Defining a species

What defines two organisms as being two different species?

A case study with black and pied stilts

Speciation of the black and pied stilts

• Generally excepted that they are two distinct species

• Black stilt - Himantopus novaezelandiae• Pied stilt – Himantopus himantopus

http://basicbiology.net/gallery/animals/pied_stilt.jpghttp://www.mattjoneswildlifeimages.com/page40.htm

Background to their relationship

• Believed to have originated from two separate colonisations from Australia pied stilts to New Zealand.

• Black stilts - potentially 1 million years ago

• Pied stilts – possilby as recent as 150 years ago

• Differences

– plumage (black vs black & white)

– other morphological difference

– behavioural differences.

Background to their relationship

• IUCN red list status

Black stilt – critically endangered

Pied stilt – least concern

• Black stilts mate for life

• Due to a very low population size - between 58-78 adult birds - will sometimes mate and hybridize with pied stilts.

Definitions of species

• Biological species concept – species defined as populations of individuals that can produce viable offspring.

– Probably the most well-known definition of a species

– Hybridisation?

• Morphological species concept – classified as the same species if organisms fit into the same morphological criteria.

– Classical definition going back to the days on Carl Linnaeus – the father of taxonomy

Definitions continued• Recognition species concept – species defined as

organisms that recognise each other as potential mates.

• Not well accepted due to hybridisation and phylogenetic

• Phylogenetic species concept – species are a group of organisms that share a single common ancestor.

• Concordance species concept – takes into account a range of diagnostic markers such as breeding, morphology, DNA markers.

• Plus many more...

How do pied and black stilts fit in?

• Morphological species concept:

• The two species are similar in many ways.

• Main difference in is the colour of plumage.

• Black stilts - larger body size, longer beaks, shorter legs, longer wings.

• But all are expected adaptations to a colder climate.

How do pied and black stilts fit in?• Phylogenetic species concept:

• Pied stilt and black stilt have been shown to have two different haplotypes for mtDNA

• i.e. two genetically different species

• Mitochondrial genome <0.001% of the nuclear genome

• Potentially not very representative

• Concordance species concept:

• The combination of morphological, mtDNA, isozyme and plumage markers indicate two distinct species

What does this all mean?

• Firstly, as I said at the start – it is generally accepted that they are two distinct species.

• It seems the phylogenetic and concordance species concepts hold more weight than other species definitions.

• Illustrates how difficult it can be to classify a species.

• Scratches the surface of the difficulties encountered by taxonomists.