Linking chromosomes to genetics

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Lecture 18 Speciation
Readings Ch. 24, Overview, 24,1, 24.2
Speciation in context
microevolution, macroevolution, cladogenesis,
anagenesis The Biological Species Concept
reproductive isolation,
prezygotic isolation (habitat, temporal,
behavioral, mechanical, gametic), postzygotic
isolation (reduced hybrid viability, reduced
hybrid fertility, hybrid breakdown) reproductive
isolation by pleiotropy
Modes of speciation
allopatric speciation, sympatric
speciation Allopatric speciation
population differentiation,
races/subspecies, sexual selection Sympatric
speciation
polyploidy, habitat differentiation
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It is useful to divide evolution into two
parts Microevolution - population genetics,
natural selection, drift, mutation, studies of
ongoing adaptation and evolution of
diversity Macroevolution - large scale changes in
organisms over time The transition between these
is the study of speciation - the formation of new
species.
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Fig. 24.2 Another pair of terms that are useful
in dividing evolutionary biology into
understandable divisions.
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What are species?
Fig. 24. 3 Well, we cant define them on the
basis of morphology (body form anatomy) because
one can find pairs of species that are very
similar, while in other cases there is a lot of
variation within a single species.
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So what are species, really?

• Many biologists use the biological species
  concept, which is based on the idea that members
  of species can reproduce succesfully with each
  other.
• Thus it is based on reproductive isolation
  (barriers) between species, and gene flow within
  species.

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Biological species are organisms that interbreed
under natural conditions
Hobbs at the Sierra Safari Zoo in Nevada - not
natural conditions! (tiger mother, lion father)
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Figure 24.4 Reproductive Barriers
Rread in detail! - species are kept apart by
reproductive barriers, which can be prezygotic or
postsygotic
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Fig. 24. 7 Strong selection can produce
reproductive isolation involving unrelated
traits because of pleiotropy. In this experiment,
adaptation to diet may have affected the
composition of cuticular hydrocarbons that are
involved in mate recognition (but the analysis
has not been done).
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Types (or modes) of speciation

• Speciation requires interruption in gene flow
  (movement of genes from one population to the
  next)
• allopatric speciation geographic barrier
  physically isolates populations at first so they
  can evolve reproductive isolation which will keep
  them apart when they later come together.
• sympatric speciation populations of organisms
  with overlapping ranges speciate without
  geographic isolation.

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Fig. 24.5
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Fig. 24.6
These two species of antelope squirrels are
perfectly separated by the Colorado River in
Arizona and southern California. They have
evolved separately until they have reached the
point of being species.
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However... Populations that have been
geographically isolated, differentiated
genetically, and then brought back into sympatry
need not have become species. Many times all you
get are genetic differences between the
previously isolated populations that do not
involve reproductive isolation. Sometimes
differentiated populations of a species fall into
a few distinct groupings but often they do not).
Sometimes such populations are unnamed, sometimes
they are named in English, in which case they can
be called a race, and some are given Latin names,
which case they are called a subspecies. The
biological reality is the presence of distinct
geographic populations - the names are only in
our heads. If properly used, such names are
convenient for describing nature.
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Rio Grande turkey, Meleagris gallopavo intermedia
Eastern turkey, Meleagris gallopavo silvestris
Intergrades occur where the two subspecies meet
(red striped)
Subspecies of the turkey
Merriams turkey
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Occelated turkey, Meleagris occeleta, Mexico,
Guatemala, allopatric from common turkey, no
hybrids reported.
Two species
Common turkey, Meleagris gallopavo
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Sympatric speciation - speciation in the same
geographic area, is a hot area of research right
now. Two mechanisms are 1. Polyploidy -
speciation due to changes in chromosome number.
Read Fig. 24.8, 24.9. 2. Habitat selection -
mating in different habitats - I will talk about
my research in this area at the end of the
semester, so I will not go into it today.
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The apple maggot fly Rhagoletis pomonella is an
example of likely sympatric speciation by habitat
isolation.
pupa in soil
adult fly
larva in fruit
egg in fruit
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Life history of species of Rhagoletis pomonella.

• One generation per year
• Adults emerge from pupa in soil to coincide with
  fruit ripening.
• Adults meet and mate on host plant.
• Pupation in soil long overwintering pupal
  diapause.

Rhagoletis pomonella
There appears to be two host races (distinct
poulations on different host plants), the
ancestral hawthorn race and the recent apple race
- apples were brought to this country about 400
years ago. These two races show partial
reproductive isolation, evolved in sympatry.
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Hawthorns are related to apples (rose family) but
not in the same genus. North American is very
rich in hawthorn species.
The native host plant of R. pomonella, a native
insect.
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Emergence curves differ between apple and
hawthorn at a site in Grant Michigan that has the
two plants growing in a completely interspersed
manner. We know this is a genetic difference.
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Apples and haws are unequally attractive to the
two races Volatile blends for apple and hawthorn
fruit Apple blend Hawthorn blend Butyl
hexanoate (0.37) Butyl hexanoate
(0.01) Pentyl hexanoate (0.05)
3-Methylbutan-1-ol (1.0) Propyl hexanoate
(0.04) Isoamyl acetate (0.4) Butyl butanoate
(0.10) 4,8-Dimethyl-1,3(E),7-nonatriene
(0.02) Hexyl butanoate (0.44) Ethyl acetate
(20.0) Dihydro--ionone (0.02)
We know this is a genetic difference as well.
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Acon 95 data
Proof that the two host races, occuring together
at the Grant site, do not make a Hardy-Weinberg
population - because there are different allele
frequency differences! We know they do
interbreed, but natural selection keeps them from
melting back together
Haw
Apple
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Conclusion The apple host race of Rhagoletis
pomonella has evolved substantial differences
from the ancestral hawthorn host race, in just
150 generations. These genetic differences
involve emerging from the ground earlier in the
year and choosing the odor of apple fruit over
that of hawthorn fruit. Allele frequencies at
molecular loci that are linked to the unknown
genes for emergence and odor choice are
consistently different between the races. These
are partial species, evolved in sympatry.