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Species and Communities
BIOLOGICAL SPECIES CONCEPT (MAYR 1970,
POPULATIONS SPECIES AND EVOLUTION)
Species
What is a species?
Population structure
Evolution of geographic variation
Gene flow and clines
Assortative mating
Local variation
Geographic isolation
Secondary contact and hybridization
Ecology of speciation
Geographic replacement
Behavior and speciation
Communities
Factors shaping patterns of
species diversity and abundance
“Species are groups of interbreeding natural populations that
are reproductively isolated from other such groups.”
PHYLOGENETIC SPECIES CONCEPT (CRACRAFT
1989, SPECIATION AND ITS ONTOLOGY)
A species is the smallest aggregation of populations
diagnosable by a unique combination of character states in
comparable individuals. The goal of the phylogenetic species
concept is to define indivisible taxa that can be used in
cladistic analysis.
Generalized model of speciation
Speciation is driven by changes in
the spatial structure of populations
• Dispersal
Genetic Divergence
A’
A
Isolation
– Where young birds settle
to reproduce
B
Isolating Mechanisms
Colonization
Reproductive
Fragmentation
Ecological
• Philopatry
– Tendency of birds to
return to their birth place
Behavioral
 Hybrids
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Dispersal processes determine effective population size
Geographic variation drives speciation
Deme - a reproductively cohesive population
Effective Population Size – number of individuals in a deme
The more individuals interbreeding the higher the
effective population size
• Geographic variation in
color and size is
characteristic of about
1/3 of American birds
Small demes evolve faster
Effective population size decreases with dispersal
distance
Other factors that decrease effective population size
Fragmentation of populations into small isolated
populations with limited dispersal
Founding of new populations by a small number of
colonists
Existence of non-monogamous breeding systems
Opposing forces explain geographic differences
•
– Changes in gene
frequencies caused by
differential fecundity and
survival
•
Assortative mating
Natural Selection
• Assortative mating
produces geographic
variation
Gene Flow
– Preferred pairing of like
types
– Genetic blending caused by
interbreeding
Proportion of red-phase Screech Owls
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Geographic isolation is an important
component of speciation
Secondary contact tests species
• Large Scale (allopatric)
– Colonization of islands
• Galapagos finches
• Hawaiian Honeycreepers
– Habitat fragmentation
• Toucans
• Small Scale (sympatric)
• Mascarene White-eye
Hybrids occur when ecological
isolation breaks down
• Reproductive
isolation limits
effects of hybrids
• Stable hybrid zones
can persist for
centuries
• Ecological isolation
is less resistant to
hybridization
Oriole hybrids
• When ecologically
similar species become
sympatric one will
usually replace the other
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Blue-winged, Golden-winged Warblers
Behavioral isolating mechanisms
promote speciation
• Speciation promoted
by
Range expansion of
Blue-winged Warblers
Lawrence’s
– Complex behaviors
– Enhanced brains
– Cultural transmission
of information
Brewster’s
What is a species?
•
Exploding Species (TREE
11:314-315, 11:509)
– Typical estimate for birds is
10,000 species
– Molecular techniques and
better field studies may
increase this to 20,000
– Authors argue that
biological species concept
needs revision
“Failure to observe
inbreeding does not mean
lineages are completely
independent; neither does
interbreeding necessarily
prohibit the continued
divergence of two lineages
in contact”
Communities
• Factors shaping
patterns of bird
species diversity
and abundance are:
– Time
– Energy
– Space
Robert H. MacArthur. 1958.
Population ecology of some warblers
of northeastern coniferous forests.
Ecology 39:599-619.
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Bird diversity reflects habitat
and prey diversity
• Vegetation structure
correlated with bird
species diversity
(Karr and Roth 1971)
• Prey size diversity
associated with bird
diversity
(Schoener 1971)
Tropical
Temperate
Other evidence of competition
•
•
•
Ecological displacement in
Caribbean hummingbirds
Altitudinal displacement in
Appalachian thrushes
Ecological equivalents in
European and North American
tits
Character displacement
indicates competition
• Character displacement
refers to morphological or
behavioral adaptations
caused by competition.
• Gant’s (1986) study of
Galapagos Finches is a
classic example of character
displacement driven by
competitive exclusion.
– When three species occur on
one island (Santa Cruz) they
have distinct bill sizes. Single
species on islands (Daphne
Major and Los Hermanos)
have intermediate bill sizes.
Patterns of species diversity
• Temperate – tropical
gradients in diversity
reflect:
– More food
– Benign climate
– Stable environment
(MacArthur 1969)
Number of landbird species
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Island biogeography
Evidence of equilibrium dynamics
80
Number of species
70
60
50
40
30
20
10
0
10
100
1000
10000
100000
Island Area (km 2)
•
The number of species on an island reflects a balance between rates
of immigration (colonization) and extinction.
Extinction rates increase and immigration rate decrease as the
number of species present on an island increases.
The intersection of the two curves for any particular island size defines
the expected equilibrium number of species (S).
•
•
– Immigration rates on islands that are distant from source areas are lower
than rates on islands close to source areas
– Extinction rates on large islands are lower than rates on small islands
(MacArthur and Wilson 1967
Sources and sinks
•
•
•
•
Habitat patches on a landscape
function as islands of varying
habitat suitability
Source habitats generate
surplus productivity (R0>1), sink
habitats cannot sustain stable
populations (R0<1)
Populations in source habitats
show less variability than
populations in sink habitats
Landscapes with larger and less
isolated patches show less
variability than landscapes with
smaller or more isolated
patches
Pulliam, H.R. 1988. Sources, sinks,
and population regulation. The
American Naturalist 132: 652-661.
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