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Speciation
1. Species definitions
2. Troublesome species
3. Pre-mating and post-mating isolation
4. Allopatric speciation (separate areas)
5. Peripatric speciation (tiny isolated subpopulation)
6. Parapatric speciation (adjacent areas with gene flow)
7. Sympatric speciation (same area)
8. Gradual versus punctuated evolution
Species definitions
Ernst Meyer proposed the biological species concept (BSC): a species is
an interbreeding population that is reproductively isolated from other such
populations.
Species definitions
Possible weaknesses of this definition:
• Doesn’t help with asexuals.
• Troublesome in prokaryotes where individuals may have “sex” with
drastically unrelated types and gain a few genes from them.
• Many populations that biologists want to consider species do have some
gene flow. Example: dog, wolf and coyote.
• Some populations become reproductively isolated but don’t show any
adaptive differences. Example: Drosophila infected with different strains
of Wolbachia.
• Hard to apply to extinct species.
Species definitions
One alternative definition was proposed by Chong-I Wu, who called it the
genic species definition. Speciation has occured when:
1. Gene pools of adaptive loci would not mix even if the barriers to gene
flow were removed, and
2. Advantageous mutations are often (not necessarily always) restricted to
the population in which they arise.
This focuses more on adaptation. It is open to most of the same criticisms
as the BSC.
One significant difference is that the BSC will call polyploids that are not
fertile with their diploid parents a new species, and Wu’s definition will not,
because there has not been time for adaptive differences to arise.
Species definitions
Paleontologists tend to use a working definition that involves visible,
consistent differences and absence of apparent hybrid forms. It is extremely
hard to compare past and present species counts, because they use different
definitions.
Drosophila has many “cryptic species” which are definitely species by the
BSC (they cannot interbreed) but are difficult or impossible to distinguish.
In some cases the main difference may be mating behavior. These would
be essentially impossible to see in the fossil record.
We may overcount fossil species that differ in bony, hard characters and
undercount ones that differ in soft tissue or, worse, behavior.
Species definitions
In asexuals “species” are also defined by appearance; two lineages are
different species if they have different morphology or behavior, and the
same species if they don’t.
This has caused trouble in bacterial genetics.
Bacteria have little
appearance, so some things classified as the same “species” turn out to be
extremely divergent genetically (i.e. laboratory E. coli versus pathogenic E.
coli).
Many asexuals may sneak in a little sex now and then, which could help to
hold a species together.
However, they sometimes have sex with drastically divergent partners; it’s
hard to argue that E. coli and the fungus Penicillum are the same species,
but E. coli seems to have gotten its penicillin-resistance gene straight from
the source.
Species definitions
• Species could be defined by degree of genetic distance
• The cutoff would be pretty arbitrary
• Few biologists are willing to consider non-interfertile sexuals the same
species even if they do have little genetic distance
• More are willing to consider interfertile sexuals different species
• Example: dog, wolf, coyote
Troublesome species
Male Drosophila which are infected with the bacterial parasite Wolbachia
can no longer reproduce with uninfected females. So a Wolbachia infection
(which is transmitted from mother to offspring, like mtDNA) causes instant
one-directional reproductive isolation.
If two different strains of Wolbachia are present, it can cause bi-directional
reproductive isolation.
This may have been involved in several Drosophila speciation events. But
it is odd to think of an infection as changing a fly’s species!
Troublesome species
Troublesome species
Ensatina salamanders in California exist as a ring with some forms on the
coast and some inland. Every population is interfertile with its neighbors,
but the ends of the ring are not interfertile with each other.
This is called a “ring species”.
Troublesome species
How many species?
The BSC might say only one, because there is gene flow throughout the
system. The genic species model might see more than one, because there
is differential adaptation, but it’s hard to put down boundaries.
This is probably a snapshot of speciation in action. Black-headed gulls also
behave like this; the ring goes all the way around the world.
Troublesome species
Rana esculenta from an earlier lecture is troublesome. It has its own gene
pool (the chromosomes derived from R. ridibunda) but also shares the gene
pool of R. lessonae.
Troublesome species
An adaptation which occurs in R. lessonae will be present in R. esculenta
as well, though it is not transmitted any further once it goes into R.
esculenta. An adaptation which occurs in R. esculenta will be inherited
only by R. esculenta; it doesn’t get back into R. lessonae.
This is not a biological species because it definitely interbreeds. It does
seem to be a genic species because it has its own pool of adaptations–but
is R. lessonae a real species, then?
The systematist who gave R. esculenta its Latin name thought it was a
species. It has a distinctive morphology and behavior.
Troublesome species
• Another troublesome amphibian: Ambystoma salamanders
• Common all-female types which use sperm only to activate the egg exist
• They may be diploid, triploid, tetraploid, or pentaploid
• In preparation for meiosis they double their chromosomes one time more
than usual
• Their ova thus have the same number of chromosomes as the adult
• They may carry chromosomes from the sexual species A. jeffersonianum,
A. laterale, A. tigrinum, A. tigrinum or A. texanum
• Often the mtDNA is from a different species than the nuclear
chromosomes
• Probably a few sperm sneak through to increase ploidy
• These animals were originally called a species: A. platineum
Reproductive Isolation
Reproductive isolation can take two basic forms, pre-mating and postmating.
• Pre-mating (before fertilization)
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–
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Different breeding season
Different mating grounds
Different mating behavior
Males and females not attractive to each other
Incompatible genitals
• Post-mating (after fertilization)
– Mating is sterile
– Hybrid offspring are inviable
– Hybrid offspring are sterile
Reproductive Isolation
There is a cost if two populations come into contact and they have only
post-mating isolation. They will mate, and the offspring will not succeed,
reducing reproductive success.
If post-mating isolation is present and the two populations are in contact,
evolution will favor establishment of pre-mating isolation.
Modes of speciation
Speciation events are classified according to whether the two new species
start out geographically separated, partly separated, or mixed together.
• Allopatric speciation–separate geographical areas
• Peripatric speciation (tiny isolated subpopulation)
• Parapatric speciation–adjacent areas with some gene flow
• Sympatric speciation–same area
Allopatric speciation
The best understood mode of speciation is allopatric meaning “separate
homelands”. A population is divided into two parts by a physical barrier.
The two parts evolve separately until they are no longer able to interbreed.
During their separation pre- or post-mating isolation may develop. For
example, one population may change its chromosome arrangement, or its
breeding season.
If they later come back into contact, they will tend to evolve pre-mating
isolation.
Allopatric speciation
Forces that can lead to reproductive isolation in allopatric species:
• Adaptations to different environments that lead to underdominance when
combined.
• Changes in major developmental
underdominance when combined.
pathways,
• Changes in gene order.
• Changes in chromosome number or structure.
• Genetic drift in key traits such as mating behavior.
again
leading
to
Peripatric speciation
• This is a variant of allopatric speciation
• A small group is isolated from the main population
• Genetic drift causes rapid genetic divergence
• It’s hard to tell after the fact whether speciation was allopatric or
peripatric
• (in other words, how important was drift?)
Peripatric speciation
(1-7 are ground finches, 8-13 are tree finches.)
Parapatric speciation
Parapatric speciation meaning “adjacent homelands” occurs when two
populations are partially separated, but there is significant gene flow.
Remember that migration will prevent populations from evolving
independently if 4N m >> 1 unless opposed by strong selection.
How can populations becomes species in the face of strong gene flow?
Parapatric speciation
• The grass Anthoxanthum odoratum can live on metal-contaminated soil
• Initially gene flow was probably high
• However, hybrids were less fit
• Modern populations on toxic soil have a different flowering season
• This may be parapatric speciation in action
• Other plants in the same situation become asexual instead
• This also cuts gene flow and can lead to speciation
Sympatric speciation
Probably less common is sympatric speciation meaning “same homeland.”
The population divides into two species without any physical barrier at all.
There are several known ways this can happen.
Sympatric speciation
If a major genetic change creates reproductive isolation, and the resulting
population can survive, this can lead to “instant” sympatric speciation.
One example is polyploidization in plants. The change from, say, 2N to 4N
causes instant isolation because hybrids would be sterile triploids (3N).
In a plant which can self-fertilize or reproduce asexually it is relatively easy
for a new species to start this way. It is more difficult in sexual outcrossers
such as mammals.
Sympatric speciation
Another possibility is that the population is divided not by a physical barrier
but by some other barrier.
The classical example is host plant in insects. If females start laying eggs
on a new food plant, they can establish a population which lives in the
same area but doesn’t interbreed much. Selection for different traits on the
different plants can then encourage even more isolation.
Sympatric speciation
• Maggot flies in North America lived on native hawthorn
• Settlers introduced apple about 200 years ago
• Female flies tend to lay eggs on plants of their natal type
• Male flies tend to look for mates on plants of their natal type
• Some genetic differentiation between hawthorn maggot flies and apple
maggot flies is observed
• This situation could lead to sympatric speciation
Sympatric speciation
Another example: some Pacific salmon have a strict two-year life cycle.
Salmon born in 2003 will spawn in 2005, their children will spawn in 2007.
Salmon born in 2004 will spawn in 2006, children in 2008.
There can be reproductive isolation between the even and odd years even
in the same river.
It is not clear that this has led to time-separated species but it seems
possible that it could. (If they later mixed in the same year, it would be
hard to tell that they evolved in separate years.)
Co-speciation
• Host species and parasite species often speciate together
• Species trees of the two groups will look very similar
• Example: gophers and gopher lice
• Reproductive isolation of hosts may isolate parasites
• Adaptation of hosts may spur adaptation of parasites
• The reverse may also happen: Wolbachia
Gradual versus punctuated
Population geneticists usually think of speciation as proceeding by a slow
accumulation of differences.
Stephen Gould and colleagues, based on fossil data, proposed an alternative
model called punctuated equilibrium.
This model suggests that there is little change in a species during its lifetime,
but that change happens rapidly when a new species arises. It is called
“punctuated equilibrium” because the fossil record seems to show long
periods of stasis broken or “punctuated” by brief bursts of rapid change.
Gradual versus punctuated
It is possible that some species evolve gradually and others in bursts.
Certainly polyploidization is a burst of change.
It is also possible that we are running into trouble comparing modern
species with fossil species. Perhaps the “equilibrium” periods have plenty
of speciation but not the kind we can see in fossils.
My own view is that punctuated equilibrium is a real pattern but it controls
evolutionary innovation, not speciation in general.
Stasis
Some organisms–examples are coelacanth, ginkgo, horseshoe crab– appear
to change very little over long periods of time.
They also generally appear only in one or a few species.
Other organisms change much more rapidly, and also tend to diversify into
multiple species.
This could be due to a developmental “program” in the static species which
is very difficult to change.
Alternatively, they may simply have a successful design with no easy-to-reach
improvements.
Stasis
David Raup has described a pattern in which during “normal” times
specialist species which adapt to a wide variety of niches are most successful.
But during mass extinctions, simple generalized organisms with a very wide
niche are the main survivors.
This would be species selection on a big scale and with a very long timespan
(Raup sees a pattern of major exinctions at approximately 26 million year
intervals).