The Origin of Species Chapter 24 Bozeman Tutorial: SpeciationBozeman Tutorial: Speciation (11:39)

Biological species concept Biological species -- Naturally interbreeding populations that produce fertile offspring. Largest unit of population in which gene flow is possible. Reproductive barriers prevent genetic mixing with other species, isolating the gene pool. Prezygotic or postzygotic depending on whether they function before or after the formation of zygotes.

Prezygotic Barriers 1. Habitat Isolation Two species living in different habitats within the same area may encounter each other rarely if at all. 2. Temporal Isolation Two species that breed at different times of the day, seasons, or years cannot mix their gametes. 3. Behavioral Isolation Species-specific signals and elaborate behavior to attract mates are important reproductive barriers among closely related species.

Temporal Isolation

Mating Behavior

Prezygotic Barriers cont. 4. Mechanical Isolation Incompatible anatomy may prevent mating. 5. Gametic Isolation Lack of gamete recognition; sperm may not survive the internal environment of the female.

Postzygotic Barriers 1. Reduced Hybrid Viability Zygote does not survive gestation period due to genetic incompatibility. 2. Reduced Hybrid Fertility Hybrid offspring are sterile; still no gene flow. 3. Hybrid Breakdown First generation hybrids are viable and fertile, but offspring of the next generation are not viable or sterile.

Mule (reduced hybrid fertility

Geographic isolation and speciation A new species may originate when the gene pool of a population is separated from the parent species. Isolated group can then undergo unique changes in allele frequencies caused by selection, genetic drift, and mutations. Two modes of speciation: allopatric and sympatric.

Allopatric speciation Allopatric speciation -- geographical barrier physically isolates the population, blocking gene flow. Include emergence of mountain ranges, movement of glaciers, formation of land bridges Also depends on the mobility of animals or the dispersibility of pollen and seeds of plants. Small isolated population is more likely to become a new species than large one (genetic drift). More likely to occur at the fringe of the parent population's range (peripheral isolate).

Adaptive radiation Allopatric speciation occurs on island chains where new populations, which stray or are dispersed from their ancestral populations, evolve in isolation. Adaptive radiation -- Emergence of many diversely adapted species from a common ancestor.

Sympatric speciation Sympatric speciation -- Reproductive isolation occurs without geographical isolation; common in plants where cell division is incomplete, resulting in organisms with multiple sets of chromosomes (polyploidy). 1. Autopolyploidy – chromosome number doubles within a species. 2. Allopolyploidy -- two different species combine chromosomes, forming vigorous hybrids.

Autopolyploidy

Potato: ranging from diploid (24) to hexaploid (72); cultivated potato varieties are tetraploid (48); many wild species are diploid but may range up to hexaploid.

Allopolyploidy

Arabidopsis thaliana (10) + Cardaminopsis arenosa (16) = Arabidopsis suecica (26)

Wheat: = 42

Debate about rates of speciation Gradualism -- Big changes occur due to the accumulation of many small changes; constant and gradual. Allele frequency changes in the gene pool lead to the divergence of species. New species branch off from ancestral species over long spans of time. Issues: Lack of transitional forms in the fossil record.

Debate about rates of speciation cont. Punctuated equilibrium -- occurs in spurts of relatively rapid change. Species undergo most modification as they first separate from the parent species, then show little change. By “spurt”, we’re talking 50,000 years. Issues: microevolutionary change is constant, and can occur without affecting morphology, which is what fossils can only show.