BISC 102 Chapter Notes - Chapter 27: Synapomorphy, Sympatry, Alpheidae

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November 3rd, 2015
Read Chapter 26: pages 503-506, 508-510
Reading Notes:
Key Concepts
Speciation occurs when populations of the
same species become genetically isolated by
lack of gene flow and then diverge from each
other due to selection, genetic drift, or
mutation
Populations can be recognized as distinct
species if they are reproductively isolated from
each other, if they have distinct morphological
characteristics, is they exploit dierent sets of
resources , or if they form independent
branches on a phylogenetic tree
Populations can become genetically isolate
from each other if they occupy dierent
geographic areas, if they use dierent habitats
or resources within the same area, or if one
population is polyploid and cannot breed with
the other
When populations that have diverged come
back into contact, they may fuse, continue to
diverge, stay partially dierentiated or have
ospring that form a new species
Introduction
gene flow makes allele frequencies more
similar among populations
if gene flow ends, allele frequencies in
isolated populations are free to diverge -
meaning that populations begin to
evolve independently of each other
Speciation is a splitting event that creates two
or more distinct species from a single
ancestral group
How are Species Defined and Identified?
genetic distinctions occur due to mutation,
selection and drift
these act on each species
independently
A species is an evolutionarily independent
population of group of populations
Four criteria for identifying species (there are
more that have been proposed) ...
1. the biological species concept
2. the morphospecies concepts
3. the ecological species concept
4. the phylogenetic species concept
Biological Species Concept
According to the biological species concept,
the critical criterion for identifying species is
reproductive isolation
this is logical because no gene flow
occurs between populations isolated
from each other
How to distinguish distinct species….
1. they do not interbreed in nature
2. fertile and viable ospring are not
produced when matings take place
to organize the mechanisms that stop gene
flow between populations, biologists
distinguish them as being:
Prezygotic Isolation (literally “before
zygote”), which prevents individuals of
dierent species from mating, or
Postzygotic Isolation (“after zygote”),
in which the ospring of matings
between members of dierent species
to not survive or reproduce
Process Example
Prez
ygoti
c
Isola
tion
Temp
oral
Populations are
isolated because they
breed at dierent times
Bishop Pines
and
Monterey
Pines release
their pollen
at dierent
times of the
year
Habit
at
Populations are
isolated because they
breed in dierent
habitats
Parasites
that begin to
exploit new
host species
are isolated
from their
original
population
Beha
viour
al
Populations do not
interbreed because of
their courtship displays
dier
To attract
male fireflies,
female
fireflies give
a specits-
specfic
sequence of
flashes
Gam
etic
Barri
er
Matings fail because
eggs and sperm are
incompatible
In sea
urchins, a
protein called
bindin allows
sperm to
penetrate
eggs.
Dierences
in the amino
acid
sequence of
bindin cause
matings to
fail between
closely
related
populations
Mech
anica
l
Matings fail because
male and female
reproductive structures
are incompatible
In alpine
skypilots
(flowering
plant, the
length of the
floral tube
varies. Bees
can pollinate
in
populations
with short
tubes, but
only
hummingbird
s can
pollinate
populations
with long
tubes
Post
zygo
tic
Isola
tion
Hybri
d
Viabil
ity
Hybrid ospring do not
develop normally and
die as embryos
When ring-
necked
doves mate
with rock
doves, fewer
than 6% of
eggs hatch
Hybri
d
Sterili
ty
Hybrid ospring mature
but are sterile as adults
A male horse
can mate
with a female
donkey to
produce a
sturdy, long-
lived mule.
The mule is
sterile
however.
Although the biological species concept has a
strong theoretical foundation, it has
disadvantages . The criterion of reproductive
isolation cannot be evaluated in fossils or in
species that reproduce asexually. In addition,
it is dicult to apply when closely related
populations do not happen to overlap with
each other geographically
The Morphospecies Concept
Under the Morphospecies Concept
researchers identify evolutionarily independent
lineages by dierences in size, shape, or other
morphological features
the logic behind morphospecies concept is
that distinguishing features are most likely to
arise if populations are independent and
isolated from gene flow
Disadvantages...
1. It cannot identify cryptic species,
which dier in traits other than
morphology, such as the meadowlarks
exam
2. the morphological features used to
distinguish species are subjective
The Ecological Species Concept
the ecological species concept defines a
species as a set of organisms exploiting a
single set of resources, having the same range
environmental tolerances, and facing the same
predators and parasites
emphasizes the role of natural selection
most useful for identifying species of bacteria,
archaea or asexual eukaryotes
these species can’t be defined by
reproductive isolation from other
species
The Phylogenetic Species Concept
The phylogenetic species concept is a
recent addition to the tools available for
identifying evolutionarily independent lineages
and is based on reconstructing the
evolutionary history of populations
widely applicable and precise
the reasoning behind this concept started with
Darwin’s claim that all species are related by
common ancestry
Darwin was suggesting that all species
form a “monophyletic group” on the
tree of like.
Monophyletic = a clade or lineage
consists of an ancestral
population, all of its descendants
and only those descendants
Monophyletic groups are identifies by traits
called synapomorphies
these traits are found in certain groups
of organisms that exists in no others
a homologous trait that one inherited
from a common ancestor, that is unique
to certain populations or lineages
Under the phylogenetic species concept,
species are defined as the smallest
monophyletic groups on the tree of life
Advantages of phylogenetic species concept...
1. it can be applied to any population
(fossil, asexual or sexual)
2. it is logical because species have
dierent synapomorphies only if they
are isolated from gene flow and have
evolved independently
Disadvantages of phylogenetic species
concept...
1. carefully estimates phylogenies are
available only for a tony (though
growing) subset of populations on the
tree of life
In actual practice, researchers use all four
species concepts to identify evolutionary
independent populations in nature
Isolation and Divergence in Allopatry
Genetic isolation happens routinely when
populations become physically separated.
Physical isolation, in turn, occurs in one of two
ways: dispersal or vicariance
a population can disperse to a new habitat,
colonize it and found a new population
a physical splitting of a habitat is called
vicariance
vicariance due to glaciation plaid a key
role in each of these speciation events
speciation that begins with physical isolation
via either dispersal or vicariance is known as
allopatric speciation
populations that live in dierent areas are said
to be in allopatry
biogeography the study of how species and
populations are distributed geographically
once populations are physically
isolated, how do genetic drift and
slecetiong produce divergence?
Dispersal and Colonization Isolate Populations
Peter Grant and Rosemary Grants witnessed a
colonization event while working in the
Galapagos
studying medium ground finches
a new species arrived on the island
(large)
through measuring and weighing - they
discovered that the new population had
much larger beaks
Potential reasons for bigger beaks...
1. genetic drift produced a colonizing
population that happened to have
particularly large beaks relative to the
source population
2. Natural selection in the new
environment could favour alleles
associated with large beaks
the general message here is that the
characteristics of a colonizing population are
likely to be dierent from the characteristics of
the source population due to the chance - that
it is genetic drift. Subsequent natural selection
may extend the rapid divergence that begins
with genetic drift
Vicariance Isolates Populations
if a new physical barrier such as a mountain
rate or a river splits the geographic range of a
species, vicariance has taken place
a phylogenetic tree of the snapping shrimp
shows that may of the species found on either
side of the Isthmus are sister species
to summarize, physical isolation of
populations via dispersal of vicariance
produces genetic isolation - the first
requirement of speciation
Isolation and Divergence in Sympatry
when populations or species live in the same
geographic area, or at least close enough to
one another to make interbreeding possible,
biologists say that they live in sympatry
(“together homeland”)
prediction was the gene flow would easily
overwhelm any dierences among populations
created by genetic drift and natural selection
in general gene flow overwhelms the
diversifying force of natural selection and
prevents speciation
Can Natural Selection Cause Speciation Even
When Gene Flow is Possible?
sympatric speciation - speciation that occurs
even though gene flow is possible, it is rare or
non-existant
new studies are starting to challenge
this view
the key realization is that even though
sympatric populations are not physically
isolated, they may be isolated in preferences
for dierent habits
Can Polyploidy lead to Speciation?
the mutation reduces gene flow between
mutant and normal, or wild-type individuals. It
doe so because mutant individuals have more
than two sets of chromosomes
this is polyploidy
mutations are any change in an organisms
DNA present; polyploidization is a massive
mutation that aects entire chromosome
By meiosis, diploid individuals produce
haploid gametes and tetraploid individuals
produce diploid gametes. These gametes
unite to form triploid (3n) zygote
Even if this ospring develops normally and
reaches sexual maturity, its three homologous
chromosomes cannot synapse and separate
correctly during meiosis
thus they are distributed to daughter
cells unevenly
end up with an uneven # of
chromosomes
the triploid individual is virtually sterile
Tetraploid and diploid individuals rarely
produce fertile ospring when they mate. As a
result, tetraploid and diploid populations are
reproductively isolated
Two general mechanisms….
1. Autopolyploid - (“same many-form”)
individuals are produced when a
mutation results in a doubling of
chromosome number and the
chromosomes all come from the same
species
2. Allopolyploid - (“dierent many-form”)
individuals are created when parents
that belong to dierent species mate
and produce an ospring where
chromosome number doubles.
Alloployploid individuals have
chromosome sets from dierent species
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