Unit 3 - Lecture 5 - Origin of Species

Why is defining a species difficult?

Because different organisms vary widely, so no single definition works for all groups.

What is the morphological species concept?

Defines species by physical traits (appearance and structure).

Main problem with the morphological species concept?

Hard to decide how much difference is “enough” to separate species.

How is DNA used to identify species?

Molecular sequence similarity / “DNA barcoding.”

What is the biological species concept?

Species are groups of populations that interbreed and produce fertile offspring.

What is reproductive isolation?

Species do not normally interbreed in nature; hybrids are rare or non-viable.

Is the biological species concept always useful?

No — cannot apply to fossils, asexual species, or species that are hard to observe.

What is speciation?

The process that produces new species through reproductive isolation and divergence.

What must happen for speciation to begin?

Gene flow must be reduced or blocked between populations.

Two main types of reproductive barriers?

Prezygotic and postzygotic barriers.

When do prezygotic barriers act?

before fertilization.

When do postzygotic barriers act?

After fertilization (during development or adulthood).

What is habitat isolation?

Species live in different habitats and rarely meet.

What is temporal isolation?

Species breed at different times (day, season, year).

What is behavioural isolation?

Different courtship rituals prevent mating.

What is mechanical isolation?

Mating is physically impossible due to incompatible structures.

What is gametic isolation?

Sperm and egg cannot fuse.

what is hybrid inviability?

Hybrids fail to develop properly or die early.

What is hybrid infertility?

Hybrids survive but cannot reproduce (e.g., mules).

What is hybrid breakdown?

Hybrids are fertile but their offspring are weak or sterile.

How can reproductive barriers evolve “accidentally”?

Through genetic change while populations are isolated.

How can natural selection strengthen reproductive barriers?

It can favour individuals who avoid mating with other species (reinforcement).

What is allopatric speciation?

Speciation caused by geographic separation blocking gene flow.

Common geographic barriers?

Rivers, mountains, oceans, landmasses.

Example of allopatric species from lecture?

Squirrels on either side of the Grand Canyon.

After geographic separation, what causes divergence?

Natural selection and genetic drift.

What happens if strong reproductive barriers evolved?

Species remain separate (no interbreeding).

What is a hybrid zone?

Region where different species meet and produce hybrids.

What can happen in a hybrid zone?

Fusion, reinforcement, or stable hybrid zone.

What is fusion?

Hybrids have high fitness → species merge back together.

What is reinforcement?

Hybrids have low fitness → selection strengthens reproductive barriers.

When do long-lasting hybrid zones occur?

When hybrids have variable fitness or exist at low frequency.

What is sympatric speciation?

Speciation occurring without geographic separation.

What is required for sympatric speciation to happen?

Some barrier to gene flow within the population.

Examples of sympatric speciation mechanisms?

• Host switching (e.g., parasites, herbivores)

• Disruptive selection

• Polyploidy, especially in plants

What is polyploid speciation?

Speciation caused by a change in chromosome number.

What is allopolyploidy?

Hybridization between two species → sterile hybrid → chromosome doubling → new fertile species.

Why does doubling restore fertility in allopolyploids?

Each chromosome now has a homologous partner for meiosis.

Why is allopolyploid speciation important?

It is common in plants and produces new species rapidly.

Why is allopolyploid speciation an exception to the strict “tree of life”?

It involves two parent species merging into a third (a hybrid branch).

What concept defines species by inter-fertility and reproductive isolation?

The biological species concept

What are prezygotic barriers?

Barriers that prevent mating or fertilization.

What are postzygotic barriers?

Barriers after fertilization that reduce hybrid viability or fertility.

What is the key requirement for allopatric speciation?

geographic separation.

What is the key requirement for sympatric speciation?

Reduced gene flow within the same region.

What are the three hybrid-zone outcomes?

Fusion, reinforcement, and stable hybrid zone.

What special speciation process is common in plants?

Allopolyploid speciation.