Linking chromosomes to genetics

1
Welcome back to IB 150...
Hobbs at the Sierra Safari Zoo in Nevada (tiger
mother, lion father)
2
Lecture 13 Natural selection
Readings Ch. 24, Overview, 24,1, 24.2
Comcluding comments on natural selection in
guppies Speciation
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) Other species
concepts
Modes of speciation
allopatric speciation, sympatric
speciation Allopatric speciation
population differentiation,
clines, races/subspecies, lab experiments Sympatri
c speciation
polyploidy habitat differentiation, sexual
selection Adapative radiation Genetics of
speciation Tempo of speciation
3
Natural selection and medicine
Postponed to end of semester

• An example using the influenza virus.

4
Guppies from a river in Trinidad Upstream
Downstream, with strong predators.
5
Different color patterns (silver vs orange) that
sexual selection has favored in different rivers
in Trinidad. Drift probably plays a role in
determining which kind of ornaments is selected
in a particular river.
6
Fig. 22.12 Many features of guppies have been
found to be subject to experiments on natural
selection - not just color.
7
The work on natural selection in guppies (and
many other forms of life) illustrates that
different populations of the same species can
become different from one another - become
differentiated - due to the action of natural
selection, sexual selection, and also genetic
drift.
8
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.
9
Fig. 24.2 Another pair of terms that are useful
in dividing evolutionary biology into
understandable divisions.
10
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.
11
So what are species, really?

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

12
Biological species are organisms that interbreed
under natural conditions
Hobbs at the Sierra Safari Zoo in Nevada (tiger
mother, lion father)
13
Fig. 24.4 - read in detail!
14
So what are species, really?

• The biological species concept is not perfect.
• Does not help us to understand largely asexual
  organisms like bacteria. In such cases a
  phylogenetic species concept (based on a tree)
  may be used.
• Does not help with fossils... Which do not
  reproduce! Here a palaeontological (basically,
  based on morphology) concept is useful.

15
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.

16
Fig. 24.5
17
Fig. 24.6
18
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! Think about the
guppy examples. One possible outcome is what are
called subspecies or races - large areas within
a species range that have some consistent genetic
feature. Sometimes such areas are unnamed, some
are named in English, and some are given Latin
names, which case they are called a subspecies.
Could be due to drift or natural selection.
19
Rio Grande turkey, Meleagris gallopavo intermedia
Eastern turkey, Meleagris gallopavo silvestris
Intergrades occur where the two subspecies meet
20
Occelated turkey, Meleagris occeleta, Mexico,
Guatemala
Two species
Common turkey, Meleagris gallopavo
21
Fig. 24. 7 Strong selection can produce
reproductive isolation involving unrelated
traits because of pleiotropy.
22
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. 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.
23
Fig. 24.8 - Autopolyploidy in plants. See also
Fig. 24.9, on autopolyploidy.
24
Adaptive radiation - bursts of speciation in new
geographic areas (good examples involve
archipelagoes like the Galapagos and Hawaiian
islands), in which the new species have adapted
to different ecological niches / habitats.
25
Three Hawaiian Drosophila species
26
How to study genetics of speciation 1. Study
the closest species you can, so you can see the
differences that occurred during speciation, not
afterward. 2. Close species are also ones in
which you can make hybrids in the lab so you can
do genetics. 3. Try to map genes for
reproductive isolation. 4. Do experiments to
explain how natural selection produced the
differences.
27
Drosophila sechellia, a model for understanding
speciation 1. In a group of very similar
species related to D. melanogaster (which has a
completely sequenced genome). 2. Lives only in
the Seychelles archipelago off the east coast of
Africa. 3. Has adapted to living only on the
rotting fruit of Morinda cittrifolia. This plant
is toxic to other Drosophila. Other Drosophila
live on garbage. 4. Prezygotic isolation is
incomplete - they prefer their own species but
will hybridize with D. simulans in the lab.
28
Drosophila sechellia, a model for understanding
speciation 5. D. sechellia can be hybridized in
the lab with D. simulans. The F1 males are
sterile, the F1 females are fertile. This allows
backcrosses to be made. Postzygotic isolation is
incomplete. 6. The number of genes involved in
producing male sterility is relatively large,
perhaps 20. In another close species, a gene for
sterility has been localized and sequenced. 7.
Work hints that the isolation has evolved by
pleiotropy - alleles that are fixed by selection
during adaptation to the unique food plant might
also produce sterility and affect mate choice. 8.
Thus, geographic isolation and adaptation to a
new ecological niche has produced a new species.
29
Fig. 24.13 Much debate has occurred on the issue
of whether speciation is gradual over tens of
millions of years, or whether it occurs rapidly
- in tens of thousands to hundreds of thousands
of years. In the rapid speciation model, rapid
change at speciation alternates with long periods
of stasis.
30
Lecture 13 Natural selection
Readings Ch. 24, Overview, 24,1, 24.2
Comcluding comments on natural selection in
guppies Speciation
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) Other species
concepts
Modes of speciation
allopatric speciation, sympatric
speciation Allopatric speciation
population differentiation,
clines, races/subspecies, lab experiments Sympatri
c speciation
polyploidy habitat differentiation, sexual
selection Adapative radiation Genetics of
speciation Tempo of speciation