3: Macroevolution

Question 1

Define gradualism

Answer 1

Steady change over time
Expected outcome from microevolution
Doesn’t mean smooth change, rather the accumulation of small stepwise changes over time

Question 2

What type of evolution results in gradualism?

Answer 2

Microevolution

Question 3

What is the ‘problem’ with gradualism?

Answer 3

Fossil record generally lacks transitional forms
New taxa appear abruptly in the fossil record
Major morphological innovations sometimes appear suddenly in the fossil record, often preceded and followed by periods of relative stasis

Question 4

Give some reasons why there are gaps in the fossil record

Answer 4

Death → inevitable
Burial with organism (mostly intact) → rare due to scavenging and decay
Sedimentation → rare in terrestrial environs
Survival of sedimentary rocks → rare, geologic processes repeatedly destroy sedimentary layers
= Fossilisation is rare
= Rarer still that transitional forms of species will be fossilised

Question 5

Describe the Punctuated equilibrium hypothesis

Answer 5

→ Rates of evolutionary change during and between speciation were different because different processes were at work
- Proposed that speciation took place in small pop.s
- And that a genetic evolution took place due to process other than natural selection
- Implies that evolution is fast during speciation and slow between speciation events
= widely discussed but not fully accepted

Question 6

Give an example of rapid evolutionary change

Answer 6

Homo erectus, 920ml brain → Homo sapiens, 1400ml brain
Took 20,000 years
For this speed of evolution, selection need only eliminate 1 in 500 individuals per gen (i.e. selection only needs to remove that smallest brains)

Question 7

Give an example of evolutionary stasis

Answer 7

Horseshoe crab
Stasis is fully compatible with selection
If selection pressures do not vary greatly over time, then net change should be minimal

Question 8

Define Convergent evolution

Answer 8

lineages that aren’t closely related by evolve similar adaptations because they live in similar environs

Question 9

Describe what it means that rates of macroevolution vary among species

Answer 9

What if stasis between bursts isn’t the absence of change, but the absence of consistent directional change?
Recent studies show rates of evo change are just as high for fossil time series with net stasis as those with net change

Question 10

Describe convergent evolution in the Anolis Lizards

Answer 10

→ 6 ‘ecomorphs’
Different microhabitats occupied by different species
Morphologically & behaviourally distinct
Are these a result of relatedness? Or convergent evolution
Phylogenetic tree, shows that each ecomorph represented on each island
= Not to do with historical contingency, more to do with environment → convergent evo

Question 11

Define adaptive radiation

Answer 11

“Adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage”

Question 12

What does adaptive radiation require?

Answer 12

Differentiation of a single ancestor into multiple species
Variation in morphological traits that allow exploitation of range of environments

Question 13

What causes adaptive evolution?

Answer 13

Ecological opportunity: many examples of diversification in the apparent absence of ecological (competitive) restraint

e.g novel environs, release from predators