Part of AS 90717

Relates to US 6317

What is EVOLUTION ?

Evolution is change over time -

Change means heritable change in a population

Time means over generations

Example: Evolution of polar bears

Polar bears feed only on the Arctic Ocean ice, available since less than 5 million years ago.

Seals swim under the Arctic ice, but have to keep breathing holes open.

Q: So where did polar bears come from?

A: they share a common ancestor with brown bears.

TIME

EvolutionarychangeSpecies A

Brown bear

Species B

Polar bear

1. Mutation (white fur)2. Hunting advantage (seals can’t see white bears so well)3. Better breeding success (more white cubs)4. Increased % white bears in population over time5. Reproductive isolation (polar/brown bear hybrids infertile)

anagenesis = change within a population;

cladogenesis = branching to produce 2 new species.

Two patterns of speciation:

Biological species concept: a species is “a population or group of populations whose members have the potential to interbreed with one another to produce viable fertile offspring, but which cannot produce viable fertile offspring with members of another species.”

i.e. this species concept is based on interfertility rather than physical similarity.

By this definition, separate species must be reproductively isolated.

Geographical isolation by itself ≠ reproductive isolation – it is not a feature of the organisms themselves.

http://www.creativegraphics.ws/main/Information/funbets/Gallery/ExtraPhotos.html www.geocities.com/ zedonknzorse/

Habitat isolation

Behavioural isolation

Temporal isolation

Pre-zygotic isolating mechanisms operate before fertilisation:

Mechanical isolation

Gametic isolation

Post-zygotic isolating mechanisms operate after fertilisation:

Hybrid breakdown

Reduced hybrid viability Reduced hybrid fertility

Two modes of speciation

Has speciation occurred during geographic isolation?

Species A

TIME

Species B

Species C

Sequential Evolution

Species A is replaced by B, then by C

Appearance of sequential evolution in the ancestry of horses(only one species survives).

1990s

Extinct horses Modern horses

Species A

TIME

Species B Species C

Divergent Evolution

Species A is replaced by two or more coexisting species.

Ancestral parrotfrom Australia

TIMEDivergent Evolution

80 mya

Ancestral parrotfrom Australia

TIMEDivergent Evolution

Kaka

tree

80 mya

60 mya

Ancestral parrotfrom Australia

TIMEDivergent Evolution

Kaka Kakapo

groundtree

80 mya

60 mya

Ancestral parrotfrom Australia

TIMEDivergent Evolution

Kaka Kakapo

groundtree

80 mya

60 mya

3 mya

Ancestral parrotfrom Australia

TIMEDivergent Evolution

Kaka Kakapo

groundtree

Kea

alpine

80 mya

60 mya

3 mya

Ancestral parrotfrom Australia

TIME

Kaka-North Island

Kaka –South Island

Divergent Evolution

Kaka Kakapo

groundtree

Kea

alpine

80 mya

0.4 mya

60 mya

3 mya

Fleming 1979

Species A

TIMERadiation

Species C Species DSpecies ESpecies B

Many descendant species in different places

Adaptiveradiationof NZwrens

Two models for the tempo of speciation

From: Arensburger et al. (2004), J. Biogeogr. 31: 1769-1783

Species A

TIMEConvergent Evolution

Species X

Species A

TIME

Species B

Convergent Evolution

Species X

Species ZSpecies C Species Y

Unrelated species look similar if they live in similar environments

Sabre-tooth tigers

Marsupial Placental

Simpson 1980:70

Convergence in unrelated, streamlined swimmers

Shark - fish Dolphins - mammals

Common features: both adapted to move in water Dorsal, pectoral fins, hydrodynamic shape

Differences: tail flukes reflect different ancestrySharks always aquatic, dolphins descended from land animals

Why does evolution keep going?

Why do we not get perfection??

Because:The world is always changing, producing new challenges and opportunities.

Continental drift

Fig 15.3A