Chapter 24 Notes: The Origin of Species

The Origin of Species

Introduction

• Speciation is the process by which one species splits into two or more species.
• Darwin was fascinated by speciation.
• It explains both the tremendous diversity and the unity of life.

The Biological Species Concept

• A species is 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 a species together genetically.

Reproductive Isolation

• Reproductive isolation occurs when biological barriers prevent members of two species from interbreeding and producing viable, fertile offspring.
• These barriers limit the formation of hybrids, which are offspring resulting from interspecific mating.
• Reproductive isolation can be classified based on whether factors act before or after fertilization.

Prezygotic Barriers

• Prezygotic barriers block 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

• Two species that occupy different habitats within the same area may encounter each other rarely, if at all.
  • Example: Apple maggot flies and blueberry maggot flies are isolated because they feed and lay eggs on different fruits.

Temporal Isolation

• Species that breed at different times of day, in different seasons, or in different years cannot mix their gametes.
  • Example: Western spotted skunks mate in the summer, while eastern spotted skunks mate in the winter.

Behavioral Isolation

• Courtship rituals and other behaviors unique to a species are effective barriers to mating.
  • Example: Blue-footed boobies mate only after a unique courtship display.

Mechanical Isolation

• Mating is attempted, but morphological differences prevent its successful completion.
  • Example: The genital openings of snails in the genus Bradybaena do not align if their shells spiral in opposite directions.

Gametic Isolation

• Sperm of one species may not be able to fertilize eggs of another species.
  • Example: Surface proteins on the sperm and eggs of different sea urchin species bind poorly to each other, preventing fusion and zygote formation.

Postzygotic Barriers

• Postzygotic barriers prevent hybrid zygotes from developing into viable, fertile adults through:
  • Reduced hybrid viability.
  • Reduced hybrid fertility.
  • Hybrid breakdown.

Reduced Hybrid Viability

• Genes of different parent species may interact in ways that impair the hybrid’s development or survival in its environment.
  • Example: Hybrid offspring of different subspecies of salamanders of the genus Ensatina do not usually complete development.

Reduced Hybrid Fertility

• Meiosis may fail to produce normal gametes, resulting in sterility if the parent species have chromosomes of different number or structure.
  • Example: A mule, the hybrid offspring of a male donkey and a female horse, is robust but sterile.

Hybrid Breakdown

• First-generation hybrids are viable and fertile, but offspring in the next generation are feeble or sterile.
  • Example: Hybrids between certain strains of cultivated rice are vigorous and fertile, but members of the next generation are small and sterile.

Modes of Speciation

• Speciation can occur in two main ways:
  • Allopatric speciation: populations are geographically isolated.
  • Sympatric speciation: populations are not geographically isolated.

Allopatric Speciation ("Other Country")

• Gene flow is interrupted when a population is divided into geographically isolated subpopulations.
  • Example: A lake may subside and form two lakes with separated populations.
• The effectiveness of a geographic barrier depends on the organisms' ability to move about.
  • Example: A canyon may be a barrier for small rodents but not for birds, coyotes, or the pollen and seeds of flowering plants.

Sympatric Speciation ("Same Country")

• Speciation occurs in populations that live in the same geographic area.
• Sympatric speciation is less common than allopatric speciation.
• It can occur if gene flow is reduced by factors such as:
  • Polyploidy
  • Sexual selection
  • Habitat differentiation

Polyploidy

• Accidents during cell division can cause polyploidy, which is the presence of extra sets of chromosomes. This process can form a new species within a single generation without geographic separation.
• Polyploidy is common in plants but rare in animals.
• Types of polyploids:
  • Autopolyploids: have more than two sets of chromosomes, all derived from a single species.
  • Allopolyploids: have more than two sets of chromosomes, derived from different species.
  • Example of Autopolyploid: A diploid cell with 2n = 6 undergoes cell division error and meiosis, resulting in a tetraploid cell with 4n. The gametes produced by tetraploids can then create a new species.
  • Example of Allopolyploid: A diploid cell from species A (2n = 6) and a diploid cell from species B (2n = 4) produce normal gametes with n = 3 and n = 2 respectively. These gametes form a sterile hybrid zygote with n = 5. A mitotic or meiotic error in a hybrid plant cell doubles the chromosome number creating a diploid cell with 2n = 10, resulting in a viable, fertile allopolyploid.

Sexual Selection

• Sympatric speciation can be driven by sexual selection.
  • Example: Speciation of cichlids in Lake Victoria was likely driven by female mate choice based on male breeding coloration.

Habitat Differentiation

• Sympatric speciation can also result from the exploitation of new habitats or resources.
  • Example: Apple maggot flies evolved in North America after switching hosts from hawthorn to apple.
  • Maggot flies mate on their host plant, resulting in habitat isolation between groups using different hosts.
  • Apple-feeding flies develop faster than hawthorn-feeding flies, resulting in temporal isolation.
  • Alleles that benefit flies using one host plant harm those using the other, causing post-zygotic isolation.

From Speciation to Macroevolution

• Differences accumulate with successive speciation events; eventually new groups of organisms form that differ greatly from their ancestors.
• Other groups shrink in size as species are lost to extinction.
• Macroevolution is the cumulative effect of many speciation and extinction events.A diploid cell from species A 2n = 6 and a diploid cell from species B 2n = 4 produce normal gametes with n = 3 and n = 2 respectively. These gametes form a sterile hybrid zygote with n = 5. A mitotic or meiotic error in a hybrid plant cell doubles the chromosome number creating a diploid cell with 2n = 10, resulting in a viable, fertile allopolyploid.