Chapter 16: Mechanism of Speciation

Mutation

________ can create polyploidy and other chromosome changes as a barrier to gene flow.

Speciation

________ begins when gene flow is disrupted and the population becomes genetically isolated.

Polyploidy

________ is common in angiosperms (50- 70 %) and ferns /allies (95 %), frequent in bryophytes and algae, and rare in fungi and gymnosperms.

Geographical speciation

________ is called allopatric because the diverging populations are found in separate (more or less discontinuous) areas.

Drift

________ can produce rapid genetic divergence in small isolated populations.

Species

the smallest evolutionary independent unit that is isolated by lack of gene flow

Evolutionary forces

mutations, selection, gene flow (migrations), drift

Morphospecies Concept

2 populations are treated as different species if they are discontinuously separate in morphology, meaning that the border between the species is marked by the absence of some combination of characteristics (discrete/continuously variable traits)

based only on the pattern (no taxon-specific processes), likely to work for a variety of organisms

Advantages of MSC

Without additional criteria, the MSC cannot handle different morphologies during the life cycle of the same species or polymorphism and cryptic species

Disadvantages of MSC

Cryptic species

species that are indistinguishable morphologically, but different in other traits

Biological Species Concept

"groups of interbreeding populations that are reproductively isolated from other such groups."

Confirms lack of gene flow (biologically intuitive)

Strength of BSC

BSC Species

potential to interbreed and produce fertile offspring determines species membership

Knowledge about reproductive isolation in nature is hard to achieve for the majority of organisms, how do we deal with fossils, assuming sexual process when most taxa are asexual, and difficult to apply to plants because many divergent populations can hybridize

BSC limitations

Phylogenetic Species Concept

the fundamental grouping criteria is monophyly

Monophyletic Group

a taxon that contains ALL descendants of a common ancestor originating in a single event

PSC Species

the smallest monophyletic group of common ancestry

can be applied to sexual and asexual organisms, can be applied to fossils, clear evolutionary perspective, testable

PSC Advantages

Time, money, careful analysis, will species change when phylogenetic hypothesizes change, how well do we know phylogeny, species, and genetics are not always correct

PSC Disadvantages

Secondary contact

two populations that have diverged in isolation from a common ancestor are reunited geographically

Allopatric model

speciation occurs when populations become geographically isolated and diverge because speciation acts on them independently

Dispersal and Vicariance

There are two types of allopatric speciation

Dispersal and colonization hypothesis

speciation events begin when small populations colonize new geographical areas

Vicariance

the formation of barriers due to environmental changes

Sister Species

the species diverged from the same ancestral node on a phylogenetic tree

Sympatric Speciation

the formation of 2 or more descendants species from a single ancestral species all occupying the same geographical location

Mechanism of Divergence

genetic drift, natural selection, sexual selection

Bottleneck Effect

A large-scale but short-term reduction in population size followed by an increase in population size

Sexual selection

divergence due to differences on how females choose mates, how males attract females, and how male compete for females

hybridization

After genetic isolation and divergence, the recently diverge populations may come back in contact and have the opportunity to interbreed

Reinforcement

Natural selection that results in assortative mating in recently diverged populations in secondary contact; it finalizes the speciation process by producing complete reproductive isolation

Assortative mating

occurs when individuals tend to mate with other individuals with the same genotype or phenotype

Prezygotic Isolation

reproductive isolation between population caused by difference in mate choice or timing of breeding, so no hybrid zygotes were formed

Habitat isolation

populations of concern occur in different habitats in the same geographical region

Seasonal/Temporal

Mating or flowering times occur at different time zones

Sexual/ethological

Mutual attraction between the seces of different species is weak or absent

Mechanical

physical attributes of animal genetalia are specific to each species or flower parts prevent transfer of pollen

Isolation by different Pollinators

In flowering plants, related species may be specialized to attract different insect as pollinators

Gametic

Incompatibility of male and female gametes preventing the formation of hybrid zygote

Postzygotic isolation

reproductive isolation between populations caused by dysfunctional development, sterility, or decreased fitness in hybrid forms

Hybrid inviability

hybrid zygotes have lower viability or are inviable

Hybrid Sterility

The F1 hybrids of 1 sex or both sexes fail to produce functional gametes

Hybrid Breakdown

The F2 backcross hybrids have lower viability or fertility

Hybrid Zone

a geographical region where interbreeding between divergent population occurs and hybrid offspring are frequent