Natural Selection and Speciation

Another way to look at it…

• Directional Selection

Fitn

ess

Trait Trait

Positive Directional Selection Negative Directional Selection

• Stabilizing selection

average trait value

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ess

Egg laying wasp that eventually kills the cactus

• Disruptive Selection

average trait value

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ess

Black-bellied seed cracker (Pyrenestes ostrinus)

• Smith, T. B. 1990. Evolution. 44(4):832

• Black-bellied seed crackers (Pyrenestes) in Cameroon

Same birds different study

Open portion of bar – Number hatched

Black portion of bar – Number survived

Two more types of selection

• Correlational – working on combinations of traits at once.

(body size)

(Wing length)

• Frequency Dependent Selection – Level of selection dependent upon most common phenotype (at the time)

GenyochromisScale eater

Scalebiters – frequency dependent selection

Right “mouthedness” Left “mouthedness”

Time

Frequency of right “mouthedness”

Frequency of left “mouthedness”

• Selection can lead to speciation

• Speciation:– Splitting of one species into two or more

species

– Transformation of one species into a new species over time

Anagenetic (gradual) versus Cladistic (punctuated) Speciation

A

AB

TIM

E

PHENOTYPICVARIATION

A

B

TIM

E

PHENOTYPICVARIATION

Species

• Many, many definitions

• Biological species definition: A group of actually or potentially breeding individuals which are reproductively isolated from all other groups.

• Doesn’t work in all situations– Fossils– Asexual organisms

Isolating mechanisms

• Prezygotic –

• Postzygotic -

• Pre-zygotic barriers include– Temporal isolation– Behavioral– Mechanical– Gametic

Prezygotic I.M.s

• Habitat isolation– Dendroica warblers in Eastern US

Modes of speciation

• Allopatric – allo = different, patri = fatherland

• Sympatric – sym = together

• Has to do with location or habitat

Speciation only occurs with reproductive isolation

Sympatric speciation

• Polyploidy in plants

• Hybrids

Adaptive Radiation

• Evolution of ecological (& morphological) diversity within a rapidly speciating lineage.

• Hawaiian honeycreepers