Speciation and Reproductive Isolation

• New species come from evolutionary processes.
• Populations that are isolated by biological barriers can spur the creation of new species.
• Geographically and reproductively isolated populations evolve independently.
• Isolation can be allopatric with geographic separation or sympatric where species share the same area.

Pre-Zygotic Barriers

• For animals to be classified as different species, reproductive isolation must exist.
• Reproductive isolation means genetic code cannot pass through reproduction for several generations.
• Morphology and DNA sharing help determine species classification.
• Types of pre-zygotic isolation include geographic, ecological, temporal, behavioral, mechanical, and gametic.

Geographic Isolation

• Physical barriers cause geographic isolation where species occur in different areas.
• Geographic isolation is a form of allopatric speciation.
• Harris's antelope squirrels on the canyon's south rim, and the closely related white-tailed antelope on the north rim, exemplify species that are geographically isolated.

Ecological Isolation

• Ecological isolation is a type of sympatric isolation.
• It occurs when species occupy different habitats in the same region and rarely encounter one another.
• Garter snake species of the Thamnophis family that occupy the same area, but live in different types of habitats are an example of ecological isolation.
• Lions and tigers could hybridize, but they live in different habitats.

Temporal Isolation

• Temporal isolation occurs when species that breed during different times cannot mix gametes.
• Temporal isolation is a type of sympatric speciation.
• Eastern spotted skunks that mate in late winter and western spotted skunks exemplify temporal isolation.

Behavioral Isolation

• Unique behavioral patterns and rituals isolate species.
• Specific behaviors identify species members and attract mates of the same species.
• Courtship rituals and mating calls are examples of behaviors that cause reproductive isolation.
• Blue-footed boobies only mate after a unique courtship display.

Mechanical Isolation

• Morphological differences in anatomy can prevent successful mating and cause mechanical isolation.
• Closely related plant species can have distinct flower appearances that attract different pollinators, leading to mechanical isolation.

Gametic Isolation

• Gametic isolation occurs when sperm from one species cannot fertilize the eggs of another species.
• Sperm is prevented from penetrating an egg via a biochemical barrier where receptor recognition is like a lock and key.
• Chemical incompatibility occurs when sperm cannot survive in the female reproductive tract.
• Sea urchins release sperm and eggs into surrounding water and form zygotes, different species of red and purple are unable to fuse.

Post-Zygotic Barriers

• Post-zygotic barriers prevent hybrid offspring from developing into viable, fertile adults.
• Reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown are all barriers.

Reduced Hybrid Viability

• Genes of different parent species may interact and impair the hybrid's development.
• Salamander species of the Ensatina genus may interbreed, but most hybrids do not complete development, and the ones that do are frail.

Reduced Hybrid Fertility

• Even if hybrids are viable and healthy, they may still be sterile.
• Chromosomes of parents differ in number or structure, and meiosis in hybrids fails to produce normal gametes.
• Horses have 64 chromosomes, donkeys have 62, and mules have 63, making them sterile.

Hybrid Breakdown

• Hybrids are fertile and viable in the first generation, however, when they mate, offspring are feeble or sterile.
• In some strains of cultivated rice, hybrids are vigorous, but the next generation is small and sterile.