Topic 7.10 Notes – Speciation

What a Species Is and When Speciation Happens

Biological Species Concept

Under the Biological Species Concept (BSC), a species is a group of organisms that can interbreed in nature and produce viable, fertile offspring.

Two key pieces:

• Viable = offspring survive.
• Fertile = offspring can reproduce.

The defining feature is reproductive isolation. If two populations cannot successfully exchange genes, they are considered separate species.

Classic example:

• Horses and donkeys can produce a mule.
• Mules are sterile → no gene flow continues.
• Therefore, horses and donkeys are separate species.

The AP exam sticks to sexually reproducing organisms when using BSC. It does not expect you to apply this concept to bacteria.

What Speciation Actually Is

Speciation is the formation of new species.

It happens when:

1. Gene flow is interrupted between populations.
2. Genetic differences accumulate over time.
3. Populations become reproductively isolated.

Once gene flow stops, each population evolves independently through:

• Mutation
• Natural selection
• Genetic drift

That independent evolution is what drives divergence.

On free-response questions, they love giving a scenario where a barrier appears and asking what happens next. The key phrase they want to see is that interruption of gene flow allows allele frequencies to change independently.

Mechanisms That Cause and Maintain Reproductive Isolation

Reproductive isolation happens through prezygotic or postzygotic barriers.

Prezygotic Barriers

These prevent fertilization from ever happening.

• Habitat isolation
  Populations live in different environments within the same region.
• Temporal isolation
  They breed at different times (season, time of day, year).
• Behavioral isolation
  Different courtship signals.
  Caribbean Anolis lizards have distinct mating displays and dewlap colors.
• Mechanical isolation
  Reproductive structures do not fit.
• Gametic isolation
  Sperm and egg cannot fuse.

These barriers block the formation of a zygote, so gene flow never begins.

Postzygotic Barriers

Fertilization occurs, but offspring are unsuccessful.

• Hybrid inviability
  Embryo fails to develop properly.
• Hybrid sterility
  Offspring is sterile (mule).
• Hybrid breakdown
  F1 generation is viable, but F2 has low fitness.

On multiple-choice, pay attention to whether the question describes failure before or after fertilization. That determines pre vs post.

Types of Speciation

There are two main patterns.

Allopatric Speciation

Occurs when populations are geographically separated.

A physical barrier such as:

• Mountain range
• River
• Island formation

Once separated:

• Mutations differ
• Selection pressures differ
• Genetic drift can be strong (especially small populations)

Over time, reproductive isolation evolves, and even if the barrier disappears, the populations no longer interbreed.

Examples:

• Hawaiian Drosophila diversifying across islands
• Adaptive radiation after colonizing new habitats

Sympatric Speciation

Occurs without geographic separation.

Populations live in the same area but become reproductively isolated due to:

• Disruptive selection (extreme phenotypes favored)
• Sexual selection (different mating preferences)
• Ecological specialization

Example:

• Apple maggot fly (Rhagoletis)
  Some shifted from hawthorn trees to apple trees.
  Different fruit preference → different breeding times → reduced gene flow.

Students often think physical separation is required. It is not.

Evolutionary Processes That Drive Divergence

Once gene flow decreases, several forces increase differences:

• Mutation introduces new alleles.
• Genetic drift changes allele frequencies randomly.
• Natural selection favors traits suited to each environment.
• Sexual selection changes mating traits.

Adaptive Radiation

Rapid diversification from one ancestor into many species.

Occurs when:

• New habitats open (new islands).
• Ecological niches become available (after mass extinction).

Speciation rates can spike during these periods.

Rates and Patterns of Evolution

Evolution does not always proceed at the same pace.

In fossil record questions, long stable periods followed by sudden shifts point to punctuated equilibrium.

Divergent vs Convergent Evolution

Adaptive radiation is a form of divergent evolution.

Convergent evolution explains why dolphins and sharks look similar but are not closely related.