BSCI Topic 7: The origin of Species--Branches in the Tree of Life

Species Definition:

1. Species difficult to define (understand complexities); were originally defined by morphology, then by reproductive isolation (biological criteria), now via phylogenetic analysis/molecular biology

Species

Group of organisms that can interbreed and produce fertile offspring.

Morphological species concept

Based on physical traits.

Biological species concept

Based on reproductive isolation.

Phylogenetic species concept

Based on shared ancestry

Dobzhansky–Muller model

evolution of reproductive isolation; Divergent alleles in isolated populations cause hybrid incompatibility; explains how hybrid incompatibilities can arise between diverging populations leading to speciation

Reproductive isolation:

• Prezygotic barriers: Habitat, temporal, behavioral, mechanical, gametic.

• Postzygotic barriers: Hybrid inviability, sterility, breakdown.

Allopatric speciation

the evolution of a new species from an ancestral one due to a geographic barrier that separates the two populations, preventing interbreeding and gene flow;

Geographic separation leads to divergence.

a. Physical division of population

b. Divergence due to founder effect, drift, selection

c. Various fates if recontact: fusion, reinforcement, stable hybrid zone, speciation via hybrids

    (understand these)

Sympatric speciation

Occurs without geographic isolation (e.g., polyploidy, habitat shifts).

Hábitat Segregation

a pattern where different populations within the same species, or different species, occupy distinct habitats or different parts of the same habitat.

Autopolyploidy

Chromosome duplication within one species.

Allopolyploidy

Hybridization followed by chromosome doubling.

Outcomes of secondary contact:

Fusion, reinforcement, hybrid zone, or new species.Fusion, reinforcement, hybrid zone, or new species.

Species separated by Pre-Zygotic Barriers

prevented from interbreeding due to mechanisms that act before fertilization can occur, such as different mating times, habitats, behaviors, or incompatible reproductive structures. These barriers contribute to speciation by ensuring reproductive isolation and preventing gene flow between different populations, allowing them to diverge into distinct species

Pre-Zygotic Barrier: i) Habitat isolation

occurs when two species favor different habitats, making it unlikely they will encounter each other and mate

Ex:) A cricket population divided by a flood, or lions and tigers coexisting in some parts of India.

Pre-Zygotic Barrier: Temporal Isolation

form of reproductive isolation where two species are kept from interbreeding because they reproduce at different times, such as during different seasons, times of day, or stages of life.

This prevents them from meeting and mating, even if they live in the same habitat, and is a key mechanism that drives speciation

Ex:) Two species of frogs that inhabit the same area may be reproductively isolated if one breeds earlier in the spring than the other.

Pre-Zygotic Barrier: Behavioral Isolation

occurs when differences in behaviors, such as unique courtship rituals or mating calls, prevent individuals from different species from interbreeding, thus maintaining reproductive isolation

Ex:) Cricket Songs: Different populations of crickets might produce distinct mating songs, ensuring that females only respond to males singing their specific tune

Pre-Zygotic Barrier: Mechanical Isolation

is a prezygotic reproductive barrier that prevents two species from interbreeding due to physical incompatibility of their reproductive structures or mating organs, such as differently shaped genitalia or incompatible flower-pollinator relationships.

Ex:) Orchids and Pollinators—The shape of an orchid's flower can be so specialized that only certain insects can pollinate it

Pre-Zygotic Barrier: Gametic Isolation

is a prezygotic reproductive barrier that prevents fertilization by making sperm and eggs from different species incompatible. It's a mechanism that maintains genetic integrity and limits gene flow between species, even if they mate

Ex:) different species of sea urchins (like the purple sea urchin and red sea urchin) that release their eggs and sperm into the water, but chemical or protein differences on the gamete surfaces prevent them from fertilizing each other.

b.  Post-zygotic

i) Reduced viability of the hybrid

ii) Hybrid offspring survives but is itself sterile (2nd generation can not reproduce)

Post-Zygotic Barrier: Hybrid Inviability

This occurs when the hybrid zygote fails to develop or survive to adulthood, often due to genetic incompatibilities between the parent species.

Ex: The hybrid offspring of a goat and a sheep may experience zygote mortality, meaning the zygote cannot develop properly and dies before birth or shortly after

Post-Zygotic Barrier: Hybrid Sterility

Here, the hybrid offspring are viable and survive to maturity, but they are unable to produce functional gametes (like sperm or eggs), making them infertile.

Ex:) Mules (Hybrid Sterility): (A well-known example is a mule, the offspring of a horse and a donkey. Mules are sterile because horses and donkeys have different numbers of chromosomes, which leads to unpaired chromosomes in the mule, interfering with the process of meiosis and preventing the production of viable gametes.

Post-Zygotic Barrier: Hybrid Breakdown

In this case, the first generation of hybrid offspring is both viable and fertile, but subsequent generations (F2 or later) become weak, sterile, or otherwise non-viable.

Ex:) Lion-Tiger Hybrids (Hybrid Breakdown): While both lions and tigers can produce a hybrid offspring (a liger or tigon), the subsequent generations of these hybrids often exhibit reduced fertility or viability, demonstrating hybrid breakdown.