Classifying buttons vs. life

April 13, 2009
 
Buttons can be classified in many ways

Buttons can be classified in many ways

If you want to teach someone about classification, a pile of assorted buttons is a good tool. It is always interesting to have two or more groups working on a button classification and see what criteria they use. Buttons make a fine model and come in enough variations to make classifying them interesting and even challenging. However, there is more to classifying life than classifying buttons. Life has another dimension.

 Buttons are here-and-now objects. Big buttons don’t have little buttons and they don’t pass on information to offspring. They have no history that we can observe and they share no ancestors. Buttons carry no information about their past. Classifying them is a good model for what Linnaeus did when he classified life. He felt, at least for most of his career, that all life had been created instantaneously and had always been and would always be just as he saw it.

Life has important differences from buttons. Darwin’s work was very important in calling attention to life’s history and to the idea of evolution, which he called descent with modification. We now have overwhelming evidence that life changes through time and all living species have a long history. Fossils tell a good deal of the story, but the “second fossil record,” the DNA in each organism, is what has allowed us discover much more of the story.

Each species had ancestors that stretch back in time. For the story of the many human ancestors, see The Ancestor’s Tale by Richard Dawkins. This inspiring book has its own Wikipedia entry, in which the ancestors it visits are listed. http://en.wikipedia.org/wiki/The_Ancestor’s_Tale Connie Barlow describes associated experiential activities for children at The Great Story website. http://thegreatstory.org/ancestors-tale.html

Two species may be descendents of a common ancestor that lived in the not-so-distant past. Biologists strive to place these sorts of close cousins in the same group, a lineage that includes the ancestor and its descendents. Other life may have shared an ancestor with our cousin species, but much further back in time, with many more lineages also sharing that far past ancestor. This may be shown on a Tree-of-Life diagram as a deeper branch from earlier in life’s history.

How can we model descent with modification? Once at a teacher’s workshop I used chocolate candy. We had plain miniature chocolate bars, bars with nuts, Hershey’s Kisses, and Hershey’s Hugs. We made a branching diagram that illustrated our hypothesis of the descent of these candies. The best part was eating the samples after we had finished with our phylogeny.

Maybe there is no suitable model for classifying life other than life itself. It certainly won’t be boring.

Advertisements

Flowers on trees

April 7, 2009

Spring is bloom time for angiosperm trees (that’s trees other than conifers and ginkgoes) in temperate climates. Trees have two basic approaches to flowering, make big showy flowers and use insect pollinators, or make lots of small, inconspicuous flowers and use the wind to transport pollen. I saw both of these lately.

Staminate and pistillate (arrow) catkins of alder bloom in early spring.

Staminate and pistillate (arrow) catkins of alder bloom in early spring.

The alder was in flower in mid-March. It had formed the buds of its inflorescences last summer, so it was ready to go when the weather warmed. Trees with wind-borne pollen must bloom before their leaves bud out. The pistillate flowers form in little catkins, marked by the arrow in the photo. The bracts between the flowers persist and enlarge as the ovaries develop. When the fruits mature in the fall, and the seeds are shed, the bracts remain. They are the structures in the back that look like miniature brown pine cones. The staminate catkins are much more conspicuous in bloom, but they fall off after they have released their pollen.

Alders in most areas of the US are likely to have bloomed by now, but their cousins, the birches, are yet to flower. Birches have similar staminate inflorescences, and these also form the previous summer. The pistillate flowers are borne on an upright catkin-like inflorescence. When the fruits are mature, the bracts and the seeds are shed, leaving a bare stem. Some of the bracts may not be shed by spring, but you can easily tell a birch from an alder by the little cone-like structures on the alder. Their cousins, the hazelnuts have similar staminate catkins, but all you can see of the pistillate flowers is a cluster of tiny red threads sticking out some bracts.

The tiny red thread-like styles of a hazelnut protrude from a cluster of bracts

The tiny red thread-like styles of a hazelnut protrude from a cluster of bracts

The pear tree, on the other hand, is very showy with its white-petaled flowers. It is a member of the rose family, whose flowers have either one carpel or several carpels distinct from one another. The pear and the apple typically have five carpels. You can see the five styles and stigmas in the photo below. The flowers of cherries, plums, apricots, and almonds look a great deal like apple and pear flowers, but the stone fruits have only one carpel. The immature anthers of the flower are pink. As they mature, the anthers split and peal back, revealing the pollen. The anthers shrink and darken when they are mature.

Be sure to observe and point out blooming trees to your children this spring. They may not notice without your help. They will likely be interested in the sequence of fruit development, once they see the flowers.  

This pear flower has five green styles and stigmas.

This pear flower has five green styles and stigmas.