Key Concepts of Biological Species Concept and Evolution

What is a species?
- A species is commonly defined as a group of populations that can interbreed and produce viable offspring.
- Example: Eastern Spotted Skunk vs. Western Spotted Skunk; although visually similar, they are different species.
The biological species concept primarily applies to sexual organisms; it is not applicable to asexual reproducing organisms like bacteria.

Reproductive barriers are mechanisms that prevent species from interbreeding. These barriers can be classified as:
1. Prezygotic Barriers
2. Postzygotic Barriers

Spatial Isolation: Species occupy different habitats.
- Example: White-throated sparrow (dense thickets) and White-crowned sparrow (open meadows).
Behavioral Isolation: Differences in mating behavior prevent breeding; for instance, different courtship rituals.
Temporal Isolation: Species breed at different times, e.g., Monterey pine and Bishop’s pine release pollen at different months.
Mechanical Isolation: Anatomical differences prevent mating even if species come in contact, such as incompatible flower shapes and pollinators.
Gametic Isolation: Sperm and egg may meet, but fertilization fails.

Definition: Barriers occur after fertilization that prevent the hybrid zygote from developing into a viable, fertile adult.

Reduced Hybrid Viability: Hybrid zygotes do not survive or grow into healthy organisms.
- Example: Ranunculus plants adapted to different habitats produce hybrids that are poorly adapted.
Hybrid Sterility: Hybrids may be vigorous but sterile due to differing chromosome numbers, preventing successful reproduction (e.g., mules, hybrids of horses and donkeys).
Hybrid Breakdown: First-generation hybrids are viable but their offspring are sterile or unhealthy. Example: Cotton plants.

Definition: Process by which one species diverges into two.
Requirements: Populations must become isolated followed by genetic divergence (mutations).
Allopatric Speciation: One population is geographically isolated from another. This leads to genetic divergence.
- Example: River separating squirrel populations leading to speciation over time.
- Example of Galápagos finches, which evolved into multiple species due to geographic isolation.

When discussing species relatedness, we refer to a common ancestor shared by different species.
Closer species in genetic terms have a recent common ancestor.
Humans are closely related to chimpanzees but distantly related to lemurs based on evolutionary history.

Convergent Evolution: Independent evolution of similar traits in unrelated species due to adaptation to similar environments.
- Example: Cacti and Hoodia plants exhibiting similar features but not closely related.
Divergent Evolution: Groups from a common ancestor evolve and gather differences, potentially forming new species.
- Example: The African golden mole resembles a shrew in appearance but is more closely related to elephants based on DNA analysis.

Gradualism: Slow, steady change over time, accumulating beneficial adaptations.
- Example: Evolution of horse toe structure over millions of years.
Punctuated Equilibrium: Rapid bursts of change followed by long periods of stability.
- Example: Resistance in bacteria and insects to pesticides.

Organisms are grouped into one of three domains: Bacteria, Archaea, and Eukarya.
Each organism is categorized successively into Kingdom, Phylum, Class, Order, Family, Genus, and Species for more precision.
Binomial Nomenclature: Each species is given a two-part name (Genus + Species).
- Example: Homo sapiens for humans.
- Note: The genus name is capitalized, and the species name is lowercase; both should be italicized when typed.

Eukaryotic cells likely evolved from prokaryotic cells through:
1. Development of Endomembrane System: Infolding of plasma membrane leading to the formation of organelles.
2. Endosymbiosis: Mitochondria and chloroplasts originated as independent prokaryotic cells engulfed by a larger host cell and formed symbiotic relationships.