Chapt.: 21- The Species Concept

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Chapt. 21- The Species Concept Species and
Their Formation
When and where did the first organism, species
appear (Chapt. 19)?
How does one species arise from another? Is a
new species always better adapted to the
environment?
The Galapagos Islands were viewed by Darwin as a
Genesis.
How does a new species arise- was not addressed
in the Origin of Species only adaptation, or
the beginning of speciation.
We must begin with the question What is a
species?. i.e. When do gene/allele
frequencies in a population change enough to
result in a new species?
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Darwins Finches
Also Fig. 21.6
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Can a mutation create a new species overnight??
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Why isnt there a continuous spectrum of change
among all species, i.e. why so many missing
links? How can variation and similarity be
selected for at the same time?
The Biological Species Concept-- a group of
isolated, interbreeding (or potentially
interbreeding) individuals that produce fertile
offspring under natural conditions. \ an
Independent Evolutionary Unit
This concept goes beyond the taxonomic species
that was identified based on phenotypic
similarity!
Key to the BSC is the idea of Reproductive
Integrity (i.e. Genetic Isolation). Ques
tions What about our interpretation of the
fossil record?
What about asexual organisms?
What about the genetic middleman, e.g. the
Rocky Mountain deermouse?
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Critical Step in Speciation-- splinter population
becomes isolated!!
What kind of isolation?
Three types of genetic isolation can occur (1)
Allopatric Speciation-- geographic, mechanical
barrier develops. (Fig. 21.4)
(2) Sympatric Speciation-- reproductive isolation
without spatial or physical separation (one
individual, or many individuals become isolated
inside the parent population) Genetic
mechanisms!! (3) Parapatric Speciation weak
isolation over small distances, (steep
environmental change) often with with gene flow
What conditions favor allopatric speciation?
Hint the same that favor microevolution
population size, gene pool, natural selection,
and Chance!!
Why are islands so ripe for Allop. Symp
speciation?
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A model for allopatric speciation within an
island system Isolation Rare Dispersal (e.g.
Founders) Competition Adaptive Radiation
Allopatric Speciation
E.g. Island Hopping and availability of new
niches minimizes competition (Fig. 21.5,
Hawaiian Drosophila).
e.g. Hawaiian Islands vs. Florida Keys! Why??
(Size-Mobility-Dispersal Isolation
Potential) Endemics! (Continental Drift?)
Sympatric Speciation--spatial separation not
needed, only reproductive isolation via genetic
mistakes or microhabitat selection.
Polyploidy extra sets of chromosomes. How??
(Pg. 211, Chapt. 9).
Non-disjunction!! Causes autopolyploidy
(4n,8n,etc.) Allopolyploidy hybrids of
different species, sterile
Can result in viable and fertile offspring! 25 to
50 of all plant species. Why plants?!!
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Plants self-fertilzation, or crosses with
others of same ploidy level, e.g. siblings
(autopolyploid) crosses between
different species result in improper pairing
of homologous chromosomes during meiosis and
sterility (allopolyploid). But, asexual
reproduction occurs and, ultimately, the
return of fertility (chromosome duplication).
e.g. Tragopogon species (sunflower family) two
introduced diploids in 1950, now two
tetraploid hybrids by 1985!! (Fig. 21.2)
Polyploid hybrids often more fit than parent
species. Why?
Also, there have been multiple origins of the
tetraploid hybrids. (Molecular comparisons of
chloroplast and ribosomal genomes)
Sympatric speciation by polyploidy rare in
animals (Why?) More often occurs by
microhabitat selection and reproductive isolation.
Parapatric speciation- more difficult to document
until molecular data became available, e.g.
flowering times change on polluted soils
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Remember-- Why is it important to have so many
different species? Diversity among species is
just as important as diversity among
individuals of a population,
i.e. interspecific vs. intraspecific diversity
both critically important!! WHY??
Morphological changes with little change
genetically are common. (Fig. 21.12)
How are different species maintained as separate
species (reproductive isolation)?--
Reproductive Barriers (Isolating
Mechanisms) (1) Prezygotic- e.g. Habitat,
behavioral, temporal, mechanical, (see Fig.
21.6) gametic (post-mating) (2) Postzygotic-
e.g. Hybrid zygote abnormal, offspring
infertility and low survival (see Fig. 21.9
for summary)
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Rates of Speciation How fast can a new species
form from an ancestor? Horseshoe crab unchanged
for 200 million yrs Hawaiian Drosophila -
hundreds of different species in 40 mil yrs
- Species richness -Life history
traits (size/mobility/mating) ( see Figs. 21.13
and 21.14) Speciation
-Environment (heterogeneous) Potential
-Generation times (short)
Evolutionary Radiation low extinction and
high speciation rates (Fig 21.16, gaping
muscles)
Speciation Evolution Punctuated
Eqiulibrium WHY IS BIODIVERSITY
IMPORTANT?? SUSTAINABLE ECOSYSTEMS?? e.g.
National Parks??