origin of species

Gene flow

occurs among individuals within a population and among populations within a metapopulation

metapopulation

a population of populations connected by migration

group of spatially separated populations of the same species that interact at some level

Biological species concept

species if can breed with one another

focuses directly on limitations to gene flow

only applies to sexually reproducing organisms

Morphological species concept

is a species if they look the same and have similar structures

critical for paleontologists because they only have morphologies

used as indication that gene flow is not occuring because otherwise the populations would be homogenous

can be challanging to tell how much variation within a population

Phylogenetic species concept

smallest possible group descending from a common ancestor in a phylogenetic tree is a species

species are monophyletic groups

General lineage species concept

each species is a metapopulation that evolves independently from other metapopulation

Members of the same metapopulation exchange alleles often enough that they can be treated as members of the same evolutionary lineage

Over time, different species behave like independent evolutionary units, following separate trajectories

microbial speciation via horizontal gene transfer consistent with this

why do different strains of the same species typically share a “core genome” but differ in the presence and absence of many other genes

because of horizontal gene transfer

Pan-genome

the the total number of all genes found in all populations within a species

Stable ecotype model

a species is a long-lived population of genetically related individuals that share a stable set of adaptations for the same ecological niche

-species are sorted into genetic and ecological clusters

-is a species concept constructed for microbes in particular

gene exchange in microbes

horizontal gene transfer

gene exchange in sexual reproducing organisms

horizontal gene transfer

how speciation happens

1. Allopatric speciation

2. Parapatric speciation

3. Sympatric speciation

Allopatric speciation

geographic separation of populations

-prevents gene flow between them

if geographic barrier removed, species can hybridize leading to genetic homogenization through introgression

hybridization wont prevent speciation in cases of reinforcement

introgression

the movement of alleles from one species/population to another

reinforcement

selection again hybrid offspring that results in speciation, given enough time

Parapatric speciation

Evolution of new species within a spatially extended population that still has some gene flow

happens as a result of isolation by distance

isolation by distance

occurs when populations in close proximity are genetically more similar than populations that live farther apart

Sympatric speciation

Evolution of new species within a contiguous population that still has extensive gene flow (no geographic barriers)

Reproductive isolating barriers that are not physical

ecological speciation

can also occur as a result of behavioral differences (populations that breed at different times or respond to different behavioral cues)

ecological speciation

evolution of reproductive barriers between the populations as a result of adaptation to different environments or ecological niches, even though no geographical barriers are present

As long as different populations are evolving away from each other faster than gene flow homogenizes them

the populations will continue to diverge, eventually becoming different species

Premating barriers

time of reproduction

Allopolyploidy

occurs when two species hybridize

If the resulting hybrid offspring have an odd number of chromosomes, they cannot reproduce sexually

rare duplication event in the hybrid’s asexually produced offspring can double the number of chromosomes, thus allowing the new species to reproduce sexually

new species cannot breed with either of the parental species that formed it