Lecture 2.7: The Origin of Species

What is a Species?

The Biological Species Concept

• A population or a group of populations whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species.
• Key Ideas:
  • They produce offspring in a “natural environment.”
    • Fertile hybrids can sometimes be produced in a zoo but are never seen in the wild.
    • Strange hybrid animals
  • If hybridization occurs between two separate species the offspring are not fertile.
    • Example: horse + donkey → mule (infertile)

The Hallmark of the Species Concept: Reproductive Isolation

See pages 445–448 in Campbell's Biology, 5th Edition for details on these types of reproductive isolation.

• Prezygotic Barriers
  • Habitat Isolation
  • Behavioral Isolation
  • Temporal Isolation
  • Mechanical Isolation
  • Gametic Isolation
• Postzygotic Barriers
  • Reduced Hybrid Viability
  • Reduced Hybrid Fertility
  • Hybrid Breakdown

Modes of Speciation

Interrupting gene flow is the key to reproductive isolation and therefore to creating new species. There are two ways to do this:

• Allopatric Speciation
  • Allo: other
  • Patric: country
  • Populations evolve into two new species in “other countries.”
  • A geographic barrier physically isolates gene flow between populations.
  • Example:
    • The north and south rims of the Grand Canyon each have a different species of squirrel (see below).
    • Presumably this speciation event occurred because of the geographic separation due to the appearance of the canyon.
    • What was once one species of squirrel is now two separate species.
• Sympatric Speciation
  • Sym: together
  • Two new species evolve in the same geographic region (“together in the country”).
  • Gene flow is prevented by chromosomal changes.
    • Occurs as an accident during cell division.
    • Extra sets of chromosomes appear: polyploidy
      • Word derivation:
        • Poly: many
        • Ploid: fold
      • Two kinds of polyploidy:
        • Autopolyploidy
        • Allopolyploidy
    • This is common in plants.

Figure 24.7, page 451, Campbell's Biology, 5th Edition

Polyploidy

Autopolyploidy

• This occurs when an individual has more than two sets of chromosomes (4n), all derived from an original species (2n).
• Word derivation:
  • Auto: self
  • Poly: many
  • Ploid: fold (chromosome)
  • Autopolyploid: having many chromosomes from their self (their species)
• How does this occur?
  • During normal meiosis the chromosome number is reduced by half (2n to n).
  • In autopolyploidy, a failure in meiosis causes the chromosomes to fail to separate, resulting in gametes with twice as many chromosomes as normal (2n).
  • If this organism fertilizes itself then it offspring will be tetraploid (4n).
• Because this offspring has twice as many chromosomes as its “parent” species, it is unable to interbreed with it and is therefore now a separate species.

Figure 24.10(a), page 454, Campbell's Biology, 5th Edition

Allopolyploidy

• This occurs when two different species contribute to a polyploid hybrid.
• Word derivation:
  • Allo: other
  • Poly: many
  • Ploid: fold (chromosome)
  • Allopolyploid: having many chromosomes from others
• How does this occur? It is not easy:
  • Interspecific hybrids (hybrids created from two different species) are usually sterile because the haploid set of chromosomes from one species cannot pair with the haploid set of chromosomes from the other species during meiosis.
  • However, the hybrids are usually very vigorous and can propagate asexually (assuming they're plants).
  • Eventually these sterile hybrids can be transformed into fertile polyploids through two different mechanisms: see the diagram below.
• Accounts for 25–50 % of all new species of plants.
• Allopolyploidy and you
  • Many of our food plants are polyploids.
  • So, while this means of speciation may sound complicated and rather bizarre, it is common in nature and the species that have been generated by it are important for our own survival.
  • Common polyploid plants:
    • Oats
    • Cotton
    • Potatoes
    • Tobacco
    • Wheat

Figure 24.10(b), page 454, Campbell's Biology, 5th Edition

Tempo of Speciation: The Punctuated Equilibrium Model

• Developed by S. J. Gould and Niles Eldredge.
• Attempts to explain the “gaps” in the fossil record.
• The idea:
  • During most of the life time of a species there is no change—things are in “equilibrium.”
  • Occasionally a dramatic environmental change “punctuates” this “equilibrium” and rapid speciation occurs.
  • This idea is in opposition to Darwinian gradualism.

Figure 24.10(a), page 454, Campbell's Biology, 5th Edition