Overview
This lecture covers the definition of species and the different species concepts, as well as reproductive barriers that lead to the formation of new species (speciation).
Speciation and Evolution
- Microevolution involves small genetic changes within a population, but members remain the same species.
- Speciation is the process by which one species splits into two or more, increasing biodiversity.
- Over long periods, speciation results in the vast diversity of life we see today.
Defining a Species: Four Concepts
- There are multiple definitions of species because a single definition does not fit all organisms.
- Four main species concepts are covered: biological, morphological, ecological, and phylogenetic.
Biological Species Concept
- Species are groups of populations that can interbreed and produce fertile offspring in nature.
- Reproductive isolation maintains species boundaries; organisms that do not interbreed are different species.
- Issues: hybridization (e.g., polar bears and grizzly bears), extinct organisms (e.g., Neanderthals), and asexual organisms.
Morphological Species Concept
- Species are defined by physical traits and appearance; useful for fossils and asexual organisms.
- Drawback: subjective decisions about which traits to use and where to set cutoffs.
Ecological Species Concept
- Species are defined by their ecological niche, including their role, diet, and habitat.
- Can distinguish similar-looking organisms based on behavior or habitat needs.
Phylogenetic Species Concept
- Species are the smallest group sharing a common ancestor, forming a unique branch on the tree of life.
- Relies on genetic and biochemical analysis, but the cutoff for differences is subjective.
Reproductive Isolation and Barriers
- Species concepts often depend on reproductive barriers that prevent gene flow between groups.
- Barriers are categorized as prezygotic (before fertilization) or postzygotic (after fertilization).
- A zygote is a fertilized egg cell.
Prezygotic Barriers (prevent fertilization)
- Habitat Isolation: species occupy different habitats within the same area.
- Temporal Isolation: species breed at different times (seasons, times of day, or years).
- Behavioral Isolation: species have different courtship rituals or mating behaviors.
- Mechanical Isolation: anatomical differences prevent successful mating.
- Gametic Isolation: gametes (egg and sperm) are incompatible and cannot fuse.
Postzygotic Barriers (after zygote forms)
- Reduced Hybrid Viability: hybrids are weak or unhealthy and do not survive well.
- Reduced Hybrid Fertility: hybrids are healthy but sterile (e.g., mule from horse and donkey).
- Hybrid Breakdown: first-generation hybrids are viable and fertile, but their offspring are feeble or sterile.
Natural Selection and Reproductive Barriers
- Natural selection favors prezygotic barriers because they prevent wasted resources on unviable or sterile hybrids.
Key Terms & Definitions
- Microevolution — Small genetic changes within a population across generations.
- Speciation — The process by which one species splits into two or more.
- Species Concept — Criteria used to define what a species is.
- Reproductive Isolation — Prevention of gene flow between groups, maintaining separate species.
- Zygote — A fertilized egg formed by the fusion of sperm and egg.
- Hybrid — Offspring from parents of two different species.
- Prezygotic Barrier — Reproductive barrier before fertilization.
- Postzygotic Barrier — Reproductive barrier after fertilization.
Action Items / Next Steps
- Review Campbell Biology Chapter 24 on species concepts and reproductive barriers.
- Understand and be able to compare the four species concepts and examples of reproductive barriers.
- Prepare for the next lecture on mechanisms of speciation.