Lecture notes
LT12 Speciation
- Institution
- University College London (UCL)
Speciation - modes of speciation allopatric, sympatric - Heliconus butterflies example, extra reading with sources cited
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SpeciationSpeciation: Process by which new species are formed – occurs when interbreeding
(gene flow) has ceased between populations where it previously existed – new species
are reproductively isolated (though in some cases, this is the mechanism by which
speciation occurs)
What parameters to use – morphological, phylogenetic, biological
Can encounter problems
- Morphological convergence
- Cryptic variation (phenotypes are almost identical)
- Intra-specific variation (eg. domestic breeds of dogs, sexual dimorphism)
- Accounting for extant and extinct diversity (synapsid mammals, hominid skulls)
Morphological species concept: species are sets of organisms that look similar
Phylogenetic species concept: species are the smallest sets of organisms that should
that share a common ancestor
Biological species concept
Based on population genetics and gene flow
- Species are groups of actually or potentially interbreeding populations which are
reproductively isolated from other such groups (Mayr, 1942)
- Founded in population genetics
- Hybridisation/gene flow can occur in nature occasionally – how much is gene flow is
permitted/tolerated to maintain species distinction
- Review conservation notes
Advantages Disadvantages
Does not reply on morphology Does not apply to asexual species
Predicts unviable interbreeding Does not consider (temporary)
Preducts greater genetic distances geographic barriers to reproduction
between species Cannot be applied to the fossil record
Hybridisation/gene flow occurs in
nature – hat is the threshold level?
Can also be defined as distinguishable groups of genotypes that remain distinct in
the face of potential or actual hybridisation and gene flow
- Similar to Darwin’s usage of species to divide biodiversity by means of gaps or
troughs in phenotype/genotype distributions
- Reproductive isolation genotypic clusters in contact might remain distinct: this
becomes a method of achieving speciation and species maintenance rather than a
definition of species state itself
,- Speciation by polyploidisation – easier to describe this way: autopolyploids though
have the same initial gene frequencies as parents but have distinct heritable traits
such as chromosome number and ploidy at individual loci – such can be species
providing that euploids form clusters more abundant than intermediates formed by
hybridisation between them
- Such species have no guarantee of permanence – 2 genotypic clusters might be
stable for a long time, yet when ecological circumstances change, gene flow exceeds
some threshold – eventually resulting in single genotypic cluster that absorbs both
species Darwin’s finches and cichlid fish (“despeciation” can be a form of hybrid
speciation as a new species has resulted from fusion of 2 old species)
What is speciation?
Process which occurs many generations
Populations become independent of each
other over time
Gene flow is reduced over time or stopped
Populations diverge: accumulation of
genetic differences role for drift and
natural/sexual selection
Processes generates continuum from
subspecies/races to species
Mechanisms of reproductive isolation
Pre-zygotic (pre-fertilisation) mechanisms
a) Temporal incompatibility: species may have different activity patterns, nocturnal,
diurnal, different breeding seasons – coral spawning times differ by couple of hours
, b) Behavioural incompatibility: species have specific calls, rituals, postures that enable
them to recognise potential mates - flower morphology of monkey flowers differ
based on type of pollinators – pollinators dictate the gene flow, effectively isolating
different species (eg. certain flowers are signals for hummingbirds only)
c) Gamete immortality: sperm and egg from 2 species fail to unite, fertilisation will be
unsuccessful – abalone sperm proteins lysin digests vitelline membrane of eggs to
enable fertilisation, divergence in structure between species prevents interspecific
fertilisation
d) Structural incompatibility: successful mating requires compatible copulatory
apparatuses, appearance and chemical make-up (eg. Carabus)
Post-zygotic mechanisms
a) Genetic Incompatibility
1. Chromosome inversions and rearrangements (can evolve due to drift or selection)
- Drift will produce mutations in two different populations – neutral evolution will
then fix any of the mutations independently in the 2 populations
- Mice in Madeira (Britton-Davidian et al., 2000) – Mountains separate populations, 6
distinct chromosomal races caused by Robertsonian fusions – no hybrids found,
would be sterile
2. 2 locus model
- By drift a random mutation can become fixed, but only if it interacts with another
locus
- When you get a hybrid, it is unknown if the interaction between 2 loci will be
maintained
- Dobszhanksy-Muller incompatibilities – model explains how incompatibilities in
hybrids emerge without affecting parental species (must involve at least 2 loci)
3. Haldane’s rule
- When one sex of F1 hybrid between species is inviable or sterile, sex is usually the
heterogametic sex (mammals: XY, XX, but birds is the opposite ZZ, ZW)
- Recessive genetic incompatibilities on X or Z are uncovered in heterogametic sex
- Faster evolution of sex-chromosomes
b) Hybrid inviability: hybrid of 2 species viable but sterile, unable to breed
c) Hybrid Breakdown: first generation may be fertile but subsequent generations
infertile or non-viable
Allopatric Speciation
Populations become geographically separated, each being subjected to different
natural selection pressurs, finally establishing reproductive isolating mechanisms
1. Moving to new environments
- Parent population expands range and occupies new parts of environment
- Expansion of range and occupies new parts of environment
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