Speciation and Hybridization

Tigon: A hybrid offspring of a male tiger (Panthera tigris tigris) and a female lion (Panthera leo).
Liger: A hybrid offspring of a male lion and a female tiger; known as one of the largest cat species.
- Hercules: A liger recognized by the Guinness World Book of Records as the world's largest cat, weighing 922 pounds; it died in 2019. Additional information about Hercules can be found online.

Examples of rare animal hybrids include zonkeys, zodongs, narduga, deepalo, wolfin, savannah cat, jaglion, geep, coywolf, and growler bear.
Key questions to consider:
- What are the parent species?
- How are they named?
- Are they fertile?
- Are they found in the wild, born in captivity, or bred in the lab?

The biological species concept emphasizes reproductive isolation but faces challenges due to interspecific hybridization.
Even with reproductive isolating mechanisms, hybridization occurs among animals, plants, and birds.
- 10% of bird species hybridize naturally.
- Many plant species hybridize naturally.
The biological species concept applies primarily to sexual species, posing challenges for defining fossilized or asexual species such as prokaryotes that reproduce via fragmentation or binary fission.

Paleontological Species Concept
- Focuses on morphology due to the inability to observe mating behavior in fossils.
- Defines species based on discrete morphological differences.
Phylogenetic Species Concept
- Defines a species as a branch on the phylogenetic tree.
Ecological Species Concept
- Views species in terms of their ecological niche.
- Groups organisms broadly (e.g., all web-footed birds or long-beaked birds).

Questions remain regarding assigning all organisms to species, particularly:
- Asexual populations (e.g., prokaryotes)
- Hybridizing organisms
- Organisms living within other organisms
The ongoing effort to address these questions leads to the frequent proposal of new species concepts.

Anagenesis: Transformation of one species into another gradually over time.
- Does not lead to diversity.
Cladogenesis: Splitting of one ancestral species into two descendant species, resulting in branching.
- Illustrated by a cladogram.
- Leads to diversity.

Allopatric Speciation
- Occurs in geographically separated populations.
- Gene flow is interrupted, leading to the evolution of new species.
- Involves an initial single population separated by a formidable barrier.
Sympatric Speciation
- Occurs within the same geographic region.
- Populations overlap but evolve into different species through chromosomal changes or non-random mating, which reduces gene flow.
Parapatric Speciation
- Occurs in adjacent areas with different habitats (e.g., forest and grassland).
- Differences in environmental conditions lead to speciation.

Example: Squirrels separated by the Grand Canyon evolved into different species due to the canyon's formation, which created a barrier.
Test:
- Start with a single population.
- Observe migration to different areas separated by a geographic barrier.
- If populations can still interbreed upon reunion, speciation has not occurred.
- If populations cannot recognize each other or mate upon reunion, allopatric speciation has occurred.

Species A on the mainland migrates to a nearby island.
Over time, the island population evolves into species B due to lack of gene flow.
If species B returns to the mainland and cannot mate with species A, allopatric speciation has occurred.

Takes place in geographically overlapping populations.
Mechanisms: Chromosomal changes or non-random mating.
These mechanisms reduce gene flow, causing populations to evolve into new species.

Leads to polyploidy, where organisms have more than two sets of chromosomes.
- Diploid: Two sets of chromosomes (one from each parent).
- Polyploidy: more than two sets of chromosomes.

Autopolyploidy: Arises from genome duplication within a single species.
- Same species (A crossing with A) results in the same number of chromosomes.
- Spontaneous genome duplication leads to tetraploidy.
- Meiotic reduction of eggs, followed by genome duplication without cytokinesis, can result in tetraploidy.
Allopolyploidy: Involves two different species (A and B).
- Hybrid (AB) is formed through interbreeding.
- Gamete formation can be unequal, leading to aborted gametes and seedlessness.
- Spontaneous genome duplication results in a hybrid with paired chromosomes, making it fertile.

Homologous Chromosomes: Chromosomes bearing the same set of genes from two different parents.
Homeologous Chromosomes: Chromosomes from different species (A and B).
Homeologous pairing is predominant in allopolyploids.

Species a(2n = 4) + Species B(2n = 6)
Meiotic error in Species A: 2n = 4 after meiosis
Normal meiosis in Species B: n = 3
After fertilization, chromosomes: n = 4 + n = 3, a hybrid with 7 chromosomes is formed.

Black mustard (Brassica nigra) crosses with cabbage (Brassica oleracea) producing Brassica carinata.
Black mustard crosses with Chinese cabbage(Brassica rapa) to create Brassica juncea.
Chinese cabbage crosses with Cabbage to create Brassica napus species.
Constructed by Korean scientist Nagaharo Yu to explain the relationship between plants or Chinos Brassica.

Ploidy Level = Number of Chromosomes/Basic Number
- Example for Potato: 48/12 = 4 = Tetraploid

Common Allopolyploids: tobacco, cotton, wheat, oats, pear, apples and strawberries.