Lecture 9: Speciation and Reproductive Isolation

Flashcards covering key concepts from Lecture 9 on evolution, speciation, and reproductive isolation, including different species concepts, mechanisms of isolation (prezygotic and postzygotic), and specific forms of speciation (allopatric, sympatric, adaptive radiation, ring species).

Speciation

When microevolutionary changes add up to a big evolutionary moment, resulting in new species evolving from pre-existing species (macroevolution).

Macroevolution

A big evolutionary moment, such as the evolution of new species through speciation, which results from the accumulation of microevolutionary changes.

Morphological species concept

Identifying species based on observing visible anatomical characteristics and their appearance.

Biological species concept (BSC)

Groups of actually or potentially interbreeding natural populations which are reproductively isolated from other groups, defining what makes up a species. Developed by Ernst Mayer in 1942.

Reproductively isolated

The state where different species cannot mate together to produce fertile offspring, a key aspect of the Biological Species Concept.

Prezygotic isolating mechanisms

Mechanisms that prevent genetic exchange between species before fertilization or before a zygote gets formed, typically by preventing sex or fertilization from occurring.

Postzygotic isolating mechanisms

Mechanisms that prevent genetic exchange between species after a zygote gets formed, typically by preventing the proper functioning of zygotes or their offspring.

Habitat isolation

A prezygotic isolating mechanism where species occupy different habitats or niches, thereby rarely coming into contact and preventing breeding; includes geographic and ecological isolation.

Geographic isolation

A type of habitat isolation where a physical barrier (e.g., ocean, mountain range, large puddle for small organisms) separates two populations, preventing gene flow.

Allopatric

Describes populations that are physically or geographically separated from each other.

Sympatric

Describes populations that are not physically separated and are in the same place at the same time.

Ecological isolation

A type of habitat isolation where, even if sympatric, species use different parts of the same environment and therefore rarely meet, preventing breeding (e.g., lions in grasslands and tigers in forests).

Temporal isolation

A prezygotic isolating mechanism where species reproduce in different seasons or at different times of the day, preventing hybridization (e.g., flowers opening at night vs. day).

Behavioral isolation

A prezygotic isolating mechanism where distinct mating rituals, courtship displays, or signals (e.g., bird dances, mating calls) prevent members of different species from attracting each other.

Mechanical isolation

A prezygotic isolating mechanism involving physical differences in reproductive structures (e.g., penis shapes in insects, flower structures limiting pollinator access) that prevent successful mating or gamete transfer.

Gametic isolation

A prezygotic isolating mechanism where chemical cues from the egg fail to attract sperm of another species or prevent foreign sperm from fertilizing the egg.

Broadcast spawners

Species that release their gametes directly into the water, relying on chemical cues and mechanisms like gametic isolation to ensure correct fertilization (e.g., abalone, corals).

Hybrid inviability

A postzygotic isolating mechanism where, when two species cross, the embryo does not develop or, if born, does not survive to adulthood.

Hybrid infertility

A postzygotic isolating mechanism where hybrids between two species survive but are infertile (cannot produce viable offspring), such as a mule (donkey-horse hybrid).

Hybrid breakdown

A postzygotic isolating mechanism where initial hybrids are viable and fertile, but their offspring (F2 generation or later) become infertile (e.g., offspring of ligers are infertile).

Allopatric speciation

The process of speciation that occurs in allopatry, where populations become new species due to physical separation by a geographic barrier preventing gene flow.

Dispersal isolation

A mechanism of allopatric speciation where a small number of individuals leave a population to establish a new, isolated population (founder effect), leading to differentiation (e.g., species on Galapagos Islands).

Vicariance isolation

A mechanism of allopatric speciation where a new geographic barrier emerges and splits an ancestral population into two or more isolated populations (e.g., a river separating beetles, continental drift separating cycads).

Missing link isolation

A mechanism of allopatric speciation in a chain of populations where an intermediate population goes extinct, creating a gap too large for gene flow between the remaining populations, leading to their isolation.

Polyploidy

A rapid mechanism of sympatric speciation, especially common in plants, where an error in meiosis results in an individual with multiple sets of chromosomes (e.g., tetraploid), which is reproductively isolated from its parent species in a single generation.

Tetraploid

An organism or cell that possesses four complete sets of chromosomes (4n) instead of the usual two, often resulting from polyploidy.

Disruptive selection

A type of natural selection that favors individuals at both extremes of a phenotypic range over intermediate phenotypes, which, when combined with assortative mating, can contribute to sympatric speciation (e.g., pea aphids specializing on different host plants).

Assortative mating

A mating pattern where individuals are more likely to mate with others who share a similar phenotype or trait, which can perpetuate differences within a population and facilitate sympatric speciation.

Adaptive radiation

Rapid speciation that occurs when an organism enters a new environment with abundant and underutilized resources, leading to the evolution of many diverse species adapted to exploit different aspects of that environment (e.g., Darwin's finches, Hawaiian tarweed, Anolis lizards of the Caribbean).

Incomplete speciation

A situation where populations in different geographical areas exhibit some phenotypic variation but can still interbreed to produce viable offspring where their ranges meet, often considered subspecies or local variants (e.g., Wild lettuce (Lactuca sp.) varieties whose ranges overlap).

Ring species

A unique case of partial speciation where a series of interbreeding populations forms a ring around an uninhabitable area, and adjacent populations can interbreed, but the two ends of the ring, where they geographically meet, are reproductively isolated from each other (e.g., Ensatina salamanders, California rat snakes).