LT12 Speciation

Speciation

Speciation: 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