Lecture 11: The Idea of Species
Why do we need a definition?
Species: fundamental unit of biology
Earliest definition: HOLOTYPE
Controversy about definition:
1. Phenetic Species Concept
2. Biological Species Concept
3. Ecological Species Concept
And more…
Species Concepts from Various Authors
D.A. Baum and K.L. Shaw - Exclusive groups of organisms, where an exclusive group is one whose members are all more closely related to
each other than to any organisms outside the group.
J. Cracraft - An irreducible cluster of organisms, diagnosably distinct from other such clusters, and within which there is a parental pattern of
ancestry and descent.
Charles Darwin - "From these remarks it will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set
of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and
more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for mere
convenience sake" (Origin of Species, 1 st ed., p. 108).
T. Dobzhansky - The largest and most inclusive reproductive community of sexual and cross-fertilizing individuals which share a common gene
pool. And later...Systems of populations, the gene exchange between which is limited or prevented by reproductive isolating mechanisms.
M. Ghiselin - The most extensive units in the natural economy, such that reproductive competition occurs among their parts.
D.M. Lambert - Groups of individuals that define themselves by a specific mate recognition system.
J. Mallet - Identifiable genotypic clusters recognized by a deficit of intermediates, both at single loci and at multiple loci.
E. Mayr - Groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.
C.D. Michener - A group of organisms not itself divisible by phenetic gaps resulting from concordant differences in character states (except for
morphs - such as sex, age, or caste), but separated by such phenetic gaps from other such units.
H.E.H. Patterson - That most inclusive population of individual biparental organisms which share a common fertilization system.
G.G. Simpson - A lineage of populations evolving with time, separately from others, with its own unique evolutionary role and tendencies.
P.H.A. Sneath and R.R. Sokal - The smallest (most homogeneous) cluster that can be recognized upon some given criterion as being distinct
from other clusters.
A.R. Templeton - The most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion
mechanisms (genetic and/or demographic - i.e. ecological -exchangeability).
E.O. Wiley - A single lineage of ancestor-descendant populations which maintains its identity from other such lineages and which has its own
evolutionary tendencies and historical fate.
S. Wright - A species in time and space is composed of numerous local populations, each one intercommunicating and intergrading with others.
Not all are mutually exclusive…
Phenetic Species Concept
• Clusters in morphospace
• Extension of how spp. defined in practice
• Works for sexual, asexual & extinct spp.
Sp. A
Sp. B
Sp. C
Problems…
Morphological diffns ≠ Biological diffns
1) Sexual Dimorphism
e.g. Black Widow Spider
2) Life Cycle Stages
e.g. Barnacle
3) Polymorphism
Can get tons of morphological variation in one
species
e.g. Cattleya orchids
e.g. Snow goose
e.g. Ensatina salamanders
4) Sibling Species
• Differ in reproduction but not morphology
• e.g. Leptobrachium smithi
L. hendricksoni
• e.g. Uca borealis
U. vocans
Biological Species Concept
• Most commonly used definition
• Reproductively isolated group of interbreeding
individuals
• Species = Gene Pool
• Must form viable offspring
• Idea predates Darwin!
• NOT better just different than phenetic def’n
How define BSC in measurable terms?
• Most spp. rarely
observed during
breeding
• Look for
intermediaries…
evidence of
interbreeding
Sympatric Species
Very closely related & present in same range
but no offspring.
Why not?
1. Premating Isolation
•
Prevent formation of hybrid zygotes
2. Postmating Isolation
•
Reduce viability/fertility of hybrids
Premating Mechanisms
Ecological Isolation – wrong place
Seasonal/Temporal Isolation – wrong time
Ethological Isolation – no attraction
Mechanical Isolation – copulation/pollen
transfer impossible
5. Gametic Isolation – gametes inviable
1.
2.
3.
4.
Ecological Isolation
• Overlap in range but niche differences
e.g. Sceloporus clarkii (trees) vs. Sceloporus magister
(open areas)
Temporal Isolation
Ethological Isolation
• Acoustic
• Visual Displays
Mechanical Isolation
• Scotch broom
• Bumblebees –
weight determines
ability to pollinate
Post-mating
Mechanisms
1. Hybrid inviability (dies/does poorly)
2. Hybrid sterility (gen’lly heterogametic)
F1 hybrids have non-functional gametes
3. Hybrid breakdown
F2 hybrids have reduced
viability/fertility
Are RIM the cause or effect of
speciation?
• EFFECT if selection against hybrids
Anatidae (surface-feeding ducks) vs. Anserini (geese)
• anatids usually have complex diverse courtship
displays
• hybrids usually fertile
Ho : courtship displays are R.I.M. that evolved
because hybrids have reduced fitness
Pair formation behav.  divergence
vs. pair maintenance behav.
• selection favours early ID
Diversity should correlate with degree
of sympatry
• risk of hybridization
Alternative hypotheses:
• sexual selection
• dominance
• predation risk
Conclusion: they are RIM now but
don’t know history
Biological Species Concept
Problems:
• "ring" species (hybrids)
• Only applicable to the present
• No evolutionary dimension
• Doesn’t apply to asexual organisms
• Temporally/Spatially separated species
Ecological Species Concept
• Because niches are discrete, can be used to
identify discrete species (phenetic clusters)
Disruptive selection
(against hybrids)
Competitive exclusion: competitive spp.
cannot coexist forever if have same limiting
resources (only 1 sp. /niche)
Leads to…
Character displacement
• Spp. that are sympatric differ more than allopatric spp.
• Theory: allows exploitation of different niches
e.g. Hawaiian Honeycreepers
Overall Difficulty
• Competing effects of selection & gene flow
• Selection – Ecological definition
• Gene flow – Biological definition