Download Lecture 2.3. The Origin of Species.

Lecture 2.3. The Origin of Species.
“In the same way, we can account for the separate islands
having each their peculiar species, either on the supposition that the same original emigration peopled the whole of
the islands with the same species from which differently
modified prototypes were created, or that the islands were
successively peopled from each other, but that new species have been created in each on the plan of the preexisting ones” (A. R. Wallace (1855), p. 188).
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Species the Fundamental Units of Evolution.
1. Theory Special Creation as
articulated by Lyell allowed
that species could vary, but
could not “depart indefinitely from the original type.”
2. We now know that species
descend the one from the Common descent according
other. BUT
to Darwin. From The Origin of
Species.
3. Species nonetheless possess a permanence and relative stability not observed
in subspecies or varieties.
4. Of the several taxonomic categories – species, genera,
families, etc.
a. Only species can be defined unambiguously, and
then only in certain cases.
b. The remainder can only be defined subjectively.
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 Biological Species Concept (BSC)
“Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." [Ernst
Mayr (1963), p. 19]
1. Promoted by Ernst Mayr, but the idea actually predates Darwin.
2. With regard to the definition, note the following:
a. "Interbreeding" refers both to mating and to the
production of fertile / viable offspring.
b. "Potentially" refers to the fact that individuals of
the same species living in different places would
interbreed if given the opportunity.
c. “Reproductively isolated" means that mating
cannot occur, i.e., for structural or behavioral reasons (pre-zygotic isolating mechanisms), or that
offspring, if produced, are sterile and / or doomed to
die before reproducing (post-zygotic isolation).
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d. "Natural" refers to the
fact that reproductive
isolation can only be
assessed with regard to
what transpires in nature.
e. What goes on in zoos
and the breeding pen is
immaterial – e.g., ligers
and tigons.
3. BSC only applies to sexual
species.
4. BSC identifies gene flow
as the cohesive force that
holds species together.
Summer and winter distributions of Baltimore and Bullock’s orioles. Note the regions of hybridization.
5. Gene flow opposes geographic variation in selective
pressure.
6. In widely distributed species, the tension between local
selection and gene flow produces clines.
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Previously classified as separate species, Baltimore and
Bullock orioles interbreed in the western Great Plains. They
are now considered geographic races of a single species.
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Rose breasted and black-headed grosbeaks also interbreed
in the western Great Plains, but manifest assortative mating
and are consequently considered “good” species.
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 Diminution of a Melanic Cline in Britain.
1. B. betularia historically evidenced clinal variation.
2. Frequency of principal melanic morph (carbonaria)
increased along rural to
urban transects.
3. Steepness of one such
cline has diminished since
1975 (Saccheri et al.,
2008).
4. Consistent with
Clinal variation in B. betularia.
Top. Environmental transect;
a. Less pollution following industrial areas shaded. Botpassage of clean air reg- tom. Carbonaria frequency
ulations in the 1960s.
along transect. Circles – 19691975; squares – 2002. Both
b. Selection against carbo- data sets only intelligible if one
considers time, location and
naria.
selection. From Saachen et al.
(2008).
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 Questions.
1. (8 pts) What is the adaptive significance of Allen’s rule illustrated in the picture below?
Allen's Rule in North American hares (Lepus spp.).
From the northern arctic hare (L. arcticus) through the
more southerly desert jackrabbit (L. alleni), members of
the genus Lepus show progressively longer extremities
(legs and ears) and leaner bodies. In this case, the cline
is composed of species as opposed to geographic races.
2. (8 pts) The picture at the
right indicates an inverse
relationship between temperature (50+ year averages) and body mass in
woodrats. Discuss.
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 Other Definitions of Species.
1. Morphological species
concept (MSC).
a. Based on anatomical
differences.
b. Look for clustering in
character space.
c. Differs from “classical”
systematics which em- The morphological species
phasizes
“conserva- concept looks for clusters in
tive” characters – e.g., character space.
flowers vs. leaves.
d. Can be applied to asexual species and fossils.
e. Fails if distributions do not segregate cleanly –
Lamarck’s problem.
f. Can incorrectly split single species into many –
think domestic breeds of cattle, dogs, pigeons,
etc.
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2. Phylogenetic species concept (PSC).
a. Morphological species
concept in a new guise
– substitutes “objective”
methods of cladistic
analysis for the biologist’s judgment.
Phylogenetic species concept
b. Species are the tips of identifies species with “tip”
phylogenetic trees – taxa.
i..e., smallest monophyletic groups.1
c. Like MSC, PSC can be applied to asexual species and fossils.
d. For most groups, the requisite data and analyses
lacking / haven’t been performed.
e. Can incorrectly split single species into many,
i.e., like morphological concept.
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As discussed later, a monophyletic group consists of an ancestral group
and all of its descendants.
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 Allopatric (geographic) speciation.
1. Formerly isolated populations remerge or speciate.
2. If populations remerge, gene flow follows – introgression.
3. Hybrid infertility / inviability (post-zygotic reproductive isolation) selects for pre-zygotic isolation and
ecological divergence.
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 Hybridization.
1. Can facilitate expansion
of species into new areas.
2. Coyotes an example.
a. Have recently extended
their range eastward.
b. Northern expansion has
proceeded more rapidly
than in the south.
Expansion of coyotes in the
northeastern United States.
Postulated wolf-coyote in the
north based on analysis of
mitochondrial DNA analysis
of eastern coyotes and
source populations.
c. In the north, but not in
south, wolf genes incorporated into coyote genome.
d. Reflected by larger body size that allows them to hunt
deer.
3. Now recognized that hybridization can produce new
species.
a. Recall the 19th century debate pre-Darwin and Mendel’s interest in the stability of hybrids.
b. See HW question on Darwin’s finch below.
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 Parapatric and Peripatric Speciation.
1. Parapatric. New species from peripheral populations.
2. Peripatric. New species from disjunct colonies.
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 Sympatric Speciation.
1. Polyploidy – “instant” speciation results from chromosomal duplication and resultant hybrid sterility.
2. Other mechanisms, e.g., behavioral changes / mating
preferences, producing reproductive isolation
The simplest form of polyploidy, autopolyploidy, involves the
production of diploid gametes and results in a doubling of chromosome number. Self-fertilization is most often required to perpetuate the new species. If outcrossing is obligate, perpetuation
requires simultaneous formation of multiple individuals. More
complex mechanisms can produce polyploid species resulting
from hybrids. In these cases, one speaks of allopolyploidy.
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 Question.
(On Daphne Island in the Galapagos Islands, Peter and
Rosemary Grant have identified the formation of what appears a new species of Darwin’s finch. The birds are descended from a single individual – probably a hybrid from
another island. They differ from
the locals morphologically and
with regard to song. Importantly, they mate solely amongst
themselves. For additional details, go to
http://www.nature.com/news/2009
/091116/full/news.2009.1089.html
A new Darwin’s finch. The
bird’s ancestor, # 5110 (almost all the birds are tagged)
had a larger bill than the indigenous medium ground
finches and a different song.
Drought reduced its descendants to a single brothersister pair whose offspring
3. (8 pts) Discuss this event have since mated only with
with regard to the modes of each other.
speciation given above and
also with regard to pre- and post-zygotic isolation.
Note: (1) young birds learn their songs (how to sing
and what to respond to) from Dad; (2) both song and
bill morphology are important cues for recognizing
conspecifics and hence, potential mates.
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 Adaptive Radiations.
1. Differentiation of a single
lineage into many.
2. Often observed on oceanic islands.
3. In the fossil record, adaptive radiations follow major
extinction events, but often delayed.
4. In both instances, multiple
speciation events a response vacant ecological
niches.
5. Island specialists often
less well adapted to new
ways of life than mainland
counterparts. May reflect
lack of
a. Genetic diversity;
b. Competition from other
species.
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Top. Tarweeds and silverswords. Bottom. Hawaiian silversword.
 Examples of Adaptive Radiations Exemplifying Ecological Divergence.
1. Within the genus Penstemon, variation in flower shape /
color allows for utilization of different pollinators.
2. Cichlid fishes of Lake Nyasa. Variation in skull / jaw
morphology facilitates specialization on different food
sources.
Above. Variation in flower
color and morphology within
the genus Penstemon correlates with pollinators. Right.
Variation in skull morphology
in cichlid fishes correlates
with diet.
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3. Early Radiation of Mammals.
Two-thirds of mammalian evolution occurred in the shadow of nonavian dinosaurs. Shown here are Jurassic mammals, their
evolutionary relationships and ecological niches. From Lee and
Beck (2015),
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 Questions.
4. (10 pts) Most hummingbird-pollinated
flowers are red and have long tubes.
How have evolutionary biologists accounted for this? (Requires outside
reading.)
5. (8 pts) Where / what is Lake Nyasa
(Malawi)? Why would the cichlid
fishes of this lake have undergone
Satellite view of
extensive adaptive radiation?
Lake Nyasa.
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 Extinction.
1. The flip side of speciation.
Over 99% of all species
have gone extinct.
2. Background level of extinction. Result of
a. Biotic interactions (ecological / evolutionary),
b. Gradually changing climates / landscapes.
c. Chance extinction
small populations.
of
Extinction rates for invertebrates since the Cambrian.
Peaks call attention to times
at which extinction rates rose
above background levels
(dots). The end Cretaceous
event (KT) resulted entirely or
in part from the Chicxulub
impact in what is now the Yucatan peninsula. After Raup
and Sepkoski (1982).
3. Mass extinctions. Result
of unique events that occurred over geologically
short time periods – recall
Cuvier’s revolutionary upheavals of the globe.
4. Regarding background extinction, ecologists have suggested an approximate balance between speciation
and extinction – recall Lyell.
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