1) The document discusses speciation in insects, including different species concepts, types of speciation, and mechanisms of speciation. 2) It describes four main species concepts: typological, nominalistic, biological, and evolutionary. The biological species concept, which defines species as groups of interbreeding natural populations reproductively isolated from other such groups, is most widely accepted. 3) Speciation occurs through the evolution of reproductive barriers between populations, including prezygotic barriers like habitat isolation and postzygotic barriers such as hybrid sterility. Disruptive selection can divide populations into distinct species.

1. INSECT SPECIATION RULES AND RECENT TRENDS
IN
SPECIATION RESEARCH
Presented by :-
S. Nagesh
TAM/2016-17
Dept. of Entomology
S.V .Ag.College, Tirupati
1

2. Contents
 Introduction – species
 Different species concepts
 Kinds of species
 Speciation
 Types of speciation
 Mechanisms of speciation
 Insects as models of speciation
 Case studies
 Conclusions
2

3. Introduction
 The foremost task of a taxonomist is to know the different
‘kinds’ of animals existing in nature.
 These ‘kinds' are actually the species.
 Various definitions have been put forward by various
workers.
3

4.  Species as “the assemblage descended from one another or
from common parents, and of those who resemble each
other (Cuvier,1829)
Georges cuvier
4

5.  Thompson (1937) “The group of individuals distinguished by
an irreducible set of constant properties and connected by
descent and genetic relationship.”
 Wilmoth (1967) “a well defined autonomous and persistent
organic unit, living in a free state of nature, and generally of
less perfect fecundity outside than inside its limits.”
5

6. Different concepts species
1) Typological species concept
2) Nominalistic species concept
3) Biological species concept
A fourth one, the Evolutionary
species concept has been added
to it by Grant (1971).
Ernst Mayr (1957) reviewed the works of others on species and
all these definitions given so far are broadly grouped into three
main concepts:-
Ernst Mayr
6

7. 1) Typological Species Concept:-
 The observed diversity of the universe reflects the existence
of a limited number “universals” or types.
 Individuals do not stand in any special relation to each other.
 If two individuals appear sufficiently different, they are
different species.
7

8.  The species can be recognized by their essential characters,
and these are expressed in their morphology.
 It is also called as the morphological species concept.
8

9. Criticisms:
 No longer accepted.
 Individuals of same species shows morphological differences
due to sexual dimorphism, age differences, polymorphism.
 In case sibling species, this concept fails because they are
perfectly good genetic species but lacking conspicuous
morphological differences.
9

10. 2) Nominalistic Species Concept (Occam ) :-
 Only individuals exist, while species are man’s own creations.
 Nature produces individuals and nothing more; species have no
actual existence in nature.
 They are mental concepts.
10

11. Criticisms:
 No biologist can agree with the idea that the species are man-
made when it is now an established fact that they are the
products of evolution.
 The basic drawback with the nominalists was their
misinterpretation of the casual relation between similarity
and relationship.
 Members of a species taxon are similar to each other because
of common heritage.
 It is not true that they belong to same taxon because they are
similar, as claimed by these workers.
11

12. 3) Biological Species Concept:-
 When it was realized in the 18th century that none of the
above mentioned concepts was applicable to biological
species.
 “Species are groups of interbreeding natural populations that
are reproductively isolated from other such groups” (Mayr,
1940).
12

13. Species has three separate functions:
1) It forms a reproductive community i.e., the individuals of an
animal species recognize each other as potential mates and
seek each other for the purpose of reproduction.
13

14. 2) It is an ecological unit species interacts as a unit with other
species with which it shares the environment.
14

15. 3) It is also a genetical unit consisting of a large
intercommunicating gene pool.
Thus, this concept is biological in its true sense and is mostly
accepted by present day taxonomists.
15

16. 4) Evolutionary Species Concept (Grant):-
 Simpson (1941) then defines an evolutionary species as a
“lineage evolving separately from others and with its own
unitary evolutionary role and tendencies”.
16

17.  Dr Alfred E. Emerson (1961) attempted to combine the
biological species concept and evolutionary species concept
and defined a true species as that ‘‘which has evolved or
evolving, reproductively isolated and genetically distinct
groups of natural populations.’’
Dr Alfred E. Emerson
17

18.  There is another very pertinent question which is often asked.
How many species shall be described?
 The logical answer is:
-Just as many as are present in nature, no more and no
less.
 About one and a half million species of animals have already
been described.
18

19. Kinds of Species
Other Kinds of Species:-
 There are many kinds of species which pertain to evolutionary
or ecological concepts.
 It is, therefore necessary for a student of Entomology to know
all such names for a clear understanding of true species.
 All such types are discussed below.
19

20.  Sibling species:- Pairs or groups of similar or closely related
species which are reproductively isolated but
morphologically identical.
 Sympatric species:-Species occupying the same geographical
area.
 Allopatric species:-Species normally inhabiting completely
different geographical areas.
20

21.  Rivas (1964) reinterpreted the concept of these two terms by
adding ‘Syntopic’ and ‘Allotopic’ for clear understanding
 Sympatric:- To be used when two or more related species
have the same or overlapping geographical distributions
regardless of whether they occupy the same macro habitat
(whether in the same locality).
 Syntopic:- To be used in reference to two or more related
species which occupy the same macrohabitat.
 These species occur together in the same locality, are
observably in close proximity, and could possibly interbreed.
21

22.  Allopatric:- To be used in reference to two or more related
species which have separate geographic distribution.
 Allotopic:- To be used in reference to, two or more related
species which occupy same geographic distribution but do
not occupy the same macro habitat.
 These species are not in close proximity,
 cannot interbreed and do not occur together in the same
locality although they may have the same geographic
distribution.
22

23. Speciation
 Speciation is the evolutionary process by which biological
populations evolve to become distinct species.
 The biologist Orator Fuller Cook coined the term 'speciation'
in 1906
23
O.F. Cook

24.  What is a species?
 “Biological species concept”: a species is a group of
organisms that interbreed under natural conditions and that
are reproductively isolated from each other.
– Reproductively isolated: don’t produce fertile hybrids.
–Natural conditions: artificial breeding doesn’t count.
For example, artificial insemination.
24

25. Reproductive Isolation
How do populations become reproductively isolated?
 Selection and genetic drift causes gene mutations and
altered allele frequencies causes isolation within species.
 To maintain as a single species, there must be gene flow
between populations.
 Matings between members of separated populations that
allow mixing of alleles.
 In the absence of gene flow, mutations in different
populations will be independent
 Allele frequencies will change independently.
25

26. Once gene flow stops: genetic divergence occurs.
The two populations gradually become genetically different.
26

27. What properties of sexually reproducing organisms
lead to the evolution of discrete species?
Two explanations exist:-
1) Ecological explanation.
2) Sexual isolation explanation.
27

28. 1) Ecological explanation:-
 The ecological explanation states that ecological niches are
discrete and that the clusters of different species exploit
different physical resources.
 disruptive selection makes hybrids that “fall between
niches" less fit.
28

29. General modes of selection
Three general modes of selection:-
A. The original population.
B. Stabilizing Selection:
Intermediate traits are
favoured by selection, resulting
in a decrease in variation.
A. Directional Selection: One
extreme trait is favoured,
resulting in a change in the
mean value of the trait.
B. Disruptive Selection: Extreme
traits are favoured over the
intermediate trait values, can
divide the population into two
distinct groups.
Disruptive selection plays an
important role in speciation
29

30. 2) Sexual isolation explanation:-
 The sexual isolation explanation states that individuals of the
same species will adapt different to the environment.
 Over time the number of differences will increase and result
in the formation of new species.
 Speciation in sexually reproductive organisms is based on the
evolution of reproductive barriers for the gene flow between
populations.
30

31. Barriers can occur before fertilization and after
fertilization.
A. Prezygotic barriers:- Occur before fertilization.
B. Postzygotic barriers:- Occur after fertilization.
31

32. A. Prezygotic barriers
 Habitat isolation:- Populations live in different habitats and
do not meet.
 Behavioral isolation:- Little or no sexual attraction between
males and females.
 Temporal isolation:- Mating occurs at different seasons or
times of the day.
 Mechanical isolation:- Structural differences in genitalia
prevent copulation.
 Gametic isolation:- Male and female gametes fail to attract
each other or inviable.
32

33. B. Postzygotic barriers
1) Extrinsic barriers:-
Ecological inviability:- Hybrids develop normally but suffer
decreased viability, as they can not find a suitable ecological
niche.
Behavioral sterility:-
 Hybrids have normal gametogenesis but suffer lowered
effective fertility because they cannot find mates.
 Hybrids might have an intermediate courtship behaviour
unattractive to individuals of the opposite sex.
33

34. 2) Intrinsic barriers:-
Hybrid inviability: Hybrids have developmental defects causing
full or partial inviability.
Hybrid sterility:
Physiological sterility: Hybrids suffer developmental defects in
their reproductive system causing full or partial sterility.
Behavioral sterility: Hybrids suffer a neurological defect that
renders them fully or partially incapable of courtship.
34

35. 35

36. Cladogenesis:-
 Cladogenesis is an evolutionary splitting event where a
parent species splits into two distinct species, forming a clade.
Anagenesis:-
 Anagenesis, also known as phyletic transformation.
 It is the process in which a species, gradually accumulating
change, eventually becomes sufficiently distinct from its
ancestral form with out branching or splitting .
Modes of speciation
36

37. Types of speciation
i. Allopatric speciation.
ii. Peripatric speciation.
iii. Parapatric speciation.
iv. Sympatric speciation.
37

38. i. Allopatric speciation
 Allopatric speciation (from the ancient Greek allos- meaning "other“
and patris- meaning "fatherland").
 It is also referred as geographic speciation.
 In allopatric speciation populations are separated by geographical
isolation.
 In allopatric speciation extrinsic factors – as great distance or a
physical barrier prevents two or more groups from mating.
 Various geographic changes can arise such as formation of
mountains, islands, bodies of water, or glaciers.
38

39.  Physical isolation is an effective barrier to gene flow and in
many cases it is an important trigger for divergence.
39

40. ii. Peripatric speciation
 Peripatric speciation is a mode of speciation in which a new
species is formed from an isolated peripheral population.
 Peripatric speciation resembles allopatric speciation.
 Nevertheless, the primary characteristic of peripatric
speciation proposes that one of the populations is much
smaller than the other.
40

41. Peripatry can be distinguished from allopatry speciation by three
key features:-
1. Strong selection caused by the dispersal.
2. Colonization of novel environments.
3. The effect of genetic drift on small populations.
41

42. iii. Parapatric speciation
 In parapatric speciation, two sub populations of a species
evolve reproductive isolation from one another while
continuing to exchange genes.
 This mode of speciation has three distinguishing
characteristics:-
1) Mating occurs non-randomly,
2) Gene flow occurs unequally, and
3) Populations exist in either continuous or discontinuous
geographic ranges.
42

43.  This distribution pattern may be the result of
unequal dispersal,
 incomplete geographical barriers,
 divergent expressions of behavior.
 Parapatric speciation predicts that hybrid zones will often
exist at the junction between the two populations.
43

44. iv. Sympatric speciation
 Etymologically, sympatry is derived from the Greek
roots sym ("together") and patriς ("homeland").
 The term was invented by Poulton in 1904.
 Sympatric speciation is the process through which new
species evolve from a single ancestral species while
inhabiting the same geographic region.
44

45.  Sympatric speciation events are quite common in plants due
to polyploidy.
 If the two organisms are closely related (e.g. sister species)
and present in the same geographic region such a distribution
may be the result of sympatric speciation.
45

46. Difference between Allopatry and
Sympatry
 Two central factors differ between sympatric and allopatric
speciation.
1. Sympatric speciation does not require large-scale geographic
distance to reduce gene flow between parts of a population.
 New species arise within the range of the parent population
2. Secondly, in sympatric speciation gene flow may continue for
a number of generations after the populations have become
separated,
 whereas complete isolation arises between populations
evolving in allopatry.
46

47. 47

48. Mechanisms of speciation
i. Assortative mating
ii. Reinforcement
iii. Selection
iv. Sexual selection
v. Polyploidy
48

49. i. Assortative mating:-
 It is a mating pattern and a form of sexual selection in which
individuals with similar phenotypes mate with one another
more frequently than would be expected under a random
mating pattern.
 Some examples of similar phenotypes body size, skin
coloration/pigmentation, and age.
49

50. ii.Reinforcement:-
 Reinforcement is a process of speciation where pre-zygotic
isolation is enhanced by natural selection
against hybrids between two populations of a species.
 The idea was developed by Alfred Russel Wallace and it is
also referred as the “Wallace effect”.
50

51. iii. Natural selection:-
 Charles Darwin popularised the term "natural selection".
 Natural selection is the differential survival and reproduction
of individuals due to differences in phenotype.
 It is a key mechanism of evolution, the change
in heritable traits of a population over time.
 This occurs because random mutations arise in the genome of
an individual organism, and offspring can inherit such
mutations.
Charles Darwin 51

52. iv. Sexual selection:-
 Sexual selection is a mode of natural selection where
members of one biological sex choose mates of the
other sex to mate with (intersexual selection), and compete
with members of the same sex for access to members of the
opposite sex (intrasexual selection).
 These two forms of selection mean that some individuals
have better reproductive success than others within
a population either from being more attractive or preferring
more attractive partners to produce offspring.
52

53. v. Polyploidy:-
 Polyploid cells and organisms are those containing more than
two paired (homologous) sets of chromosomes.
 Most species whose cells have nuclei in diploid condition,
meaning they have two sets of chromosomes, one set
inherited from each parent.
 Polyploidy is found in some organisms and is especially
common in plants.
53

54. Insects as models of evolution
 Tremendous diversity of insect taxa offer representative
“models” for the study of distinct mechanisms of speciation.
In addition, insect models often come with practical
advantages:-
 Insects can frequently be reared in the laboratory.
 Having short life cycle.
 present the possibility of hybridizing races, subspecies, or
species for genetic studies of natural variation; and enable the
study of large samples for hypothesis testing.
 Insects arecommonly used in testing of specific hypotheses
from a diversity of disciplines, including ecology,
phylogenetics, evolutionary process, behavioral ecology,
genetics, and genomics.
54

55. Case studies
55

56. 1) Premating isolation is determined by larval rearing
substrates in cactophilic Drosophila mojavensis. Host plant and
population specific epicuticular hydrocarbon expression
influences mate choice and sexual selection.
 Drosophila mojavensis and its sister species Drosophila
arizonae were taken for the study of speciation.
 These two species are distributed across the arid south-
western extent of North America and Mexico, infesting the
fermenting tissues of cacti.
 Drosophila mojavensis was feeding on Agria cactus. Some of
the individuals started exploiting the other cactus i.e., organ
pipe cactus.
Havens and Etges (2013)
56

57.  Change of host caused change in epicuticular hydrocarbons
which is a key element that is responsible fore mate
recognition.
 Change in epicuticular hydrocarbons resulted sexual isolation.
 This was estimated by laboratory raring and mating
experiments with this two species.
 Unmated and mated insects were collected and CHCs were
extracted and quantified through Shimadzu capillary gas-
liquid chromatography.
 Results showed that insects feeding on organ pipe cactus have
more CHCs than insects feeding on agria cactus and evolved
as Drosophila arizonae .
57

58. Agria catus Organ pipe cactus
Drosophila mojavensis Drosophila arizonae
58

59. 2) Insect Speciation Rules: Unifying Concepts in
Speciation Research - Speciation in Laupala crickets
 The genus Laupala (Gryllidae: Orthoptera) is a group of
cricket species native to the rain-forested slopes of the
Hawaiian islands offering rare insights into the process of
speciation.
 In Laupala, the most conspicuous axis of differentiation is
male song.
 In crickets, males sing a long-distance calling song to which
females respond and reaches male through phonotaxis.
Mullen and Shaw (2014)
59

60.  Songs are simple, consisting of long trains of pulses produced
by wing stridulation but they can vary between the species.
 These mate recognition phenotypes are therefore often likely
to be speciation phenotypes, i.e. traits whose divergence
contributes, directly or indirectly, to a reduction of gene flow
during speciation.
 This differential male song was responsible reproductive
isolation between the individuals and lead to evolution of
sympatric species i.e., Laupala kohalensis and Laupala
paranigra.
60

61.  Experimental results shown that Laupala kohalensis [∼3.7
pulses per second (pps)] and Laupala paranigra (∼0.7 pps)
are closely related species that differ in pulse rate by
approximately 25 standard deviations.
61

62. 3) The molecular forms of Anopheles
gambiae: A phenotypic perspective
 The African malaria mosquito Anopheles gambiae is
undergoing speciation, being split into the M and S molecular
forms.
 Studies suggesting that selection mediated by larval
predation promoted divergence between temporary and
permanent freshwater habitats.
Anopheles gambiae
Lehmann and Diabate (2008)
62

63.  S (Saharan)– form lies in small ephimeral, predator free rain
pools and M (Mopti)- form exploit more persistent water
bodies.
 General predators of this A. gambiae is Notonecta sp.
backswimmers and Xenopus tadpoles.
 These molecular forms show differences in oviposition based
on presence or absence of predator chemical cues.
Notonecta Xenopus
63

64.  Post zygotic barriers responsible for reproductive isolation of
these two forms are oviposition and predator vigilance.
 S- form adult can identify the predator chemical cues and
avoid oviposition in those areas.
 M- form spend less time on foraging and more time on
predator vigilance and it can easily escape from predators but
not the S- form.
 These two factors were responsible for population divergence
of Anopheles gambiae into M and S forms.
64

65. Species boundaries of M and S molecular forms of
Anopheles gambiae in Africa.
65

66. 4) Insect Speciation Rules: Unifying Concepts in Speciation
Research – speciation in Rhagoletis pomonella: THE APPLE
MAGGOT FLY
 Tephritid fruit flies in the genus Rhagoletis have been heavily
investigated as a potential case of sympatric speciation via
host-race formation.
 In the mid-1800s, a larval host shift occurred from their
native host, hawthorn (Crataegus monogyna), to
domesticated apples (Malus pumila ) in USA.
Crataegus monogyna Malus pumila
Havens and Etges (2013) 66

67.  Host-plant identification by adult female fly involves visual,
olfactory, tactile cues at the time of oviposition.
 This host shift is the key barrier to gene flow and contributed
to the formation of two “ host races” of Rhagoletis pomonella,
which are isolated as a result of a combination of :-
 host-specific mating.
 oviposition preferences.
Rhagoletis pomonella
67

68. 5) Specialized Feeding Behaviour Influences Both
Ecological Specialization and Assortative Mating in
Sympatric Host Races of Pea Aphids.
 Sympatric populations of pea aphids (Acyrthosiphon pisum
Harris, Homoptera: Aphididae) on alfalfa (A1) (Medicago
sativa) and red clover (C1) (Trifolium pratense) are highly
genetically divergent and locally adapted.
 Pea aphids rapidly assess alfalfa and clover and reject the
alternate host based on chemical cues that are perceived
before the initiation of feeding.
Caillaud and Via (2000)
Pea aphid
alfalfa clover
68

69. 1. Prealighting Behavior :-
 In the first experiment, 20–25 winged aphids of a given
genotype were released from an aerial platform into a cage
containing two pots of each host.
 Within each cage, the number of aphids on each plant was
scored after 1.5, 3, 4.5, 6, 24, and 72 h.
 Over time, an increasing host number of A1 aphids settled
on alfalfa, while C1 aphids accumulated on clover.
 During the entire first day of the experiment, it has been
observed that aphids were flying, landing on plants, and
taking off again.
 By 72 h, most individuals had accumulated on their respective
hosts.
69

70. A. Alfalfa specialist (genotype A1).
B. Clover specialist (genotype C1).
70

71. 2. Time of penetrating host tissue:-
The above results shows that there is reproductive
isolation between the two races of pea aphid i.e., alfalfa A1
and clover C1 due to host specialization.
71

72. Conclusions
 Evolution – is a continuous process.
 New species will be forming in nature continuously, those
newly formed species should be identified and documented
timely, to have accurate knowledge on species composition.
 Out of all mentioned species concepts biological species
concept widely accepted and followed.
 Among all general modes of selection disruptive selection is
responsible for speciation.
 Insects follow cladogenesis mode of speciation.
72

73.  Out of four types of speciation, sympatric speciation is
dominant in insects, this is due to host shift.
 Host shift was observed in following insects that lead to
speciation.
1) Pea aphids (Acyrthosiphon pisum)
2) Drosophila mojavensis
3) The apple maggot fly (Rhagoletis pomonella)
73

74. References
 Caillaud, M.C and Via, S. 2000. Specialized Feeding Behaviour
Influences Both Ecological Specialization and Assortative
Mating in Sympatric Host Races of Pea Aphids. The American
Naturalist. 156 (6): 606-621.
 Havens, J.A and Etges, W.J. 2013. Premating isolation is
determined by larval rearing substrates in cactophilic
Drosophila mojavensis. Host plant and population specific
epicuticular hydrocarbon expression influences mate choice
and sexual selection. Journal of Evolutionary Biology. 26 (3):
562-576.
 https://en.wikipedia.org/wiki/Speciation
 Kapoor, V.C. Theory and Practice of Animal Taxonomy. Oxford
& IBH publishing co. New Delhi. 56.
74

75.  Lehmann, T and Diabate, A. 2008. The molecular forms of
Anopheles gambiae: A phenotypic perspective. Infection,
Genetics and Evolution. 8: 737-746
 Mullen, S.P and Shaw K.L. 2014. Insect Speciation Rules:
Unifying Concepts in Speciation Research. Annual Review of
Entomology. 59:339–61.
 Shaw, K.L., Parsons, M.Y., Lesnick, C.S. 2007. QTL analysis of a
rapidly evolving speciation phenotype in the Hawaiian cricket
Laupala. Molecular Ecology. 16: 2879-2892.
75

76. 76