Concept 24.1: The biological species concept emphasizes reproductive isolation

Flashcards from Concept 24.1 of Pearson's Campbell Biology, Twelfth Edition.

Speciation

The process by which one species splits into two species

• This produced the tremendous diversity of life, and helps to explain the unity of life

• This also bridges micro- and macro- evolution

Microevolution

Changes in allele frequency in a population over time

Macroevolution

Broad patterns of evolutionary change above the species level

Species

A group of populations whose members:

• Have the potential to interbreed in nature

• Produce viable, fertile offspring

• Do not produce viable, fertile offspring with members of other such groups

Gene flow between populations holds one of these together genetically

Biological species concept

The concept that defines what a species is and how one arises

• Cannot be applied to all organisms, such as fossils or prokaryotes, because mating cannot be observed

• Furthermore, while this model emphasizes absence of gene flow, gene flow occurs between many distinct species — like grizzly bears and polar bears, on occasion

Reproductive isolation

A process that results when biological barriers impede members of two species from interbreeding and producing viable, fertile offspring, limiting the formation of hybrids

• Classified by whether factors act before or after fertilization

Hybrids

Offspring that result from interspecific mating

Prezygotic barriers

Barriers that enact reproductive isolation, blocking fertilization from occurring by:

• Impeding different species from attempting to mate

• Preventing the successful completion of mating

• Hindering fertilization if mating is successful

Habitat isolation

A prezygotic barrier that states that two species occupying different habitats within the same area encounter each other rarely, if at all

• Apple maggot flies are isolated from blueberry maggot flies because they feed and lay eggs on different fruits

Temporal isolation

A prezygotic barrier that states that two species breeding at different times of the day, in different seasons, or different years cannot mix their gametes

• Western spotted skunks mate in summer, and eastern spotted skunks mate in winter

Behavioral isolation

A prezygotic barrier that states that courtship rituals and other behaviors unique to a species are effective barriers to mating

• Many species, such as sula nebouxii, mate only after a unique courtship display

Mechanical isolation

A prezygotic barrier that states that mating is attempted, but morphological differences prevent its successful completion

• The reproductive openings of snails in the genus Bradybaena do not align if their shells spiral in opposite directions

Gametic isolation

A prezygotic barrier that states that sperm of one species may not be able to fertilize eggs of another species

• Surface proteins on the sperm and eggs of different sea urchin species bind poorly to each other, preventing fusion and zygote formation

Postzygotic barriers

Barriers that enact reproductive isolation by preventing hybrid zygotes from developing into viable, fertile adults through:

• Reduced hybrid viability

• Reduced hybrid fertility

• Hybrid breakdown

Reduced hybrid viability

A postzygotic barrier that states that the genes of different parent species may interact in ways that impair the hybrid’s development or survival in its environment

• The hybrid offspring of different subspecies of salamanders of the genus Ensatina do not usually complete development

Reduced hybrid fertility

A postzygotic barrier that states that meiosis may fail to produce normal gametes, resulting in sterility, if the parent species have chromosomes of different number or structure

• The hybrid offspring of a male donkey and a female horse, a mule, is robust, but sterile

Hybrid breakdown

A postzygotic barrier that states that first-generation hybrids are viable and fertile, but offspring in the next generation are feeble or sterile

• Hybrids between certain strains of cultivated rice are vigorous and fertile, but members of the next generation are small and sterile