Speciation

Speciation is the process that bridges microevolution and macroevolution.
- Microevolution: Refers to changes in allele frequencies in a population over generations.
- Macroevolution: Encompasses patterns of evolution above the species level, for example, the origins of new groups of organisms.
Key factors that determine a species include:
- Morphology: The structure or form of organisms.
- Physiology: The biological functions and processes of organisms.
- Biochemistry: The chemical processes within and related to living organisms.
- DNA: The genetic material that carries biological information.

Most biologists use the Biological Species Concept (BSC) as a foundational definition of a species:
- Definition of BSC: A group of populations whose members have the potential to interbreed and produce fertile offspring, provided they encounter one another in nature.
- Important attributes:
  - Members can be separated by physical barriers with a low chance of eventual contact.
  - Offspring produced must be capable of reproducing.
- Example data regarding gene flow in species:
  - 2n=64, 2n=62, 2n=63.
The key concept in the BSC is reproductive isolation:
- Definition of Reproductive Isolation: Refers to factors that prevent two species from interbreeding.
- Prezygotic Barriers: Factors that prevent the formation of a zygote.
- Postzygotic Barriers: Factors that result in reduced fertility or viability of hybrids.

The BSC has limitations and may not apply in all situations:
- Cannot be utilized for:
- Fossils: No reproductive interaction can be observed.
- Asexual organisms: These do not fit the definition focused on interbreeding.
- Some species have the capability of forming hybrids despite classification.
Other species concepts exist that address these limitations:
- Morphological Species Concept: Based on phenotypic (physical) similarities.
- Ecological Species Concept: Defined by the ecological role a species plays within its ecosystem.
- Phylogenetic Species Concept: Identifies the smallest monophyletic group in a phylogenetic tree.

Mechanisms responsible for speciation are characterized by the significance of geographic separation:
- Allopatric Speciation: Occurs when geographic separation disrupts gene flow between populations.
- Origin of the term: "Allos-" means "other"; "-Patra" refers to "homeland" or "fatherland".
- Sympatric Speciation: Takes place when other mechanisms disrupt gene flow without geographic separation.

Research findings indicate that each species of snapping shrimp has a sister taxon located on opposite sides of the Isthmus.
Areas with more geographical barriers often exhibit greater species diversity.

Speciation can occur without geographic barriers, with three primary mechanisms identified:
- Polyploidy: The condition in which a species has more than two complete sets of chromosomes, such as seen in a tetraploid melon created through treatment with colchicine.
- Habitat Differentiation: Occurs when variations in the environment allow for niche differentiation among species.
- Sexual Selection: This is when mate choice leads to speciation, as seen in studies of P. pundamilia and P. nyererei cichlids under different light settings altering their mating preferences.
- Reference for study: O. Seehausen and J. J. M. van Alphen (1998).

Hybrid zones are regions where incomplete reproductive barriers exist, resulting in the formation of hybrids:
- Definition: Occur when populations meet in overlapping areas, allowing hybrids to form.
- Impact of Hybrids: Typically, heterozygotes (hybrids) are often less fit, which hinders gene flow across the two populations.

There are three potential outcomes when hybridization occurs:
- Reinforcement: When hybrids have lower fitness, leading to the strengthening of reproductive barriers between species.
- Fusion: When hybrids allow gene flow between species and weaken the reproductive barriers.
- Stability: When hybrids persist over time, maintaining the hybrid zone without resulting in extinction of either parental species.