Biological and Evolutionary Concepts of Speciation and Reproductive Isolation

Species

A group of organisms that can interbreed and produce fertile offspring, and are reproductively isolated from other groups.

Morphological species concept

Based on physical traits (appearance).

Drawbacks of the morphological species concept

Can be subjective; doesn't work for cryptic species or variation within a species.

Biological species concept

Based on reproductive isolation (can they interbreed?).

Drawbacks of the biological species concept

Doesn't apply to asexual organisms or fossils.

Lineage species concept

Based on evolutionary history and descent from a common ancestor.

Result of speciation

Formation of new species that are reproductively isolated from each other.

Dobzhansky-Muller model

Explains how genetic incompatibilities can evolve and cause reproductive isolation.

Fixed allele

An allele that has become the only variant in a population at its locus.

D-M model lineage mutation

Different mutations become fixed, and when combined, they reduce hybrid viability or fertility.

Example of hybrid incompatibility

Horse + donkey = mule (infertile).

Allopatric speciation

Speciation due to physical separation of populations (geographic isolation).

Founder effect in allopatric speciation

A small isolated population may evolve quickly due to genetic drift and different selection pressures.

Ring species

Populations that can interbreed with neighboring groups but not with distant ones in a circular geographic range.

Sympatric speciation

Speciation without geographic separation, often due to polyploidy or niche differentiation.

Polyploidy

Having more than two sets of chromosomes.

Autopolyploidy

Chromosome duplication within a single species.

Allopolyploidy

Combining chromosomes from two different species.

Prezygotic isolation

Favored by selection because it prevents wasted reproductive effort.

Mechanical isolation

Physical differences prevent mating.

Temporal isolation

Species breed at different times.

Behavioral isolation

Different courtship behaviors prevent mating.

Habitat isolation

Species live or mate in different habitats.

Gametic isolation

Gametes cannot fuse (sperm and egg incompatible).

Postzygotic isolation

Low hybrid zygote vigor means the zygote doesn't survive.

Low hybrid adult viability

Hybrid dies before reproducing.

Hybrid infertility

Hybrid lives but can't reproduce (e.g., mule).

Hybridization

Mating between two species that produces hybrid offspring.

Hybrid zone

Region where closely related species interbreed.

Hybrid selection

Hybrids are usually selected against because they have reduced fitness.

Hybridization effect on fitness

Sometimes increases diversity or allows gene flow.

Factors influencing lineage divergence

Diet, habitat specialization, sexual selection, dispersal, and ecological opportunity.

Diet speciation

Populations specialize on different food sources and diverge genetically.

Specialists vs. generalists in speciation

Specialists (e.g., pandas, anteaters) speciate more.

Pollinator role

Transfers pollen, affecting reproductive isolation and speciation.

Sexual selection

Preference for certain traits leads to reproductive isolation.

Dispersal

Movement to new areas; increases chances of allopatric speciation.

Evolutionary radiation

Rapid diversification of species when new ecological niches open up.