Macroevolution

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Macroevolution or - the evolution of species
The Biological Species Concept A species is
a population or group of populations whose
members have the potential to interbreed in
nature to produce fertile offspring, and whose
members are reproductively isolated from other
such groups.
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But this is a hellish definition to test
species Darwin (1853) After describing a
set of forms as distinct species tearing up my
manuscript, making them one species tearing
that up making them separate, then making
them one again (which has happened to me) I have
gnashed my teeth, cursed species, asked what
sin I committed to be so punished. (Darwin was
trying to separate barnacles.)
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More practical problems 1) How do we deal with
asexual species (bacteria, many fungi, some
fish, and assorted others)? They do not
interbreed. They dont fit the definition. 2) How
do we deal with fossils? Are morphological
differences sufficient to define species? That
would not always work with living species. 3)
Does a different morphology necessarily mean a
different species? No! 4) What about
geographically separate populations? How can
we tell if they can interbreed in nature?
The species concept may answer this question.
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What happens when apparently different species
are brought together artificially (in zoos,
etc.)? Lions and tigers interbreed in zoos,
producing ligers. Bison and cattle breed on
farms, producing commercially valuable
beefalos. It only works successfully in one
direction bull with buffalo cow Why?
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Note the front shoulders of this beefalo. A
female cow cant handle birthing the calf of the
cross without dangerous distress.
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So, how do new species evolve? Through
accumulation of genetic differences. Eventually,
sufficient difference has accumulated
to reproductively isolate the two groups, even if
they should be in contact. Those differences
usually accumulate while the groups (populations)
are geographically isolated.
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An example from northern Canada There are two
major species of lemmings (collared lemming,
brown lemming). Lemmings lived in the shadow of
receding glaciers. As James Bay opened with
glacial melting, the western and eastern
populations of lemmings were separated
(geographically isolated). Differences
accumulated over time producing the two lemming
species we have today. However, there are other
mechanisms that can produce reproductive
isolation even while groups are in close
proximity. These barriers are separated into
pre-zygotic (preventing zygote formation)
and post-zygotic (affecting embryo development).
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Pre-zygotic barriers 1) Habitat isolation -
e.g., marsh versus forest warblers, or parasites
limited to different host species 2)
Temporal isolation - e.g., breeding seasons
in yellow-headed (May - early June) and
red-winged (late June - early July) blackbirds.
3) Behavioural isolation - its the species
specific mating dances that many of the 400
Hawaiian Drosophila do in courtship that
reproductively isolates them. 4) Mechanical
isolation - e.g. imagine Great Danes and
chihuahuas attempting to mate
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5) Gametic isolation - sperm (or pollen) and
egg must be chemically compatible. In plants,
pollen doesnt germinate, pollen tubes fail to
grow, In animals, membrane proteins dont
match, and fertilization doesnt occur
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post-zygotic barriers 1) hybrid inviability
- genetically programmed development is a
complicated process. Species differ in this
program. When genomes are mixed in a hybrid,
conflicts result in the embryo failing to
develop completely. They generally
dont survive. 2) hybrid sterility - the
hybrids survive to maturity, but cannot produce
viable offspring. The reason is usually
traceable to incompatible chromosomes that dont
match up in meiosis. An example horses and
donkeys mate, the offspring (mules) are viable,
but sterile
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3) hybrid breakdown - first generation
hybrids are viable and fertile, but the second
generation (or beyond) are feeble (low
survivorship and greatly reduced reproductive
output) or sterile. This occurs among different
species of cotton. In the absence of successful
barriers to hybridization - gene exchange
between species occurs. It is called
introgression. With gene flow, new species cant
form. So, think of populations in the process of
accumulating differences never being able to
accumulate sufficient differences to speciate.
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Even though various mechanisms can lead
to reproductive isolation, the most common
remains geographic isolation. The separation can
occur at three levels 1. Allopatry -
allopatric speciation 2. Parapatry - parapatric
speciation 3. Sympatry - sympatric speciation
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Allopatric speciation - A population becomes
geographically fragmented A body of water
(river, ocean) may separate them (e.g. the
lemmings). A small group may colonize an
island (e.g. Darwins finches Plate
tectonics may cause the rise of
mountains between them. - Either due to
environmental differences between sites
(differences in regimes of natural selection) or
chance events in small colonist groups
(drift) genetic differences between groups
accumulate.
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• - Eventually, sufficient differences accumulate
  to
• prevent interbreeding. At this point we say
  a
• new species has evolved.
• - Differences appear and spread more rapidly in
• small populations (drift! mutation is not
  more
• likely in small populations)
• Frequently, it is marginal populations within
• what had been a large, widespread population
  
• that become isolated (more likely to
  encounter
• different environments).
• - Adaptive radiation may occur as small groups
• become repeatedly isolated, e.g. Darwins
• finches or Hawaiian silverswords.

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A ground finch Sharp-beaked finch (Geospiza
nebulosa)
A cactus finch Small cactus finch (Geospiza
scandens)
Another ground finch with a smaller bill Small
ground finch (G. fuliginosa)
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A survey of the full set of Darwins finches
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A few of the wild and wonderful Hawaiian
silver- swords. All grow at upper elevations in
Hawaii. The most remarkable is the Haleakala
silversword, which grows in the cone of an
active volcano on Maui.
Haleakala silversword, Argyroxiphium sandwichense
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Island adaptive radiation can go in strange
directions. These are sunflowers that result
from adaptive radia- tion on the island of
St.Helena in the south Atlantic.
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Parapatric speciation - Populations are not
separated their boundaries contiguous -
Speciation can occur when a strong environmental
gradient extends across the boundary
between populations - Differences in
selection pressures must be great enough to
overwhelm any gene flow across the
boundary - Two examples distribution over a
mountainside distribution over a mine
spoil gradient
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Imagine a mountainside. It gets colder as you
climb. Temperature changes by 10C per kilometer.
One species lives at the bottom of the slope,
the other at the top. Differences in temperature
adaptations may mean that reproductive success
is very much lower in the wrong part of
the slope. Slowly, populations come to differ in
many ways, and parapatric speciation occurs.
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The other example is of grasses growing on and
off land of a mine in Wales that produced lead.
Heavy metals (lead, copper, nickel) are poisonous
to many (most) plants. Selection on mine lands
produced a variety of Agrostis tenuis that was
tolerant of lead. A part of the change
(mutation) giving tolerance was a shift in
flowering time. Thus, although tolerant
and intolerant plants grow in adjacent areas,
there is little or no gene flow between
populations, and speciation can occur.
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Sympatric speciation - Populations overlap in
distribution (sym - same patra - country).
Then how can they become reproductively
isolated? - Two accepted ways by host
specialization by becoming polyploid -
Host specialization - when host is both feeding
and mating site, a change in host can isolate
the shifted population. Example in
text Rhagoletis pomonella normally feeds and
mates on hawthorn fruits. Some switched in NY
in 1864 to feeding on apples. In 1960 some
switched again, to cherries.
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- becoming polyploid autopolyploids - double
chromosome number by non-disjunction or nuclear
fusion in meiosis. Diploid gametes
self-fertilize. Result is tetraploid. It cannot
backcross with parents, but is fertile with a
like type.
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Allopolyploidy - fertilization involving gametes
from two different species. Interspecific
hybrids are usually inviable or sterile (due to
failure of chromosome pairing in synapsis of
prophase in meiosis chromosomes arent really
homologues), but Non-disjunction in the
first generation of the hybrid can make a
viable, fertile hybrid.
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Is allopolyploidy important? Did anyone have
bread (pizza, big Mac, sandwich) for lunch? The
bread was made from wheat. Allopolyploidy
produced Triticum aestivum, or bread wheat. Its
chromosome complement is 2n42. That complement
arose by spontaneous hybridization of 2 other
wheat grasses with 28 and 14 chromosomes. 2n28
--gt n14 in gametes 2n14 --gt n7 seems
incompatible, but by non-disjunction in
meiosis of this hybrid, a fertile species with
2n42 was formed.
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Adaptive radiation occurs on continents, as
well, but continental drift and plate tectonics
brings faunas into contact, and there are then
extinctions. Here are drawings of South American
mammals driven extinct after the rise of Central
America permitted exchange. Only the armadillo
and opossum successfully moved north. A host of
larger North American mammals crossed southward,
and drove ecologically similar species extinct.
The text has a diagram of the North American
mammals that were (at least temporarily)
successful in the south.
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Here are diagrams of what they drove extinct
Probably related to condylarths, with
camel- like habits
Large, mastodon-like
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Extinction of species is as important to what
we observe today as is speciation. There is a
nominal background rate of species disappearance
(extinction). However, in the history of life on
earth there have been periods when much larger
numbers of extinctions occurred. These are called
mass extinctions. There have been 5 mass
extinctions (and humans are almost certainly
driving a 6th. Causes of some are not known. One
seems relatively well explained - the mass
extinction that occurred 65 MYBP, and eliminated
dinosaurs, making room for diversification and
enlargement of the mammals.
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The Cretaceous mass extinction was caused by
a combination of climate change and the collision
of a 17km diameter asteroid into the Yucatan
peninsula. The effects of collision were much
like the nuclear winter that would be caused by
global nuclear warfare Intense fires over much
of Mexico and the U.S., a global dust cloud
darkening the skies for months, a sudden change
in climate as a result, death of plants, and
therefore a lack of food to support the
huge vegetarian dinosaurs, leading to the death
of the meat eaters.
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We are driving another mass extinction.
Many species are going extinct each day, though
we dont even know their names, and may not have
even discovered them yet. How? We are cutting
down tropical forests in South America, Africa,
and Asia for lumber, firewood, and conversion to
agricultural land, both on the large scale and as
a result of population increase and ancient
practice of slash and burn farming. The effect of
this is not only loss of plant species, but loss
of the diversity of insect and other species
directly or indirectly dependent on the plants.
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Tempo and mode in Evolution There are two views
of the rate of apparent change in species 1) the
microevolutionary view - new species formation
results from gradual accumulation of phenotypic
(usually seen as morphological) change. 2) the
punctuated equilibrium - most of the morpho-
logical change becomes apparent when species
initially form. Populations are then very small.
Selection can rapidly move the
characteristics of the entire species, and
drift can lead to rapid change. Through the
remainder of the species history, there is
little evident change.
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The result is a history in the fossil record of
long stasis (equilibrium, constancy) punctuated
by short periods of dramatic change. Is only one
of these hypotheses correct, and the other wrong?
No! A punctuated equilibrium is evident in the
fossil record. The sudden change may represent
1000s of years of gradual change, but it looks
rapid when viewed on a geological time scale.