Ten species in one: DNA barcoding reveals cryptic species in the

neotropical skipper butterfly Astraptes Fulgerator

Paul Hebert, Erin Penton, John Burns, Daniel Janzen & Winnie Hallwachs

Proceedings of the National Academy of Sciences2004

Content: JanCritique: Emily

In

The Big Picture

The scientific method is the process by which scientists, collectively and over time, endeavor to construct an accurate (that is, reliable, consistent and non-arbitrary) representation of the world.

-Frank Wolfs

1. Observation and description of a phenomenon.

2. Formulation of an hypothesis to explain the phenomenon.

3. Use of the hypothesis to predict the existence of other phenomena.

4. Performance of experimental tests of the predictions by several independent experimenters and properly performed experiments.

Thinking Critically…

No species definition!

Instead, refers to “sequence threshold (3%) typically encountered between congeneric species pairs”

Typically?

Thinking Critically…

However, imply the use of the biological species concept

Reproductive isolation

•Genetic distinctiveness

•Caterpillar colour patterns

•Food plant usage

•Differences in adult morphology

Genetic Distinctiveness

Mitochondrial DNA is solely maternally inherited!

Females are also the “primary agents of food plant selection”

mtDNA does not account for male gene flow

What are the males up to? Hey ladies

Caterpillar Colour Patterns

“on average, FABOV’s ventral orange is slightly paler than that of SENNOV”

Seems rather subjective…

Food Plant Usage

TRIGO and CELT are the only two ‘species’ that do not share their food plants with others!

• SENNOV and FABOV share 5 food plants

• LOHAMP and LONCHO can live on each others’ primary food plants

•BYTTNER also has own food plant, but only four of them were found!

Differences in Adult Morphology

“close study of adults, sorted by their caterpillar food plant, showed subtle differences in colour, pattern, size, and wing shape”

Thinking Critically…

Have Hebert et al. made a good, scientific case for reproductive isolation among their proposed species?

Thinking critically…

Why the neighbour joining tree?

• “has the advantage of speed”

• BUT… taxonomy uses character-based, not distance- based trees

• Makes union of taxonomy and barcoding difficult

• Lacks objective criteria to delineate species

Thinking Critically…

So, Hebert gathered enough barcodes to establish the set of objective criteria to delineate species…

“sequence threshold (3%) typically encountered between congeneric species pairs”

Thinking critically…

… and then didn’t use them.

Less than 0.5% sequence divergence

“Only two taxa (CELT and TRIGO) have minimum sequence divergences (3.4% and 5.4%)”

Intraspecific vs.

Interspecific

The minimum 3% sequence divergence threshold was based

on:

Interspecific difference = at least

10x intraspecific variation

Thinking critically…

Even though 8 out of 10 of the proposed species failed to satisfy Hebert’s own species threshold…

…he wrote a paper proposing all 10 as species.

Why is my critique important?

Why is my critique important?

•Science is supposed to be objective

•DNA barcoding has the potential to be an incredibly useful tool

•By not being scientifically rigorous, Hebert et al. weaken their own arguments