Transcript 34 speciation

Speciation
- Polymorphism
- Patterns of natural selection
- Mechanisms of isolation
- Rates of evolution
Refer to chapter 17 in text
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- Polymorphism
Within a population there is a range of forms (morphs) for a variety of traits:
phenotypic polymorphism: two or more conspicuous forms.
genetic polymorphism: multiple genes affecting a form, like height.
This is the variety acted on by natural selection.
Why isn’t it better to have one trait, or the other?
Balanced polymorphism:
Two (or more) alleles cooperating is better than either one alone.
e.g.
HsHs → sickle-cell anemia
HHs
→ OK (heterozygous advantage)
HH
→ more susceptible to malaria
Note: this is microevolution,
not speciation, but provides a
testable example of natural
selection.
http://www.genetologisch-onderzoek.nl/index.php/92/biologie/
Transient polymorphism:
One version is favored sometimes, the other at other times.
e.g. industrial melanism and peppered moth
- Patterns of natural selection
stabilizing selection:
The central-most morph is most
successful, and distal forms are
reduced. Results in fine-tuned,
but potentially fragile species.
disruptive selection:
The central form is less adaptive,
and the population splits into two.
Due to competition, loss of original
resource... Easy step to speciation.
directional selection:
One extreme is favored over the
other. Generally leads to speciation
only if two populations are already
separated.
divergent evolution:
The result of divergent selection,
in which one species faces selective pressure for
two different niches:
Recognized by presence of homologous structures.
e.g. Australian apple maggot fly:
originated from the
Australian hawthorn maggot fly,
but when apples were brought in,
the new food source was optimized...
Now two distinct species exist.
adaptive radiation
is a specific case of divergent evolution,
in which many niches are available for exploitation
e.g. the post-Mesozoic explosion of mammals
after mass extinction of dinosaurs
Adaptive radiation
also happens
when organisms
colonize a new island
grouping…
This resembles the
process of the
finches in the
Galapagos islands.
convergent evolution:
A specific type of directional selection, in which two species
are long separated, but selected towards a common adaptation:
Often recognized by the presence of analogous structures.
The C4 pathway has
cropped up at least
40 separate times.
http://cas.bellarmine.edu/tietjen/Evolution/Charley/Converge36.gif
- Mechanisms of isolation
In order for actual speciation to occur,
there has to be some sort of boundary
preventing the mixing of genes…
Reproductive isolation… self explanatory.
- prevention of successful interbreeding, ….somehow.
Two main groupings:
1. Allopatric – (“other country”) Organisms are physically separated.
This could be across an ocean,
or a stream,
or a spit of land,
or adjacent puddles.
2. Sympatric – (“same country”)
Various mechanisms that isolate neighbors…
Sympatric mechanisms
prezygotic (no zygote formed)
habitat isolation: different local habitat.
e.g. water v land garter snakes
behavioral isolation: courtship, communication
boobies, Pacific killer whales, bower birds....
temporal isolation: time of day, year, cycles…
NA E spotted skunk (winter) v W (summer)
mechanical isolation: structural incompatibility…
2 monkey flowers attract different pollinators
gametic isolation: repro. tract or gamete surface
sea urchins
postzygotic (no reliably fertile offspring)
reduced hybrid viability: don’t survive to repro.
Ensatina salamanders (‘zygote mortality’ in Mader)
reduced hybrid fertility: hybrids live, but can’t repro.
horse + donkey = mule
hybrid breakdown: hybrids can’t survive over
multiple generations (‘F2 fitness’ in Mader)
rice strains
This is the blurry path to speciation.
These work for solely sexually reproducing organisms.
What other factors contribute to speciation?
Polyploidy
e.g. autoploidy, mostly in plants, due to nondisjunction in meiosis,
leads to speciation in one generation.
(It can’t breed with a diploid anymore,
but can self-pollinate until there is a population of tetraploids.)
alonefarmer.blogfa.com/post-1851.aspx
There are various invertebrates that can do this, too,
and a mouse line was made tetraploid in a lab…
Cross 2n w haploid gamete → triploid (seedless)
Hybridize 2 w different 2n, then nondisjunction = alloploidy
- Rates of evolution
Microevolution by selection or genetic drift
can happen in the course of a few generations.
Speciation by polyploidy happens in a single generation.
More commonly, the accumulation of adaptive mutations
can be very slow (thousands of years? tens of thousands?),
and if an organism is well adapted,
there may be little change in allelic frequency
over millions of years.
HOWEVER, after a disruption,
there is generally astonishingly rapid evolution
as newly freed or created niches are filled,
depending on existent mutations.
- Punctuated equilibrium
Contrast allopatric and sympatric speciation
Explain the role of morphic variability
in natural selection.
phenotypic polymorphism
allopatric
genetic polymorphism
sympatric
balanced polymorphism
prezygotic
heterozygous advantage
habitat isolation
transient polymorphism
behavioral isolation
stabilizing selection
temporal isolation
disruptive selection
mechanical isolation
directional selection
gametic isolation
divergent evolution
postzygotic
adaptive radiation
reduced hybrid viability
convergent evolution
polyploidy
reduced hybrid fertility
reproductive isolation
hybrid breakdown
punctuated equilibrium