SPECIATION

BIO 206 – Evolutionary Biology

What is speciation?

 Darwin concluded that all species of birds—indeed all species of

animals, and finally all living things—may have originated by
successive branching of lineages throughout the history of life, from
a single common ancestor.

 Modern research has affirmed that this is indeed how the enormous
diversity of organisms arose.

 The forks in the great Tree of Life were caused by speciation, the
process by which one species gives rise to two.
What is a species?

 To start, we will define species based on the biological species

concept (BSC), formalized by Ernst Mayr:
"Species are groups of actually or potentially interbreeding
populations, which are reproductively isolated from other such
groups."

 The important concept in the BSC is reproductive isolation

 If populations of organisms do not hybridize regularly in nature, or if they
fail to produce fertile offspring when they do, then they are
reproductively isolated and are considered separate species.
 If two species are reproductively isolated, then there is little to no gene
flow between them. They belong in different gene pools.
 (A) and (B), despite having different colors, belong to the same species,
Megascops asio. (C) is a different species (M. kennicottii); even if it looks similar
to (A), they cannot interbreed.
What is a species?

 However, some eukaryotes and all bacteria and archaea do

not interbreed, since they reproduce by binary fission.
 In that case, we will consider this definition: bacterial species may
consist of cells that recently descended from a common ancestor and
that have not experienced gene flow via lateral transfer

 What if the organisms are geographically separated as

opposed to being in the same location?
 If they are geographically separate they are thus also reproductively
isolated, by virtue of their geographical distance.
What is a species?

 Niles Eldredge and Joel Cracraft proposed the phylogenetic

species concept (PSC):
Species is "a diagnosable cluster of individuals within which
there is a parental pattern of ancestry and descent, beyond
which there is not, and which exhibits a pattern of phylogenetic
ancestry and descent among units of like kind."

 Species are identified by estimating the phylogeny of closely-

related populations and finding the smallest monophyletic
groups.
In the image, the terminal nodes are different species (same letter – different
populations of the species).
How are species determined?

 Morphological and other phenotypic characters (see image)

 Careful analysis of phenotypic characters are the basis for identifying
morphospecies.

 Genetic markers and phylogenetic analysis

Speciation

 Speciation is hypothesized to have three stages:

 an initial step that isolates populations,
 a second step that results in divergence in traits such as mating system
or habitat use, and
 a final step that produces reproductive isolation.

 First two steps are thought to take place over time and while
populations were located in different geographic areas; the
final phase when the diverged populations came back into
physical contact
 Of course, this isn't universally true – the third phase may not
occur, and populations might have been isolated at the same
time and the same place
Mechanisms of isolation

 Isolation is a reduction in gene flow between two populations.

 Isolation most commonly occurs as a result of geographic

factors that cause populations to become physically
separated.
 However, any factor that reduces the probability or
effectiveness of interbreeding between two populations can
cause isolation, such as changes in chromosome number.
Mechanisms of isolation: geographic isolation

 Allopatric – geographically isolated

 Physical isolation creates an effective barrier to gene flow
 Research has shown that in many cases, geographic isolation has been
an important trigger for the second stage in the speciation process

 Two mechanisms:
 Dispersal – e.g. a population rides a boat to another place and end up
colonizing it
 Vicariance – an existing range is split by the appearance of a physical
barrier, e.g. a road or a river
Isolation by dispersal and vicariance. (a) Dispersal: arrows indicate movement
of individuals. (b) Vicariance: arrows indicate an encroaching physical feature
such as a river, glacier, lava flow, or new habitat.
Mechanisms of isolation: geographic isolation

 Dispersal: Hawaiian drosophilids

 Hawaiian drosophilids are a widely diverse group and differ in area, food
sources, where they lay eggs, etc.
 Many of them are island endemics. If a population (or a gravid female)
disperse to other islands, it can establish colonies.
 The Hawaiian islands are produced by a volcanic hotspot under the
Pacific Ocean; the hotspot spews magma onto the Pacific plate and
forms islands. Continental drift then moves these islands away.
 If a population was on the new islands (thus becoming founders),
divergence occurred, resulting from genetic drift and natural selection
 Five closely-related
Drosophila hemipeza Drosophila planitibia Drosophila heteroneura Drosophila species.
The older-to-younger
sequence of
divergences on the
phylogeny
corresponds to the
older–younger
sequence of island
formation.

This pattern is
consistent with the
hypothesis that at
least some of the
speciation events in
this group were the
result of island
hopping.
Mechanisms of isolation: geographic isolation

 Vicariance: Snapping shrimp (Alpheus) on either side of the

isthmus of Panama
 The isthmus rose and created a land bridge between North and South
America millions of years ago
 The species on either side of the isthmus (Pacific and Atlantic) became
separated
 Snapping shrimp were sampled from both sides of the isthmus seemed
to be closely-related species (sister species) based on morphological
examination, with one member of each pair found on each side of the
land bridge
 Phylogenetically, species were indeed each other's closest relatives
 They were also put together in aquariums in courtship experiments:
 Males and females with greater divergence were less interested in one another
 None of the pairs produced fertile clutches
 The Isthmus of Panama

Phylogeny estimated from sequence divergence in

mitochondrial DNA. Morphological sister species
Alpheus malleator, found on the Pacific
from opposite sides of the isthmus are identified by
side of the Isthmus of Panama
letters and colors.
Mechanisms of isolation: polyploidy and other
chromosome changes

 Mutations resulting in polyploidization can produce instant

reproductive isolation between parental and daughter
populations
 E.g. when a 4n descends from a 2n they cannot produce fertile offspring
with 2n because of dysfunctional chromosome complements that result
from crosses.
Mechanisms of isolation: sympatric species

 Differences in chromosome number provide the necessary

boundary for isolation for sympatric species, or those that
overlap geographically
 Example: Butterfly species Agrodiaetus have chromosome # ranging
from 10-134
 These are important in maintaining boundaries between Agrodiaetus species
since they are sympatric

 What keeps two sympatric species separate?

 Prezygotic isolation – hybrid offspring are never formed, usually because
mating between species does not take place
 Could be reproductive structures do not fit, the female is not stimulated by
males of the other species, failure of fertilization
 Postzygotic isolation – hybrid offspring suffer from inviability, sterility, or
reduced fitness
 Hybrids can be poorly adapted, or less successful in obtaining mates, among
others
Mechanisms of isolation: other methods

 Other examples:
 Difference in pollinators
 one monkeyflower species is visited by a
hummingbird, the other by a bee; but never both
pollinators, so the pollen are not deposited on the
flower of the other species
 Temporal isolation
 the Japanese winter moth (Inurois punctigera) has
two populations that reproduce during different
times
 Anatomical reproductive incompatibility
 The snail Euhadra can be "left-handed" or "right-
handed"
 If a mutation caused "left" offspring to become
"right," they could not mate with "left" because
their genitals openings don't line up
Mechanisms of divergence

 What causes phenotypic divergence between closely related

species?

 Isolation is a critical step, but it must be followed or paralleled

by divergence for complete speciation to occur.
Mechanisms of divergence: adaptation to
different habitats

 Biotic and abiotic ecological factors are important agents for

natural selection, and populations adapt over time
 When populations of a single species occupy multiple habitats
with differing selection pressures, a frequent outcome is
phenotypic and genetic divergence, which can lead to
speciation if gene flow is limited.

 Example: Mimulus guttatus, the yellow monkeyflower, has two

ecotypes: coastal and perennial
 Experiments showed that the plants native to a certain habitat has high
fitness
M. guttatus
Left: coastal ecotype, perennial
Right: inland ecotype, annual, Researchers crossbred the ecotypes until they created
produces flowers quickly
genetic mosaics and grew them in two sites

In each habitat type the plants with the highest survival

and reproductive success were the plants native to that
habitat.
Mechanisms of divergence: assortative mating

 Traits that influence the way species choose mates can lead
to assortative mating, in which individuals with certain traits
more often choose mates that share those same traits.

 Example: Male calls and female preferences covary among

populations of a Hawaiian cricket (Laupala cerasina), to the
point that females hardly respond to the calls of the most
different population
The pulse rate of the mating call of male crickets (L. cerasina) and the pulse rate
preferred by females both vary among local populations. These differences are
genetically based. The confidence intervals around each point show that the
preference ranges of females of the most widely different populations would not
include the most divergent males.
Mechanisms of divergence

 Both environmental factors and mating preferences can act

together
Hybridization and gene flow between species

 When speciation occurs between geographically isolated

populations, interbreeding between the two sister species can
follow due to migration or geographic changes. This is
referred to as secondary contact.
 For sympatric species, interbreeding and gene flow are always
possible, with diverse and consequential outcomes.
 Hybridization occurs when recently diverged populations
interbreed.
 This is common in plants.
Hybridization outcomes: reinforcement

 The evolution of stronger prezygotic isolation because of

selection against low-fitness hybrids is called reinforcement.
 The reinforcement hypothesis predicts that when closely related species
come into contact and hybridize, a mechanism that reduces the
occurrence of hybridization will evolve.
 For example, selection might favor mutations that alter aspects of mate
choice or life history (such as the timing of breeding

 Example: Phlox drummondii and Phlox cuspidata have similar

light blue flowers, but P. drummondii produces dark red
flowers at regions of sympatry
 Because the fertility of hybrids is up to 90% lower than that of non-
hybrids, the difference in color is strongly favored by selection:
pollinators move less pollen between flowers that have different colors,
so P. drummondii produces fewer low-fitness hybrids when it has dark
red flowers rather than light blue flowers in the zone of sympatry.
Reinforcement in Phlox:
(A) Distribution
(B) Flower color difference in P. drummondii
(C) Results of common-garden field experiments, in which all color types were grown with P.
cuspidata. Both parental (light blue and dark red) and hybrid genotypes with light red and dark
blue flowers have equal fruit production but differ in the proportion of their offspring that are
hybrids with P. cuspidata
Hybridization outcomes: hybrid zones

 Hybrid zone – is where hybridization

is ongoing and hybrid offspring are
common.
 Measuring fitness across hybrid types is the
best way to study the dynamics of hybrid
zones

 Example: Mountain wildflower

Ipomopsis aggregata and I. tenuituba
and their hybrids
 At the site within the hybrid zone, no hybrid
form had lower fitness, and many had
higher fitness, than both parental species

Red: I. aggregata; Light pink: I.

tenuituba. Intermediate colors
in the graph indicate hybrids
(backcross, F1/F2 crosses)
Hybridization outcomes: hybrid speciation

 Polyploid hybrid speciation: The increased number of gene copies in

polyploids changes the expression (e.g., amount of gene product) of
many genes, and alters many phenotypic traits

Tragopogon pratensis T. dubius T. porrifolius

n=6 n=6 n=6

T. miscellus T. mirus
n=12 n=12
Hybridization outcomes: hybrid speciation

 When ploidy stays the same

following hybridization, there is
no chromosomal isolation
between hybrids and parental
species. This is known as
homoploid hybrid speciation.

 E.g. Audobon's warbler,

Dendroica audoboni, is a
homoploid hybrid, with nuclear
DNA most closely related to D.
nigrifons and mtDNA to D.
coronata.
What drives diversification?

 The formation of new species over time does not occur at a

constant rate, leading researchers to investigate the possible
causes of diversification across evolutionary history.
 In some instances, adaptive radiations result from a lineage
invading a new habitat such as an island, after which the rate
of diversification is determined by ecological factors.
 However, rapid bursts of speciation can also be caused by the
evolution of key phenotypes that permit a lineage to colonize
new ecological niches or diversify in mating preferences.
Summary

 Speciation is hypothesized as:

Isolation → divergence → reproductive isolation
 Allopatric speciation is due to physical isolation (can also
occur because of drift/founder effect); no gene flow between
the sister species
 Sympatric speciation – same or overlapping regions, requiring
strong selection, and disrupts or limits gene flow
 Differences in chromosome number, prezygotic and post-zygotic
isolation
Summary of isolation barriers

 Premating barriers: features that impede transfer of gametes

to members of other species
 Ecological isolation: potential mates do not meet
 Temporal isolation: species breed at different seasons or times of day
 Habitat isolation: species mate and breed in different habitats
 Immigrants between divergent populations do not survive long enough to
interbreed
 Potential mates meet but do not mate
 Sexual isolation in animals: individuals prefer mating with members of their
own species
 Pollinator isolation in plants: pollinators do not transfer pollen between
species
Summary of isolation barriers

 Postmating prezygotic barriers: mating occurs, but zygotes are

not formed
 Mechanical isolation: reproductive structures of the sexes do not fit
 Copulatory isolation: female is not stimulated by males of the other
species
 Gametic isolation: failure of fertilization

 Postzygotic barriers: hybrids are formed but have reduced

fitness
 Extrinsic: hybrids have low fitness for environmental reasons
 Ecological inviability: hybrids are poorly adapted to both of the parental
habitats
 Behavioral sterility: hybrids are less successful in obtaining mates
 Intrinsic: low hybrid fitness is independent of environmental context
 Hybrid inviability: reduced survival is due to genetic incompatibility
 Hybrid sterility: reduced production of viable gamates
Summary of hybridization outcomes

 Reinforcement and genetic incompatibility – selection against

hybrids
 Hybrid zone – Selection favors hybrids in transitional habitats
 Hybrid speciation – Selection favors hybrids in novel habitat
not occupied by parental species
 Polyploid
 Homoploid

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