L3 SPECIATION & PROCESS OF EVOLUTION

Species

Group of populations where members have the potential to interbreed to reproduce fertile offspring

Why is the biological species definition limited?

It doesn’t work well for:

1. Asexual organisms

2. Extinct organisms (fossils)

3. Subspecies that interbreed with some, but not others

Speciation

The process by which a new species forms from an existing one

What are three stages of speciation?

• Barrier to breeding (e.g., geographical isolation)

• Becoming different (adapt to different environments → accumulate differences)

• Two different species (populations become so different they can’t interbreed)

Reproductive Isolation

When populations can no longer breed successfully, due to genetic, behavioral, or physical barriers.

Prezygotic Barriers

Factors that prevent mating and fertilization between species.

What are 5 prezygotic barriers?

Habitat isolation, temporal isolation, behavioural isolation, mechanical isolation, gamete isolation

Habitat Isolation

Two species rarely encounter each other because they live in different habitats

Temporal Isolation

Breed at different times of the year / day / seasons, etc.

Behavioural Isolation

Different mating calls, pheromones, etc.

Mechanical Isolation

Female and male sex organs are not compatible

Gamete Isolation

Sperms of one species may not be able to fertilize eggs of another species

Postzygotic Barriers

Anything that prevents the hybrid zygote from developing into a viable and fertile adult

Hybrid Infertility (Sterility)

Hybrid offspring remains healthy, but they are sterile/infertile

Zygotic Mortality

The egg is fertilized, but zygote die in early development before birth

Hybrid Inviability

Hybrid embryo forms, but either die before birth or if born alive, it is not healthy and suffers an early death

Hybrid Breakdown (plants)

The second generation may be inviable/sterile, first generation is viable and fertile

Allopatric Speciation

New species form when populations are geographically isolated from each other.

What are the steps in the allopatric speciation model?

1. Physical barrier divides the ancestral population.

2. Genetic differences accumulate (natural selection/genetic drift).

3. Reproductive isolation has occured

Sympatric Speciation

New species form within the same geographic area as the parent population, usually in plants.

Parapatric Speciation

Neighboring populations hybridize along a barrier, developing reproductive isolation due to habitat differences

Gradualism

Evolution occurs slowly and steadily through small changes over long periods.

Punctuated Equilibrium

A new species changes most as it buds from a parent species and changes little for the rest of its existence

How do fossils support gradualism vs. punctuated equilibrium?

• Gradualism: Fossils should show small, incremental changes.

• Punctuated equilibrium: Gaps in fossil records show stasis with rapid bursts of change.

Divergent Evolution

When closely related species become increasingly different due to different environments or niches

Convergent Evolution

Unrelated species evolve similar traits independently because they live in similar environments.

How do divergent and convergent evolution differ?

• Divergent: Common ancestor, different traits → homologous structures.

• Convergent: No common ancestor, similar traits due to environment → analogous structures.