Species and speciation

• There is a wealth of evidence to support the idea that natural selection is the force that has produced all different species of organisms on Earth
• Yet in the examples of directional selection we described (evolution of antibiotic resistant bacteria, and changes peppered moths) no new  species have been produced

How can natural selection produce new species? We must first consider exactly what a species is;

• a widely accepted definition is: a group of organisms, with similar morphological, physiological, biochemical and behavioural features, which can interbreed to produce fertile offspring, and are reproductively isolated from other species
• 'Morphological' features are structural features, while 'physiological' features are the way that the body works
• 'Biochemical' features include the sequence of bases in DNA molecules and the sequence of amino acids in proteins

It is quite rare to test the ability of two organisms to interbreed. Biologists frequently rely on morphological, biochemical, physiological and behavioural differences to decide whether they are looking at specimens from one or two species. DNA sequences sometimes may be used to assess how similar two organisms are to each other.

• It can be extremely difficult to decide when these features are sufficiently similar or different to define two organisms as belonging to the same or different species
• It leads to uncertainities and disagreements about whether to lump many slightly different variations of organisms together into one species, or whether to split them up into many different species

Biologists agree that the feature which really decides whether or not two organisms belong to different species is their inability to interbreed successfully

In explaining how natural selection can produce new species, therefore, we must consider how a group of interbreeding organisms (i.e all of the same species) can produce another group of organisms which cannot interbreed successfully with the first group.

The two groups must undergo reproductive isolation. Reproductive isolation can take very different forms;

Prezygotic (before a zygote is formed) isolating mechanisms include:

• individuals not recognising one another as potential mates or not responding to mating behaviour
• animals being physically unable to mate
• incompatability of pollen and stigma in plants
• inability of a male gamete to fuse with a female gamete

Postzygotic isolating mechanisms include:

• failure of cell division in the zygote
• non-viable offspring (offspring that soon die)
• viable, but sterile offspring

The last is the most wasteful of energy and resources

The main difficulty in investigating how reproductive isolation can arise is that it takes time

• a speciation experiment in a lab would have to run for many years
• the evidence that we have for the ways in which speciation can occur is almost all circumstantial evidence
• we can look at populations of organisms at one moment in time, that is now, and use the patterns we can see to suggest what might have happened, and might be…