52 terms

Species and Speciation

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Evolutionary Species concept
a species is a lineage of . . . populations which maintains its identity from other such
lineages and which has its own evolutionary tendencies and historical fate"
key attributes that make a group of populations into a species
are their shared past evolutionary
history and their common future evolutionary fate—at least until this species itself
bifurcates to form new descendant species. Notice that this definition is inherently
phylogenetic: species is a group of populations that have a shared past and will
have a shared future on a phylogenetic tree.
drawbacks of evolutionary species concept
The evolutionary species concept defines what a species is, and what role
species play in evolutionary history, but it does not offer particularly useful
practical advice on how we should go about identifying species and drawing
species boundaries in the study of natural populations
Approaches include
the phenetic species concept, the biological species concept, and the
phylogenetic species concept
identifying species
phenetic species concept; concept takes advantage of this fact, drawing species boundaries around clusters
of phenotypically similar individuals or populations individuals or populations are highly clustered in phenotype space and use this pattern of clustering to assign species boundariesA similar process
can be applied at higher levels of taxonomic organization to delineate genera,
families, orders, and other taxonomic levels.
historically the phenetic species concept was used by
was used by numerical taxonomists—
scientists who use statistical analyses of multiple traits to classify organisms
The phenetic species concept remains in common use today, especially in the
classification of plants and microorganisms
drawbacks of phenetic concept
how to weigh the relative importance of the characters or traits used to delineate species boundaries.
Should all traits be viewed as equally important in classifying organisms, or should some traits be weighed more heavily because they are particularly important?
Early numerical taxonomists tended to assign equal weights to all characters they
measured, but this approach was quickly abandoned by some, in favor of weighing
certain characters more heavily than others
biological species concept
first introduced by Ernst Mayr,
Under the biological species concept, a
species is composed of "groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups
under the biological species concept it is
it is the pattern of gene flow, rather than the pattern of phenetic similarity, that determines species
boundaries diagnosing what constitutes a species, the biological
species approach looks directly to the evolutionary mechanism—gene flow—
responsible for the "shared evolutionary fate" that is fundamental to the concept of
species. As a result, the biological species concept is not based on attributes of the
individuals, but rather it delineates species by properties possessed by populations.
If individuals in one population are capable of mating with individuals in another
population, then individuals in both populations are part of the same species, and
they are said to share the same gene pool. If populations are reproductively isolated
from one another then the individuals in such populations are not considered to be part of the same
species.
drawbacks of the biological species concept
very hard to apply this concept to extinct species that are known only from
paleontological evidence. Although reproductive isolation can sometimes be
inferred from the distribution and form of fossils, this is not often the case.
drawbacks ctd
Another problem for the biological species
concept is the occasional hybridization events
between individuals in populations that are, for
all practical purposes, reproductively isolated.
If individuals in population 1 consistently mate with those in population 2, individuals in these populations are classified as part of the same
species. But what if matings between individuals in different populations are rare or nonexistent? The two populations can still be part of a common
species, because the biological species concept allows the populations to be potentially interbreeding.
What if the offspring produced by cross-population
matings are nonviable or infertile? In this case, we clearly have two species. But what if the offspring merely have reduced viability or reduced fertility?
How rare do cross-population matings have to be,
and how poorly must the hybrid offspring fare,
before we can say that the two populations are two separate species
drawbacks of BSC ctd
Another major limitation of the biological species definition is that it is
restricted to sexual species. With its emphasis on the reproductive isolation of
populations, the biological species concept makes little sense as a species concept
for asexual organisms. As Ernst Mayr notes, "[i]n an asexually reproducing species
every individual and every clone is reproductively isolated. It would be absurd to
call each of them a separate species"

forms that do not meet
many plants with extensive hybridization
Phylogenetic Species concept
Like the phenetic
species concept, this approach looks to character differences in order to distinguish
among species, but it does so in a different way. The basic problem in distinguishing
species remains the same: How do we determine whether two groups are behaving as
evolutionary species that are able to maintain distinct identities so that they have their
own evolutionary histories? If two groups have been separated long enough to have
diverged and produced distinguishing characters, they must have been reproductively
isolated from one another and, as evidenced by these distinguishing characters, they
must have already experienced unique evolutionary histories.
But what characters are the right characters to use in making such distinctions
According to this approach, we draw species boundaries using shared derived characters
that are unique to one monophyletic group and absent from all other populations in
the phylogeny. These characters can then be used to distinguish among species
phylogenetic species
the smallest monophyletic group distinguished
by a shared derived character
ways shared derived characters can be used to distinuisgh species
By looking at shared derived characters
that distinguish monophyletic groups, the phylogenetic species concept selects appropriate characters for classifying species.
characters that are polymorphic
within a population will not form
monophyletic clades, and therefore they should not be used to define species boundaries under the phylogenetic
species concept
By requiring that a species be the smallest distinguishable
monophyletic clade
the phylogenetic species concept also
determines an appropriate taxonomic level at which to
draw species boundaries.
critique
One of the major critiques of the phylogenetic species concept is that the
traits it uses to distinguish among species do not have to be ecologically or
physiologically significant. Thus, distinguishing traits can be minor characters of
minimal significance. As a result, the phylogenetic species concept often divides
up organisms into more fine-grained species categories than may seem appropriate,
resulting in a far greater number of species than would be delineated by other
species concept
drawback
Moreover, the phylogenetic species concept does little to ensure
that species considered separate at present will have separate evolutionary fates in
the future. Because there is no requirement of restricted gene flow, members of
two distinct phylogenetic species may be able to interbreed readily, which would
enable the two species to fuse back into one species at some point in the future
clearly no one species concept will work for all organism
The phenetic species concept looks for clusters of phenotypic
characters; the biological species concept looks at the presence or absence of
gene flow; the phylogenetic species concept relies on shared derived traits of
monophyletic groups. But most of the time, all three species concepts will readily
agree on species boundaries. Populations that belong to different species typically
show large phenotypic differences, absence of gene flow, and shared derived
traits.
modes of speciation
three models of speciation—allopatric,
parapatric, and sympatric speciation. These three models are distinguished from one
another by the relative geographic positions of populations undergoing speciation.
allopatric speciation
the process of speciation takes place in populations that
are geographically isolated from one another

The central premise underlying allopatric speciation is that reproductive isolating
mechanisms evolve in populations while they are geographically isolated. This
geographic isolation may be a consequence of a physical barrier such as a mountain
range, a river, an ocean, a desert, or some other barrier
parapatric speciation
incipient
species—diverging populations on the path to speciation—have distributions that
abut one another.
sympatric speciation
populations diverge into new species
while in the same location

Speciation that takes place in geographically overlapping populations.

This can occur by chromosomal changes and nonrandom mating. Both can reduce gene flow between organisms and cause populations to evolve to new species.

Example: Polyploidy (greater than 2 sets of chromosomes) - most common in plants.
in all allopatric speciation models
the processes of genetic drift, mutation, and
natural selection cause populations to diverge from one another
Different mutations are likely to
arise by chance in two allopatric populations, further augmenting their differences.
Finally, no two geographically isolated populations will experience exactly the
same selective conditions, and any differences in selective conditions can cause
the populations to diverge by natural selection. In the long run, these processes
may lead to multiple forms of reproductive isolation between these populations.
This is because gene flow between geographically isolated populations may be
permanently eradicated when the members of one population lose the ability to
breed successfully with members of the other population due to differences in
geographic range, genetics, behavior, or reproductive physiology.
once gene flow becomes impossible
the populations no longer share a common evolutionary fate and thus can result in the formation of new species
allopatric speciation is often subdivided into a dumbbell model and a peripheral isolate model
In the dumbbell model of allopatric speciation, an initially large
population is subdivided into new populations that are themselves still relatively
large. In the peripheral isolate model, the populations that are geographically
isolated from one another differ in size, with one large population and one or
several smaller populations
one of the most important differences between dumbell and peripheral isolate models
pertains to the role of genetic drift in driving divergence between the populations.
In the dumbbell model, the descendant populations are each relatively large in size,
making it unlikely that drift dramatically affects divergence. By contrast, in the
peripheral isolate model, a peripheral population may be founded by a relatively
small number of individuals, resulting in strong founder effects. Moreover, the net
population size in the peripheral population may be much smaller than that of the
progenitor population, resulting in accelerated genetic drift.
parapatric speciation
Parapatric speciation occurs when two adjacent populations diverge into separate
species without a geographic barrier to dispersal
The core of the
parapatric speciation idea is that some sort of cline—a spatial gradient in the
frequency of phenotypes or genotypes—exists in nature because adjacent
populations experience somewhat different selective conditions
a hybrid zone
an area in which diverging populations encounter each other, mate, and produce
hybrid offspring—is essential in the parapatric speciation processBy contrast, the allopatric and sympatric models do
not include a hybrid zone during the speciation process.
hybrid zone in parapatric speciation
it is assumed that the zone will eventually disappear completing the spciatino process. this can occur because hybrid offspring may be at a selective disadvantage. because they are not suited to life in any section of the cline. this generates selection for for genetic, physiological, or behavioral reproductive isolating mechanisms that
deter hybridization between the two populations, and which once in place may
lead to the completion of the speciation process.
not all parapatric models assume
that hybrids are at a disadvantage
The ecologically neutral dynamic equilibrium model suggests that
hybridization produces hybrids that are always inferior to nonhybrids. In contrast,
the ecologically dependent bounded hybrid superiority model assumes a genotype-byenvironment
interaction, such that in hybrid zones, hybrids may have superior
fitness to nonhybrids.
ring species
it is sometimes hard to make a clear distinction between allopatric and parapatric speciation. conside the case in which indivudials live in a series of populations that are connected to one another in a ringlike fashion
there is some gene flow but it not continuous along the ring. Indeed, as one would
expect if the ring originated at the northern tip, the amount of gene flow decreases
along both ranges as one moves south, leading to southern populations at the end
of each ridge being more genetically distinct from one another than from other
populations in the ring
sympatric speciation
populations. For evolutionary biologists, sympatric speciation is the most difficult
of the three forms of speciation to understand. The difficulty stems from the fact that, without some sort of geographic barrier or some sort of gradient in selective conditions, some other mechanism must drive a single species to split into two species. One possibility is that speciation may be driven by resource competition
reproductive isolating mechanisms and the genetics of speciation
Prezygotic isolating mechanisms prevent or deter individuals from different
populations from mating with one another, or prevent fertilization from occurring if such a mating does take place.

Postzygotic isolating mechanisms operate after fertilization and conception. With postzygotic mechanisms in place, a
mating between individuals from different populations may lead to successful fertilization, but the embryo may not survive. If it does, it may be either sterileor have dramatically reduced.
morphological species concept
if it looks likeone then it one usually only concept available for fossils hybrid gene flow could swamp out morphological differences.
genetic isolation
important to all concepts
genetic barriers
polyploidy changes in chromosome number individulas with different karyotypes are reproductively incompatible important mechanism in plant speciation
divergence
genetic drift- following founder effect drift can promote divergence. probably not a major mechanism of divergence on its own.
divergent speciation with natural selection
if important ecological resources are of only a few distinct types then ecologoical specialization results.
divergvence via sexual selection
species concepts are about reproductive isolation and gene flow assortative mating reduces gene flow between groups - promotes divergence
if they continue to diverge until they ae reproductively incompatible cannot produce fertile hybrid offspring if they are completely distinct
if they are partially distinct
if they diverged in allopatry and they meet again called secondary contact they are distinct but still able to hybridize.
hybrid possibilities
infertile no gene flow
speciation complete isolation is post zygotic.
have reduced fitness but are fertile- some gene flow speciation in progress
hybrids have same or higher fitness
gene flow may eliminate differences no more speciation event or gene flow may be limited to zone of contact
reinforcement
if hybrids are infertile or have reduce fitness. that selects for parents to avoid this mistake. post zygotic isolation that has already occurred selects for prezygotic isolation that is called reinforcement and it promotes speciation

predicts
prezygotic isolation evolves faster in sympatric species pairs

Reinforcement is the evolution of traits that reduce matings between divergent populations, due to natural selection against production of low-fitness hybrid offspring.
hybrid studies using classical genetics
• Haldane's rule
- Heterogametic sex more likely to be inviable or
sterile
- Mammals and most insects
• Males XY females XX; males heterogametic
- Birds and butterflies
• Females ZW males ZZ; females heterogametic
Probably due to alleles on sex chromosomes that interact with
autosomal loci to cause inviability. Homogametic sex has
species typical X-linked and species typical autosomal, but
heterogametic sex has species typical autosomal but
heterospecific X-linked.
in plants polypolidization
is the retention of extra sets of chromosome,
autopolyploids- add chromosomal sets from same species

allopoluploids combine chromosomal sets from different species
secondary contact
occurs when two formerly allopatric populations meet
3 outcomes no interbreeding- isolating mechanisms in place speciation is completed

introgression:no isolating mechanisms populations merge completely

partial interbreeding: some isolating mechanism in place a hybrid zone forms but hybrids are less fit. reinforcement should occur to complete the process by the evolution of additional prezygotic barriers.
dispersal
vicariance allopatric speciation
Dispersal occurs when individuals leave an area and physically move to a new habitat and colonize it, forming a new populations. Vicariance occurs when an existing population is fragmented into 2 or more isolated populations by changes in the habitat. Dispersal and vicariance produce geographic isolation, which reduces or eliminates gene flow between populations. Stated another way, geographic isolation leads to reproductive isolation
parapatric speciation hybrids
If hybrid offspring have low fitness, then reinforcement should evolve and the populations should be come separate species. If hybrid offspring have equal fitness to the parental forms, then the parental populations should coalesce over time. If hybrid offspring have higher fitness than the parental forms in a certain habitat, then a new species or a stable hybrid zone may form in that habitat.
divergence and sexual selection
Speciation requires reproductive isolation and divergence. If divergent sexual selection leads changes in the traits that certain individuals use to choose mates, then sexual selection will cause those traits to diverge rapidly, and the individuals using the diverged traits as mate-choice criteria will be reproductively isolated from the original population.
recognition species concept
the most inclusive population of individual organisms that share a common fertilization system
focuses on mate recognition systems: courtship displays
the timing of reproductive events
neuroendocrine signals
design on copulatory organs
gamete compatibility