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Transcript
Speciation
Speciation is the evolution of more than
one species (or, at least, a single
different species) where previously
there was only one.
=> species are the unit of evolution
Species Concepts
But what is a species?????
• Scientists have been arguing about
this for decades and still don't agree
on the exact definition of a species.
This is a very hotly debated issue.
• They have rather come up with whole
range (about 15) of species concepts
• In this lecture I will talk about 7 of
them.
E.g. Parts of a seahorse
Pectoral fin
coronet
1. Morphological Species concept
Linnaeus: A species is the smallest group of
individuals that are distinct and
distinguishable from all others
eye
Dorsal fin
– Morphology refers to the form and
struc ture of an organism or any of its parts.
Tail
Reproductio n
Strengths:
• Is in fact the way we recognise species
differences.
• Works well for sexual and asexual organisms
• Applies well to past species (fossils can be rela ted
to ex tant species)
e.g Coelacanth was previously only known from
fossils
Polymorphism
Weaknesses:
•Does no t connec t with genetics. Morphological
differences can be presen t withou t significant
genetic divergence!
•Polymorphism, sibling species, sexual dimorphism,
mimicry complexes
Light-morph jagu ar (typi cal ) Dark-mo rph jagu ar (rare)
Sibling species
- Differences between sexes
Argyroso mus japoni cus (RSA) Ar gyros omus coronus (Angol a)
Reprod uctiv ely isolated, yet very similar morphology
Mimicry
2. Biological Species Concept (BSC) –
Saddl ed Puf fer
Canthigaster v alentini
1 dorsal fin
Mimic Leather jac ket
Ernst Mayr, 1942:
• Most dominant concept in ecology studies
“ A species is a group of interbreeding
natural populations that are
reproductively isolated from other such
groups."
Paralut er es prionurus
2 dorsal fins!
2 pairs of spines on the
caudal peduncle
This concept assumes that Genetic
divergence is key to speciation
Genetic divergence is when local units of a popula tion
become repro duc tively isolated fro m o ther units. The
driving forces of evolution (genetic drift, na tural
selec tion and mu ta tion) may opera te in each isolated
population and thus lead to changes in gene
frequencies.
Biological Speciation is the process by which species
form when two genetically diverging species remain
isolated from each o ther.
Because genetic divergence is a gradual process, it is
impossible to say precisely when species form.
As long as gene flow continues individuals
will remain members of a species even
though t hey may be geographically distant.
Reproductive isolation is key to the BSC:
2. Biological Species Concept
Strengths
-Criterion of reproduc tive isolation can be tested,
observed, and / or inferred
-Includes genetic information
Weaknesses
- Ignores hybridization
- Does no t work well for asexual organisms
- Depen den t on g eographic isolation to achieve sp.
sta tus
- Reproducti ve iso lation evo lve s g radually
- Can no t be used for paleo species (ex tinct or fossils)!
3. Evolutionary Species Concept
Or Genealogical species concept
An evolutionary species is a lineage (an
ancestral-descendant sequence of
populations) which maintains its identity
evolving separately from others and with
its own unitary evolutionary role and
tendencies”
–Iden tity ; biologically distinct
Includes concep ts associated with BSC e.g.
Species Recognition Mechanisms
–Tendencies and historical fate;
Species have an origin
Undergo evolu tion
May disappear by ex tinction.
3. Evolutionary Species Concept:
Concept :
• Recognizes more than just genetic and
morphological differences
4. Phylogenetic Species Concept:
Concept:
A species is a cluster of organisms tha t is
diagnostically distinguishable from o ther such
clusters, and within which there is a paren tal
pattern of ancestry and descen t."
•However;
–Change within lineages does no t c rea te a new sp.
–difficult to determine “evolu tionary fate”
– how much diversity is allowed within a common
evolutionary fate?
–The stu dy of speciation is key, no t the study of
microevolution
– It rates trai ts as ances tral or deri ved and then looks
for gr oupi ngs bas ed on si mi lari ti es.
Avoids all reference to repro duc tive isolation and
focuses instea d on phylogenetic histories of
populations.
Lineage; ancestor descendant
members of a species have a common ancestor
cartilaginous
Sharks, rays and chimaeras
Coelacanth
4. Phylogenetic Species Concept:
Concept:
a s pecies is a “tip” on a phy logeny , th at is, th e s mall est s et of
organ isms th at sh ar e an a nc est or and c an b e d istin guished fr om other
such s ets. Und er this d ef in ition, a r in g spec ies is a single s pecies that
en compass es a l ot of ph enot ypic var iat ion.
Lobes
Lungfish
bony
Rays
One
speci es
5. Recognition species concept:
concept:
Problems
(1) confuses histories of traits with
histories of organisms, may divide
species into groups based on
characteristics that do not have any
clear biological relevance
(2) classifications change with more data
(3) creates taxonomic inflation (new
species names without the discovery of
new species)
6. Cladistic species concept
A speci es i s a set of organi sms (an
evoluti onar y li neage) betw een tw o
branch poi nts or betw een one branch
poi nt and an exti ncti on ev ent or a
moder n populati on (Ri dley 1993).
a)
Problem:
-Teleosts
- Adaptive radiation
- Phylogeneti c s tatus of s peci es
changes over ti me
b)
c)
3 species using this
definition, but 1 and 2
phenotipically identical
3 species, and all are
phenotypically different
1 species, but with
significant phenotypic
change over time
”A species is the most inclusive population of
bipar ental organisms t hat share a common
fertilization system" (Paterson, 1985).
Comment: Si mi lar to BSC i n vi ewi ng cons peci fi c
populati ons as a fi el d for r ecom bi nati on, but focuses on
repr oducti on faci li tati ng mechani sms wi thi n speci es
rather than on r epr oducti ve barri ers.
di sadv antage
Li ke BSC can only be appli ed to contem porari es, sex ual,
and geographi cally conti guous populati ons, hence to a
mi nori ty of natural populati ons
Other concepts…
concepts …
7. Cohesive Species concept
A species is the most inclusive group of
organisms having the potential for
genetic and/or demographic
exchangeability. (Templeton, 1989)
Atte mp ts to incorpora te strengths o f BSC, E SC, and R SC.
Cohe sion mechanisms a re genetic e xchangeabili ty (factors
that de fine gene flow) and de mograp hic exchangea bility
(facto rs tha t de fine and sprea d new gene tic variants
through gene tic drift and natu ral selection).
Difficult to use based on the fact tha t di fferen t scienti sts
may choo se to e mpha si ze di fferen t cohe sive mechanisms
Problems in defining species
2. Geographic variation
1. Polymorphisms
Refers to variation over geography
variation within a population. Different
individuals in the population have distinctly
differ ent structures, colors, biochemistry, etc.
but clearly belong to the same population since
they reproduce with one another.
populations in different areas look different
from each ot her.
One common pattern of geographic variation
is clinal variation. Clinal variation refers to a
gradual change in some featur e across
geography.
e.g. Hima ntur a ua rnak
3. Hybrid zones
Geographic areas wher e two distinctly different
for ms of organism contact each other and
interbreed.
Outside this zone, the two for ms retain distinct
differ ences from one anot her but within the
zone inter mediate for ms between the two occur
as a result of interbreeding.
e.g O. niloticus in southern Africa
As a cons equ ence o f the 2 million year isolation, there is a high level o f
geneti c divergen ce (4.5% COI gen e) bet ween the A. aequid ens
populations.
ANGOLA (n=5 2)
So uth Africa (n=33 )
Mo de Min Ma x Mo de Min Ma x
1 st Do rsa l Fin Spine Co unt
10
10
10
10
10
2 nd Do rsa l Fin Spine Co unt
1
1
1
1
1
10
1
2 nd Do rsa l Fin Ra y Co unt
27
24
28
27
26
29
Ana l Fin Spine Co unt
1
1
1
1
1
1
Ana l Fin Ra y Co unt
9
9
9
9
9
9
From Hendri ques , 2008
No s hare d
hapl otype s
Pecto ra l Fin Ra y Co unt
18
16
19
17
16
18
La tera l L ine Sca l e Co unt
72
70
77
75
71
79
Krug er 200 9
Max. Size (Fork Length)
Max. Weight
?
Length at Maturity (Fork Leng th)
Angola (1)
South Africa
(2)
960 mm
1300 mm
6.8 Kg
25 Kg
430 mm
900 mm
•
•
•
•
•
•
•
Morphological species concept
X
Biological species concept
Evolutionary species concept
X
Phylogenetic species concept
Recognition species concept can’t test
3 speci es
Cladistic species concept
Cohesive species concept
X
Angola
Species Concepts
•
RSA
•
Value of each concept depends on its use.
Various authors have argued that a
combination is the best met hod.
–
–
mor phologica l, ph ysiol ogica l, b ehav ior al
geogra phic
–
–
–
–
–
lif e h ist ory & d ev el opment
habitat & feed in g ec ol ogy
phyl ogen etic s
evolut ionar y fat e
And Genet ics etc . b arc oding
Note:
1. There i s uncer tai nty of the s peci es rank
2. The term “s peci es” can mean di ffer ent thi ngs i n
di ffer ent tax onomi c groups