How do new species form?
• Genetic difference will accumulate due to genetic drift or because selection favours different alleles in different populations
• For a species to form:
- prevent/reduce gene flow
- populations no longer interbreed
- process generates continuum where subspecies/races eventually become new species
Outline the Biological Species Concept BSC
Defines species in terms of interbreeding success
-> There is gene flow within species and restricted/no gene flow or hybridisation between species due to isolating mechanisms
What are the issues with the BSC?
- Applies to species that reproduce sexually (bacteria/archaea exchange genes sporadically)
- Definition omits geographic isolation as this may be temporary but reproductive isolation due to genetic differences is usually irreversible
- Doesn’t apply to fossil record- molecular taxonomists use Phylogenetic species concept PSC
- In nature hybridisation/introgression occurs- contemporary sexually reproducing organisms (10% bird and butterfly species, 6% mammals) can fully hybridise
- Introgression (movement of some genes across species) is potentially common
Outline the PSC
Species are recognisable geographic forms with a unique evolutionary history. Species are tips of the phylogenetic tree
Outline the types of reproductive isolation
Pre-zygotic (pre-mating)
- Ecological/habitat
- Temporal/seasonal
- Behavioural
Pre-zygotic (post-mating)
- Mechanical
- Gametic incompatibility
Post-zygotic
- Hybrid inviability
- Hybrid sterility
- Sexual selection against hybrids
Outline Pre-zygotic pre-mating isolation
• Ecological/seasonal- mates don’t meet
- e.g. coral- temporally driven isolation- gamete release separated by 1.5 to 3 hours (all released in <30 mins). Low density from first means unlikely to cross fertilise
• Behavioural isolation- don’t attempt mating
- Divergence In signals- mating calls/monkey flowers pollinated by bees or hummingbirds- adapted to specific pollinator
Outline Pre-zygotic post-mating isolation
• Mechanical
- Carabus beetles in Japan- copulatory organ differences- adeagus breaks off and damages reproductive organs
• Gametic incompatibility
- Externally fertilising marine invertebrates- cell surface proteins dictate whether sperm penetrates egg
- Abalone- sperm protein lysin dissolves vitelline membrane of conspecific eggs
- species differ in aa sequence of lysin proteins
Outline post-zygotic isolation
• Hybrid inviability
- lower survival rates
- Heliconius butterflies- predators don’t recognise H.melpomene and H.cydno pattern as warning colouration
• Hybrid sterility
- structural differences between chromosomes or conflict in gene action of parental genomes
• Assortative mating
- conspecifics favoured I’m mate choice
- fruit flies- lineages fed on different foods preferred to mate with their own lineage
How can speciation happen quickly? Give an example
Allopolyploidy- chromosomes from different parent species
- e.g. salsify flowers after accidental introduction to North America from Europe- Hybrid daughter species can’t reproduce with parent species
Outline allopatric speciation with examples
- Geographically divided populations may come back into contact
- Depending on how strong reproductive barriers have become, species may not interbreed, or hybrids may have minimal reproductive success
E.g. Isthmus of Panama 3mya separated Alpheus snapping shrimp
- No longer interbreed
- Geminate species pairs- many species more closely related to species on other side of Panama than on their own side
E.g. Darwin’s Galapagos Finches
- Ancestor species colonised one island 3mya and dispersed and colonised
- beak shapes changed according to food- feeding specialisation
- Over time females lose attraction to Male song with different beak shape (affects song)-> reduces gene flow
Outline sympathetic speciation with examples
- Genetic divergence of species in same area
- Need to invoke action of strong selection to bring about differences in ecology/mating preference- selection required to maintain reproductive barrier
E.g. apple maggot fly- adults mate on host plant and larvae develop in ripe fruits. Ancestral native is hawthorn
- allele frequencies at several loci differ significantly between apple and hawthorn derived flies- limited gene exchange ~6%
- difference in host preference and difference of ~3 weeks between mating activity
- sympatric evolution- almost speciation, with high degree of reproductive divergence
E.g. speciation in cichlids (lake-dwelling)- depth causes gradients in light, temp, O2-> affect resources, predators, parasites etc.
- gradients allow sorting of adaptive variation- can drive partial reproductive isolation
