Mechanisms of Evolution
Topics in Depth
The Theme of The Origin of Species in Mechanisms of Evolution
Stop us if you've heard this one:
There once was a man on the Beagle
This Darwin, studied finches and seagulls
Species that he collected
Hinted they were naturally selected
An idea some thought quite illegal
When Charles Darwin started his studies on the H.M.S. Beagle in 1831, he wanted to study what made nature tick…or he thought it was an opportunity to buy a sweet backpack and fill it with tons of black and white composition notebooks. Either way, Darwin was a naturalist who spent five years traveling on the ship that was sent to chart the world. While traveling the ocean blue, he kept a detailed diary of the organisms he saw at each pit stop, often collecting specimens to take home with him to England. His most famous specimens were finches. His crew was probably thankful he didn't choose to study spiders.
Darwin took birds from South America and the Galapagos Islands, off the South American coast, back to his naturalist buddies at home. They confirmed that the birds from different geographic location were different species of finches. By definition, a species is unable to reproduce with any other species. Therefore, these finches were related but distinct.
Darwin kept meticulous notes and drawings on the species he encountered.
What made these species different? Were they ever the same? Darwin believed a few South American finches must have flown off to the Galapagos. Boy, their arms must have been tired. Over time, they had become a new species of finch. Time and space had resulted in their evolution. It occurred to him that the driving force over this evolution was natural selection, where certain traits helped the finches survive in the Galapagos. Therefore, the finches with those traits lived long and prospered, passing those traits on to their offspring.
Darwin pointed out the following evolutionary concepts:
1. Natural selection is a mechanism that drives evolution.
2. Species go through common descent with modification.
In other words, all organisms were originally derived from the same ancient ancestor. If we trace this way back to the beginning of life, this was probably a single-celled organism. Say hello to your great Uncle Bacterium.
Darwin wrote numerous pages detailing his ideas about the organization and evolution of life. This page from his notes famously describes different species labeled "A" through "D" as branching off from a common ancestor, marked as "1." Darwin called this concept, "common descent with modification."
You can think of natural selection as putting a population on a black licorice-flavored diet. Not everyone can handle such a strong taste all day long. Those that can will stick around and fill up on the twisted candy—eventually having lots of licorice-loving kids. Eventually, we'll have naturally selected a room full of people who can't get enough licorice. Darwin's novel and controversial ideas on evolution, unfortunately licorice-free, were published in his book, On the Origin of the Species by Means of Natural Selection.
As smart as Darwin was, there were a few things he didn't know. It would be a long time before DNA, a heritable material, was discovered. Once DNA was discovered, we could explain that finches sport differently shaped beaks because they possess different genes for their snazzy bills. The physical traits Darwin described, or phenotypes, were influenced by an organism's genetic makeup, or genotype. We don't mean Levi's.
Not only do different genotypes make for differences among species of finches, we now also know that they make for differences within populations. For example, take a species of finches with small beaks that are perfect for eating small seeds. Some members of the population could still have a range of beak sizes that are all relatively small compared to the beak sizes of a different finch species. The small-beaked finches can reproduce with each other regardless of where they fall in this range. If they can make cute little baby birds together, then they are still the same species. These guys just have different alleles, or variants of the gene that code for beak size. To make things further complicated, some phenotypes are influenced by multiple genes. We call this a polygenic trait. Like, a trait that's so popular, all the genes want to be a part of it.
Darwin's hobby was his work. When he wasn't studying organisms for his research, he was breeding fancy-shmancy pigeons in his backyard.
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