Download BLY 303 Lecture Notes, Guest Lecture 10/27/11 (O`Brien) Patterns in

BLY 303 Lecture Notes, Guest Lecture 10/27/11 (O’Brien)
Patterns in Evolution
I.
Characteristics of a good scientific theory
A.
PARSIMONY
1.
Simplicity is good
2.
Okham’s Razor: When considering a group of conflicting
explanations, recommends choosing the most simple hypothesis
3.
Example of geocentric vs. heliocentric solar systems
a.
Problem of RETROGRADE motion of planets when instead
of moving west-to-east in the sky, occasionally a planet’s
movement reversed to east-to-west.
b.
Ptolemy’s Geocentric Model used rotating orbits that orbited
the earth
c.
Copernicus’s Heliocentric Model assumed planets rotated
around the sun
B.
TESTABLE
1.
If a theory is not falsifiable, it is not scientific
2.
Church needed to accurately predict the date of Easter Sunday:
defined as the Sunday following first the full moon following the
vernal equinox.
3.
Heliocentric Model proved to be more accurate than geocentric
II.
PHYLOGENY
A.
Defined: evolutionary history of a group of organisms
1.
Usually depicted as a branching PHYLOGENETIC TREE
2.
Where 2 branches diverge or split on “tree” indicates where an
ancestral group split into 2 (sometimes more) descendent groups
B.
The only figure in Darwin’s Origin of Species was a hypothetical tree of
life showing how lineages diverge from common ancestors
III.
Inferring Phylogenetic History
A.
Assume that changes in species characters require significant periods of
time, thus the fewer differences in characters between groups the more
recently they shared a common ancestor
B.
Problem has been what characters should be considered when comparing
taxonomic groups
1.
Historically, taxonomists could only realistically consider a few
characters before being overwhelmed with data
2.
Is leg structure (Uniramia) more or less important than
morphologies of mouth parts (Mandibulata)?
C.
Recent Revolution in taxonomic techniques
1.
DNA sequencing provides far more data than morphological
measurements, especially important with…
a.
prokaryotic Eubacteria
2.
D.
E.
b.
eukaryotic protists
Computers enable statistical comparisons to be made of very large
data sets.
Terms
1.
MONOPHYLETIC taxon or CLADE: all members share a
common ancestry
2.
POLYPHYLETIC taxon: although species may share
characteristics, they don’t share a recent common ancestor
3.
OUTGROUP
a.
A taxon that diverged from another taxonomic group before
members of the other group diverged from one another
b.
Used in statistical comparisons preparing phylogenetic
trees
(1)
A type of control
(2)
Compare how often new characteristics are acquired
c.
An outgroup should be relatively close to group being
analyzed, but does not share a common ancestor with it.
(1)
Choosing an outgroup relies on previous research
(2)
Rodents and marsupials are more distantly related
to primates than primates are to one another
4.
ANCESTRAL trait
a.
Present in common ancestor
b.
Sometimes called “primitive” or “plesiomorphic” trait
c.
Recorded as 0
5.
DERIVED trait
a.
Evolved after separation from common ancestor
b.
Sometimes called “advanced” or “apomorphic” trait
c.
Recorded as 1
Example
1.
Four species descend from 1 ancestor
2
2.
3.
F.
IV.
10 characteristics are followed (a through j)
Note that the common ancestor, Ancestor 1, has 0 for all character
states
4.
Tic marks along branches indicate the evolution of a character
state; 0 becomes 1
5.
There are 3 monophyletic groups
a.
Species 2 & 3
b.
Species 1, 2 & 3
c.
Species 1, 2, 3 & 4
6.
Species 2 & 3 group
a.
Share 7 character states: a0, b0, c1, d0, e0, f1, & j0
b.
Because they share more character states than any other
pair, they are most similar and share the most recent
common ancestor (Ancestor 3)
7.
Species 1 is more similar to Species 2 & 3 than to Species 4
a.
Species 1, 2, & 3 share 4 characteristic states. a0, b0, c1, f1
b.
Common Ancestor 2
8.
Relatedness between Species 1 & 4
a.
Share 4 states (g0, h0, i0, j0)
b.
None of the shared states are derived states
Complications
1.
HOMOPLASIOUS CHARACTER:
a.
Evolved 2 or more times in a phylogenetic tree.
b.
Does not have a unique origin
c.
Taxa that share trait did not inherit it from a common
ancestor
d.
CONVERGENT EVOLUTION: Independent origin of a
derived characteristic trait in two or more taxa
e.
EVOLUTIONARY REVERSAL: Loss of a derived
character to ancestral state (1 becomes 0)
The Method of Maximum Parsimony
A.
Two problems
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B.
C.
D.
V.
1.
How can one tell which state of a character is derived?
2.
How can one tell if a character is uniquely derived or homoplasious?
Occasionally the fossil record helps
The best estimate of the true phylogeny is the one that requires the fewest
evolutionary changes
The best phylogenic hypothesis is the one that requires the fewest
homoplasious changes
Evolutionary Processes
A.
Definition of Evolution: Change in the genome or the frequency of alleles
in a population over time
B.
Mechanisms that shift genetic frequencies in populations over time
1.
NATURAL SELECTION
a.
Differential reproductive success or FITNESS
b.
Individuals with certain heritable traits produce more
offspring than individuals without those traits
c.
Results in a change in the genetic makeup of the population
d.
Works by reducing frequency and eventually removing
alleles from population
2.
SEXUAL SELECTION
a.
Differential reproductive success as a result of variation in
the ability to obtain mates
b.
Usually females chose mates
c.
Characters the females chose may put males at risk of
predation
(1)
Elaborate horns
(2)
Bright plumage
d.
Hamilton-Zuk Hypothesis
(1)
SECONDARY SEX CHARACTERISTICS
(a)
Associated with an individual’s gender
and maturation
(b)
Not directly involved with reproduction
(c)
Often appear to be poorly adapted for
predator avoidance
i
Not cryptic
ii
Rather than provide camouflage,
many appear to call attention to
the bearer
(2)
Asked what is the selective advantage for secondary
sexual characteristics?
(3)
Proposed that females may chose traits that reflect
the health of males.
(a)
Sick males would tire during courtship dances
(b)
Loss of bright feathers or presence of dull
plumage might indicate that those birds were
infested with fleas and other parasites
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(c)
C.
Easily noticed males have avoided
predators
(4) Methods
(a)
Ranked birds (both species and individuals
within species) on degree of sexual display
i Brightness of plumage
ii Variability of song
(b) Examined birds for blood parasites
(5)
Results: Rejected the null hypothesis of there being
NO correlation between sexual selection and blood
parasites
(6)
However, experimental tests in other systems have
not always rejected the null hypothesis
3.
GENETIC DRIFT
a.
Allele frequencies change randomly
b.
Changes usually occur by appearance of neutral mutations
c.
Often seen in small, isolated populations
4.
GENE FLOW
a.
Occurs when individuals migrate into (usual mechanism) or
leave a population
b.
New individuals introduce new alleles to population
5.
MUTATION
a.
Modifies gene frequencies by adding new alleles
b.
New alleles may be beneficial, detrimental, or have no
effect on fitness
Founder Effect, bottlenecks and Genetic Drift
1.
The FOUNDER EFFECT
a.
Occurs when a new population is started a few individuals
(1)
Sometimes just one mating pair
(2)
Could be one inseminated female, such as an insect
b.
Single individuals can only carry 2 alleles at the most
c.
The genetic variability of the new population will be much
less than the population from which the founders originated.
d.
Often observed with…
(1)
…island populations
(2)
…introduced species
2.
GENETIC BOTTLENECKS
a.
Occurs in situations when the number of individuals within a
population is low.
b.
Genetic diversity within the population is low even if the
population is able to later expand to a large size
c.
Observed or associated with…
(1)
Founder effect
(2)
Introduced/invasive species
(3)
Species at risk of extinction
(a)
California condor
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(b)
3.
Siberian tigers
Genetic Drift
a.
Because the number of individuals is small in these
populations, genetic drift is a common occurrence
b.
Frequencies of alleles that are rare in the original population
can dominate even though they offer no known selective
advantage
c.
On the contrary, some alleles may be considered detrimental
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