Speciation (SL IB Biology)

Speciation increases diversity

• The theory of evolution states that species do not stay the same, but change over time; this can lead to the process of speciation
• Speciation can be defined as

The development of new species from pre-existing species over time

• Speciation has resulted in a great diversity of species on Earth
  • Theoretically, at the origin of life on Earth, there would have been just one single species
  • This species evolved into separate new species
  • These species would then have divided again, each forming new species once again
  • Over millions of years, evolution has led to countless numbers of these speciation events, resulting in the millions of species now present on Earth
• Speciation can occur when the exchange of genes, or gene flow, between populations of a species is prevented, e.g. due to them being separated on different islands
  • When gene flow stops, genetic differences can accumulate between the two populations
  • This may happen faster if different selection pressures are acting on the two populations
• A speciation split has occurred when the two populations can no longer interbreed to produce fertile offspring; at this point the two populations are said to be reproductively isolated from each other
  • Note that in order for speciation to have occurred, there must be reproductive isolation; gradual evolutionary change alone is not enough

Speciation diagram

Speciation is thought to have given rise to the huge diversity of species on Earth

Extinction reduces diversity

• While speciation increases the number of species on Earth, not all of the species that have evolved over evolutionary time still exist today; many species have gone extinct, meaning that they no longer exist
  • E.g. The passenger pigeon and the woolly mammoth
• Extinction reduces the number of species on Earth

Reproductive isolation

• Organisms that belong to the same species share the same characteristics and are able to breed together to produce fertile offspring
• Reproductive isolation occurs when changes in the alleles and phenotypes of some individuals in a species prevent them from successfully breeding with other individuals that don't have these changed alleles or phenotypes
• Examples of allele or phenotype changes that can lead to reproductive isolation include:
  • Seasonal changes
    • Some individuals may develop different mating or flowering seasons, becoming sexually active at different times of the year
  • Behavioural changes
    • Some individuals in a population may develop changes in their courtship behaviours, meaning they can no longer attract individuals of the opposite sex for mating
• These changes can occur as a result of geographical isolation of populations

Geographical isolation

• Reproductive isolation can occur when populations of a species become separated from each other by geographical barriers
  • The separated populations are said to be geographically isolated from each other
• Geographical barriers can include
  • Naturally occurring barriers such as a body of water, or a mountain range
  • Man-made barriers, such as a motorway
• Geographical isolation creates two populations of the same species between which no gene exchange can occur
• The two populations may be affected by different selection pressures, meaning that natural selection may act differently on the two populations
  • This is known as differential selection 
• Over time, the two populations may become so different that they are reproductively isolated, and speciation has occurred

Geographical isolation diagram

A mountain range can lead to geographical isolation, and eventually reproductive isolation

Bonobos & chimpanzees

• An example of a speciation event that has resulted from geographical isolation is the evolution of bonobos (Pan paniscus) and chimpanzees (Pan troglodytes)
• Chimpanzees are found to the north of the Congo river, and bonobos to the south
  • This suggests that at some point in their evolutionary past the river caused two populations of their ancestor species to become geographically isolated
  • Different selection pressures would have acted on the two populations, so differential selection occurred, resulting in differences between the two populations, e.g.
    • Chimpanzees tend to be more behaviourally aggressive than bonobos; this could have arisen due to more intense competition for resources
    • Chimpanzees have male-dominated social structures while bonobos have dominant females
  • Eventually the two groups became reproductively isolated, and were two separate species