What is a species?

A specie is:

The smallest evolutionarily independent unit
Mutation, selection, gene flow, and drift operate separately in different species
There is no gene flow

Or

A population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but do not produce viable, fertile offspring with members of other such groups.

Speciation

 An evolutionary process in which one species splits into two or more species.

Microevolution

Evolutionary change below the species level; change in the allele frequencies in a population over generations.

Macroevolution

Evolutionary change above the species level, including the origin of a new group of organisms or a shift in the broad pattern of evolutionary change over a long period of time.

biological species concept

Reproductive isolation

-Is the existence of biological factors that impede members of two species from producing viable, fertile hybrids
-Confirms a lack of gene flow

hybrid

prezygotic barrier

A reproductive barrier that impedes mating between species or hinders fertilization if interspecific mating is attempted.

postzygotic barrier

 A reproductive barrier that prevent hybrid zygotes produced by two different species from developing into viable, fertile adults.

Limitations of the Biological Species Concept

The biological species concept cannot be applied to Asexual organisms, Fossils, Organisms about which little is known regarding their reproduction

The Ensatina eschscholtzii complex

a split ring species

(spliting of the lizard in the regions)

The morphological species concept

Characterizes a species in terms of its body shape, size, and other structural features

The phylogenetic species concept

Defines a species as a set of organisms with a unique genetic history

Applies to all organisms including:
Asexual & Fossils

Disadvantages of Phylogenetic

Good phylogenies

-Not available for most organisms

-Expensive and time consuming

Allopatric speciation

A new species forms while geographically isolated from its parent population

Sympatric speciation

The emergence of a reproductive barrier isolates a subset of a population without geographic separation

Parapatric speciation

Populations that abut one another diverge, usually in response to some strong ecological gradient.

Gene flow is interrupted or reduced when a population is divided into two or more geographically isolated subpopulations

allopatric speciation

Divergence and Allopatry

•Genetic drift
-Bottlenecks, -Rarely has a large effect except when long or repeated
•Selection
-Different environments can lead to different
-Adaptations, -Behavior, -Sexual preference

Divergence and speciation take place without geographic isolation

Sympatric  Speciation

Genes that cause host preference may lead to associative mating

sympatric speciation

Polyploidy

–Is the presence of extra sets of chromosomes in cells due to accidents during cell division
–Results in reproductive barriers between different ploidylevels.
–Has caused the sympatric speciation of many plant species

An autopolyploid

•An autopolyploid
–Is an individual that has more than two chromosome sets, all derived from a single species

Allopolyploidy

A fertile individual that has more than two chromosome sets as a result of two different species interbreeding and combining their chromosomes.

Can occur in abutting populations, particularly if there is a steep ecological gradient and if divergence causes assortative mating

Parapatric speciation

•Need not evolve as a mechanism to avoid hybridization
•Instead, usually evolves as a consequence of adaptation(s) to improve survival or mating success in a particular population or species.

Prezygotic isolation

•Could evolve to prevent matingsthat produce unfit hybrids
•This would be called “reinforcement”
•The best evidence for this would be greater prematingisolation between sympatric than allopatricpopulations of the two species

Prezygotic Isolation

•Develops over time with genetic isolation of populations
•Due to divergence at one or (usually) more loci such that combinations of alleles from the two species result in low fitness
•Divergence could be due to selection or drift

Postzygotic isolation

What if there is no reproductive isolation upon secondary contact?

–Hybrid = equal fitness = coalescence
–Hybrid = higher fitness = new species