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 AuthorsD.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, 1st 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. C

Sp. B

Problems…Morphological diffns ≠ Biological diffns

1) Sexual Dimorphisme.g. Black Widow Spider

2) Life Cycle Stages

e.g. Barnacle

3) PolymorphismCan 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

1. Ecological Isolation – wrong place

2. Seasonal/Temporal Isolation – wrong time

3. Ethological Isolation – no attraction

4. Mechanical Isolation – copulation/pollen transfer impossible

5. Gametic Isolation – gametes inviable

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 evolvedbecause hybrids have reduced fitness

Pair formation behav. divergence vs. pair maintenance behav.

• selection favours early IDDiversity should correlate with degree

of sympatry• risk of hybridizationAlternative hypotheses: • sexual selection• dominance• predation riskConclusion: 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