Experimental taxonomy at home.

Ernst Mayr pioneered the biological species concept, an idea that brought taxonomy of species into line with population genetics and evolution.  The idea is that a species is defined by the genetic relationships among its members; they’re all part of one big potentially-interbreeding population.  In Linnaeus’s day people sought to classify species based on what they looked like, rather than who they could breed with.

Using appearance is a pretty good proxy for the ability to interbreed, and much of the time it’s what taxonomists still do, simply because doing the breeding experiments or measuring genetic relationships among individuals is just too time-consuming.

But there are two classes of concerns that arise.

On one hand, individuals belonging to the same species can look very different.

Sometimes juveniles are hugely different from adults, like caterpillar and butterfly, elva and eel, or juvenile vs adult lancewood.  In a New Zealand plant example, Jim Le Comte and Colin Webb (Le Comte & Webb 1981) showed experimentally that the speargrass Aciphylla townsonii is actually the juvenile form of A. hookeri.  Different juveniles seem to be a feature of New Zealand plants, but they're common elsewhere too.

Juvenile (left) and adult foliage of mataī (Prumnopitys taxifolia)

 Secondly, small genetic differences can lead to quite big visible differences in plants or animals that belong to the same species.  Some of these are simple polymorphisms, like eye colour in humans.  In Veronica amplexicaulis, a hairy form used to be distinguished as a separate species (Garnock-Jones & Molloy 1983, under the old name Hebe amplexicaulis).  But it turns out this difference is the product of two alleles of a single gene, as are occasional flower colour variants in many plants.

White and blue viper's bugloss, Echium vulgare, growing side by side (Black Birch Range, Marlborough).

Thirdly, local populations might adapt to special conditions.  On mine tailings, where toxic heavy metals pollute the soil, plants may acquire tolerance, and this could involve some differences in form or in underlying physiology, yet they still freely mate with the non-tolerant individuals nearby.  These are classified as ecotypes, but not as separate species.  The differences are maintained by strong selection, even in spite of free gene flow between the tolerant and intolerant plants.

The form of Veronica albicans that grows on the dolomite outcrop at Mt Burnett looks a little different from other populations of this variable species; it might have adapted to its substrate, yet there's no evidence that it can't exchange genes with the rest of its species.

 Fourthly, some plants and animals are able to alter their form to cope with different environments they find themselves in or to escape predators—phenotypic plasticity or polyphenism.  Some inchworm caterpillars develop different appearances to blend in with the foliage of whatever host plants they’re living on (Greene 1989).  The underwater and aerial leaves of aquatic plants can be hugely different.  Plants can have very different leaf shapes, or even leaf anatomy, depending on the amount of sunlight they’re receiving or even what season they’re in.

Eryngium vesiculosum has very different leaves in summer (above) and winter (Webb 1984).

In all four of these situations, the result is two different looking plants growing together side by side, giving the appearance of two distinct species that aren’t interbreeding.  That’s just the sort of thing that gets taxonomists and field botanists excited, because we always like to discover a new species.

On the other hand, the reverse situation can arise.  Two species can look so similar that their existence isn’t even suspected until genetic tests are done.  These are called cryptic species.

It’s important to be aware of these possibilities, and in fact to rule them out as explanations before jumping to the conclusion that the variation we’re observing is due to the existence of more than one species.  The idea that two different-looking plants growing side by side must be different species is simplistic, yet "side by side" has become a bit of a mantra in some circles.

One way to test these potential new species is by growing different-looking plants together in uniform environments—common garden experiments—and also growing genetically identical plants in different environments—reciprocal clone transplants.  These approaches were pioneered in the first half of last century by Swedish botanist Turesson and by American botanists Clausen, Keck, & Hiesey.

Veronica lanceolata in flower, Rimutaka Range.

The speedwell hebe Veronica lanceolata is widespread in the North Island of New Zealand and a few parts of the South Island.  Each region has its own form of the species and, in general, adjacent populations are quite similar.  With some familiarity, it’s possible to tell from its appearance where a plant has come from.  These differences are maintained in common garden experiments, but I don’t regard these forms as different species because they can cross freely, their flowers and fruits are very similar, the differences are quantitative rather than qualitative, they have the same chromosome number, and the changes are mostly gradual and continuous from place to place.

Each leaf is from a different population of Veronica lanceolata.

However, there are places where two very different-looking forms grow together side by side, and this is exactly the sort of situation where a simplistic "side by side" approach might lead a botanist to the view that two species are involved.  In the Ruahine and Kaimanawa Ranges, especially on limestone cliffs, there are low-growing small leaved plants growing together with bushier large-leaved plants.  Although their leaves and stems are different in size and stature, their flowers and fruits are the same, which is a bit of a clue that these plants are responding in a plastic way to their environments, that there are no underlying genetic differences, and no breeding barriers between them.  I’d always assumed so at any rate, even though some field botanists made numerous collections of both forms, mounted them as separate accessions, and labelled them to highlight the differences and the fact they grew together side by side.  The hint was implicit: these might be different species.  Perhaps fortunately, nobody had the confidence in their hunch to give them different names.

South end of the Maungaharuru Range.

A couple of summers ago I was in the Maungaharuru Range in central Hawkes Bay with my colleague Heidi Meudt from Te Papa.  We were looking for forget-me-nots along the tops for a detailed genetic study she's conducting into their taxonomy and evolution.  Along the cliffs at the south end of the range we found Veronica lanceolata growing in mostly shady sites among mosses and algae.  But when we stumbled into some sink-holes things got interesting.  Here were moist shady sites with quite large-leaved plants, very close to sunny outcrops with tiny creeping plants.  It was an ideal opportunity to test my hunch that these were just plastic responses to moisture and shade.

There wasn’t room in the garden at home for a large randomised trial, so I sampled just a couple of plants of each type from sites only a few metres apart, and brought them home to grow in pots.  I also pressed branches of each, to record and preserve how they had looked in the wild.

Collection 2834, small and large leaved plants just after potting, Feb 2011.

Collection 2836, a small leaved plant just after potting, Feb 2011. The white plastic labels are 13 mm wide.

They’ve been growing now for about 18 months, and some, but not all, of the changes are quite dramatic.  For 2384, the small-leaved plant now has somewhat bigger leaves, but it's still distinctly smaller than the large-leaved plants.  For 2836, leaves are now up to 20 mm long, whereas they were about 5 mm before.

Both surfaces of the largest leaves from each of the three plants (two from 2836 small), Sep 2012.

So the results are a bit mixed, and this shows how important it is to use large samples, not just a couple of plants, and to randomise the trial properly.  The plants have exhibited some phenotypic plasticity but that doesn't account for all the differences.  Note also the two very different leaf shapes from the same plant of 2836: more evidence of plasticity.  Maybe both phenotypic plasticity and ecotypic differentiation are happening in this population.  Next time I'm in a position to collect a bigger sample and repeat this experiment I will do so.  In the meantime, I can try crossing the small- and big-leaved plants this summer.  My expectation is the offspring will be fully fertile.

With these speedwell hebes, the differences in growth form and leaf shape are striking, but they aren’t sufficient to compel rejection of the hypothesis that they’re the same species, because there are two different and simpler explanations—phenotypic plasticity and ecotypic differentiation—for that pattern.  My simple experiment hasn't clearly demonstrated which is happening, because the experimental design and sampling are insufficient.  But it's important to note also that these are quantitative differences—leaf shape and size—just the sorts of things that often vary in natural populations.

I'm sure it’s possible to test species status scientifically and explicitly.  That means starting with a testable hypothesis.  It’s better to start with the hypothesis that the two entities are conspecific, because any differences are evidence to the contrary that would compel us to reject the hypothesis.  If instead we start with the hypothesis that they’re different species, it’s hard to imagine how many similarities between them would compel us to reject that idea.  And if we start with the hypothesis that there are two species, and then seek evidence to support the hypothesis, then we're not doing science, at least not as it was formulated by Karl Popper.

References

Garnock-Jones, P.J.; Molloy, B.P.J. 1983.  Polymorphism and the taxonomic status of the
Hebe amplexicaulis complex (Scrophulariaceae).  New Zealand Journal of Botany 20: 391–399.

Greene, E. 1989. A diet-induced developmental polymorphism in a caterpillar. Science 243: 643-646.

Le Comte, J.R.; Webb, C.J. 1981.   Aciphylla townsonii — a juvenile form of A. hookeri (Umbelliferae).  New Zealand Journal of Botany 19: 187–191.

Webb, C.J. 1984.  Heterophylly in Eryngium vesiculosum (Umbelliferae). New Zealand Journal of Botany 22: 29–33.

Wednesday wildflower: bluebell

Bluebells, Hyacinthoides non-scripta,
in woods at Wokingham, England

The English bluebell, Hyacinthoides non-scripta, is a familar and much-loved spring flower.  In woods throughout Britain it comes up and flowers as part of a cycle of spring flowers on the forest floor, along with lesser celandine (Ranunculus ficaria) and greater stitchwort (Stellaria holostea).  Each of them blooms for about a week, and then the leaves appear on the trees and the understory becomes dark and shady.

In Britain and in gardens here too, Hyacinthoides non-scripta has often hybridised with the Spanish bluebell, H. hispanica, which has larger flowers and appears to be a more vigorous plant.  The hybrids have wider leaves, more flowers on a stalk, and pedicels longer than the flowers.

Drifts of bluebells in spring can take your breath away.  A few weeks ago when I wrote that groundsel was probably the first weed I learned the name of, I certainly wasn't thinking of bluebell as a weed.  My first plant memory is of bluebells in the woods near Stafford Castle.  We'd gone for a family walk (I was probably 4 because we moved away from Stafford before I was 5, but it's possible I was 3).  I remember there was a tree house, and great drifts of bluebells under the trees.  There's a scene in the movie Ryan's Daughter, which reminded me of that sight (bluebells at 22 seconds into the trailer at the link).

Bluebells and greater stitchwort, Wokingham

Bluebells, Hyacinthoides hispanica or H. xmassartiana, Wellington Botanic Garden

Bluebells are grown commonly enough in New Zealand gardens. Occasionally they can be found in the wild, perhaps establishing from bulbs discarded with garden waste by people who dump their garden rubbish at the roadside instead of at the tip (or better, composting it).  I'm always pleased to see them (bluebells, that is), and regard them as a wildflower rather than a weed.  I've seen quite a few patches this week.

Wild bluebells, Hyacinthoides xmassartiana, Norway St steps, Kelburn, Wellington.

This flowers below are from a roadside clump in Highbury, Wellington, in a roadside weedy patch that has provided subjects for other entries in this blog.  The flowers are pale and arranged on all sides of the stalk, features on H. hispanica, but they have white pollen, a feature of H. non-scripta.  Almost certainly they are derived from hybrids, and should therefore be called H. xmassartiana.

Bluebells, Hyacinthoides xmassartiana, Highbury, Wellington.

In Scotland, Scandinavia, and Australasia the name bluebell is also given to harebells, or members of the Campanulaceae, such as Campanula rotundifolia (blåklocka in Sweden) and Wahlenbergia.

Wednesday wildflower: dove's foot cranesbill

I love the quaint English names a lot of our weeds have; this week's is a beauty.  Cranesbill as a common name for the genus Geranium is a reference to the elongated styles on the fruits; dove's foot refers to the leaf of this species.

Geranium molle, Raroa Road, Highbury, Wellington

There's another common name mix-up around geranium: the garden geraniums are classified botanically in the genus Pelargonium, although some true geraniums are also cultivated.

Geranium molle flowers, Highbury.

Dove's foot cranesbill is widespread in its native range of Eurasia and North Africa, and it's widespread in New Zealand too (Webb et al. 1988) and other parts of the world like the USA. It is distinguished from other species by its spreading sepals (C below), the short claw (narrowing at the base) of the petals (E below), and the hairless and wrinkled mericarps (1-seeded partitions of the fruit, F below).  The individual inflorescences are leaf-opposed (A below), which indicates the inflorescence is actually terminal and the new continuing stem arises from its axillary bud.

Geranium molle. A, stem and upper leaves; B, stem hairs; C, young flower; D, older flower, E, petals showing the short claw, underside at left; F, mericarps; G, flower after pollination and petal fall; H, basal leaf, abaxial surface; I, basal leaf, adaxial surface; J, stem leaf, abaxial surface; K, stem leaf, adaxial surface; L, ripe mericarps; M, unripe mericarps.

Geranium is quite a large genus in New Zealand.  The database mantained by the taxonomists at New Zealand's largest plant collection, Landcare Research, records 7 native and 14 naturalised or casual species, to which the NZ Plant Conservation Network database adds G. incanum (naturalised).  Webb et al. (1988) recorded 7 native and 8 naturalised.  The increase partly results from new taxonomic revisions that have promoted some varieties to species rank, new introductions, and perhaps a widening of the criteria for what is considered naturalised and casual.

Cranesbills have a link with famous British scientist John Dalton.  Dalton was colourblind, suffering a condition now sometimes known as Daltonism, and reported that the colour of cranesbill flowers was sky blue.  When others disagreed, he realised he wasn't seeing colour as they did.  He left his eyes to science, and genetic analysis has shown he lacked a functional copy one of the three opsin genes that are needed for full human colour vision.

Reference.

Webb, C.J.; Sykes, W.R.; Garnock-Jones, P.J. 1988.  Flora of New Zealand Vol. 4.  DSIR, Christchurch.

Wednesday wildflower: miner's lettuce

Claytonia perfoliata, basal leaves

Miner's lettuce, Claytonia perfoliata, is wild foragers' fare.  Its succulent leaves can be lightly steamed or eaten fresh in a salad.  The plants are annual, and I first noticed the seedlings about 6 weeks ago.  Lately there have been a few early flowers.

The little white flowers form in clusters in the centre of roughly circular bracts.  Each flower has two sepals, five petals, five stamens, and three stigmas on top of the ovary.  The flower stalks elongate as the flowers get older.  The seeds are small, shiny, and black.

Claytonia perfoliata.  Basal leaves, A, adaxial, B. abaxial; C. perfoliate bract with flowers; D inflorescence and side view of flowers, showing the paired sepals.

The genetic revolution in classification has affected few plants more than it's affected miner's lettuce and its relatives.  It used to be classified with Portulaca in the Portulacaceae.  It was a big surprise to discover that the cactus family arose within that lineage, and this brought about the suggestion that the two families should be combined.  However, more recently, an alternative has been proposed: the Portulacaceae can be broken up into a number of smaller families that can sit alongside Cactaceae.  One of these is the now very much smaller version of Portulacaceae (New Zealand has a couple of naturalised species of Portulaca), but miner's lettuce and some other naturalised and native New Zealand plants ended up in the Montiaceae.  I'll certainly cover Calandrinia menziesii and Montia fontana as future Wednesday Wildflowers if I get suitable material, but for now here are some native species in Montiaceae.

Montia australasica, a common and variable alpine species found in New Zealand and Australia, at Rastus Burn, Remarkables Range, Otago.  Some botanists prefer to divide it into at least 7 species.

Montia fontana subsp. fontana is a native aquatic, here growing in a stream at Sandy Bay, Enderby Island.  M. fontana subsp. chondrosperma seems introduced, and grows on soil.

Hectorella caespitosa, here at Rastus Burn, Remarkables Range, Otago, used to be classified with its close relative Lyallia kerguelenensis (from Kerguelen Island) in its own family, Hectorellaceae.  Now their relationships are better known, they're placed in Montiaceae.  Some argue the two should be in the same genus (Lyallia caespitosa would then be the name for our species).

Spring flowers

Wellington has a reputation for being windy, and it's worst in Spring and Autumn.  The last week has been pretty extreme, with gusts of 170kmh recorded at Mt Kaukau and 130 kmh in Kelburn.  At 200m, our house is in between, and the new roof stood up to the trial well, although the house did shake a bit with the big gusts.

Today it's breezy and sunny, and I walked through the botanic gardens.  There didn't seem to be any damage there, though trees had been blown over in other parts of the city.

Stormy sky, Wellington looking north.  The straight edge of the harbour is the Wellington Fault, always on our minds.

Tomorrow another front will pass, followed by a cold southerly change with snow on the Rimutaka mountains across the harbour.  So today, I'm enjoying the brief sunny respite.

Prunus blossom in the scented garden.

A mauve Corydalis in the rock garden.

Bluebells, Hyacinthoides hispanica

Edgeworthia chrysantha, relative of Daphne.

Wednesday wildflower: lilac oxalis

The war on Oxalis is unwinnable, but that doesn't stop gardeners from trying.  Over several years I think I've eliminated it from a tiny enclosed flower bed beside the front door, but in the rest of the garden it's rampant.

Oxalis incarnata, Kelburn, Wellington

O. incarnata, lilac oxalis, is probably the commonest species of Oxalis around Wellington, though we have a couple of others that will feature in coming months.  Originally from South Africa, it's found throughout New Zealand, but is less common in most of the South Island.  The leaves have three leaflets, and with their notched tips they look like the traditional shamrock.  However the name shamrock is more usually applied to one or other of the many clovers, Trifolium.

Oxalis incarnata leaves, Kelburn

It's a pest because it reproduces asexually, through little fleshy bulbils (B, below) that detach from the roots and stems to establish new plants.  Sexual reproduction in this species is thought not to happen in New Zealand.  The reason takes a bit of explaining.  Oxalis flowers come in three types, called long-style, mid-style, and short-style.  In a long-style flower, the stamens are short and mid length (C below).  In a short-style flower, the stamens are long and mid.  And in a mid-style flower, the stamens are long and short (Darwin 1877).  Pollen from each type of stamen grows best in a style of the same length, and in addition, the plants have a genetic self-recognition system, which means these flowers can't self-fertilise.  And it seems that by chance only one type, the long style form, got introduced to New Zealand.  So although they flower profusely, the seed pods don't form, and the plant must rely on its bulbils to spread (Webb et al. 1988).

Oxalis incarnata.  A, flowers; B, a bulbil; C, section through a flower, enlarged.

While many are nasty weeds, some species of Oxalis are useful.  New Zealanders know the tubers of O. tuberosa as yams, but they're not true yams (yams are Dioscorea, a monocotyledon, and O. tuberosa is more correctly known as oca in its native South America).  Oxalic acid is toxic, or at least irritant, but useful for cleaning up and bleaching mouldy timber.  A few species are ornamentals.

References.

Darwin, C. 1877.  The different forms of flowers on plants of the same species.  Murray, London.

Webb, C.J.; Sykes, W.R.; Garnock-Jones, P.J. 1988.  Flora of New Zealand Vol. 4.  DSIR, Christchurch.

Pittosporum cornifolium (again) and P. kirkii

Just about a year ago I posted a photo of Pittosporum cornifolium in fruit.  Perhaps I should have looked more closely, because right now it's in flower as well, so it probably was then too.  The flowers are dark red, like P. crassifolium and others, at least on the outside of the petals, but unlike them P. cornifolium has narrowly acute petals.

Pittosporum cornifolium, cultivated plant at Victoria University

P. kirkii has petals like that, but they're yellow, the sepals are reflexed, and the tip of the branch has large papery bracts:

Pittosporum kirkii, cultivated plant at Victoria University

The diversity of flowers and sexual systems in New Zealand Pittosporum is amazing; it suggests a diversity of pollinators and a comparative study would make a great thesis for someone.  There's a detailed account of P. cornifolium here.