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30% of species are in 6 families

Asteraceae : 25,000 Orchidaceae: 18,000

Fabaceae: 17,000 Poaceae : 9,000

Rubiaceae: 7,000 Cyperaceae: 4,000

Why is the Asteraceae (& a few other families) so big andwhy are so many other families very small?

THE ASTERACEAE

an extended family

THE ASTERACEAEtheir place in the hierarchy and why

KINGDOM: Land plants or embryophytesPHYLUM: Flowering plants or angiosperms

or Magnoliophyta CLASS: EudicotsORDER: AsteralesFAMILYGENUSSPECIES

The Asterales:

Corolla tube initiation early;Anthers basifixed, free from corolla;

pollen grains trinucleate;Carpels(2-3), inferior, styles long;seed integument <7 cells thick;antipodal cells ephemeral

KINGDOM: Land plants or embryophytesPHYLUM: Flowering plants or angiosperms

or Magnoliophyta CLASS: Eudicots ORDER: AsteralesFAMILY: AsteraceaeGENUSSPECIES

The Asteraceae

�Herbs & woody, trees & vines;�leaves normally alternate but can be opposite, often highly

dissected, margins various;�inflorescence capitulate, involucrate;�flowers actinomorphic & zygomorphic

(polysymetric/monosymetric;�Calyx reduced often to pappus of hairs;

�Corolla may be of two types on same capitulum;�Anthers 5, often fused;

�G inferior, style bifid, 2 fused ovaries.

Willis & Yule,

1922, Nature

‘Hollow curves’ are found throughout biology

There are c.13,000 flowering plant genera of which 34% contain one species and there are just 2 genera with

more than 2,000 species

AstragalusEuphorbiaPiperCarexSolanum

34% of genera monotypic

In large genera the amount of difference between the species is

often exceedingly small.

The species of the larger genera resemble varieties, more than do the species of the smaller genera.

But there are many non-biological hollow curves

• Sizes of cities in California

• Distribution of wealth

• Number of atoms of elements in the solar system

• Surnames in telephone directories

Oxford telephone directory

Surnames starting with A

0

50

100

150

200

250

300

0 50 100 150 200 250 300

Number of entries

co

un

t

AllenAdams

Possible causes of biological hollow curves

•Evolutionary processes

•Taxonomic practice

•Random & non-biological processes

Evolutionary processes resulting in hollow curves

Why are there small groups?

1 – Extinction of relatives could explain small, taxonomically isolated groups e.g. Ginkgo biloba

2 – Unique morphology in a large groupe.g. Euphorbia chamaecyse and few other species have stipules and so could be put in a separate genus

Why are there big groups?

Adaptive radiatione.g. Lupins in South America where the genus is diversifying rapidly as new ecological opportunities appear with the creation of the Andes

Taxonomic practices resulting in hollow curves

• Vascular tissue Tracheophytes

• Seeds Spermatophytes

• Carpel Angiosperms

• Legume Leguminosae/ Fabaceae

• Capitulum Compositae/Asteraceae

• Cyathium Euphorbia

• Vertebrae Vertebrates

• Mammary Glands Mammals

• Amnion Amniotes

It is argued that clear, unambiguous morphological characters are sparsely and unevenly distributed

throughout the evolutionary tree

Scotland et al., 2003

Simultaneous Broken Tree (SBT)model devised by

Scotland & Sanderson in 2004

Scotland & Sanderson 2004

In 2004 Scotland & Sanderson re-examined the problem and started with an imaginary system with no biological processes.

The same had been tried in 1957 by MacArthur. He took a stick and broke it randomly into small bits. He continued to do this until he had lots of bits of different lengths. This was the Simultaneous Broken Stick (SBS) model.

Scotland & Sanderson 2004

Scotland & Sanderson reasoned that evolution is a tree not a stick

and rather than breaking a stick they broke a branching tree that represented evolution.

This is the Simultaneous Broken Tree (SBT)model.

Scotland & Sanderson 2004

In this hyppthetical model phylogenetic tree the number of branches is [n + (n-2)] where n = number of terminal branches. This means that c.50% of branches are terminal.

Imagine randomly placing clear taxonomic characters on this tree. c.50% of the characters will be on the terminal branches because c.50% of the branches are terminal.

As the tree grows so does the likelihood of lots of little groups and few big ones

So we now have 2 models (SBS & SBT) and one set of data. We can now see how closely the models fit the data and try to explain any differences.

Scotland & Sanderson 2004

Number of species in a genus

Scotland & Sanderson, Science 2004

1000

200

200

500

0

0

0

01

1

1

1 79

1120

500

1200

Birds

Orchids

Composites

Legumes

Co

un

t

Genus size (number of species)

A

Scotland & Sanderson 2004

If we look at the number of species in the genera in different groups of plants we find the data given in the middle column

where monotypes are single species genera (given as a fraction of the total number of genera in the family)

and where dominance is the number of species in the largest genus (given as a fraction of the total number of species in the genus)

Scotland & Sanderson 2004

If we look at the number of species in the genera in different groups of plants as predicted by the Simultaneous Broken Stick model we find the data given in the left-hand column.

There are too few monotypes because of extinctions in reality.

The biggest genus is too small because adaptive radiation produces a few really big genera in reality.

Scotland & Sanderson 2004

If we look at the number of species in the genera in different groups of plants as predicted by the Simultaneous Broken Treemodel we find the data given in the right-hand column.

There are too many monotypes because taxonomists try to avoid monotypes (lumpers) in reality

The biggest genus is too big because taxonomists try to avoid large groups (splitters) in reality

Results

Neither SBT nor SBS fit the data but the departure from the data of both models is however in opposite directions so the truth should lie in between.

Explanation for hollow curves in biology

• Evolutionary process (tree)• Taxonomic practice (diagnostic characters)• Random processes (character evolution)

Scotland & Sanderson, 2004, Science: 303:643.

Practically, when naturalists are at work, they do not trouble themselves about the physiological value of

the characters which they use in defining a group,

or in allocating any particular species. If they find a character nearly uniform, and common to a great

number of forms, and not common to others, they use it as one of high value.

Hence the hollow curves

Charles Darwin – Origin of Species

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