October 21, 2010 Leave a comment
Today, I did my first reading/teaching presentation from The Complete Idiot’s Guide to College Biology. Below are a couple of excerpts from what I read today.
From Chapter 16: Darwin, Natural Selection, and Evolution
Evolution, a change in a population over time, can be a controversial concept, and things were no different when Darwin first proposed his theory of how evolution happens. Since that time, we’ve identified several other ways by which evolution can occur. Scientists have synthesized natural selection and genetics and worked out a way to identify if evolution is happening in a population.
The Historical Context of Darwin’s Ideas
Charles Darwin was born on February 12, 1809, into a society with fixed ideas about the role of divinity–specifically the Christian God–in nature. Darwin’s destiny, as it turned out, was to address nature’s role in nature, rather than God’s. He was not completely comfortable in some respects with that destiny, but this man was born with his ear to the ground, listening to Nature’s heartbeat. He was born to bring to us a greater understanding of how nature fashions living things.
Yet, he did not emerge into a howling wilderness of antiscientific resistance. Scientists and philosophers who had come before him had posited bits and pieces of what would become Darwin’s own theory of how evolution happens. But it required Charles Darwin to synthesize those bits and pieces–some of them his own, gathered on the significant voyage of his lifetime–to bring us a complete idea of how nature shapes new species from existing life.
Alfred Russel Wallace: The Unknown Darwin
Alfred Russel Wallace developed the theory of evolution by natural selection at the same time as Darwin. His road to enlightenment came via his observations on another island chain, the Malay Archipelago. Like Darwin, Wallace was a naturalist savant, and on this archipelago alone, he managed to collect and describe tens of thousands of beetle specimens. He, too, had read Malthus and under that influence had begun to formulate ideas very similar to Darwin’s. The British scientific community of the nineteenth century was a relatively small world, and Wallace and Darwin knew one another. In fact, they knew each other well enough to co-present their ideas about natural selection and evolution in 1858.
Nevertheless, Wallace did not achieve Darwin’s profile in the field of evolution and thus today does not have his name inscribed inside a fish-shaped car decal. The primary reason is likely that Darwin literally wrote the book on the theory of evolution by means of natural selection. Wallace, on the other hand, published a best seller on the Malay Archipelago.
From Chapter 13: DNA
DNA, as the central molecule of heredity, is key to many aspects of our lives (besides, obviously, encoding our genes). Medicines and therapies are based on it. TV shows and movies practically feature it as a main character. We profile it from before we’re born until after we die, using it to figure out what’s wrong, what’s right, what’s what when it comes to who we are, and what makes us different and the same.
But it wasn’t so long ago that we weren’t even sure that DNA was the molecule of heredity, and it was even more recently that we finally started unlocking the secrets of how its genetic material is copied for passing along to offspring.
The History and Romance of DNA
The modern-day DNA story is dynamic and fascinating. But it can’t compare to the tale of the trials, tribulations, and downright open hostilities that accompanied our recognition of its significance.
Griffith and His Mice
Our understanding started with mice. In the 1920s, a British medical officer named Frederick Griffith performed a series of important experiments. His goal was to figure out the active factor in a strain of bacteria that could give mice pneumonia and kill them.
His bacteria of choice were Streptococcus pneumoniae, available in two strains. One strain infects and kills mice and thus is pathogenic, or disease causing. These bacteria also have a protein capsule enclosing each cell, leading to their designation as the Smooth, or S, strain. The other strain is the R, or rough, strain because it lacks a capsule. The R strain also is not deadly.
Wondering whether or not the S strain’s killer abilities would survive the death of the bacteria themselves, he first heat-killed the S strain bacteria. (Temperature changes can cause molecules to unravel and become nonfunctional, “killing” them.) He then injected his mice with the dead germs. The mice stayed perky and alive. Griffith mixed the dead, heat-killed S strain bacteria with the living, R-strain bacteria and injected the mice again. Those ill-fated animals died. Griffith found living S strain cells in these rodents that had never been injected with live S strain bacteria.
With dead mice all around him, Griffith had discovered that something in the pathogenic S strain had survived the heat death. The living R strain bacteria had picked up that something, leading to their transformation into the deadly, pathogenic S strain in the mice. It was 1928, and the question that emerged from his findings was, What is the transforming molecule? What, in other words, is the molecule of heredity?
Hershey and Chase: Hot Viruses
A fiery debate tore through the ranks of molecular biologists and geneticists in the early twentieth century, arguing about whether proteins or DNA were the molecules of heredity. The protein folk had a point. With 20 possible amino acids, proteins offer far more different possible combinations and resulting molecules than do the four letters (nucleotide building blocks) of the DNA alphabet. Protein advocates argued that the molecule with the most building blocks was likely responsible for life’s diversity.
In a way, they were right. Proteins underlie our variation. But they were also fundamentally wrong. Proteins differ because of differences in the molecule that holds the code for building them. And that molecule is DNA.
Like what you’ve read? Read the rest in The Complete Idiot’s Guide to College Biology.