Week 5 Flashcards
what is the species concept?
Species consist of interbreeding populations that evolve independently of other populations
morphosepcies concept
Species defined by morphological differences and similarities
Cryptic species (look the same species but aren’t the same) make this hard
phylogenetic species concept
Species are identified by estimating the phylogeny of closely related populations and finding the smallest number of monophyletic groups
biological species concept
Reproductive isolation
Organisms that only mate together, do not often hybridise and produce fertile offspring
bacteria and archaea and problems with defining species
Best defined by their ecological context and what environment they can thrive in which is dependent on genes
speciation
begins when gene flow is disrupted and populations become genetically isolated.
physical isolation as a barrier to gene flow
Causes reproductive isolation
Allopatric speciation
Dispersal (part of a species moves to an island etc.) or vicariance (a divide like a mountain range appears)
polyploidy and chromosome changes as a barrier to gene flow
Changes in chromosome number isolate populations genetically
Speciation triggered by changed in chromosome number is especially important in plants
adaptation to different habitats as a mechanism of divergence
Natural selection can cause populations to diverge based on ecological differences
assortative mating as a mechanism of divergence
Assortative mating is when individuals choose mating partners according to traits and this can cause reproductive isolation
what is hybridisation?
when recently diverged populations interbreed
Prezygotic and postzygiotic isolation
Polyploid hybrid speciation
Homoploid hybrid speciation
Hybridisation can have a variety of outcomes depending on the fitness of the hybrids relative to parental fitness. Can cause:
Species of hybrid individuals
Formation of a stable hybrid zone
Reinforcement
what is reinforcement (related to hybridisation)
occurs when hybrid offspring have low fitness and natural selection leads to assortative mating and the prezygotic isolation of populations.
what drives diversification?
Ecological controls
Range size
Intrinsic properties of organisms
allopatric speciation by dispersal: drosophila
Distinct species on each volcanic island
These islands were never connected
This has happened by allopatric speciation driven by dispersal
Knowledge of geology is really important
allopatric speciation by vicariance: Panamanian snapping shrimp
Isthmus - land with water on both sides
Multiple species in Caribbean Sea and multiple in Gulf of Panama
Isthmus used to be submerged but water levels went down when ice age formed lots of ice at poles about 3mya
Closest relative is on the other side of the barrier (Jordon’s rule) - pair-wise speciation event
Deep water species diverge earlier than shallow water species - shallow water species have had less time in allopatry - could still cross land bridge for longer than deep water species
sympatric speciation
Don’t have a physical barrier - how do these species still diverge?
Strong reproductive isolation needs to occur
Need to convincingly show that the two species have always been together with an overlap
Got to be dealing with sibling taxa, cannot be an allopatric phase
A group starts to specialise on something - maybe an area with different nutritional value
Assortative mating - female prefers mates in same area of nutritional value as her - disruptive selection which should lead to divergence
lord Howe island sympatric speciation
Split between two Howea palms happened after island formed
Island is too far from other islands for allopatric speciation to occur
Peak flowering time differs between palms (temporal separation) - reinforcement by assortative mating
One species began to specialise on a type of soil (different nutrition) and this caused difference in flowering time
genetic drift as a mechanism of speciation
Case study: ring species of salamander
Divided by a valley
Cross-valley pairs are separated allopatrically
Two of the subspecies have encountered again but are still genetically diverse
Collected salamanders from each contact zone between species - what percentage seemed to be hybrids? Varied between pairs - 11%-89%
Is level of hybridisation (level of genetic admixture) due to cological divergence by natural selection or random by drift?
No strong relationship between climatic and vegetation dissimilarity and degree of admixture
Strong negative correlation between levels of hybridisation and nDNA distance and this may have just built up over time because of the mutations accumulating in the separated species
natural selection as a method of speciation
Red Crossbills
Feed on pinecones
North America
Populations vary in bill morphology
Medium bill depths have highest survival
Optimal bill depth for feeding efficiency is about the same as survival
Feeding efficiency is correlated with survival (optimality model)
Looked at feeding efficiency and bill depth eating four other types of conifers
Fitness peaks and fitness valleys between all the kind of coniferous cones
Different populations have evolved to become specialised on particular conifer species
Hybrids are unlikely to survive as their bill sizes do not suit any type of cone so survival decreases
mechanisms of speciation: divergence
Reinforcement: natural selection increases reproductive isolation between populations ie. selection against hybrids
Prezygotic vs postzygotic
Green-eyed tree-frog
Northern and southern and isolated southern populations
Rainforest species - arid cycles force frogs into refuges
When it gets wetter again, they have secondary contact
What is selecting against hybridisation?
N male with S female —> all failed (postzygotic)
S male with N female —> survived
Differences in male calls from different areas may be prezygotic isolation
Female choice experiment: northern females were not very discriminate, southern females preferred males from the south
And isolated southern frogs never choose northern males (strong reinforcement of prezygotic selection)
mechanisms of speciation: hybridisation
If it is a fit hybrid, it is selected for
Common in plants (responsible for 70% of plant diversity) - because of polyploidy
Very uncommon in animals (3% of diversity)
monophyletic group
clade eg. All mammals and their common ancestor
paraphyletic group
has the common ancestor but not all of the descendants
polyphyletic group
includes some but not descendants and does not include the common ancestor
polytomy
section of phylogeny in which the relationships cannot be fully resolved to dichotomies
parsimony analysis
tree with fewest evolutionary changes
reversals
a trait is lost
what traits can we use?
Homologous traits, use lots of traits but now DNA is mainly used
what DNA can we use?
Neutral genetic markers (non-adaptive), not under strong selection, we know the mutation rate
