3.1C Species diversity Notes

Speciation: the formation of new species through evolution

DefinitionSpeciationSpeciation is the process by which new species evolve from an existing population due to isolation and adaptation.

Over time, genetic changes accumulate, and populations diverge until they can no longer interbreed, resulting in the formation of a new species.

How Speciation Occurs

1. Speciation happens when populations of the same species become isolated and experience different selective pressures in their environments.
2. The process involves:
3. Isolation – A population is split into separate groups, preventing gene flow between them.
4. Adaptation – Each group adapts to its new environment, undergoing natural selection.
5. Genetic Divergence – Over time, genetic differences accumulate, making reproduction between the groups increasingly difficult.
6. Reproductive Isolation – Eventually, the two populations become so different that they can no longer interbreed, forming a new species.

Types of Speciation

Allopatric Speciation (Geographical Isolation)

1. Occurs when a population is physically separated by a barrier such as mountains, rivers, or oceans.
2. Each group evolves independently, adapting to different environmental conditions.
3. Over time, genetic differences accumulate, leading to reproductive isolation.

ExampleDarwin’s Finches in the Galápagos Islands evolved into multiple species after different groups became isolated on separate islands and adapted to different food sources.

Sympatric Speciation (Same Location, Different Niches)

1. Happens without physical separation—populations evolve separately in the same area due to differences in behavior, feeding, or reproduction.
2. Can be caused by polyploidy (chromosome duplication) in plants, sexual selection, or ecological specialization.

ExampleCichlid fish in African lakes have speciated within the same body of water by adapting to different depths, food sources, and mating behaviors.

Parapatric Speciation (Partial Isolation Along a Gradient)

1. A population is partially separated by a gradient (e.g., climate, soil conditions), where individuals at opposite ends experience different selection pressures.

ExampleMetal-tolerant plants in polluted soils near mines evolved separately from non-tolerant plants, despite growing in close proximity.

Reproductive Isolation: The Final Step in Speciation

Once populations are genetically distinct, they develop barriers to reproduction, preventing them from interbreeding.

Prezygotic Isolation (Before Fertilization)

1. Behavioral Isolation – Different mating behaviors (e.g., songs, dances) prevent interbreeding.
2. Temporal Isolation – Different breeding seasons prevent gene flow.
3. Mechanical Isolation – Differences in reproductive structures prevent mating.

Postzygotic Isolation (After Fertilization)

1. Hybrid Inviability – Offspring do not survive.
2. Hybrid Sterility – Hybrids survive but cannot reproduce (e.g., mules, which result from a horse-donkey hybrid but are sterile).

NoteUnderstanding speciation helps explain biodiversity, evolution, and the relationships between organisms in different ecosystems.