypes are AA and AO (A dominant), BB
BO (B dominant), OO (O dominant), AB (AB dominant)
Ex.3: Taster gene (T,t)
Two alleles for this gene (T,t) the ability to taste is dominant to
the ability not to taste a substance
o Traits showing continuous variation Great variety of phenotypes
One grade or category of phenotypes would grade into the next
category
Traits showing continuous variation usually have a polygenic basis
Polygenic: two or more genes interacting to produce the phenotype
These traits usually affected more by environment factors than
single genes are
o Formula for calculating the number of genotypes in a polygenic system
#genotypes = ( (a) (a+1) / 2 )G
A= # of alleles of a gene
g = number of genes we are considering simultaneously
Ex. R,w
# = (2x3/2)1 = 3
There are 3 genotypes (RR, Rw, ww)
Ex. A,B,O
(3x4/2)1 = 6
AA, AB, BB, BO, AO, OO
Ex. R,w : T,s
# = (2x3/2)2 = 9
2 alleles for each gene, 10 genes
# = (2x3/2)10 = 59,049 genotypes
2/6/07
A, a
o A is dominant to a
o We need to know the natural selection advantage of the allele to know which
one would be predominant in a specific environment. They express
phenotypically in different circumstances
o Two different situations
A mutant, a normal
A normal, a mutant
Not expressed phenotypically in heterozygous
Expresses only in homozygous
Genotype and Phenotype
o Categorization of phenotypic variation: discontinuous and continuous
o Formula to estimate genotype variability
o Heritability
Definition: relative amount of phenotypic variation due to genotypic
variation (causes)
P=phenotype, G=genotype, E=environment
P= G + E
VP = V G + V E
Heritability index (h2) = VG/VE = G/P
o Blood type has heritability index because it depends on genotype not
environment
o Stomach has very low heritability index because it depends on how much you
eat
Traits with high indexes are not necessarily the most important
Ex. behavior has low index
o Population can differ in the heritability index because of the environmental
factor
o **Effectiveness of natural selection in causing evolution is directly related to
the heritability of traits**
o Evolution occurs as the level of genes, has to be correspondence because
phenotype and genotype
For selection to cause evolution, must be relation between phenotype
and genotype
Population Genetics; Hardy-Weinberg Theorem
o Gene pool is the total store of genes or alleles in a population or species
Generations gene pool is determined by parents gene pool:
o Population community of individuals in which matings typically occur
o Evolution change in allele frequency over time
o Allele frequency proportion of a particular allele relative to all alleles of a
gene
o Hardy-Weinberg Theorem
Both allele frequencies and genotype frequencies do not change over
time if certain conditions or functions are met
Establishes the situation in which evolution will not occur (evolution
depends on the change in allele frequencies)
Conditions/Assumptions
1) Assume that there is random mating within a population
(everyone has equal probability to mate with everybody else in
population no direction/decision in pairing of individuals)
2) infinitely large population size
3) no natural selection
4) no migration (no one leaves, no one enters)
5) no gene undergoes mutation
One or more of these is always being violated because evolution is
evolving every generation
Violation of assumptions 2-5, leads to allele frequency changes (i.e.,
evolution)
Violation of any of the assumptions, leads to genotypic frequency
changes
Non-random mating does not change allele frequency, only
genotypic frequency
The effect of non-random mating on genotype and phenotype frequencies over
time
o Positive assertive mating (i.e., inbreeding) where individuals will mate
because they assort themselves based on similar phenotypes (most likely) or
genotypes
Matings between individuals of similar phenotypes and genotypes
Ex. (red heads mate with red heads)
o Negative assertive mating (I.e., out breeding)
Mating with individuals not related to you
o Interest of non-random mating: though does not cause evolution, can be
important in situations where there is natural selection based on genotype
Nonrandom mating
Positive assortment
Negative assortment
Genotype
Frequencies
Increase
homozygousity
Decrease
heterozygousity
Decrease
Allele Frequencies
No change (does not
lead to evolution)
No change
homozygousity
Increase
heterozygousity
Mechanisms of Evolution
o Four forces or mechanisms of evolution
o They individually or in combination can cause changes of allele frequencies in a
group/population over time
Mutation
Gene flow
Genetic drift
Natural selection
o Non-random mating is not a mechanism of evolution (may change genotype freq,
but not allele freq)
o Forces of evolution
How they influence allelic variability
How each influences this within one population and then compare
populations within a species
Mutation
The only way of getting new variance of a gene within a species is
by mutation. Mutations are either harmful or neutral to the organism
Persist within a pop/species because prove to be advantageous in
natural selection (has to occur in sex cells)
o Mutation: populations become dissimilar to one another
Over time, as result of natural selection, pop reaches summit of
adaptation and wants to suppress this mutation system
o Errors in replication are fundamental in variation and
evolution
Gene Flow: exchange of alleles between populations (i.e., ad mixture
between gene pools or populations)
Either new alleles can be introduced as a result of gene flow or can
change the frequencies of existing alleles
Can be characterized by contact between neighboring populations,
can be the diffusion of alleles across large geographical areas
without the population moving (just mating contact at the
boundaries, or neighboring contact of populations)
Can also be caused by migration or large scale colonization
Can be introduction of new alleles because of this genetic contact
Populations become more similar to one another (if have two
separate populations and they combine as result of gene flow,
reduce variation over time)
Forces of evolution
Allelic Variability
Mutation
Gene Flow
Genetic Drift
Natural Selection
Within pop
between pop
Increase
increase
Increase
decrease
Decrease
increase
Decrease
increase
Gene flow inhibits or retards speciation
o Species a group(s) of actually potentially interbreeding organisms that
are reproductively isolated from other such groups
Different species cannot interbreed because of the unbalanced
number of chromosomes (ex. human mating with chimps)
o Horse and donkey can interbreed to form a mule but then
the mule is sterile
o There are exceptions to this species rule
Process of speciation
o Consider species to consist of four populations: A, B, C, D
o species results when one pop of a species can no longer interbreed with
the other pops
o this reproductive isolation occurs when one species (A) becomes
geographically isolated from the other populations (B,C,D)
o upon isolation, the isolated species may become genetically different from
the others
o the gene pools of population A and B-D begin to diverge due to the
mechanisms of evolution
original species not made of B-D, and the other species made up of
only species A
o the gene pool divergence is of such magnitude that members of A either
actually or potentially cannot mate with members of B through D, then a
new species has emerged
o now have species 1: made of B,C,D and a species 2: made of A
o key to this process of speciation is geographical isolation of species
this geographical isolation blocks gene flow
reproductive barrier between two species so they can no longer
reproduce
Genetic Drift
o The most difficult of the methods of evolution
o Genetic drift is evolution due to random factors
Violates Hardy-Weinberg Theory of infinitely large population size
o Sampling phenomenon with genetic drift - The larger the sample size, the
closer you should be to expected results
o Formula for computing the potential magnitude of evolution by genetic
drift
. (sigma) = SQRT[ (p)*(q) / 2(N) ]
o . = potential magnitude of genetic drift
o P,q = allele frequencies for a gene
o N = pop size
o Sigma = estimated magnitude of allele frequency change by
genetic drift
Ex. N1=50 and N2=500,000
o .p= 0.8
o .q = 0.2
o .1 = .04
o This means that p must equal between 0.76-0.84
o This means that q must equal between 0.24-0.16
And p+q must equal 1
o For N2, p must be between 0.7996-0.8004 and q must be
between 0.2004-0.1996. N2 = 0,0004
The larger the population, the smaller the population drift
All populations evolve every generation by genetic drift but the
magnitude of evolution by genetic drift is greater in smaller
populations than in larger populations
o In each generation, all genes are subject to genetic drift and each gene is
affected independently from one another. For any particular gene, genetic
drift (random change in allele frequency) continues until one allele of the
gene has become fixed and all other alleles of that gene become lost from
the gene pool
o Example of genetic fixation situation where one allele has frequency of
one (or unity) and there are no other alleles of that gene (have been
removed or lost from population)
o Effect of genetic drift on allele variability on a population
Simplification (decrease) of variability within a species over time
o End result of genetic drift is all genes homozygous
Between populations, the variability increases because both pops
are experiencing genetic drift (different alleles drifting in each
population) the populations will diverge in their gene pools as a
result
o Founder Effect (bottle-neck effect of population)
Change in allele frequency due to sudden reduction in population
size
Has nothing to do with natural selection (like genetic drift has
nothing to do with natural selection either)
Table below: was there intense natural selection against A and B
allele
o Relevance: E. Asians are putative ancestors to the Indians
and South Americans
o Among islands, populations may differ in their genes
because of genetic drift and not because of natural selection
Founders effect is a subsequent effect of genetic drift
A
East Asian 0.2
Indigenous 0.2
N.
American
Indians
Indigenous 0
S.
Americans
B
0.2
0
O
0.6
0.8
0
1.00
Natural Selection
o Differential reproductive success (I.e., difference among individuals in the
number of their offspring)
o Fitness relative reproductive success (an attempt to quantify natural
selection)
o The individual organism is the unit or target of natural selection. The
population, or species (depending on level of organization we are looking
at), is the unit of evolution
o The ability for natural selection to cause evolution is directly related to the
heritability of those traits
Recall: H2 = G/P
o Natural selection (H2) acts directly on ones phenotype (P in above
formula) and only indirectly on genotype (G in formula)
o Adaptation is the result of natural selection. Is an increase in the
frequency of traits in a population or species that allows organisms to
survive their environmental stresses
Individual organisms do not adapt, it is the group
All adaptation says is that evolution has occurred (it is the long term
effect of natural selection)
o **Fundamental theorem of Natural Selection** (not in text)
The opportunity for natural selection to occur is directly related to
genetic variability between a species/population (terms are used
interchangeably here)
Natural selection is opportunistic. That is, it acts on existing
phenotypic (directly) and genotypic (indirectly) in a population.
Therefore, as pops may differ in their gene pool, they may adapt
differently to the same environmental stress (there are multiple
ways of solving certain environmental stresses)
o Effect of Natural selection on allele variability
Decrease within population- there will be simplification of gene pool
over time
Between populations increase, will become dissimilar over time
(exposed to different stresses or even if exposed to same stress,
may adapt differently to solve stress.
Two Theories of the Mode of the Origin of a Species:
o Gradualism: evolution is a continuous process, and natural selection is
principle in speciation.
Tenants of the Theory:
o One population becomes geographically isolated from other
populations, and no gene flow occurs between the isolated
population and the other populations.
o The populations gene pools diverge by natural selection.
o If the isolated populations gene pool is different enough
from the other populations gene pools, and reproduction can
no longer occur between them, then a new species has
emerged.
o The rate of speciation is related to the intensity of
natural selection. The greater the intensity of natural
selection, the quicker the rate of speciation?
o Punctuated Equilibrium: Evolution is a discontinuous process, and
evolution involves (is characterized by) periods of rapid speciation
followed by long periods of stasis (i.e., no meaningful evolutionary
change) within a species. It characterizes the mode of origin of a species.
A small population becomes geographically isolated from the other
larger populations within the species, and gene flow is blocked
between the isolated small population and the other populations.
Gene pools diverge mainly by mutation and genetic drift.
The population gene pools become so divergent from one another,
than individuals from the populations can no longer interbreed and
the two species diverge. The small population becomes
reproductively isolated from the larger populations, and a new
species emerges.
Punctuated Equilibrium suggests that all morphological change or
differences between the parent and offspring species occurs at the
speciation event. Natural selection does little to change a species
once it has emerged. Once a species originates, there is little
subsequent change by natural selection.
Overview: Evolution is not future goal oriented. Natural selection results in adaptations of
a species. The fact that you cannot predict evolution does not mean that it is a random
process. You can not predict the future environment or mutations, so you cannot predict
the future of evolution.
Explanation of gaps in the fossil record. Why arent there more intermediate
types in the fossil record?
Interaction of mutation and natural selection
o Polymorphism: the situation in a species or population where there are 2
or more alleles of a gene, and at least 2 of the alleles have a frequencies
greater than or equal to 1 percent. The mutation rate for any gene is very
low, so it cannot get an allele up to 1%, so something else has to be
acting on the allele to reach the threshold of 1%. Polymorphism is
explained by natural selection acting on mutations. In our species,
approximately 28% of our genes are polymorphic.
Lets say there are 50,000 genes in our genome and 28% are
polymorphic. This means there are approximately 14,000
polymorphic genes. Genetic variability allows a species to go
through natural selection and adapt.
Example: A, B, O blood system: The genotypes are AA, A0, AB,
BB, BO, and OO, and these produced proteins. The proteins that
can be produced are A,A, A and B, B, B, and none for the OO. If
you are a type A blood at birth, you have the anti-B antibody. This
means it produces antibodies against the B protein. If you are type
AB blood, you produce no antibodies. If you are type B blood, you
produce antibodies against the A protein. If you are type OO blood,
you produce antibodies against both A and B proteins.
o Balanced Polymorphism: situation where there are 2 or more alleles of a
gene in a population or species, and at least 2 of the alleles have
frequencies equal to or greater than 1%, and the polymorphism is due to
heterozygotes having a higher fitness (natural selective advantage) than
both homozygotes.
Example: Red blood cell variants and Malaria:
o Sickle Cell Allele
o What is sickle cell? Hemoglobin binds oxygen, and it is a
protein that is comprised of 2 alpha and 2 beta polypeptide
chains. Each alpha polypeptide chain has 141 amino acids,
and each beta chain has 146 amino acids. Sickle cell is a
result of a single point mutation affecting 1 amino acid.
o Despite natural selection disadvantage to the sickle cell
allele, the allele is maintained in high frequencies at
polymorphic levels from 5-15% in some population.
o Malaria:
o (Plasmodium)
o Malaria is caused by a parasite, which is transmitted from an
infected individual to an unaffected individual by a mosquito.
When it gets into your system, it gets into the red blood cells.
o Relationship between Sickle Cell Allele and Malaria
o Evidence
Geographical correlation between the distributions of
polymorphic levels of the sickle cell allele and Malaria.
Clinical and experimental studies show that
individuals heterozygous for the sickle cell allele have
a lower rate of malarial infection compared with
individuals homozygous for normal hemoglobin.
o Why heterozygosity for sickle cell allele is advantageous?
Heterozygotes for the sickle cell allele show selective
destruction of red blood cells. Red blood cells
uninfected by the parasite (Malaria) deliver oxygen to
tissues. Red blood cells that are infected by the
parasite sickle, and then kill the parasite. The sickleshaped red blood cell provides a lethal environment
for the parasite. Therefore, heterozygotes for sickle
cell can conjointly suppress parasitic proliferation and
deliver oxygen to the tissues.
In a Malarial environment, heterozygotes for sickle
cell allele have a natural selective advantage (fitness)
over homozygotes for normal hemoglobin and for
homozygotes for sickle cell. Therefore, the sickle
cell allele is maintained at relatively high
frequencies, and this is an example of balanced
polymorphism.
o Evolution of relationship between sickle cell allele and malaria :
o Spread of slash-and-burn agriculture is responsible for the
selective advantage of the sickle cell allele
Slash-and-burn agriculture was introduced in
rainforest areas of Africa approximately 3,000-4,000
B.C. The agricultural landscape created the
conditions for proliferation of mosquitoes. Mutations
for the sickle cell allele arise periodically but under
non-agricultural conditions are selected against.
However, proliferation of mosquitoes subsequent to
the introduction of agriculture, resulted in the sickle
cell allele becoming selectively advantageous in
heterozygotes. The allele increased to polymorphic
levels. Therefore, the spread of agriculture is
responsible or related to the selective advantage of
the sickle cell allele.
o Other red blood cell anomalies and Malaria
o Red blood cell variants of Thalassemia and deficiency of the
enzyme G-6-PD (Glucose 6 Phosphate Dehydrogenase):
Thalassemia is an abnormality in the production of
hemoglobin. Homozygosity for Thalassemia is lethal,
and heterozygotes for Thalassemia show a range of
responses or phenotypes, from sever anemia to
phenotypic normality.
G-6-PD is an enzyme involved in the utilization of
glucose. The gene for G-6-PD is on the xchromosome. Individuals with G-6-PD deficiency are
phenotypically normal.
The selective advantage of heterozygosity in
Thalassemia and absence of G-6-PD is inferred by
the geographical correlation between their
distributions and the distribution of malaria. Also,
clinical evidence suggests that Thalassemia red blood
cell and G-6-PD deficient red blood cell are hostile
environments for the malarial parasite.
o Conclusion of studies on red blood cell variation and malaria .
o Natural selection is opportunistic. It acts on the existing
phenotypic and genotypic variability in a population.
Populations may differ in the mutations related to the red
blood cell: alleles for sickle cell, thalassemia, and G-6-PD
deficiency. As each red blood cell variant is naturally
selectively advantageous in a malarial environment, each
will be perpetuated. Therefore, the interaction of mutation
and natural selection means that different populations can
show different adaptations to the same environmental stress.