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