Unit Speciation: Structure
Unit Speciation: Structure
Unit Speciation: Structure
Structure
Introduction
Objectives
Concept of Species
Mechanisms of Speciation
Sympatric Speciation
Geographic Speciation
Rassenkreis and Speciation
Genetic Repatterning during Isolation
Isolating Mechanisms
Pre-mating Isolating Mechailisms
Geographical Isolation
Ecological Isolation
Ethological Isolation
Mechanical Isolation
Post-mating Isolating Mechanisms
Interspecific Sterility
Hybrid Sterility
Genetic Drift
Summary
Terminal Questions
Answers
1 31 INTRODUCTION
In Unit 11 and 12 of the course on Evolution we explained to you that adaptations
arise by gradual changes in genotypes, monitored by natural selection. We also
pointed out that natural selection promotes different types of adaptations in different
environmental conditions. Since environmental conditions tend to change from time
to time, and from place to place, it is obvious that changes in genome also occur so
that resulting phenotype is best adapted to the changed environment. Essentially the
diversity of organisms and their characters are a result of natural processes. In this
unit we extend this concept further and explain the origin of species. Also we shall
ask the question 'What is species?' Species, as we shall show later are important that
they represent a significant level of integration in living nature. W e shall define the
concept of biological species and then look into the process of speciation. Speciation
is important in evolution because it adds t o the diversity in nature, and in certain
occcasions leads to progressive evolution.
Ernst Mayr, for whom, species and species problem have been the main concern of
research, points out that speciation, the multiplication of species, that is division of
one parent species into several daughter species is one process responsible for the
evolutionary diversity of organic world. Darwin no where in his book Origin of
Species defined the word species precisely although he interpreted speciation in
terrns of reproductive isolation (Darwin again did not use the term reproductive
isolation in his book.) Darwin did en~phasisethat occupation of an unique ecological
niche by each species could be a major characteristic of speciation. In this unit we
shall briefly analyse the species concept laying emphasis on biological species and
discuss in detail different types of mechanisms of speciation. Further we shall briefly
look into how isolation brings about genetic repatterning and proceed to discuss the
mechanisms of isolation with well chosen examples. As an illustration of how isolating
mechanisms develop gradually, we shall explain the concept of ring species - the
evolution of distinct new species. Finally there will also be a discussion on a
phenomenon called genetic drift which however has no causal connection with
speciation process but explains that in peripheral isolates gene frequencies drift due
24 $@.smapling error.
Objectives
This unit should enable you to:
comprehend the species concept and define species,
8 describe the major types of speciation,
e explain the formation of ring species by the gradual development of isolating
mechanism,
e explain the concept of-genetic repatrerning during isolation process,
e classify different types of isolating mechanisms, and
e discuss large scale change in gene frequencies in small populations - the genetic
drift phenomenon.
* Sympatric speciation,,+
Peripatric speciation
26 . * Allopatric speciation..
. . ..
As we shallsee later, peripatric and allopatric speciation can be grouped into a single
type, namely geographic speciation. We shall now discuss the different types of
speciation.
Sympatric speciation can be regarded as speciation where parent species gives rise to
a daughter species without the individuals of a species being separated by space or
territory. Both instantaneous and gradual models of sympatric speciation have been
proposed. Barring one mode of instantaneous speciation by a mechanism known as
polyploidy, other modes of sympatric speciation have remained quite controversial.
Polyploidy is quite common among plants. (For a detailed discussion on polyploidy
refer to Unit 10 of Block 2 of LSE-03 of Genetics course). A cross between two
diploid plants could result in a tetraploid hybrid. The hybrid would remain largely
reproductively isolated from its diploid parents. The reason for such isolation is that
due to back-crossing if a triploid individual were to be formed, it will produce a high
proportion of nonviable gametes. There is also a possibility that interbreeding
between deploid and tetraploid forms or between different tetraploids may give rise
to other polyploids.
Polyploidy is rare or virtually non-existent in sexually reproducing organisms which
include almost all animals. In sexually reproducing organisms, if a single mutational
event or chromosomal change such as polyploidy results in reproductive isolation, the
organism will not successfully reproduce unless there is close inbreeding. Among
animals, close inbreeding is quite uncommon. But in a group of hymenopterous
parasites chalcidoidea, there is mating between brothers and sisters that emerge from
a single host. Such matings have facilitated a high species diversity in this group. But
instances of instantaneous speciation in animals is a very rare phenomenon.
Speciation in general is a gradual process and whether or not such speciation can
occur within the confines of a single interbreeding population is a debatable point.
Nevertheless, the possibility of such a speciation is not totally ruled out, as we would
see from the following discussion.
It has been pointed out that sympatric speciation may occur whenever disrbptive or
diversifying selection is active (For a detailed discussion on disruptive selection refer
to Unit 11 of Block 3 of Evolution). For example, take a population of individuals
I
with genotypes A A and A'A', each of which is adapted to live specifically on plant
species 1 and 2 respectively. The heterozygote AA' is not well adapted to either
I
species of plants. Essentially it means that each homozygote would have a higher
I fitness if it mated assortatively or non-randomly. By assortative or non-random
mating it is meant that males and females of similar phenotypes (hence genotypes)
tend to mate with each other. Such an assortative mating would very much minimise
I the production of unfit heterozygous progeny. Also the selection process would tend
to establish two different populations, each composed of a distinct genotype. You
'
may recall from your studies on Unit 11 that this is precisely the role of disruptive or
I
diversifying selection. Another locus B may be conferrring the assortative mating trait
I on the two genotypes. This locus may influence the mating behaviour or impel the
I organism to choose a specific host species, to find a mate and lay eggs. Genotypes
I BB and Bb may mate and lay egg on host 1 and bb mates and lays e4gs on host 2.
You may observe here that difference in the selection of specific host species isolates
the two genotypes from one another and a reproductive isolation sets in. In many
groups of phytophygous insects such as treehoppers, it is observed that closely related
species are confined to different host plants for feeding and breeding.
I Boundary
Peripheral population
I
Peripheral population
Peripheral populations (13:2b) (individuals living in the periphery of a population)
may experience less gene flow than central populations. But still they will form an
integral part of the species. Only when the population is a peripheral isolate (13.2~)
and not just a peripheral population, that it has the possibility of becoming a new
species (13.24.
A peripheral isolate can be formed in different environments. For instance it may be
formed on an island off the coast of a large subcontinent. The finches that Darwin
observed during his voyage on the ship H.M.S. Beagle were evolved from peripheral
isolates on the Galapagos island, off the west coast of South America.
Before we start discussing in detail the various mechanisms of isolation that in turn
were instrumental for the formation of a new species, we shall look into two concepts:
(1) The concept of ring species -an example which shows beautifully how isolation
af population develops gradually resulting in distinct new species, and (2) the concept
of genetic repatterning during isolation. And prior to that, attempt the following
SAQs.
SAQ 2
Fill in the blanks with suitable words from the text.
i) To qualify for the title of specizs the individual must be .........................
......................... from other groups.
ii) .................................................. refers to the speciation process where
the individuals of a species are not separated by space or territory.
iii) ........................ is a common mechanism of speciation in plants.
iv) Sympatric speciation may occur wherever .........................
............................ isaactive.
v) It is essential that a population which is a prospective new species is
............ .:. ........,. ......................... from the parental species.
vi) .........................speciation refers to speciation by populations of parent
species which occupy quite separate territories. .
vii) Speciation by small populations isolated on the periphery of the distribution of
.. .
parent population is ..............................................
In the west coast of USA there are two high mountain ranges, the Coastal Range
and the Sierra Nevada. A t their northern ends, near the Canadian border they are
united (Fig. 13.3). Proceeding south, they are separatcd by a hot and arid desert,
Further south, close t o the Mexican border, these high mountains meet again. There
is an amphibian species Ensatina eschscholtzii at the place marked as A in Fig. 13.3,
It is supposed that this original species split into two populations, B and G, which
then moved down the two respective mountain ranges. On one range the populations
can be seen to change gradually from B to C to D to E and on to F. On the other
mountain range the population G, like B derived from A, gradualljl moved south
changing into H, I, J and K. While these changes were taking place the two series
were isolated from each other by the desert, and isolating mechanisms were gradually
developing. Since this amphibian Ensatina has poor locomotor powers, it must have
taken hundreds of thousands of years for them to proceed from the Canadian border
to the Mexican border. I t stands t o reason that populations B and G will have the
. least amount of isolating mechanisms. The isolating mechanisms will be more
between D and I. They will be greater between E and J. Remember, both the series,
BCDE and GHIJ, are derived from a single species, Ensatina eschscholtzii,
designated in Fig. 13.3 as A. But when they reach the Mexican border, the two
populations F and K are able to overlap, as seen in the cross-hatched area, without
mating. They have passed the test of sympatry. F and K are now two distinct species.
SimiJar ring species have been shown to occur in sea-gulls forming a circumpolar ring
around the world.
COASTAL R A N NEVADA
- ,
Tim +
Fig. 13.4 : Genetic revolution. '
SAQ 3
Ans\ker in about 50 words each.
i) Explain briefly the term ring species.
................................................................................................>........
I
I
ii) Explain the concept of genetic repatterning during isolation.
.........................................................................................................
!
In this section we are to discuss the means by which populations get separated or
isolated from each other, first gain the status of'Sub-species and finally evolve
mechanisms which prevent them from mating with local populations to be called as
a distinct species. In ghort, we are to discuss the mechanisms that are responsible for
keeping populations from the access of each other that in turn leads to,origin of new
species. You are aware that local populations generally interbreed among themselves '
with only rare cases of outbreeding. Therefore, the genotypes of different populations
of a single species may show differences in some or many loci. The resulting
phenotypes could be classified into sub-species based on the differences in parts.
These subspecies because of their proximity to other populations tend to meet the
other members with the result that a single generally intermediate and variable
population is formed. This would result in the loss of the status of subspecies for the
population. However, if a sub-species is isolated over a long period of time and the
breeding with its relations is prevented, then by continuing to accumulate the genetic
differences it may lose its ability to interbreed with the parental species. Under such
circumstances, the sub-species gets elevated to the status of a species, satisfying Mayr's
definition of a species. When two groups are geographically separated from each
other, it is often difficult to determine whether they do not interbreed any more. In
other words, could they be referred to as allopatric species? Once they move into the
same territory, fail to interbreed and form intermediates, they may said to have
passed the test of sympatry or they could be regarded as sympatric species. In many
cases it has been noticed that once the species status has been established, selection
tends to promote those characters which act as a deterrent for the formation of
hybrids, since usually hybrid progeny have a reduced fitness.
George Romanes, an American neurologist who evinced a very keen interest in
evolutionary problems once wrote, "without isolation or the prevention of
interbreeding, organic evolution is in no case possible". Many recent studies on origin
of new species have proved that Romanes was largely true in his assessment of the!
problem. Let us now look into the details of mechanisms of isolation.
Ernst Mayr classified isolating mechanisms into two major types. The pre-mating
isolating mechanisms and the post-mating ones. The difference between the two types
of mechanisms is as follows. Pre-mating isolating mechanisms prevent the occurrence
of mating and the post-mating ones ensure failure of such matings once they occur.
In the discussion on pre-mating isolating mechanisms, besides the biological barriers
evolved by the organisms which prevent effectively mating between the individuals,
isolation by space will :Jso be included. We shall also examine how the ecology,
seasons and even the behaviour of individuals could be instrumental in bringing about
the isolation of populations leading to species formqtion. We shall also look into the
details uf post-mating isolating mechanisms which prevent the formation of successful
hybrids, thereby ensuring the distinctness and identity of a species.
Further each species has developed specific and elaborate mating call and this again
prevents breeding between syrnpatric forms. Finally even if mating takes place
between species, the development fails beyond the embryonic stage. Thus the
seasonal isolation has led to sterility barrier and the three species have no chance of
forming. hybrids.
A detailed study of the courtship behaviour of six species of Drosophila showed that
courtship and mating could be divided into six phases. If there is incompatibility at
any one of these six phases, the potential mates break off and the courtship is
discontinued. Under laboratory conditions, the interspecific crosses have not been
successful and the courtship was terminated even in the first stage. What is more
interesting here is that even to a trained observer differences in courtship behaviour
exhibited by different species may appear to be trivial and insignificant. But the
species recbgnise the specific signals and respond suitably. In certain other forms
differences in courtship behaviour between species could be very pronounced, The
courtship dances of the different species of Uca (shore crab) could be recognised from
a distance. This is also true of mating dances of salamanders, turtles and birds.
Earlier it appeared that Rana pipiens consisted of a single species. This assumption
34 was based on morphological considerations. But today researches have shown that
there are a number of species and the frogs have no difficulty in recognising their
own species. Both the frogs and researchers could distinguish the characteristic
mating song of each species. Some species of birds which show only minor
morphological differences can be easily differentiated by their songs.
Apart from the species specific dances and songs evolved by organisms to attiact their
mates, specific scents are produced by certain organisms for purposes of species
recognition. These scents, otherwise known as pheromones have been proved to elicit
selective response from males when left with females of two closely related species
in the same area. A study by B. Patterson in Scandinavian valley has shown that
nearly thirty seven species of moths belonging to a single genus live together without
any interbreeding among them. The author has concluded that despite all minor
morphological differences among the species, conspecific matings are assured
because of the specific scent produced by each species. Essentially the above account
tells you that specific behaviour patterns play a vital role in species recognition and
serve as a powerful isolating mechanism.
Mechanical isolation appears to play a more important role in the speciation of plants.
Since many plants are aided by insects and birds in cross pollination, a morphological
compatibility is required between the plants and the pollinating agents. You may
recall the example that has bcen cited in unit 11 in which the flowers of different
species of Pedicularis plant are pollinated differently by bumble bees. Further, queen
bees with their long mouth parts pollinate the nector producing species of Pedicularis
and the other species of the plants are pollinated sternotribically o r nototribically.
We shall now briefly look into each one of these sterility mechanisms with plant and
animal examples.
SAQ 4
1) Match the following:
-
Column1 -- Column I1
1
I
d) Discuss thc role played by courtship behaviour and scents in the isolation
process.
....................................................................................................
....................................................................................................
....................................................................................................
....................................................................................................
e) Distinguish the terms interspecific sterility and hybrid sterility.
.................................................................................................
............................,.,..,......*..*,,............................)..........................
~ " . ~ " " ~ . ~.......................................................................
"~~".~~".~~~~~
....................................................................................................
SUMMARY
In this unit you have learnt:
The concept of species and the definition that the species are a group of
interbreeding natural populations that are reproductively isolated from other such
groups. The biological species concept is more realistic than the typological or
nominalistic species concept. It explains species as a distinct unit of time, with a
capacity to change continuously over loiig periods of time. In short, species has an
evolutionary capacity.
0 The different modes of speciation. (a) The sympatric speciation refers to speciation
among populations living together. (b) Allopatric speciation refers to speciation in
populations separated by space and (c) Peripatric speciation refers to speciation of
individuals isolated as a peripheral population.
e Concept of ring species in which a population becomes isolated, changes gradually,
passes the test of sympatry and becomes a distinct species.
e The concept of genetic repatterning in which a new population develops from the
early colonisers of the island by a process of genetic revolution. Reduction in
variability, elimination of homozygote recessives by inbreeding process,
overdominance resulting in heterozygote superiority and non-allelic epistatic
interactions are sorne of the genetic events that break the genetic cohesiveness of
populations. Such events render the founder population more plastic and pliable
arid move it into a new species with better adaptations.
The different types of isolating mechanisms-the geographical, ecological,
mechanical, ethological and reproductive (both pre-mating and post-mating) all of
which promote the formation and distinctness of species.
That genetic drift is a process that may operate in small populations bringing about
large scale changes in gene frequencies. Also known as Sewall Wright effect, in
small populations the drift is due to sampling error.
I
1 3 . 1 TERMINAL QUESTIONS
1 Briefly comment on the three types of species concept.
..
..........................................................................................................
, 2) Distinguish the terms sympatric, peripatric and allopatric speciation. , '
3) What do you understand by the term ring species? Illustrate your answer with
suitable example.
13.12 ANSWERS
Self-assessment Questions
1) i) T, iii) T, iii) F, iv) T.
2) i) Reproductively isolated
ii) Sympatric speciation
Speciation
iii) Polyploidy
iv) Disruptive selection
v) Geographically isolated
vi) Allopatric
vii) Peripatric speciation.
3) i) It is quite possible that the action of selection could differentiate the different
sub-species further resulting in a circle or group of races. The terminal
members of such a ring or circle sufficiently different from others evolve into
a species - the ring species.
ii) When a new population develops from early colonisers of the islands they
undergo a variety of genetic changes by way of reduced variability,
elimination of recessive homozygotes, reduction in genetic load, superior
fitness of heterozygotes and non-allelic epistatic interactions, all of which
result in a genetic revolution or more precisely the genetic repattering.
1 4) i) a) iii), b) i), c) ii)
ii) a) The concept of geographical isolation refers to the restriction of organisms
to certain specific geographical regions with suitable ecological features.
For instance oceanic islands are not inhabited by amphibians. Pacific
ocean receives many parallel streams of fresh water each of which has its
own species or sub-species of fishes which live isolated.
b) Habitat preference do promote speciation process. In Swiss moth
Nemeophila plantagenis the habitat preference of the two races appears
to con'tribute to the speciation process. One race of the moth lives at an
altitude of about 2700 meters and the other race below 1700 meters.
Although the two races breed in the laboratory the hybrid population is
confined at an altitude of 2200 meters.
c) The five species of cypress trees of the genus Cupressus are divided into
10 groups. Each one of these could be called as sub-species. Very rarely
hybrids are formed between two groups although the groups live close
1 enough. The reason appears to be that the groups shed their pollens at
1 different times or season, preventing the occurrence of cross pollination.
d) Courtship behaviour patterns which are very specific for each species act
as a deterrent for the members belonging to different species. Mayr
opines that where there is courtship behaviour pattern the engagement
may be broken if the pairs do not belong to the same species. Similarly
specific scents produced by certain organisms to attract their mates help
in the process of species recognition.
e) Interspecific sterility: Organisms belonging to two different species may
mate but may not produce any offspring.
I Hybrid sterility : The interspecific cross may result in a sterile F1
offspring.
F
5) i) Selection, mutation, genetic recombination and gene migration.
ii) Genetic drift
iii) Sampling error
iv) Sewall wright
v) Shifting balance.
i Terminal Questions
! 1) Three types of species concept have been proposed.
I
a) The typological species concent, as suggested by Plato, reduces the immense
variety of organisms in nature to a few "types". It proposed that individuals
might vary but belonged to a single type.
b) The nominalistic species concept regarded species as man-made abstraction.
c) The biological species concept, a more realistic and objective approach to the
problem of species conceived species as members'belonging t o a reproductive
community and as an ecological unit. It also regarded species as a genetic
unit where individuals are held as a large, strongly united gene pool.
2) Syrnpatric speciation refers to speciation where parent species give rise to a
daughter species without the individuals of the species being separated,
Speciation by populations of parent species which occupy quite separate
territories is the allopatric speciation and the most common mode of species
formation. Speciation by small populations isolated on the periphery of the
distribution of the parent population can be described as peripatric speciation.
3) Ring species refers to a concept of speciation in which a widely distributed species
breaks up into partially isolated species and then acted by selection and other
factors results in a circle or group of races or Rassenkreise. For the example refer
to section 13.4.
4) a) Geographic isolation
b) Geographic isolation
c) Geographic isolation
d) Ecological isolation
e) Ecological isolation
f) Ecological isolation
g) Ethological isolation
h) Ethological isolation
i) Ethological isolation
j) Mechanical isolation
k) Reproductive isolation
1) Reproductive isolation