AP Biology 7.10 Speciation Study Notes - New Syllabus Effective 2025
AP Biology 7.10 Speciation Study Notes- New syllabus
AP Biology 7.10 Speciation Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the conditions under which new species may arise
Key Concepts:
- Speciation
7.10.A – Describe the conditions under which new species may arise
🌱 What is Speciation?
- Speciation is the process by which new species form.
- It happens when one population splits and evolves so much over time that they can no longer interbreed successfully.
🧪 Biological species: A group of organisms that can interbreed and produce fertile, viable offspring.
🔄 What causes speciation?
For speciation to happen, there must be reproductive isolation. That means two groups stop exchanging genes.
This can happen due to:
🚧 1. Geographic Isolation (Allopatric Speciation)
- A physical barrier (like a mountain, river, or island) separates populations.
- Each group adapts to its own environment, and genetic differences build up.
- Eventually, they become so different they can’t mate anymore.
🗺️ “Different places = different paths”
🧬 2. Genetic Isolation (Sympatric Speciation)
- Happens without physical separation.
- Instead, reproductive barriers happen due to:
- Behavioral isolation (e.g., different mating calls/dances)
- Temporal isolation (breeding at different times)
- Polyploidy (extra chromosomes common in plants!)
- Habitat use (e.g., same area, but different niches)
🌸 Seen often in plants & insects
💥 Other Contributing Factors:
- Mutations – create new traits
- Natural selection – selects for different traits in each population
- Genetic drift – random changes in small populations
- Sexual selection – preference for specific traits drives change
🧠 Key Point:
Reproductive isolation (geographic or genetic) is the key condition for new species to arise.
7.10.A.1 – Reproductive Isolation & Speciation
🧠 Key Concept:
Speciation happens when two populations become reproductively isolated, meaning they can no longer mate and produce fertile, viable offspring.
This isolation prevents gene flow, and over time, the two groups evolve independently until they become separate species.
❌ Types of Reproductive Isolation:
1. Prezygotic Barriers (before fertilization)
Prevent mating or fertilization from happening.
Examples:
- Habitat isolation – live in different environments
- Temporal isolation – breed at different times
- Behavioral isolation – different courtship behaviors
- Mechanical isolation – physically can’t mate
- Gametic isolation – sperm can’t fertilize egg
2. Postzygotic Barriers (after fertilization)
Mating/fertilization happens, but the offspring doesn’t develop properly.
Examples:
- Reduced hybrid viability – offspring die early
- Reduced hybrid fertility – offspring are sterile (e.g., mule)
- Hybrid breakdown – next-gen hybrids are weak/infertile
🔁 Over Time…
If isolation lasts long enough:
- Mutations, natural selection, and genetic drift cause populations to diverge genetically.
- Eventually, they become distinct species.
🧬 Summary:
Speciation = result of reproductive isolation
No gene flow → populations evolve separately → new species form
7.10.A.2 – Biological Species Concept
🧬 What Is a Species?
According to the Biological Species Concept (BSC), a species is defined as:
🧠 A group of organisms that can interbreed and produce viable, fertile offspring.
- ✅ Viable = alive and healthy
- ✅ Fertile = able to reproduce again
🔁 Key Requirements:
- Interbreeding – Members of the same species can mate with each other.
- Gene Flow – They can exchange genetic material (alleles).
- Offspring Are Fertile – Their children can also reproduce.
❌ Does Not Include:
- Organisms that can’t reproduce sexually (e.g., bacteria, some plants)
- Fossil species (we can’t test their ability to interbreed)
- Groups that look similar but can’t produce fertile offspring
(e.g., horse × donkey = mule, which is sterile)
💡 Why It Matters:
Helps scientists identify when speciation has occurred.
If two populations can’t produce fertile offspring, they are considered separate species.
🧪 Example:
A lion and a tiger can mate → produce a liger, but it’s usually infertile.
➡️ So lions and tigers = different species under the BSC.
7.10.B – Rate of Evolution and Speciation Under Different Conditions
⏱️ How Fast Do New Species Form?
The rate of evolution and speciation (formation of new species) can vary depending on environmental and genetic conditions.
⚡ 1. Punctuated Equilibrium
- Quick bursts of change followed by long periods of little or no change
- Often happens after sudden environmental change, like natural disasters, or when a small group becomes isolated.
Example: A volcanic eruption isolates a few organisms → they evolve rapidly into a new species.
🌱 2. Gradualism
- Slow & Steady Change
- Evolution happens gradually over long periods of time through accumulation of small changes.
Example: Fossil records that show slow, step-by-step changes in structure.
🌍 Ecological Conditions That Affect Rate:
| Condition | Effect on Evolution |
|---|---|
| 🌡️ Rapid environmental change | Speeds up evolution |
| 🧬 High mutation rates | More variation = faster change |
| 🧑🤝🧑 Small population size | Increases genetic drift |
| 🌐 Geographic isolation | Leads to faster speciation |
| 🌿 Stable environment | Slower evolutionary change |
🔁 Summary:
- Evolution doesn’t happen at the same speed for every species.
- Fast or slow rates depend on how quickly environments change and how populations respond.
- Both gradualism and punctuated equilibrium are valid patterns observed in nature.
7.10.B.1 – Punctuated Equilibrium vs Gradualism
📊 Two Patterns of Evolution Over Time:
⏸️ 1. Punctuated Equilibrium
- Evolution happens in rapid bursts
- These bursts are separated by long periods of stasis (no change)
- Usually triggered by sudden environmental changes or isolation of small populations
Example:
A natural disaster isolates a group → they evolve rapidly into a new species
➡️ Fast evolution + long stable periods
🐢 2. Gradualism
- Evolution happens slowly and steadily over a long time
- Small genetic changes accumulate in the population over thousands to millions of years
Example:
Gradual changes in horse limb structure in fossil records
➡️ Slow, continuous change
🧠 Summary Table:
| Pattern | Speed | Trigger | Example |
|---|---|---|---|
| Punctuated Equilibrium | Fast (in bursts) | Sudden environmental shift | Cambrian explosion |
| Gradualism | Slow & steady | Long-term natural selection | Evolution of whale ancestors |
7.10.B.2 – Divergent Evolution & Adaptive Radiation
🔀 What is Divergent Evolution?
Divergent evolution happens when one ancestral species splits into multiple different species, each adapted to different environments or niches.
➡️ These species look different over time due to different selective pressures, even though they share a common ancestor.
🌟 Example:
- Darwin’s finches on the Galapagos Islands:
- All evolved from a common ancestor
- Each species adapted to eat different types of food (seeds, insects, etc.)
- Different beak shapes = evidence of divergent evolution
What is Adaptive Radiation?
Adaptive radiation is a burst of speciation that occurs when:
- A species enters a new environment
- Or new niches become available (like after a mass extinction)
- Leads to rapid diversification
- Multiple new species evolve in a short geological time
🧠 Summary:
| Term | Meaning | Key Point |
|---|---|---|
| Divergent Evolution | Species evolve differently from a common ancestor | Caused by different habitats or niches |
| Adaptive Radiation | Fast speciation when new environments/niches are open | Rapid diversification into many species |
7.10.B.3 – Convergent Evolution
💡 What Is Convergent Evolution?
Convergent evolution happens when unrelated species evolve similar traits because they live in similar environments or face similar selective pressures—not because they share a recent common ancestor.
➡️ It’s like “evolution solving the same problem in the same way, but in different species.”
🔬 Why It Happens:
- Similar environmental challenges (e.g., needing to swim fast or stay camouflaged)
- Natural selection favors similar adaptations, even if organisms come from different evolutionary paths
🐬🦈 Example: Convergent Evolution in Burrowing Mammals
| Trait | African Golden Mole (Placental Mammal) | Australian “Mole” (Marsupial) |
|---|---|---|
| Body Shape | Streamlined for digging | Streamlined for digging |
| Front Limbs | Large paws for burrowing | Large paws for burrowing |
| Eyes | Very small or reduced | Very small or reduced |
| Nose | Tough skin pad | Tough skin pad |
| Common Ancestor | 🧬 ~140 million years ago (not mole-like) | 🧬 ~140 million years ago (not mole-like) |
➡️ Conclusion: Despite looking alike, these animals evolved similar features independently due to similar environments (digging underground), not shared ancestry.
🧠 Summary:
| Feature | Convergent Evolution |
|---|---|
| Relationship | Distantly related organisms |
| Trait similarity reason | Similar selective pressures |
| Common ancestry? | No recent common ancestor |
| Result | Similar phenotypes |
7.10.C – Mechanisms That Drive Speciation
🧬 What Is Speciation?
- Speciation is the process by which one species splits into two or more distinct species.
- This happens when populations become reproductively isolated and evolve separately.
🔑 Key Processes That Drive Speciation:
1. Reproductive Isolation
Prevents gene flow between populations. It can be:
- Prezygotic barriers (before fertilization):
- Habitat isolation
- Temporal isolation
- Behavioral isolation
- Mechanical isolation
- Gametic isolation
- Postzygotic barriers (after fertilization):
- Reduced hybrid viability
- Reduced hybrid fertility (e.g., mule)
- Hybrid breakdown – offspring of hybrids are weak or infertile
2. Genetic Divergence
Occurs due to mutations, natural selection, genetic drift, or sexual selection.
Over time, genetic differences accumulate between isolated populations.
🌎 Modes of Speciation:
| Type | Description | Example |
|---|---|---|
| Allopatric | Populations are physically separated (e.g., by a mountain or river) | Squirrels on opposite sides of Grand Canyon |
| Sympatric | Populations evolve into new species without geographic separation | Polyploidy in plants |
| Peripatric | A small group breaks away and becomes isolated | Island colonization |
| Parapatric | Neighboring populations diverge while maintaining contact zones | Grasses along a mine gradient |
🧠 Summary:
- Speciation is driven by:
- Reproductive isolation
- Genetic divergence
- Environmental pressures or random processes
🧬 It’s how biodiversity increases over time!
7.10.C.1 – Sympatric vs. Allopatric Speciation
🧬 What is Speciation Again?
Speciation = the formation of a new species from an existing one.
It happens when populations stop interbreeding and accumulate genetic differences over time.
🗺️ Two Main Types of Speciation:
1. Allopatric Speciation (“Allo” = other, “patric” = place)
- 🧱 Geographic isolation splits a population.
- No gene flow between the two groups.
- Over time, genetic divergence (due to natural selection, drift, mutation) creates reproductive isolation.
- Eventually → they become two separate species.
✅ Example:
A river splits a population of squirrels → over generations, each side evolves differently.
2. Sympatric Speciation (“Sym” = same)
- 🌍 Occurs without a physical barrier.
- Populations live in the same area, but something else prevents interbreeding:
- 🧬 Polyploidy (common in plants)
- 💃 Behavioral isolation
- 🍎 Habitat preference (e.g., apple vs. hawthorn flies)
- 🎯 Sexual selection
✅ Example:
Certain cichlid fish in African lakes evolve different colors → females only mate with specific colors → new species evolve in the same lake.
🧠 Summary:
| Type | Barrier Type | Location | Example |
|---|---|---|---|
| Allopatric | Physical | Different areas | Mountain divides a population |
| Sympatric | Biological | Same area | Polyploidy in plants 🌸 |
7.10.C.2 – Mechanisms of Reproductive Isolation
🧠 What Is Reproductive Isolation?
Reproductive isolation means two populations can’t successfully interbreed — either because they never mate or because their offspring don’t survive or reproduce.
This isolation is what allows speciation to happen!
✨ Two Main Categories:
1️⃣ Prezygotic Barriers (Before fertilization)
These barriers prevent mating or fertilization between species.
| Type | What It Means | Example |
|---|---|---|
| Habitat Isolation | Live in different habitats, so don’t meet | One snake in water, one on land |
| Temporal Isolation | Breed at different times (day, season) | Flowers blooming in different seasons |
| Behavioral Isolation | Different courtship behaviors | Bird songs or mating dances differ |
| Mechanical Isolation | Physical incompatibility of reproductive parts | Flower shape doesn’t match pollinator |
| Gametic Isolation | Sperm and egg can’t fuse | Sea urchin sperm won’t fertilize other species’ eggs |
2️⃣ Postzygotic Barriers (After fertilization)
These barriers happen after mating – they affect the offspring.
| Type | What It Means | Example |
|---|---|---|
| Reduced Hybrid Viability | Hybrid embryo doesn’t fully develop or survive | Offspring dies early |
| Reduced Hybrid Fertility | Hybrid is healthy but can’t reproduce | Mule (horse × donkey) 🐴 |
| Hybrid Breakdown | First generation is fine, but later ones are weak/infertile | Some plant hybrids |
🧬 Summary:
- These reproductive barriers help keep gene pools separate ➡️ maintain speciation and diversity.
- Without them, populations would mix and evolution wouldn’t lead to new species.