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Speciation
Discussion How does a new species arise?
First…what is a species? Biological species concept
population whose members can interbreed & produce viable, fertile offspring
reproductively compatible
Western MeadowlarkEastern Meadowlark
Distinct species:songs & behaviors are different enough to prevent interbreeding
Distinct species:songs & behaviors are different enough to prevent interbreeding
But that doesn’t capture every situation
Consider Ensatina salamanders. How many species? Which ones are different species?
Species Definitions Other definitions include:
Morphological or typological - They conform to the same body plan.
Phylogenetic or evolutionary - Share a common ancestor and a unique evolutionary history.
Ecological - Share a specific niche, unique to them and them alone.
“Species” is a human language box. Never forget that nature exists on a continuum!
Discussion Which definitions work or don’t work to
determine whether or not you’re examining different species if you’re studying… Bacteria in a lab petri dish? Hooved mammals in the modern-day
arctic? Dinosaurs? Ancient algae?
How and why do new species originate? Species are created by a series of evolutionary
processes populations become isolated - no gene flow between
them geographically isolated and/or reproductively isolated
isolated populations evolve independently
Isolation allopatric
geographic separation sympatric
still live in same area
An obstacle to mating or fertilization
PRE-zygotic barriers
behavioral isolation
geographic isolation ecological isolation temporal isolation
mechanical isolation gametic isolation
Geographic isolation Species occur in different areas
physical barrier allopatric speciation
“other country”
Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel
Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel
Ammospermophilus spp
Ecological isolation Species occur in same region, but occupy different habitats so rarely
encounter each other reproductively isolated
2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial
2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial
lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest
lions & tigers could hybridize, but they live in different habitats: lions in grasslands tigers in rainforest
Temporal isolation Species that breed during different times of day, different
seasons, or different years cannot mix gametes reproductive isolation sympatric speciation
“same country”
Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer
Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer
Behavioral isolation Unique behavioral patterns & rituals isolate species
identifies members of species attract mates of same species �
courtship rituals, mating calls reproductive isolation
Blue footed boobies mate only after a courtship display unique to their species
Blue footed boobies mate only after a courtship display unique to their species
sympatric speciation?
firefly courtship displaysfirefly courtship displays
courtship display of Gray-Crowned Cranes, Kenya
courtship display of Gray-Crowned Cranes, Kenya
courtship songs of sympatricspecies of lacewings
courtship songs of sympatricspecies of lacewings
Recognizing your own species
Mechanical isolation Morphological differences can prevent successful
mating reproductive isolation
Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.
Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen.
Plants
sympatric speciation?
Mechanical isolation For many insects, male &
female sex organs of closely related species do not fit together, preventing sperm transfer
lack of “fit” between sexual organs: hard to imagine for us… but a big issue for insects with different shaped genitals!
Damsel fly penises
Animals
Gametic isolation Sperm of one species may not be able to fertilize eggs of
another species mechanisms
biochemical barrier so sperm cannot penetrate egg receptor recognition: lock & key between egg & sperm
chemical incompatibility sperm cannot survive in female reproductive tract
Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.
Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.
sympatric speciation?
POST-zygotic barriers Prevent hybrid offspring from
developing into a viable, fertile adult reduced hybrid viability reduced hybrid fertility hybrid breakdown
zebroid
Reduced hybrid viability Genes of different parent species may
interact & impair the hybrid’s development
Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.
Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.
sympatric speciation?
Mules are vigorous, but sterile
Reduced hybrid fertility Even if hybrids are vigorous
they may be sterile chromosomes of parents may differ in number
or structure & meiosis in hybrids may fail to produce normal gametes
Donkeys have 62 chromosomes(31 pairs)
Horses have 64 chromosomes(32 pairs) Mules have 63 chromosomes!
Hybrid breakdown Hybrids may be fertile & viable in first
generation, but when they mate offspring are feeble or sterile
In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile.On path to separate species.
In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile.On path to separate species.
sympatric speciation?
Niles EldredgeCurator
American Museum of Natural History
Rate of Speciation When considering speciation events over
geological time: Does speciation happen gradually or rapidly, uniformly or unevenly? Gradualism
Charles Darwin Charles Lyell
Punctuated equilibrium Stephen Jay Gould Niles Eldredge
Gradualism Gradual, constant
divergence over long spans of time
big changes occur as the accumulation of many small ones
events can increase or decrease speciations worldwide, but overall speciation proceeds fairly regularly
Punctuated Equilibrium Rate of speciation is not
constant Organisms are in
“stasis” for much of their history, with little or no change
When speciation occurs, it tends to be in a rapid burst
Species undergo rapid change when they 1st bud from parent population
Time
Discussion Based upon what you know of
evolutionary history, where do you fall: gradualism or punctuated equilibrium supporter?
Speciation Rates Regardless of whether punctuated
equilibrium or gradualism holds, speciation rates vary by species and circumstance Speciation can occur over a scale of
millions of years, or much more rapidly! Polyploidy in plants increases
speciation rate to, in some cases, only a few years
Polyploidy and Hybrid Speciation Unlike in animals, in plants, duplicating
the genome (polyploidy) isn’t fatal. Plants hybridize more often and more
readily than animals on average
Sometimes in plants, a diploid hybrid is sterile, but a triploid or tetraploid hybrid isn’t due to the
mechanisms of chromosome alignment in their haploid life
phase.
Polyploidy and Hybrid Speciation Polyploid offspring may reproduce with other
polyploids, or re-reproduce with a parental type, or may self-fertilize (oh plants, you so crazy)
But by any of the three mechanisms, polyploids wind up reproductively isolated from the parental population, but produce non-sterile offspring = they’re a new population that evolution will be acting upon! A new species, in as little as a generation!
Polyploidy and Hybrid Speciation This has been observed in species like
the Evening Primrose, Raphanobrassica, Hemp Nettle, and the Maidenhair Fern.
Speciation Rates In all species, when a new habitat or
new niche becomes available, speciation rates tend to increase
Adaptive radiation - ecological & phenotypic diversity in a rapidly multiplying lineage
Discussion Scientists generally break it down into
two main reasons why this causes a burst in speciation events. What do you think they could be?
Speciation Rates Ex: Darwin’s finches
Ex: An explosion in bivalve species diversity after the loss of brachiopods in the “Great Dying,” or Permian extinction 250 mya
Extinction But, of course, extinction rates also
fluctuate Higher in times of environmental stress
Million years ago
% of familiesextinct
Discussion A population’s ability to respond to
environmental changes is dictated, in part, by its level of genetic diversity.
Which do you think is most resistant to extinction and why: high-genetic-diversity or low-genetic-diversity?
Lines of EvidenceMorphological,
Molecular, and Other
Lines of Evidence Modes of investigation into evolutionary history
include Morphological Molecular Developmental (which is part Morphology, part
Molecular Biology) Geographical* Geological* Active change
* - Not addressed in notes - read up on basic definition of biogeography, fossil record at home
Discussion What is the relationship between:
Recency of two populations’ last common ancestor
Amount of similarity between populations
Degree of relatedness between populations
…and WHY?
Morphological Evidence Morphology = body form
Shared deep body structures are evidence of shared ancestry, but appearances and functions aren’t necessarily… why not?
Anatomical record Homologous structures
similarities in characteristics resulting from common ancestry
Homologous structures Similar structure Similar development Different functions Evidence of evolutionary
relationship
spines
tendrilssucculent leaves
colored leaves
Homologous structures
leaves
needles
Homologous Structures Produced by divergent evolution
Your typical “population divided, evolves in two separate directions” scenario
Structure present in ancestor passed down to descendents
Analogous structures
Separate evolution of structures similar functions similar external form different internal structure &
development different origin no evolutionary relationship
Solving a similar problem with a similar solutionSolving a similar problem with a similar solution
Analogous Structures Flight evolved in 3 separate animal groups
evolved similar “solution” to similar “problems”
Analogous Structures Fish: aquatic vertebrates Dolphins: aquatic mammals
similar adaptations to life in the sea
not closely related
Those fins & tails & sleek bodies are
analogous structures!
Analogous Structures Analogous structures produced by
convergent evolution or parallel evolution Convergent evolution: Two
separate, asynchronous (different times, different ecospaces) evolutionary lineages develop a similar trait/solution Example: pillbugs and pillmillipedes
both develop similar defenses, but didn’t inherit them from a so-defended shared ancestor
Parallel Evolution Like convergent evolution, but the two species evolve at
the same time and/or in the same ecospace filling similar ecological roles in similar environments, so
similar adaptations were selected
marsupial mammalsmarsupial mammals
placental mammalsplacental mammals
Vestigial structures Modern structures that have reduced or no
function remnants of structures that were functional in
ancestral species deleterious mutations accumulate in genes for
non-critical structures without reducing fitness eyes on blind cave fish
are a kind of homology
Vestigial organs Hind leg bones on whale fossils and
modern whales
Why would whaleshave pelvis & leg bonesif they were always
sea creatures?
Vestigial structures Spurs or tiny leg bones in snakes
Vestigial structures Arrector pili, post-caudal tail, appendix
in humans
Molecular Evidence Evidence from genes & proteins The most powerful and commonly-used
these days, in part because the data set is so vast and in part because it’s easily quantifiable
“Conservation” What does it mean to say a
homologous sequence or structure is “highly conserved?” Means it’s extremely similar or identical
amongst the organisms that inherited it Conserved sequences = useful
evidence in uncovering ancestry
Conserved Structures Example, metabolic pathways = highly
conserved across all domains of life (archaea, bacteria, eukarya)
A remnant of life’s common ancestry
Bacterial metabolic enzymes - notice, more of them are common to all 3 domains than are at all unique
Conserved Structures Example: Structural evidence supports
the relatedness of all eukaryotes More than just the nucleus is
conserved… linear chromosomes, membrane-bound organelles, and endomembrane systems are as well
Molecular record Molecular evidence elegantly demonstrates the
relatedness of all life universal genetic code! The ultimate “conserved
sequence” - the whole darned thing! DNA, RNA, proteins - genome, transcriptome, proteome
Closely related species have sequences that are more similar than distantly related species DNA & proteins are a molecular
record of evolutionary relationships
Closely related species have sequences that are more similar than distantly related species DNA & proteins are a molecular
record of evolutionary relationships
Discussion “The more similar genetic loci two
populations share, the more related they are”
WHY would this be??
Conserved Sequences Think of a conserved sequence (which
can be as little as a single base pair) as being a genetic homologous structure
Conserved Sequences Suppose an ancestral population has
the sequence
AAGTCTTTAGCTAGCTGGCTGT
at a particular locus.
Over time, it accumulates mutations. Demo!
AAGTCTTTAGCTAGCTGGCTGT
AAGTCTTTATCTAGCTGGCTGT AAATCTTTAGCTAGCTGGCTGT
AAATATTTAGCTAGCTGGCTGT
AAATCTTTAGCTAGCTGTCTGTAGGTCTTTATCTAGCTGGCTGT
AAGTCTTTATCTAGCTGGCTGG
CAGTCTTTATCTAGCTGGCTGG
AAGTCTTTATCTAGCTGGGTGG
AAATATTTCGCTAGCTGGCTGT
AAATATTTAGCCAGCTGGCTGT
AAATCTCTAGCTAGCTGTCTGT
AAATCCTTAGCTAGCTGTCTGT
P
F1
F2
F3
What % of DNA do these two share?Are they closely related?
What % of DNA do these two share?Are they closely related?
Can you spot any conserved sequences among the modern (F3) species?
Discussion Suppose you have this information for locus
ß135 in three similar species.
Species A: AGCTTCGATTGCTAGCTA
Species B: AGCTACGATTGGTAGCTA
Species C: AGCTACGACCTTGGTAGCTA
Who’s most related? Who shares the most recent LCA?
It works for proteins, too!
Number of amino acid differences betweenhemoglobin (146 aa) of vertebrate species and that of humans
Number of amino acid differences betweenhemoglobin (146 aa) of vertebrate species and that of humans
100 20 30 40 50 60 70 80 90 100 110 120
LampreyFrogBirdDogMacaqueHuman
328 45 67 125
Why does comparingamino acid sequencemeasure evolutionary
relationships?
Molecular Evidence An organism’s evolutionary history is
documented in its genome! How many similarities are shared between
populations? DNA hybridization experiments
Track SNPs (single nucleotide polymorphisms), conserved sequences, common loci, duplicated genes
Analyze pseudogenes (“vestigial genes”) Even analyze whole genomes…
Genome sequencing What can data from whole
genome sequencing tell us about evolution of humans?
Primate Common AncestryChromosome Number in
the Great Apes (Hominidae)
orangutan (Pogo) 48gorilla (Gorilla) 48chimpanzee (Pan) 48human (Homo) 4646
Chromosome Number in the Great Apes
(Hominidae)
orangutan (Pogo) 48gorilla (Gorilla) 48chimpanzee (Pan) 48human (Homo) 4646
Hypothesis:Hypothesis:Change in chromosome number?Change in chromosome number? If these organisms share a common ancestor, then is there evidence in the genome for this change in chromosome number
Hypothesis:Hypothesis:Change in chromosome number?Change in chromosome number? If these organisms share a common ancestor, then is there evidence in the genome for this change in chromosome number
Could we have just lost a pair of
chromosomes?
Chromosomal fusionTestable prediction:Testable prediction: If common ancestor had 48 chromosomes (24 pairs),then humans carry a fused chromosome (23 pairs).
Testable prediction:Testable prediction: If common ancestor had 48 chromosomes (24 pairs),then humans carry a fused chromosome (23 pairs).
Centromere
Telomere
Ancestral Chromosomes Fusion Homo sapiens
Inactivated centromere
Telomere sequences
Chromosome Number in the Great Apes
(Hominidae)
orangutan (Pogo) 48gorilla (Gorilla) 48chimpanzee (Pan) 48human (Homo) 4646
Chromosome Number in the Great Apes
(Hominidae)
orangutan (Pogo) 48gorilla (Gorilla) 48chimpanzee (Pan) 48human (Homo) 4646
Test of the Human Genome“Chromosome 2 is unique to the human lineage of evolution, having emerged as a result of head-to-head fusion of two chromosomes that remained separate in other primates. The precise fusion site has been located in 2q13–2q14.1, where our analysis confirmed the presence of multiple subtelomeric duplications to chromosomes 1, 5, 8, 9, 10, 12, 19, 21 and 22. During the formation of human chromosome 2, one of the two centromeres became inactivated (2q21, which corresponds to the centromere from chimp chromosome 13) and the centromeric structure quickly deterioriated.”
“Chromosome 2 is unique to the human lineage of evolution, having emerged as a result of head-to-head fusion of two chromosomes that remained separate in other primates. The precise fusion site has been located in 2q13–2q14.1, where our analysis confirmed the presence of multiple subtelomeric duplications to chromosomes 1, 5, 8, 9, 10, 12, 19, 21 and 22. During the formation of human chromosome 2, one of the two centromeres became inactivated (2q21, which corresponds to the centromere from chimp chromosome 13) and the centromeric structure quickly deterioriated.”
Hillier et al (2005) “Generation and Annotation of the DNA sequences of human chromosomes 2 and 4,” Nature 434: 724 – 731.
Chr 2
Ancestral Chromosomes Fusion Homo sapiens
Inactivated centromere
Telomere sequences
Human Chromosome #2 shows the exact point at which this fusion took place
Human Chromosome #2 shows the exact point at which this fusion took place
Discussion If you want to analyze the evolutionary
history of an order, a phylum, a kingdom, etc., what kinds of genes do you think would be most productive to study?
Molecular Clocks Some genes show a constant rate of
mutation/evolution They can be used as molecular clocks and
used to calculate the time since divergence, calibrated against branches whose dates are known from the fossil record
Divergence time (millions of years)Nu
mb
er
of
mu
tati
on
s
90
60
30
30 60 900
Molecular Clocks Example: Use of molecular clocks
demonstrates that HIV leapt from simians to humans in the 1930s
Year
HIV
Range
Adjusted best-fit line(accounts for uncertaindates of HIV sequences)
0.20
0.15
0.10
0.05
01900 1920 1940 1960 1980 2000
Ind
ex o
f b
ase
chan
ges
bet
wee
n H
IV g
ene
seq
uen
ces
Evo-Devo: Morpho+DNA Evidence Comparative embryology reveals anatomical
similarity not visible in adults Ex: all vertebrate embryos have similar
structures at similar stages of development gill pouch in fish, frog, snake, birds, human, etc.
Pharyngealpouches
Post-analtail
Human embryoChick embryo (LM)
Evo-Devo Major changes in body form can
result from changes in the sequences and regulation of developmental genes Genes that program development
control the rate, timing, and spatial pattern of changes in an organism’s form as it develops
Ex: A change in the rate of gene expression produces very different skulls from the same genes
Chimpanzee fetus -> adult
Human fetus -> adult
Evo-Devo Ex: A change in spatial expression of
the Hox gene produces body parts in a new location, without a change in coding genes
Hox gene 6 Hox gene 7 Hox gene 8
About 400 mya
DrosophilaArtemia
Repeated Hox expression extends body
Evo-Devo Among MANY other evolutionary
applications, these means that modern organisms have many ancient genes still in place that can be used to study the ancestral form Those genes are just producing
different phenotypes now because of changes to regulatory sequences
Active Change Examples of ongoing change
Artificial selection Antibiotic resistance Industrial melanism
Artificial selection Artificial breeding can use variations in
populations to create vastly different “breeds” & “varieties”
“descendants” of the wolf
“descendants” of wild mustard
Selective breeding
the raw genetic material (variation) is hidden there
Selective breedingHidden variation can be exposed through selection!
Antibiotic Resistance
Industrial Melanism Classic Peppered Moth study: Dark vs.
light variants
Year % dark % light1848 5 951895 98 21995 19 81
“Tree Thinking”Phylogenetics,
Cladistics, Systematics
“Tree Thinking” Phylogeny is the evolutionary history of
a species or group of related species Systematics uses fossil, molecular, and
morphological data to infer evolutionary relationships and classify organisms
Depict these relationships in branching cladograms or phylogenetic trees
Order Family
Pantherapardus(leopard)
Genus Species
Canislatrans(coyote)
Taxideataxus(Americanbadger)
Lutra lutra(Europeanotter)
Canislupus(gray wolf)
Felid
ae
Carn
ivora
Pan
thera
Taxid
ea
Mu
stelidae
Lu
tra
Can
idae
Can
is
Phylogenetic Trees A phylogenetic tree is a hypothesis
about evolutionary relationships Moves forward in time from the root Each branch point represents a shared
common ancestor (usually not labeled) Sister taxa represented by the tips of
the branches
Branch point:where lineages diverge
ANCESTRALLINEAGE
This branch pointrepresents thecommon ancestor oftaxa A–G.
This branch point formsan unresolvedpattern of divergence, sharedby taxa D, E, and F.
Sistertaxa
Taxon A
Taxon B
Taxon C
Taxon D
Taxon E
Taxon F
Taxon G
Lizards and snakes
Crocodilians
Ornithischiandinosaurs
Saurischiandinosaurs
Birds
Commonancestor ofcrocodilians,dinosaurs,and birds
Discussion – Who is most related? Where’s the LCA?
Phylogenetic Trees - Limitations & Rules Show patterns of descent,
not phenotypic similarity. Don’t necessarily indicate how long ago species diverged, or how much it’s changed since then.
Taxa DID NOT evolve from any sister taxa!
Cladistics Trees are constructed based on
homologies, physical or molecular Cladistics groups organisms by
common descent. A clade = a group of species that
includes an ancestral species and its descendents
To be valid, a clade must be monophyletic - include all descendent species, and no non-descendents
(a) Monophyletic group (clade)
Group
A
B
C
D
E
F
G
Cladistics Invalid clades are
Paraphyletic – Includes the ancestor but not all of its descendents
Polyphyletic - Includes some species that do not share the ancestor in question
(a) Monophyletic group (clade) (b) Paraphyletic group (c) Polyphyletic group
Group
Group
Group
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A
B
C
D
E
F
G
Valid Clades Are reptiles monophyletic, paraphyletic,
or polyphyletic? Should “reptile” be a valid
classification?
Cladistics When comparing it to its ancestor, a
species/clade displays various homologies. Shared ancestral characters - originated
in an ancestor, shared with all members.
Shared derived characters - An inherited novelty unique to that clade.
Cladistics Can use either morphological or
molecular homologies Generally, a chart with + or 1 for “has
it,” 0 or - for “doesn’t”
To construct a tree, examine shared and derived characters, and follow the principle of parsimony, also called Occam’s Razor: the simplest explanation is usually the best.
TAXA
Vertebralcolumn
(backbone)
Four walkinglegs
Hinged jaws
Amnion
Hair
(a) Character table
CH
AR
AC
TE
RS
La
nc
ele
t(o
utg
rou
p)
La
mp
rey
Ba
ss
Fro
g
Tu
rtle
Le
op
ard
0
0
0
0
0
1
0
0
0
0
1
1
0
0
0
1
1
1
0
0
1
1
1
1
0
1
1
1
1
1
Discussion – Practice!
Variations Cladograms can be constructed to
show time or distance since divergence, amount of difference…
Trees can take different shapes: angled splits, square splits, circular…
Branch lengths indicate time
Mouse
Human
Chicken
Frog
Zebrafish
Lancelet
Drosophila
Present
CENOZOICMESOZOICPALEOZOIC
Millions of years ago542 251 65.5
Tree Shapes
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