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Species and CommunitiesSpecies

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

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.

BIOLOGICAL SPECIES CONCEPT (MAYR 1970, POPULATIONS SPECIES AND EVOLUTION)

PHYLOGENETIC SPECIES CONCEPT (CRACRAFT 1989, SPECIATION AND ITS ONTOLOGY)

“Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.”

Generalized model of speciation

Genetic Divergence

A A’ B

Isolation

Colonization

Fragmentation

Isolating Mechanisms

Reproductive

Ecological

Behavioral

Hybrids

Speciation is driven by changes in the spatial structure of populations

• Dispersal– Where young birds settle

to reproduce

• Philopatry– Tendency of birds to

return to their birth place

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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

Dispersal processes determine effective population size

Deme - a reproductively cohesive population

Effective Population Size – number of individuals in a deme

The more genetically distinct individuals interbreeding the higher the effective population size

Therefore, the effective population size decreases with shorter juvenile dispersal distances

Small demes evolve faster (often by chance)

Geographic variation drives speciation

• Geographic variation in color and size is characteristic of about 1/3 of American birds

Opposing forces explain geographic differences

• Natural Selection– Changes in gene

frequencies caused by differential fecundity and survival

• Gene Flow– Genetic blending caused by

interbreeding

Proportion of red-phase Screech Owls

Assortative mating

• Assortative mating produces geographic variation– Preferred pairing of like

types

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Geographic isolation is an important component of speciation

• Large Scale (allopatric)– Colonization of islands

• Galapagos finches• Hawaiian Honeycreepers

– Habitat fragmentation• Toucans

• Small Scale (sympatric)• Reunion Gray White-eye

Secondary contact tests species

• Reproductive isolation limits effects of hybrids

• Stable hybrid zones can persist for centuries

• Ecological isolation is less resistant to hybridization

Hybrids occur when ecological isolation breaks down

• When ecologically similar species become sympatric one will usually replace the other

Oriole hybrids

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Blue-winged, Golden-winged Warblers

Lawrence’s Brewster’s

Behavioral isolating mechanisms promote speciation

• Speciation promoted by– Complex behaviors

– Enhanced brains

– Cultural transmission of information

What is a species?• Exploding Species (TREE

1996 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

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. Populations on single species islands (Daphne Major) have intermediate bill sizes.

Other evidence of competition• Ecological displacement in

Caribbean hummingbirds

• Altitudinal displacement in Appalachian thrushes

• Ecological equivalents in European and North American tits

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

• The number of species on an island reflects a balance between rates of immigration (colonization) and extinction.

• (A) Extinction rates increase and immigration rate decrease as the number of species present on an island increases.

• (B) 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

Evidence of equilibrium dynamics

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

• Fragmentation effects: 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.