Speciation
Ch 22
Speciation
• Darwin titled his great book, On the Origin of Species.
• In Minnesota (2009), about ¼ of high school biology teachers see
creationism as a viable explanation for the origins of life on Earth.
• A 2009 Gallup poll reported that about 60% of American believe
that humans are not the result of evolution. The same poll found
that 16% of high school biology teachers believe that God created
humans in their present form sometime during the last 10,000
years
• The Creation Museum, opened in 2007 in Petersburg, KY,
presents the Earth as 6,000 years old, with humans and dinosaurs
co-existing.
• Florida’s State Dept of Education adopted NEW standards in
2008 for teaching science, using the word “evolution” for the
FIRST TIME. The adoption passed by only 4-3 and is already
under pressure to allow “alternative” explanations.
• Speciation is highly controversial!!
SPECIATION - Definition
• Folk concept (morphological): All societies give
groups names like dogs, cats, humans, flowers, fish.
Dogs beget dogs, fish beget fish, and people beget
people.
• BIOLOGICAL SPECIES CONCEPT (BSC): a
community of populations (isolated from each other)
that occupies a specific niche in nature and that can
potentially reproduce among themselves.
• CHALLENGES: not always testable (16,000 lakes in
Wisconsin, each with bluegill sunfish. Are they
separate species or the same? Limited to sexual
reproduction. Bacteria exchange genes among
themselves and among unrelated organisms.
• The question, What is a species? is not always
answerable.
Other Concepts
• PHENETIC: smallest groups consistently distinct and
distinguishable by ordinary means, mostly
morphological. If it looks like a duck…Cryptic species,
however, often look very much alike.
• PHYLOGENETIC: smallest group that possesses at
least one diagnostic character fixed in reproductive
units, i.e. ability to do photosynthesis or swim. But may
not look alike.
• CONCLUSION? Different definitions work for
different organisms. species are not fixed and are
not extensions of human behavior or feelings.
Intermixing genetic material is far more fluid and
common that human-centered thinking allows.
Post-mating
Pre-mating
Isolation allows for microevolution to lead to divergence.
Reproductive isolation leads to speciation
Classical isolating mechanisms must eventually be fixed through genetic means, or the
organisms can remain different populations of the same species.
Mechanical Barriers
— Reproductive
•
Female tract
twist
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Penis twist
•
•
•
•
Duck reproduction was studied
primarily by Patricia Brennan at Yale in
2007.
She asked, “Why do duck males have
such large phalluses?” (i.e. 40 cm long
when uncoiled)
Waterfowl (ducks, loons, swans, etc)
often resort to forced copulation,
sometimes involving 2 males and 1
female where males bite and hold the
neck of females during copulation.
Only the most “fit” male ducks can
successfully mate with females, an
example of intra-male competition, as
aspect of sexual selection.
Or an example of female choice since
females can relax their oviducts
(analogous to vagina) if they choose to.
Other species cannot successfully mate
with female water birds.
Considered by Brennan as a malefemale “competitive race” to control
reproduction.
Examples
• POSTMATING
• PREMATING
• Darwin’s finches, • Drosophila
melanogaster mate
Camarhynchus
with D. simulans,
psittacula females
but the male hybrids
won’t choose
are sterile. ONE
mates with beaks
GENE is
as large as those
responsible (nuclear
of C. pauper, a
pore gene)
closely related
species.
Mechanisms of Speciation
• ALLOPATRIC –
geographic reproductive
isolation
• PARAPATRIC adjacent with limited
gene flow until complete
reproductive isolation
• SYMPATRIC speciation within a
panmictic population
(i.e. “interbreeding”)
Ultimately, genetic incompatibility is required for speciation. Genetic incompatibility can
be at the level of the gene or with chromosomal structure. In the Dobzhansky-Muller
Model, simple mutations in key genes can result in one population of an ancestral species
become reproductively incompatible with another population of the same ancestral
species. If that happens, the two populations cannot interbreed successfully (to lead to
viable offspring).
Gene
flow
blocked
Any additional
mutations will
accumulate in one
population but not the
other, leading to
microevolution
divergence.
Why is reproductive
isolation important?
Without reproductive
isolation, gene flow
occurs and ALL offspring
have combinations of
the same alleles and
remain the same species
Modern genetics and the theory of evolution by natural
selection explains the existence of “hybrids”, something
that’s quite mysterious otherwise. When reproductive
isolation is incomplete — incomplete divergence and
speciation — then limited gene flow occurs, but selection
(fitness for survival) is often against the hybrids
• Cattleya mossiae
(left) and Laelia
purpurata (rt) can
interbreed
• But they produce
infertile offspring
• They’re closely
related species
undergoing
“speciation”,
diverging from a
common ancestor.
• Hybrids signify
gene flow
between 2
species
Non hybridizing
species
• Lions and tigers can
interbreed, but more
distantly related
species cannot
interbreed.
• Wolves, coyotes,
dogs (and several
bear species) can
(and do) interbreed
in nature as well as
in captivity
• As divergence
continues for longer
periods of time,
speciation becomes
more obvious to
everyone
Chromosomal fusions often occur. In this
case, two bat species differ from their
ancestral species because of two different
chromosomal fusions. Whole chromosomal
arm translocations are the least disruptive
because the breakage occurs at the
centromere (called centric fusions). The
impact on fertilization and meiosis of a
hybrid, however, is deadly.
We’ve talked about how chromosomal
differences between human and chimp
DNA is not in the gene sequences (about
1.5% difference), but in chromosomal
arrangement.
Human chromosome #2 has split during
chimpanzee evolution into 2 different
chromosomes. It isn’t known when this
happened during the 6 x 106 years since the
human and chimp lineage began diverging,
but the earlier it happened, the earlier
reproductive isolation would have
occurred.
The human-chimp line of primate evolution
shows some frequency in centric fusions in
certain chromosomes because of the
presence of highly repetitive rRNA genes,
possibly leading to breaks because of the
possibility of pairing due to homology and
then crossing over.
In 2005, Stephen Scherer published a paper on
regions of chimpanzee-human DNA “flipping” or
inversions, another potential source of early
reproductive incompatibility (after which
mutations and microevolution continued to the
point where humans and chimps today have
DNA that differs by < 2%).
Scherer’s group identified 1576 presumed
inversions between the human and chimp
species, 33 of which are larger than 100,000
base pairs (human genes average about 60,000
base pairs in length). Comparing chimp DNA
with the gorilla genome (phylogenetic analysis)
demonstrated that at least half the chimp
inversions occurred after the chimp-human
divergence (during human evolution). Some of
the human inversions are polymorphic (humanhuman differences), suggesting ongoing human
evolution. One human inversion is associated
with human colorectal cancer.
Human Evolution I
• Where controversy lies
• Even among people who
agree that evolution is the
over-arching theory for
understanding organisms
(here at Hunter, for
example)
• Human behavior “feels”
different than behavior of
About other animals
4
million• Genetic evidence is that
years
human evolution has
ago
speeded up since
mutation in FoxP2 gene
and evolution of speech
New
Hominid
Species
reported
April 8,
2010
•
• Remains of a 4-ft boy ≃ 1.8 – 1.95 million yrs
old
• Unclear whether it’s genus is Homo or
Australopithecus
• Changes that are decidedly Homo related start
occurring 2.6 million years ago, including shaping
stone tools
• Homo-like face (small protrusion and teeth) but
brain
size and limbs like Australopithecus
http://video.nytimes.com/video/2010/04/08/science/1247467554351/australopithecus-sediba.html
Primate family tree
Given the network nature of organismal evolution and
divergence, should there be a MISSING LINK??
Pan paniscus & Pan
troglodytes
Homo sapiens & Homo
neanderthalensis
Newly reported species
of Australopithecus
sediba
• Doonesbury
2006
Darwin’s finches relate
evolution to developmental
biology and gene expression
(and facial development in
particular).
During embryonic development, all
vertebrate faces look remarkably similar. In
2005 Tabin et al. evaluated two species, the
large ground finch and the cactus finch. In
situ hybridization analysis — using miRNA
linked to fluorescent probes — indicated a
different level of expression of Bmp4. The
authors wanted to test whether a spatial or
temporal change in Bmp4 could account for
the relative size & shape beak differences.
Bmp4 expression levels control beak depth and height.
To test this, they
misexpressed Bmp4
throughout the chick
frontonasal prominence
and converted the
narrower, short chick
beak into a much broader
bigger beak. Bmp4
promotes chondrogenesis
and beak growth.
Natural selection of
individuals with different
levels of Bmp4 expression
leads to beak evolution
and speciation
Helms J A et al. Development 2005;132:851-861
• From 25 October 2013 [Science, 342: 440]
• Study identified enhances that “fine tune expression
of genes during craniofacial development in mice.”
• The authors conclude that variation in enhancers
may contribute to “the spectrum of facial variation in
human populations.”
• Craniofacial enhancers are located in the human
genome regions associated with craniofacial birth
defects such as cleft lips and cleft palates.
• Noncoding regions may also contribute to changes in
expression of the Bmp4 gene.
1. Plot the number of species of
Darwin’s finches and the
number of islands in the
Galàpagos archipelago as a
function of time.
2. Are the data consistent with
the hypothesis that isolation
of populations on newly
formed islands is related to
speciation in this group of
birds? Why or why not?
3. If no more islands form in the
Galàpagos archipelago , do
you think that speciation of
geographic isolation will
continue to occur among
Darwin’s finches? Why or why
not? What additional data
could you collect to test your
hypothesis (without waiting to
see if speciation actually
occurs)?