Download IB104 - Lecture 32 - Speciation

This week a major ocean sailing race got underway, the Volvo Ocean
Race, which has six 70-ft crewed monohulls racing around the world in
nine legs. This is the “American” entry, sponsored by Puma.
Third Exam scores/grades
In counter-current flow systems (widely used in industry), as
exemplified by fish respiration, the blood that is richest in oxygen
encounters water that is richest in oxygen. That way, it can
continue to absorb oxygen throughout the gills. Mammals are not
able to do this nearly as efficiently.
IB104 - Lecture 32 - Speciation
Reading - Chapter 18.9-11
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Median
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Taxonomy. Biologists classify organisms into species, group them into
genera, then higher levels of the taxonomic system introduced at the start
of this class. Broadly speaking, “species” means “a kind of something”,
e.g. species of metals or chairs or pasta. For biologists this is also
roughly the meaning of the term when applied to asexually reproducing
organisms like bacteria, archaea, and many protists. But for sexually reproducing organisms like most eukaryotes and
essentially all animals it is a little more complicated. Since the evolution
of the extraordinary diversity of organisms of this planet involves the
formation of new species, or speciation, we have an entire lecture about
it. So formally, speciation is the formation of two or more new species
from a single original species. It is the ultimate source of all the
biodiversity on the plant. 1
Biological species concept
Species numbers
The most widely employed species concept for
animals is one articulated by a German
evolutionary biologist working at Harvard,
Ernst Mayr: “Species are groups of
interbreeding natural populations that are
reproductively isolated from other such
groups”
This definition leaves open a major problem,
which is what about “potentially interbreeding”
populations that are currently isolated from
each other purely by distance? A classic
example is humans, which for thousands of
years formed isolated populations on different
continents, yet were clearly potentially
interbreeding, as is now happening.
Over 1 million species have been named by taxonomists, although
preciously little is known about the vast majority of them.
Estimates of total species numbers range widely, from 3 to 10 million,
for example from insecticide fogging experiments in the tropics.
The vast majority of these undescribed species are microorganisms,
fungi, and invertebrate animals like nematodes and insects.
Nevertheless, new vertebrate species are
still occasionally described, such as this
new bovine from Vietnam in 1994. And
sometimes biologists recognize that one
“species” is actually two separate
genetically distinct, non-interbreeding
species, e.g. African elephants split into
forest and savannah forms (see handout).
Premating isolation
Failure to interbreed, or not having the potential to interbreed, means that
differences in courtship or mating behavior or physiology prevent most
inter-specific mating. This is known as pre-mating isolation, for example:
Frog calls – males call and females hear
Cricket calls – males call and females hear
Dragonfly colors – males are colorful and females see
Moth pheromones – females produce pheromone, males smell
Marine invertebrate sperm and egg recognition – simultaneous
coordinated release into the oceans, followed by specific recognition of
eggs by sperm and vice-versa
Frog calls
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Cricket calls
Dragonfly colors
Moth pheromones
Giant clam spawning
Coral spawning
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Postmating isolation
How species diverge
Hybrids between distant species are inviable, even if one forces mating.
Hybrids between closely related species are commonly sterile, and rarely
produced, except in unusual situation like zoos.
Even fertile hybrids might have ecological disadvantages in the wild.
It is impossible to select directly for postmating isolation, because any
allele that disposes a hybrid towards sterility or inviability is lost with
that individual.
Therefore postmating isolation evolves as an incidental consequence of
other genetic changes, and especially quickly in spermatogenesis and
oogenesis, leading to hybrid sterility. With the passage of lots of time, the
genes involved in delicately balanced developmental programs diverge
enough that inter-species hybrids don’t even develop, they are inviable.
Most premating isolation probably evolves the same way, for example as
a byproduct of rapid changes due to sexual selection changing how the
sexes recognize each other, or as a result of adaptations to new
environments.
Mule
Liger
Geography of speciation
Allopatry (“different place”) – populations are geographically isolated
from each other, which obviously depends greatly on the biology of the
species. Given enough time they will inevitably diverge genetically,
perhaps eventually enough to become two species.
Sympatry (“same place”) – populations are not geographically isolated
in any major way, yet diverge genetically because of divergent selection
to exploit different niches, habitats, etc. Parapatry (“adjacent place”) – when divergent populations or species
abut each other, persumably after diverging in allopatry. They might
fuse, they might remain as is with a hybrid zone between them, or they
may diverge enough to coexist.
Allopatric speciation
This is commonly thought to the the major way that animals speciate.
Given enough time, populations isolated by some kind of geographic
barrier or simply distance, will inevitably diverge genetically. This results from all the factors we considered in the last lecture on
population genetics. For example, founding of a small new population
will involve genetic drift, as would population bottlenecks in the separate
populations. Random mutations will be different between the two
populations. They will likely encounter different pathogens, parasites,
and competitors. If sufficiently far apart, they will also experience
different habitats and climates, and hence be subject to different selection
pressures. Sexual selection will be acting independently in the two
populations, perhaps leading to behavioral changes.
All of the above are capable of leading to the genetic differentiation that
underlies both postmating and premating isolation.
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Speciation on
archipelagos
(groups of islands)
Allopatric speciation is most
convincingly evident in
archipelagos, where a single
colonizing species has the
opportunity to lead to
multiple additional species
as individuals very
occasionally colonize other
islands, and eventually, once
differentiated, recolonize the
original island and co-exist
with the first species, etc.
The Hawaiian islands and
honeycreepers. The main
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A few individuals of a
species on the mainland
reach isolated island 1.
Speciation follows genetic
divergence in a new habitat.
Later in time, a few
1
individuals of the new
species colonize nearby
island 2. In this new
habitat, speciation follows
genetic divergence.
Speciation may also
follow colonization of
islands 3 and 4. And it
may follow invasion of
island 1 by genetically
different descendents
of the ancestral species.
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3
4
2
1
2
3
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Honeycreepers (I’iwi)
±50 species
Photo by Eric VanderWerf
Megalagrion damselflies - 22 species
Laupala crickets - 37 species
Hawaiian islands are only 5 Myr old,
but the island chain is 50 Myr old.
Drosophila flies
±800 species
FOUNDER SPECIES
Sympatric speciation
This is generally thought to be a less common route to speciation in
animals, although a growing number of biologists think it could be
widely important. The difficulty is understanding how two populations
can diverge genetically while remaining in the same geographic area and
hence are likely to continue to interbreed leading to gene flow
homogenizing their gene pools. The answer may be that a fairly radical behavioral shift to a new niche
can lead to rapid reductions in hybridization, and selection for adaptation
to this new niche can maintain genetic divergence despite some
hybridization.
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The apple maggot fly, Rhagoletis pomonella
Rhagoletis pomonella
One possible example of sympatric speciation involves a “true” fruit fly
in North America, family Tephritidae, which normally lives on
hawthorns, but recently colonized a new host, the apple, which was
introduced to North America from Europe/Asia about 200 years ago.
There is no reason to believe that apple plants in North America were
ever geographically isolated from hawthorns, so this colonization and
genetic divergence appears to have occurred in sympatry, and relatively
rapidly. Because these flies mate on their hosts, it is conceivable that a
rapid change in their behavior mediated this event.
Today these “host races” or “incipient species” remain morphologically
identical, that is, they look exactly alike, but they behave differently.
They preferentially seek out hawthorn or apple fruits, mate on them, lay
their eggs on them, and even time their emergence from the pupal stage
to coincide with when their fruit will be most abundant.
Some of the work on this story has
been done here, by my colleague
Stewart Berlocher. We are sequencing
the genome of this fly to examine the
genetic differences between these two
“species” more precisely. So far there
are no absolute genetic differences
between them, only frequency
differences, e.g. allele A is 80% in
hawthorn flies, but 5% in apple flies. Interbreeding and hybridization
continue at an estimated 5% per year,
which normally should lead to fusion
of the gene pools, so divergent
selection must be keeping them
different, e.g. hybrids are penalized.
Mating on a hawthorn tree
Siting on an apple
Parapatric speciation
Adjacent populations might
evolve into distinct species
while maintaining contact along
a common border, although the
other two possibilities (fusion,
and stasis) are also possible.
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