Biological Species Concept: Challenges (HL) (HL IB Biology)

Biological species concept

• The biological species concept states that a species is:

A group of organisms that can interbreed to produce fertile offspring

Difficulties applying the biological species concept

• The biological species concept relies on the idea that certain characteristics related to reproduction are unique to each species, and so prevent individuals of different species from successfully breeding together. These characteristics include:
  
  • Compatible mating rituals and seasons
  • Compatible sex organs
  • Gametes that can fuse together to produce a viable zygote

  • Gametes with the same haploid chromosome number that can match up to form homologous pairs of chromosomes

• Many groups of organisms do not reproduce using sexual reproduction, or are capable of reproducing without a mate, so the biological species concept cannot be applied

• Examples of asexual reproduction methods which challenge the biological species concept include:
  • Parthenogenesis
    • Occurs in some fish, some lizards, and some invertebrates, e.g.
      • Sharks
      • Komodo dragons
      • Aphids
    • A new individual is formed without the need for a male gamete; the female reproduces on her own
      • This is sometimes described as 'virgin birth'
    • The offspring are usually all female, and can be clones of the mother, or contain 'shuffled' combinations of the mother's alleles
  • Binary fission
    • Occurs in bacterial cells
    • Cells grow, replicate their contents, and then divide into two daughter cells 
    • Clones are produced
  • Vegetative propagation
    • Occurs in some plants
    • A new individual might develop from a runner that develops from the parent plant, or when a bulb splits into several new bulbs
      • E.g. strawberries form runners, while daffodils can develop new bulbs from one parent bulb
    • The offspring are clones

Vegetative propagation diagram

Some plants can clone themselves using runners that develop from their roots; this makes it very difficult to apply the biological species concept

Horizontal gene transfer

• Another process that presents a problem when applying the biological species concept is horizontal gene transfer
  • This is the passing of genetic information from one cell to another within the same generation
    • Gene transfer is usually 'vertical', i.e. it passes from one generation to the next
  • This occurs in bacteria
  • Genes can be transferred within a species, and even between different species

Horizontal gene transfer in bacteria diagram

Bacteria can transfer genetic material ‘horizontally’ between members of the same generation; this event is known as ‘conjugation’. This makes it difficult to apply the biological species concept to bacteria

• While the term 'horizontal gene transfer' is generally used to describe the passing of DNA between bacterial cells, it is also possible for DNA to be transferred between other different organisms, e.g.
  • Viruses to bacteria
  • Viruses to eukaryotes
  • Bacteria to eukaryotes
• This means that many organisms contain genetic information from species that are, evolutionarily speaking, only very distantly related, adding further complication to the attempt to classify organisms