Overview
This lecture explains the biological concept of species, how new species form (speciation), and the roles of reproductive isolation in this process.
What is a Species?
- Organisms of the same species can interbreed and produce fertile offspring.
- All breeds of domesticated dogs are a single species, as are all breeds of domesticated cats.
- Different species cannot usually produce fertile offspring (e.g., a zonkey, a hybrid of donkey and zebra, is sterile).
Speciation and Isolation
- Speciation is the process by which a new species develops.
- Populations must be reproductively isolated for speciation to occur.
- Isolation separates gene pools so evolutionary mechanisms act independently on populations.
Types of Speciation
- Allopatric speciation occurs when a geographic barrier (mountain, river) physically separates populations.
- Sympatric speciation happens without geographic barriers; populations are isolated by other means within the same area.
Reproductive Isolation Mechanisms
Prezygotic Barriers (Before Fertilization)
- Behavioral isolation: Different mating behaviors, such as bird songs, prevent interbreeding.
- Temporal isolation: Species breed at different times or seasons.
- Habitat isolation: Species live in the same region but occupy different habitats.
Postzygotic Barriers (After Fertilization)
- Offspring of different species may be sterile (e.g., zonkey).
- Hybrids can be weak and may not survive long.
- Genetic incompatibility may prevent hybrid embryos from developing.
Key Terms & Definitions
- Species — group of organisms able to interbreed and produce fertile offspring.
- Speciation — development of a new species.
- Allopatric speciation — speciation due to geographic separation.
- Sympatric speciation — speciation without geographic separation.
- Prezygotic barrier — reproductive barrier that prevents fertilization.
- Postzygotic barrier — barrier that occurs after fertilization, affecting hybrid offspring.
Action Items / Next Steps
- Explore additional types of reproductive isolation beyond the examples given.
- Watch the suggested videos on classification and natural selection.