Ecology & evolution - divergence and speciation

Why are the lines between species ‘fuzzy’

Creating species is creating discrete entities from a continuous process.

Entities can be even less distinct in groups that have both sexual and asexual reproduction.

Why is it important to have defined species groups

They are fundamental units of biodiversity.

Needed in the study of evolution - measuring completion of speciation process.

Needed in ecology - looking at species interactions / community structures, defining things as inter- vs intra-specific

Needed in conservation - management and environmental policy need clear defined groups, ability to define a species as endangered, those at sub-species status may be ignored for funding and initiatives

What are the different species concepts

Phenetic

Biological

Ecological

Phylogenetic

What is the phenetic species concept (example of when it was used)

Set of organisms phenotypically similar, and different from other sets.

Useful when looking at fossils that have no genetic information.

Was used to group the Conodonts (extinct jawless marine vertebrates) into species. But initially done with small sample size, and when more data was added showed that they weren’t actually differentiated into distinct groups.

Problems with the phenetic species concept (with examples)

There’s polymorphism leading to discontinuous variation in every species → does having different traits split organisms into different species. E.g. the black-bellied seed crackers which had diff. beaks so could be classified as diff. species, but showed no sexual selection to do with beak size.

Plasticity - one species can have multiple forms

Cryptic species - multiple species with ‘one’ form, look very similar, but could be genetically different. E.g. one moth species, with genetic sequencing, was found to be three

Pros of phenetic species concept

Practical - easy to observe and measure

Applicable to all taxa

What is the biological species concept

Based on reproduction. Groups of actually/potentially interbreeding populations, reproductively isolated from other groups, producing fertile offspring.

What are the mechanisms of isolation in the biological species concept (split into two categories)

Pre-mating: temporal, ecological, behavioural, mechanical

Post-mating: zygote mortality, hybrid inviability and sterility, sterile offspring

Problems with the biological species concept

Difficult to apply in practice

Breeding experiments not feasible - e.g. hard to determine hybrid sterility

Cannot be easily applied to asexually producing organisms

Some ranges do not overlap (allopatric) and do not breed, therefore cannot test sterility/inviability etc.

Disproved by existence of theoretical ‘ring species’ where neighbouring populations all interbreed and exchange genes, but the groups on the two opposite ends cannot. By biological species definition they are all the same species, but the two groups are also separate species → dilemma.

Pros of the biological species concept

Mechanism-based concept

Key concept for sexual organisms

What is the ecological species concept

Lineage/set of organisms that occupies a niche/adaptive zone minimally different than that of any other lineage in its range

Problems with ecological species concept

Needs a framework to define what the adaptive zone / niche is (used mainly for prokaryotes which can be grouped into ecological clouds)

Assumes you can find discrete niches with gaps between them in nature

Assumes organisms are independent of their environment - which is known to be untrue, like with niche construction

Pros of the ecological species concept

Applies to sexual and asexual forms

Allows for some gene flow among groups if they maintain ecological differences

Abiotic ‘adaptive zones’ relatively easy to measure

What is the phylogenetic species concept

Distinct group that shares a common ancestor, forms a single branch on the tree of life

Monophyly - consisting of an ancestor and all of its descendants

Problems with phylogenetic species concept

Needs a highly resolved phylogenetic tree

Difficult to know where to draw the line

Pros of phylogenetic species concept

Practical guide to delimitation (creating boundaries)

Based on evolutionary concept

Could apply to sexual organisms and fossil species

General correct approach when defining a species

Unified Species Concept - looking at multiple lines of evidence, and looking at all the different properties emphasised by the alternate concepts to decide on boundaries

Ways to study speciation

Theory, experiments, empirical evidence (paleontology, biogeography, phylogenetics, genetics)

Different ways of deciding when speciation has completed

Complete reproductive isolation

Genotypic clustering

Lineage sorting

Geographical / ecological clines (model representation, steep clines means speciation is complete)

What remains the biggest cause of adaptation

Natural selection

Mutations, drift, gene flow can only constrain / speed up adaptation

Common combination that leads to speciation

Isolation - which reduces/stops the gene flow, and divergent selection - which can lead to speciation even when gene flow is happening

Modes of speciation

Allopatry

Sympatry

Parapatry

Features of allopatry

No gene flow between organisms, divergent selection in different habitats

Vicariant allopatry - caused by barrier between two populations, and there are different mutations across the populations

Peripatry allopatry - two isolated populations one being much smaller, the small population experiences large genetic drift effect

Features of parapatry

Speciation among adjacent populations

Can form as ‘stepping-stones’ - limited gene flow among discrete populations

Can form as a cline - less limited gene flow along an environmental cline, but ends with two discrete populations

Features of sympatry

Free gene flow, requires very strong divergent selection

Interactions between what two factors give rise to different ways of speciation

Between gene flow and divergent selection

What is caused by no gene flow, just divergent selection

Allopatric speciation

Examples of vicariant allopatric speciation

Diptera mating experiments → different lines of flies exposed to artificial selection and put back together. Majority resulted in behavioural isolation in as little as 5 generations

Snapping shrimp Alpheus → experienced divergence after emergence of Isthmus of Panama, led to reproductive isolation and mtDNA divergence

Consistent species distribution across barriers - sister species created either side of geographical barriers

Examples of peripatric allopatric speciation

Island colonisation → sole representatives of a genome evolve on their own little island. Sister species distributed across archipelagos, from large populations to small ones on small islands.

What speciation is caused by some/medium gene flow and strong divergent selection

Parapatric speciation

Why is there weak evidence for parapatric speciation

Other forces often result in similar patterns, hard to attribute to parapatric speciation.

Species ranges also often change

Hard to rule out allopatric speciation

What is the evidence for parapatric speciation

Ring species - adjacent populations with some gene flow between them, eventually create two distinct species at either end

Ecotones (merging of two biomes) - little Greenbul bird lives on forest-savannah ecotone, and adjacent populations are highly diverged with potential for speciation

Historical observation of M. guttas - some of its population exposed to copper mine waste, adjacent populations developed copper tolerance, leading to reproductive isolation and speciation

What speciation is caused by high gene flow and very strong divergent selection

Sympatric speciation

Is sympatric speciation actually feasible

Thought to be very difficult, requiring very large divergent selection - but theoretically possible, seen in genomic data and models

Criteria for speciation to be labelled as sympatric

Species must occupy the same / overlapping geographical area(s) (be sympatric)

Substantial reproductive isolation

Sympatric taxa must be sister groups

Biogeographic and evolutionary history must make the occurrence of an allopatric phase very unlikely

What is the evidence for sympatric speciation

Around 10% of Lord Howe island flora arose by sympatric speciation - used molecular based phylogenies to distinguish sympatric speciation from double colonisation

Host shift of the apple maggot fly, Rhagoletis pomonella - changed host from hawthorn to apple tree, leading to assortative mating (non-random preferences) including mating on the same tree where they were larvae. Led to reproductive isolation. There was some hybridisation but offspring were maladapted to both habitats

What do most speciation events require

An allopatric phase

What are the other forms of speciation

Hybridsation

Polyploid speciation

Drift

What is hybridisation

Offspring of an immigrant individual and resident population are hybrids.

Successful breeding of the hybrids together, despite intense inbreeding, can lead to a population with an extreme progeny that is reproductively isolated from both parental species, eventually ecologically separated

Observed in birds where speciation came about in 3 generations

What is polyploid speciation

Polyploidy individuals (carrying more than 2 sets of chromosomes) are immediately rendered reproductively isolated from diploid counterparts

Common in plants but rare in animals

What is drift

Random fluctuations in allele frequencies

Severe drift, like the founder effect or genetic bottleneck, can break evolutionary inertia in large populations.

Some other models include genetic revolution and transilience, but not very well supported empirically or theoretically