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Give Peas a Chance

by MC Doc W

HAPPY FUN QUIZ!!! •  1. Natural selection depends on the fact that

members of any population vary, and some of that variation is _________________.

•  2. A branching diagram that shows a hypothesis of common ancestry is a __________.

•  3-4. Briefly explain the difference between artificial selection and natural selection.

•  5. Write down one question you still have about evolution. I’ll answer it to the best of my ability.

Darwin’s Dilemma. . . • Darwin’s theory of natural selection was

actually not widely accepted during Darwin’s own lifetime! – Natural selection depends on variation and on

heredity to work. – The problem is that neither Darwin not anyone

else understood how those worked. Why should organisms vary? How could they pass on that variation?

Our story begins with a young lad named Johann, born in 1820, in the small town of Hyncice nad

Odrou, in what was then the Austrian Empire and is now the Czech Republic.

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In 1843, Johann took vows as a

monk in the Abbey of St.

Thomas, in the city of Brno. As part of his vows,

he took a new name, and

became Brother Gregor. . .

Brother Gregor studied at the University of Vienna, and came back to Brno to

teach in the monastery school. In his spare time, he

was an amateur scientist, who was interested in many problems, but especially the

problem of heredity: just how did living things make

their offspring look like them?

In 1856, Brother Gregor got his abbot's permission to try some experiments in plant breeding, using peas

(Pisum sativum). Peas were a good

choice for his experiments: they’re

easy to grow, you can eat them when you’re done—and

they come in a number of different varieties, a few of which are shown

here. Top: Purple-flowered and white-flowered peas Bottom: Inflated and constricted pea pods.

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Pea flowers are unusual in having their sex organs completely enclosed by the central petals. . . meaning

that a pea flower normally self-fertilizes.

Brother Gregor—full name: Brother Gregor

Mendel—found, however, that he could crossbreed specific pea plants by cutting open

the flowers, transferring pollen from one flower to another with a fine

brush, and then sealing the flowers up again

with a bit of wax.

Mendel's Peas •  Mendel picked several varieties of pea that were

true-breeding. –  In other words, if left alone, they would always produce

more of the same variety, for as long as you cared to grow them.

•  But what might happen if you cross-bred two different varieties? Would the offspring look like one parent, or like a mixture of both, or perhaps something quite different?

Mendel ended up raising 29,000 pea plants over the course of eight years, in this greenhouse (now destroyed).

Since we can't look at all 29,000 let's examine one set of his crosses, between purple-flowered and white-flowered pea plants. . .

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When he crossed a purple-flowered pea with a white-flowered pea, all of the offspring had purple flowers. No exceptions!

X

But when he took two of the hybrid

peas and crossbred them. . . white-

flowered peas and purple-flowered peas appeared together in the

second generation.

X

X

By the way, there's a standard

abbreviation scheme for

keeping track of the

generations in an experiment

like this.

X

X

P (parental)

F2 (second

filial)

F1 (first filial)

Mendel Concluded:

•  There must be some sort of "particles" or "elements" inside the pea cells that make them look the way they do. – Others had proposed that inheritance was a sort of "blending"

of traits. But Mendel found no evidence for that. •  The trait of purple flower color is dominant to the trait of

white flower color, which is said to be recessive—because in a hybrid, the purple "element" somehow covers up the white "element".

•  However, the white "elements" can be passed on— although hidden, they may reappear in future generations.

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The same was true for the other six pairs of traits that Mendel studied. . .

DOMINANT RECESSIVE purple flowers white flowers

round seeds wrinkled seeds yellow seeds green seeds inflated pods constricted pods

unripe pods green unripe pods yellow flowers along stems flowers at ends of stems

tall plant dwarf plant