14 Macroevolution Flashcards
What is the difference between gradualism and the fossil record?
Gradualism
- Steady change over time
- Accumulation of small changes
Fossil record
- Lacks transitional forms
- New taxa and morphological innovations appear abruptly - followed by periods of relative stasis
- Takes samples from the gradual change
Why are apparent bursts of change observed?
Fossil record is incomplete and extremely unlikely to find transitional forms
What is required for fossilisation?
Death
Burial with organism intact - rare due to scavenging and decay
Sedimentation - rare in terrestrial environments
Survival of sedimentary rocks - geologic processes destroy sedimentary layers
So fossilisation is rare
What alternative model for evolutionary change did Gould and Eldredge suggest?
Punctuated equilibrium
- Proposed that speciation took place in small populations
- Genetic revolution 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
What would the punctuated equilibrium theory need to be able to cope with?
Periods of rapid divergence
e.g. Homo erectus to Homo sapiens brain
- For this speed of evolution selection need only eliminate 1 in 500 per generation
- Yes selection can explain rapid change
Periods of stasis
- Stable environment needed
- If selection pressures do not vary greatly over time, net change should be minimal
- Stabilising selection
- Yes stasis is fully compatible with selection
What is stasis?
Relatively no morphological change over millions of years
How do evolutionary rates change with net stasis and net change?
For net stasis there is not consistent directional change over geological time but the total amount of change is the same
What is divergent evolution?
Lineages split and separate
What is convergent evolution?
Lineages that are not closely related evolve similar adaptations because they live in similar environments
What are the Anolis ecomorphs?
Six ecomorphs
- Different habitats occupied by different species
- Morphologically and behaviourally distinct
- Species with the same microhabitat are morphologically similar
- Repeated evolution of ecomorphs on different islands - are phylogenetically clustered but phenotypically over dispersed (convergent)
What is adaptive radiation?
The evolution of ecological and phenotypic diversity within a rapidly multiplying lineage
- Requires differentiation of a single ancestor into multiple species and variation in morphological traits that allow exploitation of range of environments
What causes adaptive radiation?
Ecological opportunity
- Apparent absence of ecological (competitive) constraint
Novel environments
- Species diversify into new niches
Ecological release from antagonists
- End Cretaceous dinosaur extinction
- Vacant niches filled by mammals
Morphological expansion of the Acanthomorpha
- Spiny finned teleost fish
- Low morphological diversity before Cretaceous-Paleogene boundary
- Extinction of competitors
- Expansion of morphospace into vacated niches
