Download SPECIES AND SPECIATION

Species Concepts
This idea goes all the way back to Darwin where he
used visible “gaps” in morphology to delimit
species.
“I believe that species come to be tolerably welldefined objects, and do not at any one period
present an inextricable chaos of varying and
intermediate links.”(1859: pg 177)
“We shall have to treat species in the same manner as
those naturalists treat genera, who admit that
genera are merely artificial combinations made for
convenience. This may not be a cheering prospect;
but we shall at least be freed from the vain search
for the undiscovered and undiscoverable essence of
the term species…”(1859: pg 282)
 Based on judgments about the similarities among
organisms
 challenge is to make it mechanistic and testable
 want to accurately reflect evolutionary history of
organisms.
 we don’t really know whether such a thing as a species
actually exists in reality
 All three assume two things in common
1) no gene flow- species form a boundary for the
spread of alleles
2) species have their own evolutionary pathway
 Proposed by Dobzhansky and Mayr, elucidated by
Mayr as …
 “ Species are groups of interbreeding natural
populations that are reproductively isolated from
other such groups.”
 Definition implies:
no hybridization or hybrids fail to form fertile
offspring
lack of gene flow
PROBLEMS:
1.
Can not always tell if two groups of individuals are
reproductively isolated
If two groups are separated by geographical barriers
there is no way to know if they are reproductively
isolated
2.
3.
4.
Many plants hybridize freely; we will discuss
hybridization later in some detail
Cannot test it in fossil forms
Irrelevant to asexual populations
 Also called Evolutionary Species Concept
 This is the systematists contribution to the idea of a
species
 “A species is a single lineage of ancestral descendant
populations of organisms which maintains its identity
from other such lineages and which has its own
evolutionary tendencies and historical fate”
 Focuses on the idea of monophly (a set of species are all
descended from one common ancestor)
 A monophyletic group contains all of the known
descendants of a single common ancestor. There are
no parallel branches or interconnecting branches
(such as in hybridization)
 Fig 16.1 species are on the tips of the trees. Circles
represent the monophyletic groups
Notice that …
(1) Common ancestor does not
continue on as a species and
(2) every “species” divides to
form only two new “sister
species”
You do not
see branching
such as this.
or this
 RATIONALE
 can only form separate species if the populations
have diverged from one another in isolation
 The original species will always form two new
species and cease to exist itself.
 Appeal is that it is testable
 Species are identified (named) on the basis of
statistically significant differences in the traits
used to estimate the phylogeny (ancestry)
 Populations must have been independent long
enough for diagnostic traits to emerge
 Phylogenies are only available for a handful of
groups
 Very tiny differences, even a single DNA substitution
may be used as a trait that separates groups
 Could end up doubling the number of species
 Very difficult to interpret when new species actually
becomes a new species
 Define species based on the morphological differences.
Commonly used with fossils.
 This definition does not demand proof of reproductive
isolation or phylogenetic relationships
 Used when we do not have tests for reproductive isolation
or well-estimated phylogenies
 Assignment to species is often arbitrary and cannot
distinguish cryptic species
• ones which are strongly divergent based on nonmorphological characters.
• Things such as song, temperature or drought
tolerance, habitat use, or courtship displays
 Today used mostly by paleontologists
 For at least some instances there is good evidence that
fossil Morphospecies may indicate real species
differences
 In 1930 the Red wolf appeared to be a Morphospecies being
intermediate in appearance between the gray wolf and the
coyote, all 3 appearing to be distinct.
 Studies have shown that the red wolf is actually a hybrid
between gray wolves and coyotes. Therefore its
intermediate characteristics are the result of hybridization
and not independent evolution. This makes the Red wolf
not a distinct species for most biologists because…
 Neither the BSC or the PSC allow for hybridization
 However, it is still considered a separate species and the
morphospecies is the only one of these 3 definitions that
works.
 Allopatric model- speciation occurs in populations
that have been physically isolated from one another
 Sympatric model - Populations can diverge without
geographical separation, with low to moderate gene
flow between them if…
1. Selection for divergence is strong
2. Mate choice is correlated with the factor
that is promoting divergence
 Parapatric model– Strong selection for divergence
causes the gene frequencies to diverge along a gradient
 Peripatric model a subset of the allopatric model
involving colonization
Parapatric
speciation
Involves 3 steps
1.
2.
3.
Isolation of members of a population from one
another
Genetic divergence of the separated populations
Renewed sympatry of the populations with
reinforcement of the genetic differences which have
arisen
Physical Isolation
 Necessary to prevent gene flow which would keep
populations homogenized
 may occur when small populations become isolated at
the periphery of a species’ range.
 If selection is strong and gene flow is low divergence
could then occur rather rapidly
1.
2.
By dispersal and colonization
Dispersal to novel environment such as rafting a
portion of a population to an island
By Vicariance events
 Involves Founder effect = Peripatric Speciation (Mayr)
 small group of individuals cut off from the original
population colonizes a new habitat
 drift and selection on genes involved in mating and
habitat use leads to divergence
 closely related species should be found on adjacent
islands
 some of the phylogenetic branching sequence should
follow island formation
 using mitochondrial DNA it was shown that four
closely related species were found in the expected
pattern
 Figure 16.7 page 613
Figure 15.7 page 593
 Events which split a species into two or more isolated
ranges and prevents gene flow between them (or at
least greatly reduces it)
 Can be slow processes like rising of a mountain range,
long term drying trend etc
 or rapid like a lava flow that splits non-flying insect
populations
 A land bridge opened as the isthmus closed about
3 million years ago
 Found 7 pairs of closely related morphospecies of
snapping shrimp. One member of each pair on
each side of the land bridge
 The pairs from either side of the bridge are shown
to be sister species (each other’s closest relative)
believed to share the same common ancestors
which split to form each pair
5
Also, interestingly, shrimp
populations would have been isolated
in a staggered fashion as the land
bridge gradually formed in stages
Species 6 and 7 live in the deepest
water and were cut off first
1-5 were in shallower water and
diverged later
Figure 16.8 pg. 614
Genetic Divergence
 Vicariance events and dispersal events only provide
conditions for speciation
 Usually you also need to have genetic drift and/or
selection work on mutations in these isolated
populations in order to get genetic divergence.
 Sexual selection may also lead to genetic divergence
Secondary Contact (return to
sympatry)
After return to sympatry
Possible Outcome #1 after secondary contact
 Fully fertile hybrids form – no speciation has
actually occurred while in allopatry.
 Hybrids thrive and interbreed with both parental
populations, any divergence is erased.
Possible Outcome #2 after secondary contact
 Reinforcement of parental forms as two recently
diverged species.
 The two groups are considered now to be two
species.
 If populations have sufficiently diverged while in
allopatry, their hybrid offspring should have markedly
reduced fitness when compared to individuals in both
parental populations.
 Parental populations will reduce their fitness if they
produce hybrid offspring, therefore this should favor
assortative mating within each new species.
 Selection that reduces the frequency of hybrids is
called reinforcement
 The final stage of speciation, that of
establishing reproductive isolation by
reinforcement can occur in any number of
ways.
 These are called pre-zygotic isolation
mechanisms
 Temporal isolation – individuals of different species do not




mate because they are active at different times of day or seasons
of the year
Ecological isolation- Individuals mate in their preferred habitat,
and therefore do not meet individuals of other species.
Behavioral isolation- potential mates from incipient species
meet but choose members of their own species
Mechanical isolation – copulation is attempted but transfer of
sperm does not take place
Gametic incompatibility – sperm transfer takes place but egg is
not fertilized
 Plot of genetic similarity versus the degree of interbreeding
for various sister species of Drosophila. A value of 0 on the Y
axis indicates free interbreeding, 1 indicates no interbreeding.
Figure 16.12 pg 625
 Post-zygotic mechanisms may lead to hybrid
offspring which are sterile or infertile
 Zygotic mortality- egg is fertilized but zygote does not
develop
 Hybrid inviability– Hybrid embryo forms but of
reduced viability
 Hybrid sterility – hybrid is viable but adult is sterile
 Hybrid breakdown – F1 hybrids are viable and fertile
but F2 and backcrosses to parents are inviable or sterile
Chapter 16
Possible Outcome #3 after secondary contact
Creation of a new species through
hybridization.
Formation of a new third species
from the hybrid formed.
Hybrid is fertile but cannot back
cross to either parent.
The role of Hybridization
Hybridization is a common
occurrence in plants
 At least in some cases the outcome of
these hybridization events determines
the outcome of the speciation event

 In newly colonized areas or in new habitats, hybrids may
have higher fitness than the parents
 These hybrids can mate with siblings and backcross to
their parents the result is a variety of hybrid gene
combinations. This is called introgression.
 If certain of these combinations is best suited to a new
habitat, a third species may arise that is somewhere
intermediate between the parental species.
 However, it is also possible that, if the hybrids have equal
or greater fitness than either parental population,
complete introgression may occur. The result is one
species somewhat like the one that existed prior to
geographic separation
Possible Outcome #4 after secondary contact
1.
When parents and hybrid are equally fit…
The zone is wide.
Hybrid traits are found with highest frequency at the
center of the zone.
Gene frequency changes are dominated by drift.
Width of the zone is determined by
a) distance of dispersal in each generation
b) How long zone has existed
2.
When hybrids are less fit than purebred
individuals (parents)…
Fate of the hybrid zone depends on the
strength of selection against them.
(a) Strong selection leads to reinforcement,
with a very narrow and short-lived hybrid zone.
(b) If selection is weak, the hybrid zone is
wider and longer lived
A balance develops between formation of the
hybrids and the selection pressure against the
hybrids.
3.
When Hybrids are more fit than purebreds
Depends on the extent of the environment in
which the hybrids are at an advantage
New species results if hybrids are more fit in
areas outside the range of the parental species
If the advantage is at the boundaries between
the two parental species then will form a stable
hybrid zone (Parapatric speciation)
Often found in areas called ecotones where
markedly different plants and animals meet
 Between the basin and mountain subspecies
 Hybrids shown to be more fit than parents in transitional
zones
 Showed that the hybrid zone is maintained because the
hybrids have superior fitness in the transitional zone
 Potential practical problems
 Some crop species are closely related to weed species
 Herbicide resistance genetically engineered into a crop
plant could be transferred to weed plants through
hybridization
 How much genetic variation is necessary to produce a
new species?
 The BSC requires that no hybridization whatsoever
occur, however.....
 Fertile hybrid offspring can be found even when the
parental populations are markedly different from one
another.
 Current research focuses on the number, location or
nature of the genes which distinguish closely related
species in an attempt to uncover past speciation
through hybridization
Requires development of reproductive
isolation while individuals are still in contact
and gene f low is still possible
1.
2.
3.
Polyploidy
Genetic divergence (drift, natural selection etc.)
Sexual selection
Polyploidy, or the condition of having extra sets of
chromosomes can lead to genetic isolation of
populations.
Polyploids have 3n, 4n, 5n, 6n etc. numbers of
chromosomes rather than the normal diploid number.
This condition is detected by looking for two factors



1.
2.
In plants at least, chromosome numbers greater than n=14
are considered to be of polyploid origin
Related individuals will have chromosome numbers which
are multiples of some basic number, for example, in
Chrysanthemum different species have 2n numbers of
18,36,54,72, and 90
Autopolyploids
Allopolyploids
 The union of unreduced gametes from genetically
and chromosomally compatible individuals, that may
be thought of as being from the same species.
 During meiosis In the autoploids the chromosomes
pair up into quadrivalents and mostly get uneven
segregation of chromosomes. Called aneuploid
gametes (have either too many or too few
chromosomes).
 This leads to reduced fertility or sterility
Unreduced gametes
 Polyploids are derived from a hybrid between
unreduced gametes of two different diploid
species.
 Genetic or chromosomal incompatibility arises.
 In meiosis, there is a natural formation of
homologous pairs with twice as many pairs as
either parent.
 Get balanced segregation and normal gametes
 Typically these hybrid polyploids have near normal
fertility.
Unreduced
gametes
Two different
species
 Because cannot back cross to either parent the
resulting gametes are sterile
 Limits the individuals they can cross with to other
allopolyploids
If species cannot self fertilize then...
May have difficulty finding each other and
cannot out-compete the original parent
populations. But…
 If they do interbreed with each other, or are selfcompatible, they are chromosomally isolated and can
begin to diverge from parent populations immediately.
 Sexual selection promotes divergence efficiently
because it affects gene flow directly
 There is an example in the book of Drosophila
flies that may have diverged due to sexual
selection
 In the Beak of the Finch we read about cases of
sexual selection which can help prevent
hybridization in the finches.
 Recent studies are suggesting that sexual
selection is an important and necessary factor
for sympatric speciation