Transcript Macroevolution: the evolution of species

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.
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.)
More practical problems:
1) How do we deal with asexual species (bacteria,
many fungi, some fish, and assorted others)? They
do not interbreed. They don’t 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.
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?
Note the front shoulders of this beefalo. A female
cow can’t handle birthing the calf of the cross without
dangerous distress.
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.
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).
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 - it’s 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
5) Gametic isolation - sperm (or pollen) and egg
must be chemically compatible. In plants, pollen
doesn’t germinate, pollen tubes fail to grow,… In
animals, membrane proteins don’t match, and
fertilization doesn’t occur
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 don’t
survive.
2) hybrid sterility - the hybrids survive to maturity,
but cannot produce viable offspring. The reason
is usually traceable to incompatible chromosomes
that don’t match up in meiosis. An example:
horses and donkeys mate, the offspring (mules)
are viable, but sterile
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 can’t
form. So, think of populations in the process of
accumulating differences never being able to
accumulate sufficient differences to speciate.
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
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.
Darwin’s 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.
- 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. Darwin’s
finches or Hawaiian silverswords.
A ground finch
A cactus finch
Sharp-beaked finch
(Geospiza nebulosa)
Small cactus finch
(Geospiza scandens)
Another ground finch
with a smaller bill
Small ground finch
(G. fuliginosa)
A survey of the full set of Darwin’s finches
A few of the wild and wonderful Hawaiian silverswords. 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
Island adaptive radiation can go in strange directions.
These are ‘sunflowers’ that result from adaptive radiation on the island of St.Helena in the south Atlantic.
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
Imagine a mountainside. It gets colder as
you climb. Temperature changes by 10°C
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.
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.
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.
- 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.
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 aren’t really homologues),
but…
Non-disjunction in the first generation of the
hybrid can make a viable, fertile hybrid.
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 2n=42.
That complement arose by spontaneous hybridization
of 2 other wheat grasses with 28 and 14 chromosomes.
2n=28 --> n=14 in gametes
2n=14 --> n=7
seems incompatible, but by non-disjunction in meiosis
of this hybrid, a fertile species with 2n=42 was formed.
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.
Here are diagrams of what they drove extinct:
Probably related to
condylarths, with camellike habits
Large, mastodon-like
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.
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.
We are driving another mass extinction. Many
species are going extinct each day, though we don’t
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.
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 morphological 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.
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.