Speciation is the evolution of one species into two or more distinct species. There are several modes of speciation, including allopatric speciation which occurs when populations become isolated by geographic barriers. Over time, reproductive isolating mechanisms evolve between isolated populations, preventing interbreeding and leading to the formation of new species. Isolating barriers like time, distance, and geography play important roles in driving speciation by limiting gene flow between diverging populations.

1. Chapter six
Speciation
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2. Outlines
Introduction
Speciation
Patterns of speciation
Species concepts
Modes of speciation
Rates of species
Isolating mechanism of speciation
Types of evolution
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3. Introduction
 Species - a group of organisms with the ability to interbreed
and produce fertile offspring in the natural environment.
 A species is a single lineage of organisms that maintains its
identity from other such lineages and has its own
evolutionary tendencies and historical fate (Wiley 1978)
 A species is a group of organisms that maintains a
distinctive set of attributes in nature.
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4. Speciation
 Speciation is the evolution of one or more species from a single
ancestor species.
 It can occur when members of a population become isolated from each
other.
 Once two populations are reproductively isolated, they are considered
separate species.
 Speciation is the process whereby new species are formed.
 Speciation leads to macroevolution
 Macroevolution refers to the evolutionary changes that create new
species and groups of species.
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5. Speciation
 Creates multiple “daughter” species from
one ancestor
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6. Cont…
 The process by which one genetically-cohesive
population splits into two or more reproductively-
isolated populations.
 Requires the disruption of gene flow and the evolution
of reproductive isolating mechanisms (RIMs).
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7. Speciation involves barriers to reproduction
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8. Patterns of speciation
 There are two patterns of speciation as evidenced by the fossil record
• Anagenesis
 It is the accumulation of changes associated with the transformation of one
species into another.
 Evolution taking place in a single group (a lineage) with the passage of
time.
 Does not promote biological diversity
• Cladogenesis
 The splitting of one lineage into two new species arise through
cladogenesis.
 It is also called branching evolution, is the budding of one or more new
species from a parent species.
 promotes biological diversity by increasing the number of species.
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10. Cont…
Anagenesis
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Cladogenesis

11. Species concept
 Biological species concept:
 The biological species concept defines a species as members of
populations that actually or potentially interbreed in nature not
according to similarity of appearance.
 Species are groups of actually or potentially interbreeding
natural populations, which are reproductively isolated from other
such groups (Mayer, 1940).
 A species is a reproductive community of populations (
reproductively isolated from others) that occupies a specific
niche in nature (Mayer, 1982)
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12. Cont…
 Species are the members in aggregate of a group of
populations that breed or potentially interbreed with each
other under natural conditions (Futuyma).
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13. Cont…
 Ecological Species Concept:
 A species is a lineage that occupies a niche minimally
different from that of any other lineage in its range and
which evolves separately from all lineages outside its
range.
 Evolutionary species concept:
 A species is a single lineage of ancestor-descendant
populations which maintain its identity from other such
lineages and which has it own evolutionary tendencies
and historical fate.
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14. Cont…
 Phenetic species concept:
 A species is a set of organisms that look similar to each
other and distinct from other sets.
 Phenotypic similarity is all that matters in recognizing
separate species.
 Phylogenetic species concept:
 A species is the smallest diagnosable cluster of individual
organisms within which there is a parental pattern of
ancestry and descent.
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15. What causes speciation?
 Speciation, or the evolution of reproductive isolation,
occurs as a by-product of genetic changes that accumulate
between two previously interbreeding populations of the
same species.
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16. How does speciation occur?
 There are several different ways in which the evolution of
reproductive isolation is thought to occur.
 These can be generalized into a series of events, or steps.
 The “Steps” in a speciation event:
 Step 1: gene flow between two populations is interrupted
(populations become genetically isolated from each other)
 Step 2: genetic differences gradually accumulate between the
two populations (populations diverge genetically)
 Step 3: reproductive isolation evolves as a consequence of
this divergence (a reproductive isolating mechanism evolves
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17. The Rate of Speciation
 A comparison of gradualism and punctuated equilibrium.
 Gradual speciation is the gradual divergence of populations
until they have reached the levels of specific distinctness.
 During punctuated equilibrium, long periods of equilibrium
in which species exist essentially unchanged are;
 punctuated by relatively short periods of evolutionary
change during which phenotypic characteristics may change
rapidly.
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18. Cont…
Comparison between gradualism and punctuated equilibrium
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20. Modes of Speciation
 Speciation means the formation of species.
 The alternative methods of speciation have been proposed
as follow:
Allopatric speciation,
Sympatric speciation,
Parapatric speciation, and
 Peripatric speciation (altational speciation).
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21. Modes of Speciation
 Allopatric Speciation:
 Speciation with geographic isolation.
 Evolution of reproductive isolating mechanisms between populations
that are geographically separated.
 Geographic separation is defined by spatial restriction of gene flow,
not just physical distance.
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22. MODEL OF ALLOPATRIC SPECIATION
ANCESTRAL
POPULATION
GEOGRAPHIC
BARRIER
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23. .
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24. Cont…
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25. Peripatric Speciation:
 Speciation in a peripheral isolate.
 Due to rare dispersal or colonization events.
 Also referred to as Founder Effect speciation.
 In small populations DRIFT and NATURAL SELECTION
may cause rapid divergence from the parental population
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ANCESTRAL
POPULATION
PERIPHERAL ISOLATE

26. Cont…
.
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27. Sympatric speciation
 Sympatric speciation involves the formation of different
species that are not geographically isolated from one
another.
 Pattern in which speciation occurs in the absence of a
physical barrier
 Adaptation to local environments, and sexual selection
are mechanisms that can promote sympatric speciation.
 A species achieves reproductive isolation and evolves in
the same geographic location as its ancestral species
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28. Cont…
 polyploidy (extra sets of chromosomes) is a major
factor in sympatric speciation in plants
 autopolypoidy – multiple sets from one parent
species; example – plants around Agent Orange sites
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29.  Primula kewensis arose via
allopolyploidy in an interbreeding even
of P. floribunda and P. verticiliata in
1898 at the Royal Botanic Gardens in
Kew, England
 Each of the parental stocks have a 2n =
18.
 P. kewensis is 2n = 36 because of
chromosomal duplication events after
the hybrid cross – was observed 3 times
 P. kewensis now a popular garden plant.
Example, Primrose Speciation
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31. Parapatric speciation
 It occurs when populations are separated not by a
geographical barrier, such as a body of water, but by an
extreme change in habitat.
 While populations in these areas may interbreed, they
often develop distinct characteristics and lifestyles.
 It is extremely rare.
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33. Summery
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34. Isolation Mechanisms
 Isolation is an important factor contributing to the
process of evolution.
 It is the segregation of the population of a particular
species into smaller units which prevent interbreeding
between them.
 Isolation aids in splitting of the species into separate
groups.
 In consequence of being separated from one another,
organisms have developed different characteristics, due to
which they have become separate species.
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35. Types of Isolation
 Evolutionary biologists have recognized the following
types of isolations which cause speciation.
1. Isolation by Time:
 Also known as Seasonal or temporal isolation
 The populations exist in the same regions but are sexually
mature at different times, so that potential mates remain
unable to mate.
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36. 2. Isolation by Distance (Spatial Isolation)
 Sheer distance apart may also act as an isolating factor for a
species which occupies a great range of area, which is
unbroken by effective barriers.
 Its example is wrens (birds) of South America.
 Wrens are found all over the continent but the wrens of one
region differ from those of the other in colour patterns, size,
proportions and habits.
 It shows that without any barrier, sheer distance apart tends
to produce local races (subspecies).
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37. Cont…
. Variation among wrens due to Spatial Isolation
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38. 3. Geographical Isolation
 It is the most common type of isolation and occurs when an
original population is divided into two or more groups by
geographical barriers such as a river, desert, glacier, mountain or
ocean.
 All of which prevent interbreeding between them, then in the
course of time different mutations may become incorporated in
the gene pools of the different groups.
 Certain additional kinds of geographical barriers are the volcanic
formation of a mountain on land, mountain ranges with deep
valleys between and land masses as islands in sea.
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39. Cont…
.
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40. Cont…
.
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41. Cont….

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42. 4. Reproductive Isolation
 In sexually reproducing organisms species can be defined as
Mendelian populations between which the gene exchange is prevented
by reproductive isolation. .
 In simple terms isolating mechanisms are those which prevent
successful reproduction between members of two or more populations
(viz., closely related species) that have descended from the same
original population.
 Evolution of reproductive isolating mechanisms prevents nascent
species from interbreeding.
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43. Types of reproductive isolating mechanisms
 Most modern evolutionists such as Mecham (1961), Mayr
(1948, 1970), Stebbins (1966,1971), etc., have classified
the reproductive isolating mechanisms into two classes.
 These are:
premating or prezygotic isolating mechanisms and
Postmating or postzygotic isolating mechanisms.
 Prezygotic Mechanisms prevent mating formation of
viable or zygotes.
 Postzygotic Mechanisms prevent hybrids from passing on
their genes.
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44. Cont…
 premating or prezygotic isolating mechanisms prevent
wastage of gametes (germ cells) and so are highly
susceptible to improvement by natural selection;
 postmating or postzygotic isolating mechanisms do not
prevent wastage of gametes and their improvement by
natural selection is indirect
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45. A. Premating or prezygotic isolating mechanisms
 Mechanisms that prevent interspecific crosses (i.e.,
fertilization and zygote formation).
 Causes of prezygotic isolating mechanism:
 1. Habitat isolation :
 Two closely related species may occupy different ecosystems
within a geographical region.
 The populations live in the same regions but occupy different
habitats, so that potential mates do not meet.
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46. Cont…
 Example 1: Central California populations of Rana spp.
◦ The Red-legged Frog (Rana draytonii) tends to breed in large
ponds.
◦ The Yellow-legged Frog (Rana boylii) breeds almost exclusively in
fast-moving streams.
 Their habitat preferences contribute to their reproductive isolation.
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47. Cont…
 Example 2: European populations of Turdus spp.
The Common Blackbird (Turdus merula) lives and breeds in forest.
The Ring Ouzel (Turdus torquatus), a close cousin, lives and breeds on
moors.
Even when forest and moor abut, the two species do not interbreed.
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48. 2. Seasonal or temporal isolation
 The populations exist in the same regions but are sexually mature at different times,
so that potential mates remain unable to mate.
 Example 1: Closely related Rana species in California Coastal
Ecosystems
◦ The Red-legged Frog (Rana draytonii) breeding season lasts from ~
November - late April.
◦ The Yellow-legged Frog (Rana boylii) breeding season lasts from ~
late April - June.
◦ The breeding seasons may overlap in some areas.
◦ The combination of ecological and temporal isolation prevents
hybridization.
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49. Cont…
 Example 2: Closely related Fruit Flies in Hawaii
 Drosophila persimilis breeds in early morning.
 Closely related Drosophila pseudoobscura breeds in
the afternoon.
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50. 3. Ethological (Behavioral) isolation :
 Species with complex courtship behaviors usually exhibit
stereotyped "call and response" signals between male and
female before actual mating takes place.
 These rituals prevent wasted mating effort that would halt
gene transmission by inviable or infertile hybrids.
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51. Cont…
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52. 4. Mechanical Isolation
 Cross fertilization or pollination is prevented or restricted
by differences in structure of reproductive organs
(genitalia in animals, flowers in plants).
 So that, copulation is attempted but no transfer of sperm
takes place.
 Morphological differences between species prevent
hybridization.
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53. Cont…
 Example 1: Snail Shell Coiling
In some snail species, the direction of shell coiling
is controlled by a single (maternal effect) gene.
Left-coiling snails cannot mate with right-coiling snails.
Such mutations could quickly lead to further differentiation and,
possibly, speciation.
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54. 5. Gametic Isolation
 In this case, sperm and ova of the two species are
chemically incompatible, and will not join to form a
zygote.
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55. B. Postmating or postzygotic isolating
mechanisms
 Fertilization takes place, hybrid zygotes are formed, but
these are inviable, or give rise to weak or sterile hybrids.
1. Hybrid inviability or weakness:
 Sperm and egg from the two species may combine, but the
genetic information is insufficient to carry the organism
through normal development.
 The embryo dies after a few cleavages, or some time
before birth/hatching.
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56. Cont…
 Example: Drosophila spp.
◦ Despite their superficially similar appearance, D.
melanogaster and D. simulans have incompatible alleles
for nuclear pore proteins.
◦ Dysfunction of this vital gene results in inviable hybrids.
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57. 2. Developmental hybrid sterility:
 Hybrids are sterile because gonads develop abnormally, or
meiosis breaks down before it is completed.
 Example 1: Tigers (Panthera tigris) and Lions (Panthera leo)
◦ Tigers and Lions are sister taxa, but separate for millions of
years.
◦ Their hybrid offspring are viable and robust, but sterile.
◦ Chromosomes are not homologous, so do not migrate
normally at meiosis.
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58. Cont…
 male tiger x lioness --> tigon
 male lion x tigress --> liger
 reciprocal cross offspring are somewhat different
◦ maternal mitochondria input.
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59. Cont…
 Example 2: Horse (Equus caballus) and
Donkey (Equus asinus)
◦ Horses and donkeys have been separate
species for millions of years.
◦ Their hybrid offspring are viable and
robust, but sterile.
◦ male horse x female donkey --> hinny
◦ male donkey x female horse --> mule
◦ As above, reciprocal cross offspring are
somewhat different
 maternal mitochondrial input
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60. 3. Hybrid Breakdown
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 Two related species can hybridize, and their F1 offspring are fertile.
 But successive generations (F2 and beyond) suffer lower viability or
fecundity.
 Thus, they cannot become an established population.
 Example: Rice cultivars
◦ Cultivars of domestic rice have been artificially selected for centuries.
◦ Some are closely related enough to hybridize.
◦ F1 hybrids are fertile and viable.
 F2 generation is stunted and sterile.

61. Cont…
 Hybridization between closely related species can have one of several
possible outcomes.
◦ Species Reinforcement
 Hybrids have lower fitness than either parent species.
 Reproductive isolation is maintained due to lack of hybrid
survival/reproduction.
◦ Species Fusion
 Two species in a hybrid zone may have weak reproductive isolating
barriers.
 The two species may, over time, eventually share a common gene
pool.
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62. Cont…
◦ Species Stability/Hybrid Equilibrium
 Hybrids are continually produced by the two parent
populations in a hybrid zone.
 A narrow hybrid zone can foster constant hybridization with
reduced hybrid survival..
◦ Hybrid Speciation
 Hybrids may be reproductively superior to parent
populations.
 Positive assortative mating among hybrids can lead to hybrid
speciation.
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63. Patterns of Evolution
 Evolution over time can follow several
different patterns.
 Factors such as environment and predation pressures can
have different effects on the ways in which species
exposed to evolve.
 As a result there are three main types of evolution:
1. Divergent evolution,
2. Convergent evolution, and
3. Parallel evolution.
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64. 1. Divergent evolution
 process where species become increasingly different over time.
 adaptive radiation is a form of divergent evolution where a number
of diverse species form from a single ancestral one.
 eg. Galapagos finches observed by Darwin
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65. Cont…
 Homologous structures also shows that divergent
evolution
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66. 2. Convergent evolution
 The appearance of apparently similar structures in
organisms of different lines of descent.
 The development through evolution of similar features by
organisms with distinctly different ancestors.
 A common example of this is the evolution of wings in
insects and birds.
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67. Cont…
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68. 3. Parallel Evolution
 is the independent evolution of similar traits, starting
from a similar ancestral condition.
 Frequently this is the situation in more closely related
lineages, where several species respond to similar
challenges in a similar way.
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69. Summery
,
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