Image of three butterflies, one of which appears to be a hybrid of the other two.

Micro- or macro-

Genetic changes create variation. But do they really create new species?

Darwin argued that species evolved from pre-existing ancestors. Changes in genes provided a mechanism for that to happen. The theory is good, but how strong is the evidence?

There is little doubt about ‘micro-evolution’ – adaptation within a species driven by genetic change.

But ‘macro-evolution’ – formation of new species by DNA change – is more likely to be questioned by evolutionary sceptics. Despite generations of selective breeding, all dog breeds may still be members of one species, even if practical difficulties sometimes inhibit successful mating...

Slow but sure

Evolution is a slow process, so witnessing it would be difficult. Generally, it’s an inference: two organisms look alike and their genes are similar, so it’s inferred that they are related and shared a common ancestor. Mutation creates genetic variation, and habitat (or drift) does the rest.

Sometimes, though, it’s possible to see signs of speciation taking place. One example is the Heliconius butterfly family. Speciation usually results from the splitting of a single lineage, but sometimes two species may fuse to create a third. Heliconius heurippa, for example, seems to be a hybrid of H. melpomene and H. cydno. Interestingly, mate preferences maintain the three separate species: males only go for wing patterns characteristic of their own species.

It is also possible to infer the evolutionary origins of a species by comparing it genetically to its relatives. The parental origins of many hybrid plant species, for example, have been identified cytogenetically or genetically.

More generally, detailed studies of habitats have revealed genetic divergence between organisms of the same species in different locations. A variety of selective pressures seem to be promoting a split into different species.

An alternative explanation is that every species was created independently, and can evolve but within limits. A mussel always stays a mussel. Similar organisms look alike because they were created with similar sets of genes.

Scientifically, that theory has little going for it. It would involve separate species-creation events – by currently unknown mechanisms – for every species that ever lived.

Razor sharp

In such cases a rule of thumb known as Occam’s (or Ockham’s) razor is often applied. This was a method put forward by 14th-century thinker William of Ockham. When presented with two possible explanations for a problem, choose the more simple – the one that makes the fewest assumptions (‘shave off’ the more contorted explanations).

Occam’s razor is not intended to be a ‘golden rule’ that can be used in every situation, however. More evidence supporting or contradicting a model would be preferable.

Lead image:

Heliconius heurippa (left) appears to be a hybrid of H. melpomene (top right) and H. cydno (bottom right).

Juan G Montañés (photography) and Mauricio Linares (production)


About this resource

This resource was first published in ‘Evolution’ in January 2007 and reviewed and updated in December 2014.

Genetics and genomics, Ecology and environment
Education levels:
16–19, Continuing professional development