Lectures 31-33

Question 1

Who noticed that the fossil record is characterised by long period of stasis and periods of rapid evolution?

Answer 1

1970s
Stephen Jay Gould

‘Punctuated Equilibrium’
- tried to explain but many disagreed with his ideas

Question 2

Evidence for stasis and rapid evolution

Answer 2

Cheetham and Jackson
Evolution of Bryozoa

• shelly marine organisms - very good fossil record - lots of data with minimal breaks
• species show very clear evidence for bursts of rapid morphological change and periods of stasis

Question 3

Why is it harder to explain stasis rather than rapid evolution?

Answer 3

Lots of evidence for microevolutionary studies for rapid morphological evolution

• like cichlids, maize and dog breeds
• artificial selection

Question 4

5 possible explanations for evolutionary stasis

Answer 4

1) Natural Selection is feeble
2) Evolution is fickle
3) Lack of genetic fuel
4) Lack of ecological opportunities
5) Spread of Fit genotypes is limiting

Question 5

How can natural selection be feeble - why can it explain evolutionary stasis

Answer 5

Perhaps - often weak selection in natural population - not strong enough to generate rapid change

BUT
- seems unlikely as common estimates of strong selection in extant populations

Question 6

How can evolution be fickle (in explanation of stasis)

Answer 6

Strong selection is not commonly sustained in the same direction for long periods

• observe rapid evolution in short term but may be reversed in subsequent generations
• no observable change in fossil record

Like finches - drought and beak size - return after period of drought ended

Question 7

How can lack of genetic fuel explain stasis?

Answer 7

Maybe selection commonly limited by lack of available genetic variation

• but artificial selection can commonly induce changes way beyond the variation seen within a single population
• maybe but not definitely

Question 8

How can a lack of ecological opportunities explain stasis?

Answer 8

Maybe static because live in stable environments - little pressure to change
- conversely rapid evolution is often seen when species colonise new environments such as lakes or islands

Question 9

Spread of fit genotype is limiting - how can this explain stasis

Answer 9

Rapid evolution in one population may not lead to overall change in the species if those genotypes do not spread out across the species range
- shifting balance theory
except selection not consistent across space as opposed to through time

Question 10

Anolis radiation in the Caribbean

Answer 10

Different morphologies are adapted to living in different parts of the habitat - large species in tree crowns and tiny skinny on twigs

• differences repeated on different islands
• morphologically similar to others on different islands - but genetically closer to ones of island
• strong support for a repeated pattern of phenotype-environment correlation

Question 11

Anolis lizards and utility of traits

Answer 11

long legged species run much faster
- struggle on small twigs - shorter legged species are more sure-footed

• grass-bush anoles have a different strategy - jump