Evolutionary Biology Flashcards
Fitness
The ability of an individual to survive and reproduce relative to conspecifics
Adaptations
Traits that increase the fitness of an individual relative to individuals that lack the traits
Pleiotropic gene
A gene that influences multiple traits at once
Natural selection
A mechanism that can lead to adaptive evolution, whereby differences in the phenotypes of individuals cause some of them to survive and reproduce more effectively than others
Result of selection
Usually reduces genetic variability
What is microevolution?
The change in allele frequencies that occurs over time within a population, due to mutation, selection, gene flow and genetic drift
What is gene flow?
The transfer of genetic variation from one population to another
Macro evolution
Major evolutionary change, especially with regard to the evolution of whole taxonomic groups over long periods of time
What are the 3 definitions of species?
- The biological species concept
- The phylogenetic species concept
- The morphospecies concept
Biological species concept
Involves reproductive isolation: if individuals from two populations cannot produce offspring or their offspring are fertile, they are reproductively isolated and can be considered as good species
Shortcomings of biological species concept
- Many populations cannot be tested for reproductive isolation as they are geographically separated
- Irrelevant to asexual taxa
- Difficult to apply to many plants where populations are clearly divergent but hybridisation occurs routinely
Phylogenetic species concept
Based on the principle of monophyly. Monophyletic groups contain all the known descendants of a single common ancestor. Species are defined as the smallest diagnosable monophyletic group. Assumes that these y
units have been isolated for sufficiently long enough that each possesses diagnostic traits
Shortcomings of phylogenetic species concept
- Phylogenies are only available for a limited number of species and different characters produce different phylogenies
- Different species may be diagnosed if they contain small genetic differences, yet these differences may not affect whether the taxa interbreed
- Subjective
The morphospecies concept
Species defined on the basis of consistent morphological differences
Shortcomings of the morphological species concept
- Some species show polymorphic morphology
- Some groups can be very small and have few measurable morphological features
- Difficult to apply to cryptic species (species which are morphologically identical but cannot interbreed so are considered different species)
What is the best way to define a species
Combine usage of biological species concept and phylogenetic species concept
What is DNA barcoding?
A certain fragment of the mitochondrial genome CO1 is used as the DNA barcode. It varies a lot between species and very little between individuals of a species
Sympatric speciation
Evolution of new species from two groups of individuals inhabiting the same geographic region
What must interactions be for coevolution to occur?
Reciprocal
What species of bat produces social calls at 45kHz?
Pipistrellus pipistrellus
What species of bat produces social calls at 55kHz?
Pipistrellus pygmaeus
What is the name for the type of habitat running alongside a stream or other moving body of water?
Riparian habitat
What is taxonomy?
The scientific classification of organisms according to resemblances and differences
What is nomenclature?
The method of naming species of animals and plants scientifically
What is the scientific term for the Latin name of a species?
Linnean binomial name
What must there be between populations for speciation to occur?
Reproductive isolation
What are the two barriers to gene flow?
Prezygotic (before the formation of the zygote)
Postzygotic
What are the 4 types of prezygotic barriers?
- Temporal isolation or habitat isolation (potential mates do not meet)
- Behavioural / sexual isolation (no mating occurs)
- Mechanical isolation (copulation occurs but no transfer of male gametes takes place)
- Gametic incompatibility (gamete transfer occurs, but egg is not fertilised)
What are the 4 postzygotic barriers?
- Zygotic mortality soon after fertilisation
- Hybrid inviability (F1 hybrid has reduced viability)
- Hybrid sterility (F1 hybrid has viability but reduced fertility)
- F2 breakdown (reduced viability or fertility in F2)
What are the three states of speciation?
- Population isolation
- Genetic and ecological divergence
- Reproductive isolation during secondary contact
What is character displacement?
When characters continue to diverge during secondary contact (reinforcement)
What is assortative mating?
Individual with similar phenotypes mate with one another more frequently than would be expected under a random mating pattern
What is genetic drift?
Changes in allele frequencies within a population due to chance variation in the survival and/or reproductive success of individuals
What effect does sexual selection have on speciation rate?
Increases speciation rate
What does molecular clock evidence suggest is the time required for reproductive isolation?
3 million years
What characteristics of a habitat favour an increased speciation rate?
Prevent gene flow Low dispersal rates Strong sexual selection High availability of vacant niches Genetic bottlenecks
What is unique about many cichlid species?
Unique jaw morphology
What are two pieces of evidence of allopatric speciation?
- Geographical distribution patterns
2. Correspondence with present or past barriers
Two types of allopatric speciation
- Vicariant speciation
2. Peripatric speciation
What is vicariant speciation?
Two widespread populations divided by emergence of an extrinsic barrier e.g. snapping shrimps
What is peripatric speciation and what is it also known as?
A ‘colony’ diverges from a widespread ‘parent’ and acquires reproductive isolation. Also known as ‘Founder Effect speciation’
What is an example of peripatric speciation?
Hawaiian Drosophila
Many species endemic to single islands
Studied by James Bonacum who used sequence differences in DNA to estimate the phylogeny of closely related species
What were the predictions of Bonacum when studying Drosophila species in Hawaii?
- Closely related species should occur on adjacent islands
- Phylogeny should correspond with island history, with the most recently diverged species appearing on the most recently formed islands
What is parallel speciation?
Repeated independent evolution of the same reproductive isolating mechanism
What do models of sympatric speciation often involve?
- Disruptive selection
- Assortative mating
- Fitness benefits to extreme forms in new niches
Case study 1 of sympatric speciation: Periwinkles
- Littorina saxatilis
- A number of different morphs found across Europe
- Low vagility (dispersal) over lifetime but overlap between morphs
- Rare intermediate forms present in transition zones
What is a morph?
A group of a single species with unique characteristics genetically adapted to a particular set of environmental conditions
Two main periwinkle morphs
- Thin-shelled, wide aperture: found on vertical surfaces, in splash zone, very wave resistant
- Thick-shelled, narrow aperture: found on boulders mid-shore, crab resistant
What type of speciation would periwinkles have been speciated under?
Parapatric speciation - population’s separated by an extreme change in habitat
Case study 2 of sympatric speciation: Lord Howe Island palms
- Two types of palms
- One type survives better in calcareous soils
- The other survives better in volcanic acidic soils
- Disruptive selection due to soil type
- This is followed by assortative mating: in the early flowering season only plants growing in volcanic acidic soils will be in flower and able to mate
When does sympatric speciation occur most easily?
When traits under disruptive selection and assortative mating are correlated genetically
Case study 2 of sympatric speciation: True fruit flies
- Rhagoletic pomonella
- Apple and hawthorn races of maggot flies
- Host-specific
- Indistinguishable, but reproductive isolation seems to be occurring
- Mating times on apple and hawthorn differ by about 3 weeks, and adults that mate on the two plants emerge at that time
- Genetic differences occur in larvae and adults
- Gene flow between races <10%
What are the two ways sympatric speciation could occur in true fruit flies?
- A genetic preference for host plant species arises in both sexes, and this is also correlated with assortative mating
- Moving to a new host plant with different timing of life cycle could drive ecological isolation ecological isolation and promote speciation
Why is allopatric speciation unlikely to be driving the speciation in cichlid fish in Cameroon and Nicaragua?
- The lakes are small
- Shores are uniform and free of barriers
- Lakes are conical and last changes in water levels are unlikely to have isolated populations
- Crater rims restrict gene flow from the outside of the lakes
- Recent evidence suggests hybridisation may have contributed to diversification
What genes are involved in divergent selection in cichlids and what do they facilitate?
rho, rdh5 and rp1l1b
Facilitate the adaptation of scotopic (twilight) vision to the darker conditions experienced by the benthic ecomorph
The absorption spectrum of the H5 allele of rho is shifted towards the blue wavelengths
Specialised modes of speciation
- Single gene mutations
- Hybridisation (special relevance to plants)
- Polyploid speciation (special relevance to plants)
Example of single gene mutation causing reproductive isolation
Euhadra (land snail)
Single gene controls left or right-hand coiling
Left and right handed snails cannot mate
Instant reproductive isolation
Example of hybridisation causing speciation
Gilia malior and G. modocensis
Inbreed by selfing
Hybrid intermediate in length of lateral branches and stem length and intersterile with parents
If new forms achieve higher fitness in new niches than parental species, speciation could occur
What are the three possible outcomes of interbreeding by hybridisation?
- Reinforcement / genetic incompatibility: reduces frequency of hybrids through process of assortative mating (as hybrid offspring are less viable)
- Selection favours hybrids in novel habitats
- Selection favours hybrids in transitional zones (e.g. periwinkles)
What is polyploid speciation and what is so special about it?
Polyploid organisms have more than 2 sets of chromosomes
Speciation by polyploidy can occur instantaneously by a single genetic event. Caused by a failure in the reduction of chromosome numbers during meiosis
It is the only universally accepted form of sympatric speciation
Example of polyploidy speciation
Chrysanthemum species
Initial n = 9
Species with up to n = 90
Allopolyploidy
Chromosomes donated by two parental species
Autopolyploidy
Chromosomes acquired from a single species
Coevolution
A process of reciprocal evolutionary change in interacting species (Thompson 1994)
Three types of reciprocal interactions between organisms?
- Mutualism (both benefit)
- Parasite/predation (prey suffers)
- Competition (both organisms suffer)
Models of coevolution
- Coevolution escalates indefinitely (evolutionary arms race)
- A stable genetic equilibrium
- Continuous cycles in genetic composition of both species
- Extinction of one or both species
Evolutionary arms race example: bats and moths
- Bats hunt insects with the aid of ultrasound
- Numerous insect taxa have independently evolved ears to detect these ultrasonic calls
- These ears are simple ear drums known as tympana and can be found in various body places depending on taxa
- If moths detect echolocation call but it is weak, the moth will change its flight pattern to avoid detection
- If the echolocation call is at close range, the bat will have already detected the moth, so it performs a POWERDIVE or powerspiral
- Moths can also send back their own acoustic signals which potentially interfere with echolocation or startle the bats
- Calls are APOSEMATIC
- Bats may now use quieter calls to avoid the moth hearing or lower sensitivity than the range of the moth’s ears (allotonic frequency hypothesis)
Parasitism coevolution example: Cuckoo brood parasitism
- 59/141 species of cuckoo are parasitic
- Lay eggs in the nest of other bird species
- The parasite nestling may eject host’s eggs or nestlings, kill them or outcompete them
- Some host species do discriminate against parasite eggs or eject them
- Cuckoo population’s comprise gentes each of which lays eggs that resembles those of their preferred host
- Pied wagtails and meadow pipits are parasitised in the UK, but not in Iceland. Icelandic population’s of these birds accept cuckoo eggs whilst British species reject them, suggesting a specific counter adaptation against cuckoos
What is diffuse coevolution?
Relates to groups of prey that coevolve with groups of predators
Example of evolutionary arms race: plants and herbivores
- Passion flower plants produce chemicals to deter herbivores
- These are ineffective against Heliconius whose larvae eat the leaves
- Butterflies compete for host plants, and lay bright yellow eggs to deter egg laying by conspecifics
- Females avoid laying on plants that already bear eggs
- Several species of passion flowers have independently evolved buds, stipules or foliage nectar glands that mimic Heliconius eggs, deterring butterflies from laying
What is a symbiotic relationship?
A mutualism which is permanent and can span over lifetimes e.g. lichen, plant pollinators
What is an example of a plant-animal mutualism?
Pseudomyrmex ants on acacia trees
Trees produce Beltian bodies to feed the and ants and nectar at nectaries, and have thorns in which the ants live
Ants protect the trees from grazing by stinging herbivores
What does polytomy show?
You do not know the relationship between species branching from that position
What is a homologue?
The same organ under every variety of form and function - similarity due to common ancestry
What is an analogy?
A superficial or misleading similarity
Independently evolved to carry out the same function (e.g. bird wings and bat wings)
What are the two types of homologies?
- Plesiomorphy
2. Apomorphy
Apomorphic similarity
Novel similarity that evolved in the last common ancestor of a group of species. Evidence of intermediate common ancestry
Plesiomorphic similarity
Inherited from a more distant ancestor and is not evidence of intermediate common ancestry
Homoplasy
A character shared by a set of species but not present in their common ancestor
How to select a tree from real data
Maximum parsimony: algorithm which distinguishes homoplasies from homologies
What does parsimony favour?
The tree that maximises character congruence (best describes most characters in the dataset. Uses the fit of characters.
The fit of a character to a tree is the number of evolutionary changes necessary to describe the character across the considered species
When was the Earth formed?
4.54-4.44 billion years ago
How to estimate the age of lineages which do not have a fossil record (which is highly incomplete)?
The molecular clock
What is a molecular clock?
An abstraction of the observation that the number of mutations we can count when comparing two species is proportional to the time since they shared a common ancestor
Example of using molecular clock to date event
Change in oxygen levels in the Earth’s atmosphere 2.5 Ga known as the Great Oxidation Event caused by cyanobacteria which photosynthesised (this lineage emerged 3 Ga)
When is the oldest certain fossil evidence for life? What is it?
3.4 Ga
Prokaryotic cells from a shallow-water, oxygenated, brightly lit and rich in sulphur environment
Microbes produced stromatolites (thin microbial films trapped mud and over time these layers were built up into layered rock)
What study suggested the earliest form of life may have lived as far back as 4.28 Ga?
Microfossils discovered in hydrothermal vent precipitates in Quebec
What life was discovered from 3.4 Ga?
Microfossils of sulphur-metabolising cells in Western Australian rocks (Wacey et al.)
What are the three defining features of living organisms?
- They can replicate themselves. Replication needs energy
- They contain proteins to catalyse reactions
- They store and transmit information (genotype) and express it (phenotype)
- They evolve
Who was HIV “patient zero”?
Gaetan Dugas
Represented as the sole responsible for the HIV epidemic in the US
How did life evolve on Earth? Oparin-Haldane theory
- 3 stages
1. Produced amino acids and compounds like hydrogen cyanide and formaldehyde which underwent further reactions to produce nucleotides (pre-biotic soup)
2. The organic building blocks assemble into polymers. Could happen if the first polymers formed on the surfaces of clay crystals, which reduces their chance of being broken down by hydrolysis
3. The polymers start self-replication. Early self-replicators may have been entirely RNA which underwent translation. RNA can both store genetic information and act as a catalyst
Problem with stage 1 of Oparin-Haldane theory
We now think that the early atmosphere may have been dominated by CO2 and N2 rather than CH4 and NH3
In these neutral gases organic compound synthesis is much lower
What lives in geothermal vaults?
Autotrophic bacteria - obtain reduced carbon from CO2
Chemical oxidation of iron compounds may also provide energy
What are alkaline vents?
- Different from black smoker vents
- Occur in serpentine rock and are not superheated
- Chemicals similar to those in Miller experiments found here
- Cell sized pores
What is the name of the hypothesis of life originating from elsewhere and arriving on a meteorite?
Panspermia hypothesis
What evidence is there for the panspermia hypothesis?
Complex molecules found in the large meteor that formed in the Sudbury crater in Canada 1.85 Ga
Organic molecules found on meteorites and comets
Evolution of photosynthesis
2.3 Ga moderate levels of oxygen present in atmosphere
Organism evolved aerobic respiration (produces 15x more energy)
Archaea
Include thermophilic forms
Probably arose from sulphur-reducing ancestor
None is completely photosynthetic
Earliest eukaryotic fossil
Single-called algae
2.1 Ga
What are the two most important new adaptations which evolved in eukaryotes?
- Sexual reproduction
2. Evolution of organelles
Two disadvantages of sexual reproduction
- Recombination can destroy adaptive complexes of genes
2. Half the offspring are male, so the rate of increase for an asexual genotype is double that of a sexual one
Advantages of sexual reproduction
- May safeguard against the accumulation of deleterious mutations
- Creates more variation - individuals better able to exploit new niches (tangled bank hypothesis)
- Organisms may change their genomes faster to win races against ever-changing challenges (Red Queen hypothesis)
Allopolyploidy
Chromosomes donated by two parental species
Autopolyploidy
Chromosomes acquired from a single species
Coevolution
A process of reciprocal evolutionary change in interacting species (Thompson 1994)
Three types of reciprocal interactions between organisms?
- Mutualism (both benefit)
- Parasite/predation (prey suffers)
- Competition (both organisms suffer)
Models of coevolution
- Coevolution escalates indefinitely (evolutionary arms race)
- A stable genetic equilibrium
- Continuous cycles in genetic composition of both species
- Extinction of one or both species
Evolutionary arms race example: bats and moths
- Bats hunt insects with the aid of ultrasound
- Numerous insect taxa have independently evolved ears to detect these ultrasonic calls
- These ears are simple ear drums known as tympana and can be found in various body places depending on taxa
- If moths detect echolocation call but it is weak, the moth will change its flight pattern to avoid detection
- If the echolocation call is at close range, the bat will have already detected the moth, so it performs a POWERDIVE or powerspiral
- Moths can also send back their own acoustic signals which potentially interfere with echolocation or startle the bats
- Calls are APOSEMATIC
- Bats may now use quieter calls to avoid the moth hearing or lower sensitivity than the range of the moth’s ears (allotonic frequency hypothesis)
Parasitism coevolution example: Cuckoo brood parasitism
- 59/141 species of cuckoo are parasitic
- Lay eggs in the nest of other bird species
- The parasite nestling may eject host’s eggs or nestlings, kill them or outcompete them
- Some host species do discriminate against parasite eggs or eject them
- Cuckoo population’s comprise gentes each of which lays eggs that resembles those of their preferred host
- Pied wagtails and meadow pipits are parasitised in the UK, but not in Iceland. Icelandic population’s of these birds accept cuckoo eggs whilst British species reject them, suggesting a specific counter adaptation against cuckoos
What is diffuse coevolution?
Relates to groups of prey that coevolve with groups of predators
Example of evolutionary arms race: plants and herbivores
- Passion flower plants produce chemicals to deter herbivores
- These are ineffective against Heliconius whose larvae eat the leaves
- Butterflies compete for host plants, and lay bright yellow eggs to deter egg laying by conspecifics
- Females avoid laying on plants that already bear eggs
- Several species of passion flowers have independently evolved buds, stipules or foliage nectar glands that mimic Heliconius eggs, deterring butterflies from laying
What is a symbiotic relationship?
A mutualism which is permanent and can span over lifetimes e.g. lichen, plant pollinators
What is an example of a plant-animal mutualism?
Pseudomyrmex ants on acacia trees
Trees produce Beltian bodies to feed the and ants and nectar at nectaries, and have thorns in which the ants live
Ants protect the trees from grazing by stinging herbivores
What does polytomy show?
You do not know the relationship between species branching from that position
What is a homologue?
The same organ under every variety of form and function - similarity due to common ancestry
What is an analogy?
A superficial or misleading similarity
Independently evolved to carry out the same function (e.g. bird wings and bat wings)
What are the two types of homologies?
- Plesiomorphy
2. Apomorphy
Apomorphic similarity
Novel similarity that evolved in the last common ancestor of a group of species. Evidence of intermediate common ancestry
Plesiomorphic similarity
Inherited from a more distant ancestor and is not evidence of intermediate common ancestry
Homoplasy
A character shared by a set of species but not present in their common ancestor
How to select a tree from real data
Maximum parsimony: algorithm which distinguishes homoplasies from homologies
What does parsimony favour?
The tree that maximises character congruence (best describes most characters in the dataset. Uses the fit of characters.
The fit of a character to a tree is the number of evolutionary changes necessary to describe the character across the considered species
When was the Earth formed?
4.54-4.44 billion years ago
How to estimate the age of lineages which do not have a fossil record (which is highly incomplete)?
The molecular clock
What is a molecular clock?
An abstraction of the observation that the number of mutations we can count when comparing two species is proportional to the time since they shared a common ancestor
Example of using molecular clock to date event
Change in oxygen levels in the Earth’s atmosphere 2.5 Ga known as the Great Oxidation Event caused by cyanobacteria which photosynthesised (this lineage emerged 3 Ga)
When is the oldest certain fossil evidence for life? What is it?
3.4 Ga
Prokaryotic cells from a shallow-water, oxygenated, brightly lit and rich in sulphur environment
Microbes produced stromatolites (thin microbial films trapped mud and over time these layers were built up into layered rock)
What study suggested the earliest form of life may have lived as far back as 4.28 Ga?
Microfossils discovered in hydrothermal vent precipitates in Quebec
What life was discovered from 3.4 Ga?
Microfossils of sulphur-metabolising cells in Western Australian rocks (Wacey et al.)
What are the three defining features of living organisms?
- They can replicate themselves. Replication needs energy
- They contain proteins to catalyse reactions
- They store and transmit information (genotype) and express it (phenotype)
- They evolve
Who was HIV “patient zero”?
Gaetan Dugas
Represented as the sole responsible for the HIV epidemic in the US
How did life evolve on Earth? Oparin-Haldane theory
- 3 stages
1. Produced amino acids and compounds like hydrogen cyanide and formaldehyde which underwent further reactions to produce nucleotides (pre-biotic soup)
2. The organic building blocks assemble into polymers. Could happen if the first polymers formed on the surfaces of clay crystals, which reduces their chance of being broken down by hydrolysis
3. The polymers start self-replication. Early self-replicators may have been entirely RNA which underwent translation. RNA can both store genetic information and act as a catalyst
Problem with stage 1 of Oparin-Haldane theory
We now think that the early atmosphere may have been dominated by CO2 and N2 rather than CH4 and NH3
In these neutral gases organic compound synthesis is much lower
What lives in geothermal vaults?
Autotrophic bacteria - obtain reduced carbon from CO2
Chemical oxidation of iron compounds may also provide energy
What are alkaline vents?
- Different from black smoker vents
- Occur in serpentine rock and are not superheated
- Chemicals similar to those in Miller experiments found here
- Cell sized pores
What is the name of the hypothesis of life originating from elsewhere and arriving on a meteorite?
Panspermia hypothesis
What evidence is there for the panspermia hypothesis?
Complex molecules found in the large meteor that formed in the Sudbury crater in Canada 1.85 Ga
Organic molecules found on meteorites and comets
Evolution of photosynthesis
2.3 Ga moderate levels of oxygen present in atmosphere
Organism evolved aerobic respiration (produces 15x more energy)
Archaea
Include thermophilic forms
Probably arose from sulphur-reducing ancestor
None is completely photosynthetic
Earliest eukaryotic fossil
Single-called algae
2.1 Ga
What are the two most important new adaptations which evolved in eukaryotes?
- Sexual reproduction
2. Evolution of organelles
Two disadvantages of sexual reproduction
- Recombination can destroy adaptive complexes of genes
2. Half the offspring are male, so the rate of increase for an asexual genotype is double that of a sexual one
Advantages of sexual reproduction
- May safeguard against the accumulation of deleterious mutations
- Creates more variation - individuals better able to exploit new niches (tangled bank hypothesis)
- Organisms may change their genomes faster to win races against ever-changing challenges (Red Queen hypothesis)
What do organelles allow?
Compartmentalisation of processed and division of labour within cells
What bacteria did mitochondria originate from?
Proteobacteria
What bacteria did chloroplasts come from?
Cyanobacteria
When did the first multicellular algae evolve?
0.9 Ga
When did the first multicellular animals evolve?
565 Mya
When is the Phanerozoic Eon?
541 million years - present
The current geologic eon where abundant plant and animal life has existed
When was the Archaen Eon?
3.8-2.5 Bya
When was the Proterozoic Eon?
2.5 Bya - 542 Mya
The Cambrian explosion
Cambrian period started about 543 Mya
Major explosion of skeletonised marine forms at about 530 Mya
May have been very rapid (5-30 My)
What modern phyla and classes of marine life appeared around the Cambrian explosion?
Crustacea Annelids Mollusca Vertebrata brachiopods
Was the Cambrian explosion really explosive?
Molecular clock evidence suggests that diversification of many clades was much earlier (900 Mya)
Bilateral animals such as arthropods or flatworms may have existed before Ediacaran fauna
What caused the Cambrian explosion?
- Increase in the availability of oxygen
- Many marine habitats became filled because new modes of locomotion evolved
- Increase in predator diversity may have fuelled evolution of defenses
- Genetic factors that determine body form may have caused diversification
How might genetic factors have caused the Cambrian explosion?
- Homeotic/Hox genes regulate the transcription of other genes and the resultant proteins determine what happens during animal development by creating chemical gradients in cells
- Gene duplication events may have allowed new body forms to evolve through modifications in developmental processes
Proof of Hox gene arising during Cambrian explosion
- Extant onychophorans and arthropods analysed
- All 10 arthropods Hox genes in place in last common ancestor of arthropods and onychophorans
- Cambrian arthropods and lobopodians probably had full set of arthropod Hox genes
- There have been no new Hox genes but shifts in where Hox is expressed can affect numbers and kinds of appendages
What are the two models of macroevolution?
- Phyletic gradualism
- Punctuated equilibrium
Both occur in fossil record
Particular patterns may be associated with particular taxa
What is phyletic gradualism?
A model of evolution which theorised that most speciation is slow, uniform and gradual
What is punctuated equilibrium?
A model of evolution which suggests there are periods of rapid evolutionary change followed by long periods of stasis (involves peripatric speciation)
What are the two requirements when testing punctuated equilibrium?
- Need to know the phylogeny of the clade so to determine ancestral and descendent species
- Ancestral species must survive long enough to coexist with new species in the fossil record (this must occur through a splitting event CLADOGENESIS)
What is anagenesis?
Morphological change on the ancestral form without speciation
Example of gradual phyletic change
Marine Foraminifera
Protozoan with calcium carbonate shell that fossil uses readily
Study involved 8 My of morphological change and speciation in the Gulf of Mexico (beginning 66 Mya)
Four morphospecies occur sequentially
Canonical scores show gradual change over time
Example of punctuated equilibrium
- Morphological change and speciation in cheilostome bryozoa from the Caribbean
- 15 Mya to extant species
- Defined 19 living and fossil morphospecies
- Ancestral and descendent species coexist, suggesting speciation
What species are examples of ‘living fossils’?
- Ginkgo tree very similar to 40 My fossil
- Horseshoe crabs virtually unchanged for 150 My
Why does stasis occur in some species?
- Lack of genetic variability preventing the evolution of new forms (unlikely)
- Oscillating selection keeping average morphology in check (dynamic stasis). Also stabilising selection
What does the background rate of extinction mean?
The probability of a family or class going extinct is constant
What percentage of total extinctions does background extinction account for?
96%
When were the 5 mass extinctions?
- Terminal Ordovician (c440 Mya)
- Late Devonian (c365 Mya)
- End Permian (c250 Mya)
- End Triassic (c200 Mya)
- Cretaceous Palaeogene (K-P) (c65 Mya)
Terminal Ordovician extinction
c440 Mya
Affected benthic and planktonic marine faunas across all latitudes
Eliminated 85% marine species, 55% genera
Linked to glaciation at South Pole
Two phases of Terminal Ordovician extinction
1st mainly affected nektonic and planktonic species. Stagnant conditions and changes in sulphur dynamics affected deep-water benthic and nektonic organisms
2nd less selective. The transgression of anoxic water onto the continental shelves caused extinction in shallower habitats
Late Devonian extinction
c365 Mya
70% of all species went extinct
Took place in a series of pulses over 1 million years
Devonian reefs (largest in Earth’s history) greatly affected. Sponges and corals suffered, cool water species survived
Causes of Late Devonian extinction
Global oceanic anoxia from spread of deoxygenated deep water
Deep and cold water species might have been tolerant of low oxygen conditions
Global cooling may also have taken place
Appears to have been a period of asteroid/comet impact (6 of 10 known craters date to Late Devonian)
Causes controversial
End Permian extinction
- Closest that multicellular life came to extinction
- 50% of all families extinct
- 90-96% of all species extinct
- Possibly only lasted 60,000 years
- Extinction highest in species with narrow geographic ranges
- Prominent in marine environments
- Reefs disappeared for 7 million years
- Trilobites went extinct
End Permian extinction causes
- Several theories linked to the formation of Pangaea
- Associated outpourings of magma that flowed on Earth’s surface (created Siberian traps)
- These caused large scale changes to the amounts of heat, CO2 and SO2 in the atmosphere, and added rock to Earth’s crust (dated 251 My)
- Major drop in sea level (up to 40% decline in ocean-shelf habitats)
- Chemical composition of water changed with CO2 levels increasing
- Reduced circulation resulted in anoxia
- Terrestrial taxa affected by climate change - continental climate (hot summers and cold winters)
- Siberian traps cause global warming
- Trap-related temperature increase (5 degrees) caused release of methane from ice, increasing temperature by further 5 degrees - sufficient for wipeout
- Eruptions may have ignited massive coal deposits
End Triassic extinction
c200 Mya
23% terrestrial families and 18% marine families
Reef-building corals, bivalves, brachipods, ammonites hit hardest
Most mammal-like reptiles went extinct
Causes of End Triassic extinction
- Possibly the result of a meteor impact in Quebec
- Major sea level fall, anoxic ocean conditions
- Volcanic activity was high in the Central Atlantic Magmatic Province - increase CO2 and sulphur dioxide
Cretaceous-Palaeogene (K-P) extinction
c65 Mya
60-80% species went extinct
Pterosaurs and dinosaurs wiped out (except birds)
Plankton line suggests marine plankton became scarce
Forest communities replaced by ferns
Causes of Cretaceous Palaeogene extinction (K-P)
- Almost certainly an extra-terrestrial body
- High levels of element iridium found in rock deposits dated to 65 Mya around the world
- Shocked quartz and microtektites also suggest meteorite impact
- Discovery of 180km crater at Chicxulub in Mexico. Asteroid may have been 10km wide
- Asteroid dated to 65.06+-0.18 Mya
What would the impact of the meteorite have caused? (K-P extinction)
Acid rain Earthquakes and volcanic eruptions Enormous tidal wave Wildfires Cooling from dust clouds Darkness from ash in atmosphere
Long-term consequences of major extinction events
Extinctions seem to be followed by a period of evolutionary diversification
What were the trends that made organisms more likely to survive extinctions?
- No evidence of selective extinction according to body size
- Bivalve species with wide geographic ranges most likely to survive
- Many species went extinct by chance
How many eukaryotic species have been named so far but how high is the actual estimate?
1.2 million species named
Could be up to 8.75 million
What is the Anthropocene?
A proposed epoch dating from the commencement of significant human impact on the Earth’s geology and ecosystems
Could the next mass extinction be happening now
What is the current rate of extinction loss?
100-1000x the background extinction rate
How fast do the continental plates move?
2cm per year
When had Pangaea formed by?
End of Permian period (c250 Mya)
Remained intact into early Triassic period
When did Pangaea begin to break up? What was then formed?
Late Jurassic
c150 Mya
2 main land masses:
Laurasia and Gondwana
What is the legacy of Gondwana?
Fossils of triassic reptiles found in continents today that were joined in Gondwana
Flightless ratite birds found in all southern continents share common ancestor in Gondwana
When did South America separate from Africa and Australia begin to separate from Africa?
Late Cretaceous
c70 Mya
When was the Eocene epoch and what was the distribution of the continental plates?
c50 Mya Bering land bridge between Eurasia and North America North and South America still unlinked India remains isolated Radiation of major mammal clades
What is Wallace’s line?
- Fauna of New Guinea is more similar to that of Australia than it is to that of Borneo
- Continental shelves separated by deep ocean trench
- Lower sea levels joined the land masses on either side of the trench previously
What are 3 different species distributions?
- Endemic: limited to a particular region
- Cosmopolitan: when a species can be found on all continents (including Antarctica)
- Disjunct: species that are not confined to a single area but are distributed in more than one region with a gap between them
What are the 6 major biogeographic (faunal) regions?
Nearctic Neotropical Ethiopian Palearctic Oriental Australian
Feature of the Quaternary period
Began 2.5 Mya
There have been cooler periods (glacial periods) interspersed with warmer interglacial periods
There is periodicity to glacial cycles
What could glaciation be caused by?
The Milankovich cycles sometimes overlap, which could contribute to glaciation
Effects of glaciations on distribution patterns
As weather becomes cooler, animals migrate away from poles and plant ranges contract
Whole communities can change
When was the last ice age?
Glacial maxima
20,000 ya
European refuge points in Italy, Iberia, Balkans
Glacial refugia and speciation
Tropical forests became fragmented during ice ages
Is their high species diversity partly the legacy of their former isolation favouring evolution of different species during fragmentation?
Species diversity does seem to be highest in modern vestiges of refuges
Late quaternary megafauna extinctions
Species heavier than 44 kg were especially prone to extinction during last interglacial (100,000-10,000 years ago)
Effects strongest in North America, South America and Australia
Hypotheses of megafauna extinctions in late Quaternary period
- Climate change, resulting in loss of open, steppe-like habitats - similar earlier climate change during interglacials in previous 900,000 years did not have associated problems
- Overhunting by humans - extinctions in Australia and N Eurasia occurred several tens of thousands of years before humans arrived
What is the kin selection theory?
Predicts that parents should invest considerable time an energy in offspring because offspring continue the existence and spread of parental genes
