Evolution on Darwin’s 200th Birthday

Feb 12th, 2009 | By | Category: Evolution, Featured Articles, Lead Article

Happy 200th birthday, Darwin.

Darwin’s major accomplishment was to condense a lot of thought on the origins of life into two basic concepts: new traits arise randomly (mutation) and the most adaptive of these new traits would become dominant in the population (natural selection)–forming the first cohesive theory of evolution.

For proof, in these early days, we had Darwin’s observations on the Galapagos Islands and the fossil records showing the rise of new traits in the living population to match changes in or introductions to new environments.

Building off of Darwin’s ideas of natural selection and mutation generating new traits came Mendel, and his conception of genetics, a systematic way by which traits are passed from parents to children. Mendel’s genes passed unchanged from parent to child cause traits of living things. An individual has two copies of each gene, one from each parent. If you have a mixture of genes for a trait, one of these genes can dominate over the other, hiding the weaker recessive gene’s trait for the generation.

Watson, Crick, Wilkins and Franklin‘s discovery of the structure of DNA in the 1950’s gave genes a physical manifestation—understandable with fairly simple chemistry. The central dogma of biology followed shortly after, in which DNA encoding for genes is transcribed into messenger RNA and in turn proteins that cause the traits first observed by Darwin hundreds of years before.

Human understanding of life has come in these spurts, separated by decades of consolidation and grappling with new data or new ways of thinking about biology. We’re, right now, in midst of another spurt in our understanding of life.

Until about a decade ago, we only really knew DNA–the long ordered strands of the four basic letters of life–in patches and spurts, with little sense of the overall map of any living thing. With enormous effort and cost, we sequenced the human genome–the vast majority of all the DNA in a human cell. We discovered that a human being has about twenty-thousand pairs of Mendel’s genes.

In the past few years, sequencing DNA has become shockingly less expensive. What once cost four billion dollars can now be done for a few thousand. And the price is dropping dramatically every few years. As a result, we now have sequenced the genomes of many other organisms–fish, cows, dogs, mice, opossums, frogs, chickens, chimps to name just a few.

If we think about Darwin’s traits as tasks a living thing must accomplish–eating, carrying oxygen and so on–and Mendel’s genes as means of accomplishing these tasks–a beak, a histone protein to wrap DNA around, hemoglobin in red blood cells–by comparing the DNA sequences for given traits (a gene’s locus) in different organisms, we can see how evolution has adapted each organism to its environment and lifestyle.

Storing DNA should be little different for a fish, frog, mouse or human. The task is as ancient as eukaryotic life. Let’s look at the genetic locus coding for a protein responsible for this DNA-storing trait, comparing how close various other organism’s DNA is to human’s sequence.

histoneh3(click for a larger version)

Each row represents the equivalent gene in another species (zebrafish, xenopus frogs, chickens, opossums, mice, dogs and macaque respectively.) The higher the blue, the closer the DNA sequence matches that of humans; given that our common ancestor with these organisms were millions, hundreds of millions for the majority, of years ago, this is a very high degree of sequence conservation. The task hasn’t changed (the goals of the trait) so the gene hasn’t changed much either.

Now let’s look at the gene responsible for the trait of carrying oxygen in our blood, the beta-chain of hemoglobin. For a fish or a frog, the demands of the task of capturing and carrying oxygen are dramatically different than those for a purely land-dwelling animal; we’d expect the DNA sequence for the equivalent gene in these animals to be quite different from human’s.

Indeed, that’s the case:

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Today, on Darwin’s 200th birthday, I encourage you to browse around, comparing human genes to those of our distant and close relatives in the animal kingdom. (Check out Opsin 1, for color vision, or the NMDA neurotransmitter receptor for some other fun genes.) You are living in a golden age of biology, in which our understanding of life is jumping by leaps and bounds every year. Within your lifetime already, we’ve gained astonishing abilities to peer into the nature, structure and function of life. Despite the centuries of advancement, Darwin’s (and Mendel’s) carefully crafted ideas of evolution and genetics have not only endured, but provided an invaluable map to understanding this vast new collection of data. Be proud.

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