Download Natural Selection

Evolution
Chapter 11
Organisms and Environment
• Populations of organisms adapt through time to
the environment in which they live
• Environment “selects” adaptations that better
able an organism to survive and reproduce
• Environment does not create the adaptations in an
individual (a common misconception)
• Thus the ecology and evolution are linked in the
lives of organisms
Evolution - change in the allele frequency of a
population through time
often seen as changes in
morphology in populations
• Why did scientists ever come up with
this idea of evolution?
• noted significant changes in earth’s
structure and in living organisms
• wondered why there are so many different
species; some resembling each other more
closely than others
• Any scientific explanation must work
through natural processes
“The purpose of science is to search out and
build explanations of the natural world that
are based on natural mechanisms.”
(Perspectives in Biology)
Interesting book….
Evolution's Captain by Nichols Peter
Un-Fig. 03.3
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Large ground finch
(seeds)
Woodpecker finch
(insects)
Cactus ground finch
(cactus fruits and flowers)
GALÁPAGOS
ISLANDS
40 km (25 mi)
Vegetarian finch
(buds)
Evolution
• Role of evolution in science:
• central tenet in biological science
• biology doesn’t make sense without evolution
• the occurrence of evolution is not questioned in
science
evolution is to biology as gravity is to physics
• Only the mechanisms of HOW it works is discussed
and debated in the scientific community
• Lamarck, Wallace, Darwin all provided the mechanisms.
• Wallace’s and Darwin’s explanation of mechanism has
survived scientific scrutiny and is called natural selection.
Theory of Natural Selection
1. Variation exists among organisms in a population
2. Some of these variations are hereditary
3. Populations produce more offspring than
environment can support
4. Individuals best adapted (better fitness) to
environment will leave more offspring than those
that are not as well adapted (less fit)
• Populations of organisms adapt through time to the
environment in which they live
• Environment “selects” adaptations that better able an
organism to survive and reproduce
Result of Natural Selection
• This can produce a change in allele
frequency in a population over time
…our definition of evolution.
• How much change is necessary before
the change is recognize as real, and not
due to chance?
• Sounds like a question answerable via
statistics?...well hang on partner and lets
see about that
Understanding Genetic Function
What is the genetic material?
What is a gene?
What does a gene do specifically?
What is an allele?
How are the diploid and haploid
conditions reflected in your genetics?
• How does inheritance actually work?
•
•
•
•
•
DNA structure
Genetic Material
• DNA
• organic macromolecule
• nucleotides are the building
blocks
• nucleotides contain
• sugar
• phosphate
• base (only variable
portion): ATCG
• nucleotides connected to
form a strand
strands connected to each
other to form a double
stranded molecule
Genetic Material
• genetic information
• encoded by the order of nucleotide bases
• gene
• piece of DNA
• instructs the cell to make an protein
• structural
• enzyme
• allele
• different forms of a gene
• Example: gene for pea pod color
• green pod allele
• yellow pod allele
Alleles
Genetic Material
• alleles occur in pairs WHY?
• chromosome contains like genes
• chromosomes arranged in pairs
• egg donor’s chromosome
• sperm donor’s chromosome
• Example: gene for pea pod color
• green pod allele (G)
• yellow pod allele (g)
• GENOTYPE = allele arrangement
• How many genotypes for one gene if only two alleles?
• GG (homozygous)
• gg (homozygous)
• Gg (heterozygous)
• alleles inherited singly WHY?
• gametes (through meiosis) reduce diploid to haploid
Genetic Material
• Possible combinations in offspring
•
•
•
•
•
•
GG X GG
GG X gg
gg X gg
Gg X GG
Gg X gg
Gg X Gg
• Can we predict genotypes of offspring?
G
g
G GG
Gg
g
gg
Gg
Population Genetics
• Hardy-Weinberg Principle
• allele frequency tends to remain constant in a
population (thus in equilibrium)
• Population
a localized group of potentially interbreeding individuals belonging to
the same species
• Species
a group of population that potentially can interbreed
• Gene pool
sum of all the alleles of all the genes of all the individuals in the
population
Hardy-Weinberg Law (also known as HW Equilibrium)
assumes allele equilibrium if the population has the
following five conditions:
1. random mating within population
2. no selection for or against any specific allele, which
would alter gene pool (natural selection)
3. no genetic drift (the population is large enough not to
be influenced by chance)
4. no gene flow into or out of the population (population
is isolated from other populations)
5. no mutations are occurring (mutations alter gene pool
by changing one allele into another)
If one or more if these conditions are NOT occurring, then what?
• no allele equilibrium, thus alleles change in frequency
through time…evolution
Population Genetics…
• Hardy-Weinberg Principle
• assumes 2 alleles for each gene
• where frequencies of G + g = 1
• if alleles remain constant…
• since genotype of diploid contains 2 alleles for each
gene…
• then (G + g)2 = genotype frequency in population
• and expanding binomial gives G2 + 2Gg + g2
• hard to keep G separate from g so lets use p=G and
q=g; thus….
p2 + 2pq + q2 = 1
• where p2 = GG, 2pq = Gg, and q2 = gg
Chi-square analysis
• tests to evaluate if results (observed)
differ from what was expected
• one must know expected values
χ2 =
Σ
(observed – expected )2
expected
Determining HW Equilibrium
• A scientist has studied the amount of polymorphism in the alleles
controlling the enzyme phosphoglucomutase (PGM) in a desert
fern.
• From one population, 520 individuals were sampled. The scientist
found the following genotypes represented:
•
•
•
AA = 42
Aa = 145
aa = 333
• From these data calculate the diploid genotype frequencies and
allele frequencies for PGM in this population.
• Use the appropriate statistical test (Chi-square) to determine if
this population is in Hardy-Weinberg equilibrium
• What is your strategy?
•
•
•
1. calculate allele frequency
2. plug frequencies into HW binomial to obtain expected #’s
3. Chi-square analysis to determine if observed differ from expected
Determining if HWE Exists
AA = 42
Aa = 145
aa = 333
1. Determine genotype frequencies
AA (p2) = 42/520 = 0.080
Aa (2pq) = 145/520 = 0.279
aa (q2) = 333/520 = 0.640
2. Determine allele frequency
Total #of PGM alleles in population
= 520 X 2 since individuals are diploid = 1040
Number of A alleles in population:
2X42 + 145 = 229
Frequency of A
A (p) = 229/1040 = 0.220
Number of a alleles in population:
2X333 + 145 = 811
Frequency of a
a (q) = 811/1040 = 0.780
Determining if HWE Exists
A (p) = 0.220
a (q) = 0.780
3. Determine Expected Genotype Frequencies (under HWE)
AA = p2
= (0.220)2 = 0.0484
Aa = 2pq
= 2 X 0.220 X 0.780 = 0.343
aa = q2
= (0.780)2 = 0.608
4. Determine expected # individuals (under HWE)
AA
= 0.0484 X 520 = 25
Aa
= 0.343 X 520 = 178
aa
= 0.608 X 520 = 316
Determining HW Equilibrium
5. Test HWE with Chi-square analysis
compare #’s (not frequencies)
χ2 =
Genotype
Σ
Observed
(O)
(observed – expected )2
expected
Expected
(E)
O-E
(O-E)2
AA
Aa
aa
Sum
(O-E)2/E
Determining HW Equilibrium
5. Test HWE with Chi-square analysis
compare #’s (not frequencies)
χ2 =
Genotype
Σ
(observed – expected )2
expected
Observed
(O)
Expected
(E)
O-E
(O-E)2
(O-E)2/E
AA
42
25
17
289
11.560
Aa
145
178
-33
1089
6.118
aa
333
316
17
289
0.915
X2calculated
= 18.593
Determining HW Equilibrium
6. Evaluate X2 and draw conclusion
• A statistical analysis tests the null hypothesis
• Null hypothesis is that no difference occurs between compared
groups (HWE)
• To do this:
•
•
•
a statistic is calculated (called the X2calculated in our example)
the calculated statistic (called the X2critical in our example) is compared to a
critical statistic which is found on a table
if X2calculated is less than X2critical
•
•
•
Null hypothesis is supported; thus the differences that appear are simply due to chance and are
not real (not statistically different groups)
HWE exists, in our example
if X2calculated is greater than X2critical
•
•
Null hypothesis is rejected; thus the differences that appear are real (statistically different
groups)
HWE does not exist, in our example
critical value
Determining HW Equilibrium
6. Evaluate X2 and draw conclusion
Genotype
Observed
(O)
Expected
(E)
O-E
(O-E)2
AA
42
25
17
289
11.560
Aa
145
178
-33
1089
6.118
aa
333
316
17
289
0.915
X2calculated
= 18.593
X2calculated = 18.593
X2critical = 5.99
Accept or Reject Null Hypothesis?
REJECT
Is population in Hardy Weinberg Equilibrium?
NO
So what has caused the difference?
(O-E)2/E
If population is NOT in Hardy-Weinberg Equilibrium, what
five processes are responsible?
1. non-random mating within population
2. selection for or against any specific allele, which would
alter gene pool (natural selection)
3. genetic drift (the population is large enough not to be
influenced by chance)
4. gene flow into or out of the population (population is
isolated from other populations)
5. mutations are occurring (mutations alter gene pool by
changing one allele into another)
Practice Problem
A botanist is investigating a population of plants whose petal color is
controlled by a single gene whose two alleles (B & B1) are codominant. She
finds 170 plants that are homozygous brown, 340 plants that are homozygous
purple and 21 plants whose petals are purple-brown. Is this population in
HWE? (don’t forget to do the proper statistical test)
Gen Freq
Allele
Freq
HW Gen
Freq
HW #
BB
170
0.320
B
0.340
0.116
61
BB1
21
0.040
B1
0.660
0.449
238
B1B1
340
0.640
0.436
231
sum
531
1.000
1
531
1.000
Practice Problem
A botanist is investigating a population of plants whose petal color is
controlled by a single gene whose two alleles (B & B1) are codominant. She
finds 170 plants that are homozygous brown, 340 plants that are homozygous
purple and 21 plants whose petals are purple-brown. Is this population in
HWE? (don’t forget to do the proper statistical test)
Genotype
Observed
(O)
Expected
(E)
O-E
(O-E)2
(O-E)2/E
BB ( brown)
170
61
109
11803
192
BB1 (purple-brown)
21
238
-217
47214
198
B1B1 (purple)
340
231
109
X2calculated = 441.531
X2critical = 5.99
Accept or Reject Null Hypothesis?
Is population in Hardy Weinberg Equilibrium?
11803
X2calculated
51
441.531
Effects of Natural Selection on the population
Stabilizing
selection
Directional
selection
Disruptive
selection
What is the source of variation?
Mutation
• chemicals and radiation (UV) are mutagens
• incorrect copying
• change base pair sequence
• bonds between DNA bases, repair leads to change in
sequence
Recombination
• viruses transfer DNA from one host to another
• sexual reproduction
• different DNA from two parents
• crossing over
• independent assortment of homologous pairs
The joint action of mutation and
recombination and selection is sufficient to
explain evolution!
Natural Selection
• Variation that is heritable is genetic
• Source of variation is
• random mutation of nucleotide base sequence
• recombination (meiosis)
• independent assortment (meiosis)
• Natural selection can lead to the
appearance of new species given enough
time and acting upon enough traits
Natural Selection
• Nature selects those that survive and reproduce - this is natural
selection
• Natural selection is THE major force in changing allele
frequencies
• Artificial selection - used by Darwin to clarify natural selection
Major Evolutionary Advances
• Life - 3,800 mya
• Prokaryotic cell/autotrophic
• Eukaryotic cell - 1,400 mya
• Multicellar plants - 1,000 mya
• Vascular tissue - 430 mya
• Needed on land - why?
• Seeds - 350 mya
Place on the time scale, the
following events:
1. Earth
2. Prokaryotes
3. Eukaryotes
4. Multicellular life
5. Vascular tissue
6. Seeds
7. Flowers
• Flowers - 140 mya
Earth
Forms
5,000
mya
Prokaryotic
Cells
4,000
mya
Eukaryotic
Cells
3,000
mya
2,000
mya
Multicellular
Plants
1,000
mya
Vascular
tissue
Seeds
Flowers
Numbers of Species on Earth
14 mil
15,000,000
number of
species
however, this
number could
be as high as
112,000,000
10,000,000
5,000,000
1.75 mil
0
total
identified
total estimated
to exist
Species Diversity on Earth
1,000,000
750,000
number of
species
500,000
250,000
0
protists
viruses
insects other higher
plants
fungi
bacteria
animals
Where are these
species?
• Tropics
• 7% of land mass
• 50% of species
Species Concepts
1. Morphological species
• the smallest groups that are consistently distinct
by their morphology
• practical approach
• useful with paleontological specimens
Species Concepts
2. Biological species
• groups of actually or potentially interbreeding natural
populations which are reproductively isolated from other
such populations
• more useful with animals than plants
• genetic isolation (reproductive isolation) critical
• prevent gene flow….
• given enough time the two populations may become
distinct from one another - so distinct...
• that when the populations come back into contact,
reproduction is not possible
Redbud - a small
Geographic Isolation = shrub that has
Reproductive Isolation attractive flowers
(early spring) and
produces nice shade
Eastern Redbud (Cercis canadensis L.)
California Redbud (Cercis occidentalis Torr. ex Gray)
Continental Drift
• The continents are moving, along with the sea
floor, at about 2 inches/year. They don't travel
very far over a human life span, but the
distance adds up over millions of years.
• This animation shows the movement of the
continents over the past 250 million years. It
starts when dinosaurs roamed the earth. At
that time, the continents were all together,
forming one land mass called Pangaea.
• http://www.tectonics.caltech.edu/outreach/ani
mations/drift.html
Continental Drift
• The continents are moving, the sea floor as well, at
about 2 inches/year. They don't travel very far over a
human life span, but the distance adds up over millions
of years.
• This simulation, which is based on current data, shows
the movement of the continents over the past 140
million years. (Note that time is given in the units "Ma,"
which means "millions of years ago.")
• 140 million years ago, dinosaurs roamed the earth. At
that time, the continents were all together, forming one
land mass called Pangaea. Over the next 140 million
years, this land mass broke apart and the pieces
travelled to their current positions.
• http://www.tectonics.caltech.edu/outreach/animations/d
rift2.html
Species Concepts
3. Phylogenetic species concepts
• Based upon reconstructing the evolutionary
history of populations
• character-based concepts
• history-based concepts
Hybrids
• Combining unlike gene sets
• Why can’t two unlike
species form fertile
offspring?
• Problem arises at meiosis
(gamete formation)
• Doubling of chromomsomes
can solve this problem
• Haploid - N
• Diploid – 2N
• Polyploidy – 3 or more sets
•
•
•
•
•
Triploid – 3N
Tetraploid - 4N
Pentaploid – 5N
Hexaploid – 6N
etc
Hybrids
Allopolyploidy
• hybrid between two
different species
(interspecific hybrid)
• sterile because meiosis does
not occur
• doubling of chromosomes
(autopolyploidy) can cure
this problem
• how common is this?
• 70% flowering plants
• 80% of grasses
• read about origin of wheat
(Triticum aestivum) in text
Widespread sterile hybrid
• Horsetail Equisetum X ferrissii
• reproduces by vegetative propagation
Origin of species – how?
1. Allopatric Speciation
• geographically isolated
populations
• gene flow prevented
Origin of species - how?
2. Sympatric speciation
• geographic isolation
NOT required
• polyploidy is
mechanism
• autopolyploids
• allopolyploids
Plant Evolution:
a case study in Astrolepis
desert ferns
• an odd place for ferns,
it would seem!
• several genera
representing a few
hundred species
worldwide
Astrolepis distribution
• in most deserts
of the New
World
• concentrated in
North America
Astrolepis morphology
• pinnately compound
leaves
• species distinguished by:
• leaflet
• size
• lobing
• scale covering
Number of species?
Weatherby (1943)
Tryon (1956)
Notholaena sinuata var. sinuata
Notholaena sinuata var. integerrima
Notholaena sinuata var. cochisensis
Hevly (1963)
Notholaena
Notholaena
Notholaena
Notholaena
Notholaena
Notholaena
sinuata ssp.
sinuata ssp.
sinuata ssp.
sinuata ssp.
integerrima
cochisensis
Cheilanthes
Cheilanthes
Cheilanthes
Cheilanthes
Cheilanthes
sinuata
crassifolia
integerrima
cochisensis
beitelii
Mickel (1979, 1988)
sinuata var. sinuata
sinuata var. robusta
madriensis var. madriensis
madriensis var. madriensis
Techniques in study
1. chromosome counts
• during prophase I
• determine levels of
polyploidy
Meiosis
Diploid spore
mother cell
Tetrad of haploid
homospores
Squash allows
chromosome number to
be determined!
Count the X & O
Squash of cell in prophase I
Techniques in study cont’d
2. allozyme electrophoresis
direction of
migration when
current is applied
origin
(where plants
are placed)
hybrid?
parent
parent
A
B
starch gel
Techniques in study cont’d
2. allozyme electrophoresis
• after
electrophoresis
• different forms of
the enzyme
(isozymes) migrate at
different speeds
(due to differences
in amino acids and
thus electrical
charge)
gel, after
electrophoresis,
stained for specific
enzyme
hybrid?
parent
A
parent
B
Techniques in study cont’d
• Enzyme systems with good
2. allozyme electrophoresis
markers for Astrolepis
• phosphoglucoisomerase
• How many enzymes are tested?
PGI
• How many markers do you
need?
• triosephosphate
isomerase
• Increasing marker number
TPI
increases reliability.
• phosphogulcomutase
PGM
• shikimate
dehydrogenase
SkDH
Data: stained gel
• band patterns additive
between proposed
parents
• thus the parentage of
hybrid is known and
can then be named
Results & Conclusions
• hybridization
is common in
Astrolepis
• hybridization
plays
prominent role
in speciation
within
Astrolepis
A. sin uata
SSS
SS
A. c ochisens is
A. beitelii
CCCC CCC CC
BBB
A. w indhamii
BMS
CMS
A. intege rrim a
CCM
CMM
A. c ra ssifolia
Missing
MM
BMM
BBM
BB
Remember the leaflet morphology?
Results &
Conclusions
•
Thus, morphology
seems to “fits”
genetic species
concept once more
information about
genetics is known