APS11006 Principles of Evolution- Thomas Lectures Flashcards
Microevolution
Changes within a species
Macroevolution
Patterns of evolutionary change above the species level
How does speciation occur?
- Biological species and reproductive isolation
- Low gene flow and disruptive selection
Definition of biological species
“A species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring- but do not produce viable, fertile offspring with members of other such groups”
Reproductive isolation
- Existence of biological factors (barriers) that impede members of two species from interbreeding and producing viable, fertile offspring
- Geographic, behavioural and temporal isolation (breeding times)
Which type of selection is expected to be associated with speciation?
- Disruptive selection
- Drives two or more peaks in fitness
- Pushes means or groups of populations apart, causing divergence in phenotypes
- When populations become so phenotypically different that they cannot interbreed, this is speciation
- Disruptive selection can be countered by gene flow
Gene flow
Exchange of genes between populations as a result of movement and interbreeding of individuals
How does gene flow counter disruptive selection?
- One island favours yellow, the other favours orange
- If individuals can move freely between islands and interbreed, mixed populations occur, not divergent populations
Allopatric speciation
- Speciation with geographic isolation
- Long term isolation can result in phenotypic divergence due to different selection pressures and random genetic drift
- If populations cannot interbreed, speciation has occurred
Organism’s dispersal ability
The extent to which an organism can overcome a barrier
Sympatric speciation
Splitting of an ancestral species into two or more reproductively isolated groups with geographic isolation
Causes of reproductive isolation
- Geographical isolation
- Behavioural isolation
- Temporal isolation
- Disruptive selection
Sympatric meaning
Occurring within same or overlapping geographical location
Rhagoletic polmonella- North American apple maggot fly
- Native plant is hawthorn, usually mates on or near
- Apple trees introduce, sympatric to hawthorn
- Habitat isolation is a pre-zygotic barrier
- Apples mature faster, so timing of feeding and therefore mating is another pre-zygotic barrier
Rate of evolution- gradualism
- Steady change over time, no jumps in characteristics
- Expected outcome from microevolution
- Accumulation of small stepwise changes over time, each generation producing a small change
Why are there gaps in the fossil record?
- Fossil record represents between 1 and 5% of species that have existed
- Fossilisation requires death (inevitable), burial with organism mostly intact (rare due to scavenging and decay), sedimentation (rare in terrestrial environments) and survival of sedimentary rocks (rare as geological processes repeatedly destroy sedimentary layers)
Rate of evolution- punctuated equilibrium
- Proposed by Stephen J Gould and Niles Eldredge
- Rates of evolutionary change during and between speciation events are different because different processes are at work
- Not fully accepted due to vague mechanisms
Can selection explain stasis?
- Horseshoe crabs and Nautilus have remained morphologically unchanged for tens of millions of years
- How have they managed this given that selection happens all the time?
- Stabilising selection
Patterns of evolution
- Divergent
- Convergent
Convergent evolution
- Lineages that are not closely related evolve similar adaptations because they live in similar environments
- E.g., aardvarks, anteaters, pangolins (share diet), morphology has converged, forming long snouts good for digging into termite mounds
Anolis lizard
- 200 mainland species, 150 Caribbean
- Classed into six ‘ecomorphs’: crown-giant, twig, grass-bush, trunk-crown, trunk, trunk-ground
- Morphologically and behaviourally distinct based on their microhabitat
What causes adaptive radiation?
Ecological opportunity
Example of adaptive radiation
Rapid diversification of mammals after mass extinction of dinosaurs
Morphological expansion of the Acanthomorpha
- Spiny finned teleost fish
- Low morphological diversity before Cretaceous-Palaeogene boundary
- After extinction of competitors, morphospace expanded into vacated niches
Key innovation
Evolution of some particular trait that opens up opportunity to exploit different niches e.g. toe pads in Anolis facilitate exploitation of arboreal niches
Types of extinction
- Contemporary e.g. local and species
- Geological e.g. background, mass
How many species are threatened with extinction?
- 1 million
- Average of 25% of species in any taxonomic group
Local extinction
- Loss of population from a particular area
- E.g. wolves from Britain
Barn owl trust
Protect species that are threatened in UK but safe on global scale
Species extinction example
- Passenger pigeons, used to be billions (flock of them crossed US prairies for 14 hours)
- Were hunted to extinction very rapidly
- Martha, the last surviving pigeon, died in Cincinnati Zoo in 1914
What % of all species that have existed have gone extinct
99
Extinction in fossil record
- Background, sum of all normal species terminations during a defined time
- Extinction events, times when many species go extinct for a shared reason, typically regional
- Mass extinctions, times of geologically rapid global disappearance of much of life
Why is it difficult to measure extinction rates using fossil record?
- Incomplete
- Classification of species from fossil record is challenge
- Sometimes higher taxa are used
Raup and Seposki, 1992
- Wrote series of papers on marine fossil record
- Explored patterns spanning 600million years
- Measured s number of families gone extinct per year
- Good estimate for background extinction rates
End-Ordovician mass extinction
- 444 mya
- Declines in nautiloids, trilobites, brachiopods, crinoids, bryzoans, corals
- None completely wiped out
- Caused by glaciation
- Around 700 genera lost
Late Devonian mass extinction
- 372 mya
- Declines in trilobites, brachiopods, bivalves, corals, nautiloids, sponges, crinoids, and various fish
- Complete loss of ostracoderms and placoderms
- Caused by LIP shifts associated with volcanic activity, and ocean anoxia
End-Permian mass extinction
- 252 mya
- Largest event of all
- Declines in brachiopods, crinoids, synapsids
- Loss of blastoids, trilobites, rugose and tabulate corals and pareiasaurs
- Caused by LIP and ocean anoxia
End-Triassic mass extinction
- 201 mya
- Declines in bivalves, ammonoids, gastropods
- Loss of conodonts and basal archosaurs
- Caused by LIP and ocean anoxia
End-Cretaceous mass extinction
- 66mya
- Declines in bivalves, gastropods, foraminifera
- Loss of dinosaurs, pterosaurs, plesiosaurs, mosasaurs, ammonites, belemites
- Caused by meteor impact in Mexico and LIP
Example of a species becoming cosmopolitan
- Lystrosaurus after end-Permian mass extinction now had almost no predators and barely any herbivore competition
- Was at one time one of the most successful vertebrates on Earth
How can measuring past extinction events, including mass extinctions, help us
to understand the current biodiversity crisis?
Characteristics of survivors of mass extinctions may indicate trajectories of recovery if the current extinction crisis proceeds unchecked.
Significance of mass extinctions for modern conservation concerns
- If a mass extinction removes certain species from ecosystems, the scaffold ecosystem may be available after crisis for new species to occupy
- But, if most components of ecosystem are removed by larger event, recovery may involve the construction of new ecosystems
- Those that survive have big impact on future
Species senescence
- Now disproved theory
- Suggests that species undergo lifecycles like individuals do
- Birth, development, reproduction, death
- Orthogenesis, change in organisms was not due to natural selection, but due to unchecked directional trends within a lineage
‘Example’ of senescence
- Irish Elk
- Evolved on an irreversible trajectory towards larger and larger antlers
- Irish Elk went extinct when antlers became too large
- Could no longer hold up heads and got tangled in trees
- Inevitably went towards larger and larger antlers?
More likely explanation for extinction of Irish Elk
- Sexual selection favoured large antlers and large body size
- Natural selection favoured smaller size in response to rapid change towards warmer, more temperate climates at the end of the last ice age
Competition and the Red Queen
- Competition theory of extinction named after Red Queen from Through the Looking-Glass
- “Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast”
Environment and the Court Jester
- Environmental theory of extinction
- Random, unpredictable changes to physical environment such as climate change, tectonic events, or even bolide impacts (meteors)
Planktonic foraminifera
- Exceptionally well preserved fossil record
- Microorganisms that live in oceans, shells are deposited in sediment after death
- Change in morphology over time has been well documented
- Clues to extinction drivers
Broad sense species selection
- Aggregate traits e.g, body size
- Can take an average of all the individuals across the species to compare to other organisms
Strict sense species selection
- Emergent traits e.g., range size
- Traits that can only be measured across the whole species, no average can be taken as the geographic cover of a species is shared by all individuals of that species
