Lecture Notes: Speciation and Reproductive Isolation

VOCABULARY-style flashcards covering key terms and definitions from the lecture notes on the origin of new species, reproductive isolation, and related concepts.

Gene flow

Exchange of alleles between populations that holds the gene pool of a species together.

Barrier to gene flow

Any factor that reduces or prevents gene flow between populations, allowing divergence.

Reproductive isolation

Biological barriers that prevent interbreeding between species or produce non-viable/sterile hybrids.

Speciation

Process by which one species splits into two or more species, increasing biodiversity and sharing ancestry.

Microevolution

Changes in allele frequencies within a population over time.

Macroevolution

Broad evolutionary changes above the species level.

Biological Species Concept

A species is a group of populations that can interbreed in nature and produce viable, fertile offspring, and cannot interbreed with other species.

Morphology

Body form used to distinguish between species.

Prezygotic barriers

Barriers that prevent fertilization from occurring before or during mating.

Habitat isolation

Two species occupy different habitats and rarely encounter each other.

Temporal isolation

Different breeding times (time of day, season, or year) prevent interbreeding.

Behavioral isolation

Species-specific courtship rituals and behaviors prevent mating between species.

Mechanical isolation

Morphological differences impede successful mating.

Gametic isolation

Sperm of one species cannot fertilize the eggs of another species.

Postzygotic barriers

Barriers that act after fertilization to prevent viable or fertile offspring.

Reduced hybrid viability

Hybrids have lower viability or survival in the environment.

Reduced hybrid fertility

Hybrids are sterile or have greatly reduced fertility.

Hybrid breakdown

First-generation hybrids are viable/fertile, but subsequent generations are weak or sterile.

Allopatric speciation

Speciation that occurs when populations are geographically isolated, interrupting gene flow.

Sympatric speciation

Speciation without geographic separation, often via non-geographic barriers.

Polyploidy

Presence of extra sets of chromosomes due to errors in cell division.

Autopolyploidy

Polyploidy where extra chromosome sets come from a single species.

Tetraploid (4n)

An organism with four sets of chromosomes; can arise from autopolyploidy.

Triploid (3n)

An organism with three sets of chromosomes; often sterile.

Allopolyploidy

Polyploid with chromosome sets from different species, formed by hybridization.

Tragopogon (goatsbeard)

Example where diploid species interbred to form new tetraploid species.

Triticum aestivum

Bread wheat; allohexaploid (6n) example of polyploid crops.

Habitat differentiation

Sympatric speciation via exploitation of a new habitat or resource.

Sexual selection

Differential mating success based on traits like coloration, driving sympatric speciation.

Lake Victoria cichlids

Many species driven by female mate choice based on breeding coloration.

Hybrid zones

Regions where individuals of different species meet and mate, producing hybrids.

Reinforcement

Natural selection strengthens reproductive barriers to reduce hybridization.

Fusion

Gene flow weakens barriers, and the parent species may fuse into one.

Stability

Hybrids continue to be produced in stable hybrid zones; barriers persist but hybrids survive.

Environmental change and hybrid zones

Environmental shifts move hybrid zones or create new ones by altering species’ contact zones.

Intrinsic barriers

Biological barriers that prevent interbreeding, not just physical separation.

Allopatric speciation evidence (Isthmus of Panama)

Geographic separation of populations (snapping shrimp) led to diverging sister species.

Patterns in the fossil record

Appearances, stasis, and abrupt changes; punctuated equilibria vs gradualism.

Punctuated equilibria

Rapid bursts of speciation with long periods of little change.

Gradualism

Slow, steady accumulation of changes over time.

Speciation rates

Rates vary; average around 6.5 million years; ranges from 4,000 to 40 million years.

Genetics of speciation: single-gene effects

Some cases: one gene can control barriers (e.g., shell spirality in Euhadra snails).

Mimulus and the yup gene

Alleles at the yup locus affect flower color and pollinator preference, reducing interbreeding.

Helianthus and polygenic isolation

Isolation in sunflowers can involve many genes (at least 26 chromosome segments) influencing barriers.

Macroevolution

Large-scale evolutionary change resulting from many speciation and extinction events.

Speciation by changes in ploidy

Speciation driven by changes in chromosome number (polyploidy).

Hybridization and speciation in sunflowers

Hybridization can lead to rapid speciation and reproductive isolation.

Time course of speciation examples

Some events occur rapidly; others over millions of years; varied intervals between events.

Isolation without geographic barriers

Reproductive barriers can arise intrinsically, without physical separation.

Speciation without geographic separation (examples)

Sympatric speciation via polyploidy, habitat differentiation, or sexual selection.

“Mystery of mysteries” (Darwin on New Species)

Darwin described observing newly formed species as a profound mystery.

Galápagos Islands and speciation

Islands with unique species formed relatively recently, noted by Darwin.