Overview
This lesson explores what defines a species, how new species form through reproductive isolation, and the mechanisms of speciation using examples from ligers, mules, Darwin's finches, and dog breeds.
What Defines a Species
- Humans (Homo sapiens) are the only surviving member of genus Homo; extinct relatives include Homo erectus and Homo neanderthalensis.
- A species is a group of organisms that can interbreed and produce fertile offspring.
- Producing babies is not enough; those offspring must also be able to reproduce.
- Two animals from different species can sometimes produce offspring, but those offspring are typically sterile (e.g., ligers, mules).
Reproductive Isolation
Reproductive isolation occurs when two populations of the same species can no longer mate successfully. This is required for speciation.
| Type | Timing | Examples | Mechanism |
|---|
| Post-zygotic | After fertilization | Ligers (lion + tiger), mules (donkey + horse) | Offspring are sterile, not viable, or cause maternal death |
| Pre-zygotic | Before fertilization | Behavioral changes (different bird songs), temporal isolation (diurnal vs. nocturnal), geographic barriers | Prevents mating from occurring in the first place |
- Post-zygotic: Parents can form a zygote, but the lineage ends there due to sterility or other issues.
- Mules result from donkey sperm fertilizing horse eggs; mismatched chromosomes prevent sex cell production.
- Pre-zygotic: Isolation happens before an egg is fertilized through behavioral, temporal, or geographic factors.
Types of Speciation
| Type | Meaning | Cause | Example |
|---|
| Allopatric | "Different countries" | Geographic isolation (distance, physical barriers) | Darwin's finches on different Galapagos islands |
| Sympatric | "Same country" | Reproductive isolation without geographic separation | Hybrid finch lineage on Daphne Major island |
- Allopatric speciation occurs when populations are separated by rivers, mountains, or distances, preventing gene flow.
- Different environmental conditions lead to different traits (e.g., thick coats in cold vs. sweat glands in warmth).
- Natural selection propagates traits that improve survival; Charles Darwin identified this mechanism through his Galapagos observations.
- Sympatric speciation happens when populations diverge without geographic separation, through behavioral or other reproductive barriers.
Darwin's Finches - Grant Study
- Peter and Rosemary Grant studied finches on Daphne Major island since 1973.
- In 1981, a hybrid ground finch-cactus finch arrived from another island; larger with a wide beak and unique song.
- The immigrant finch mated with a local ground finch; researchers tracked descendants for 28 years.
- After a severe drought (around generation 4), only two surviving descendants mated with each other.
- Their descendants developed a distinctive song different from other local finches.
- By December 2009, this lineage became genetically isolated from other island finches, demonstrating rapid sympatric speciation.
- This contradicts Darwin's belief that speciation was imperceptibly slow.
Dog Breeds Example
- Different dog breeds (corgis, greyhounds, chihuahuas, Irish wolfhounds) remain the same species because they can produce fertile offspring.
- Dog breeding represents artificial selection rather than natural selection.
- Pre-zygotic obstacles exist for extreme pairings (e.g., chihuahua and wolfhound) due to physical difficulties in mating and birthing.
- If isolated populations of different-sized dogs were separated, gene flow would stop, leading to reproductive isolation.
- Over time, they could become different enough to form separate species.
Key Terms & Definitions
- Species: Group of organisms that can interbreed and produce fertile offspring.
- Hybrids: Offspring from cross-breeding two distinct species; typically sterile.
- Evolution: Change in heritable characteristics of a species across generations.
- Speciation: Process by which new species form.
- Natural selection: Propagation of traits that enhance survival and reproduction.
- Gene flow: Transfer of genetic material between populations.