Lecture 14 - macro evolution

Question 1

What is macro evolution?

Answer 1

if speciation is underpinned by natural selection then are the patterns that we see at a macro evolutionary level on a large scale?

Question 2

Describe rates of evolution

Answer 2

• steady change over time
• expected outcome from micro evolution
• doesn’t mean smooth change, but rather the accumulation of small stepwise changes over time

Question 3

What is the problem with macroevolution?

Answer 3

• the fossil record generally lacks transitional forms
• new tax’s appear abruptly in the fossil record
• major morphological innovations sometimes appear suddenly in the fossil record, often proceeded and followed by periods of relative stasis

Question 4

Why do we observe apparent bursts of change?

Answer 4

• the fossil record is incomplete
• transitional forms extremely unlikely to be found in the fossil record

Question 5

Why are there gaps in the fossil record?

Answer 5

fossilisation requires:
- death - inevitable
- burial with organisms (mostly) intact - rare due to scavenging and decay
- sedimentation - rare in terrestrial environment
- survival of sedimentary rocks - rare, geologic processes repeatedly destroy sedimentary layers

Fossilisation is rare

Question 6

Is fossilisation common?

Answer 6

no - rare

Question 7

What is the punctuated equilibrium?

Answer 7

• they proposed that rates of evolutionary change during and between speciation were different because different processes were at work
• proposed that speciation took place in small populations
• and that a genetic revolution took place due to a process other than natural selection
• implies that evolution is fast during speciation and slow between speciation events
• widely discussed but not fully accepted

Question 8

Can we explain the fastest rates of evolution and the slowest?

Answer 8

any explanation needs to be able to cope with:
- periods of rapid divergence
- periods of stasis

Question 9

What is an example of rapid change?

Answer 9

homo erectus - 920ml brain
homo sapiens - 1400ml brain

Question 10

How can you explain rate of change so fast?

Answer 10

• for this speed of evolution, selection needs to eliminate 1 in 500 individuals per generation (i.e. only need selection to remove the smallest brain).

SELECTION CAN EXPLAIN RAPID CHANGE

Question 11

Can selection explain stasis?

Answer 11

• stasis is fully compatible with selection
• if selection pressures do not vary greatly over time, then net changes should be minimal

Question 12

How can rates of (macro) evolution vary among species?

Answer 12

what is stasis between bursts is not the absence of change, but instead the absence of consistent directional change over geological time

• recent studies show that rates of evolutionary change are just as high for fossil time series with net stasis as those with net change

RATES OF MACRO EVOLUTION VARY AMONG SPECIES - E.G. BEAK EVOLUTION IN BIRDS

Question 13

What are features of macro evolution?

Answer 13

• convergent evolution
• adaptive radiation

Question 14

What occurs with divergent evolution?

Answer 14

lineages split & separate

Question 15

What occurs with convergent evolution?

Answer 15

lineages that aren’t closely related evolve similar adaptions because they live in similar environments

Question 16

Describe the Anolis of the Caribbean

Answer 16

• 200 mainland species
• 150 Caribbean species

Anolis ecomorphs:
6 ecomorphs:
- twig
- crown-giant
- grass-bush
- trunk-crown
- trunk
- trunk-ground

Different micro habitats occupied by different. Morphologically & behaviourally distinct

Question 17

What is historical contingency?

Answer 17

• related species share evolutionary history
• closely related species tend to be similar

Question 18

How can we measure evolutionary history?

Answer 18

by measuring body and different ratios across the body

Question 19

What is adaptive radiation?

Answer 19

the evolution of ecological & phenotypic diversity within a rapidly multiplying lineage

Question 20

What does adaptive radiation require?

Answer 20

• differentiation of a single ancestor into multiple species
• variation in morphological traits that allow exploitation of range of environment

Question 21

What causes adaptive radiation?

Answer 21

ecological opportunity - many example of diversification in the apparent absence of ecological (competitive) constraint

Question 22

What is an example of island colonisation?

Answer 22

Darwin’s finches

Question 23

What are Hawaiian honeycreepers?

Answer 23

• example of divergence
• very diverse
• example of adaptive radiation - due to lack of ecological constraint

Question 24

Define ecological release from antagonists

Answer 24

• lack of predators allows species to identify
• end Cretaceous extinction of the dinosaurs
• Vacant niches filled by mammals

Question 25

Describe the morphological expansion of the Acanthomorpha

Answer 25

• spiny finned teleost
• low morphological diversity before the Cretaceous-Palaeogene boundary
• extinction of non-Acanthmorpha competitors
• expansion of morphospace into vacated niches

Question 26

What can encourage adaptive radiation?

Answer 26

evolve some trait (e.g. phenotypic or morphological change) that allows for new opportunities to be exploited

Question 27

What are the take home messages?

Answer 27

• selection & adaption shape macro evolution patterns
• lack of transitional forms and gaps in the fossil record are entirely consistent with evolutionary theory and the rarity of fossilisation
• adaptive radiation theory suggests that much of the diversity of life is the result of adaptive divergence in the presence of ecological opportunity.