Evolution in a nutshell
By AMY ADAMS
The HMS Beagle launched on its second survey expedition from England’s Devenport Harbor on Dec. 27, 1831, with a crew of 73. Charles Darwin, a 22-year-old gentleman with a penchant for collecting beetles, was aboard to serve as naturalist. Though the trip was planned to last two years, it spanned five; Darwin spent more than three of those years on land exploring parts of South America, the Galapagos Islands, Tahiti, Australia and New Zealand.
The great diversity of animals and plants — especially in the Galapagos — amazed him. When he reflected on what he’d seen during his travels, Darwin had a novel thought: Perhaps a species changes over time to adapt to its environment.
He saw signs of shifting species everywhere he looked, including the fossil record that contained seemingly ancestral forms of modern animals. The idea that populations weren’t immutable shocked people of his time and continues to raise consternation today.
Back in England, Darwin’s thoughts returned to the Galapagos, especially the finch populations, which were unique to each island. He theorized that a single ancestral population settled each island and then adapted to that island’s environment. Hundreds of thousands of years later the result was animal populations that natives could identify on sight as belonging to a specific island.
Darwin surmised that a process he called natural selection led to adaptations such as the ones he saw in the finch populations. According to his idea, animals with adaptive traits produce more offspring than other animals. If a bird with a particular beak shape could eat more seeds and was therefore healthier, that bird was also more likely to reproduce successfully and pass along its handy beak.
Darwin collected a great many specimens and recorded pages and pages of notes, but it wasn’t until 1859, 20 years after he first theorized about natural selection, that he would consolidate all of his observations in The Origin of Species.
Connecting Darwin’s dots
Darwin died in 1882, almost 100 years before molecular genetics research would reveal the how behind his ideas. A crucial element in the theory of evolution by natural selection is that animals can pass along their physical traits. It turns out that DNA is where the recipe for those physical traits resides.
Every population has somewhat varied individuals because the population contains many versions of each gene. For example, a finch with a particular version of a growth-promoting gene might grow ever so slightly larger than a finch with a different version of that gene. In a habitat that favors larger birds, the larger finches have more chicks, and as a result the growth-promoting version of the gene shows up in a greater proportion of the birds. Those larger offspring interbreed and over time the entire population becomes larger.
New traits can also pop up in a population. Each time a cell divides there’s a chance for a mutation to sneak in and get passed on to offspring. Sometimes those mutations make no difference whatsoever. Other mutations are deadly. Still other mutations might produce slight variations, like a growth-promoting gene that produces an even larger finch, or a growth gene that gets used during a different time in development, producing a normal-sized bird with particularly large wings.
In some environments that big-winged bird might fail completely, but on the right island that bird might become a reproductive ace, passing along that trait to future offspring, thereby altering the look of that population.
Natural selection reigns as evolution’s most celebrated force, but it’s not the only player. Any event that influences a population’s genetic diversity shapes the evolutionary trajectory. Some of the most important forces that drive evolutionary change or alter genetic diversity are:
Natural selection. Individuals that are more suited to an environment produce more offspring. Eventually, the trait that made those individuals so fruitful becomes common in the population.
Random mutation. Mutations happen at random throughout the genome. Some of these mutations help the organism flourish and produce more offspring to propagate that mutation. Other mutations are neutral, but persist in the population by chance. Either way, the mutation and its associated trait become common in that population.
Genetic drift. In some cases a trait might be neither helpful nor harmful, but for statistical reasons, more organisms with that feature reproduce and the trait ends up becoming common.
Bottleneck. A big storm or dramatic event can kill off a large portion of a population. The remaining individuals are left to rebuild the species. If those survivors include a few individuals that had unusual features, that feature will become common in the renewed population.
Founder effect. If a few individuals migrate together or are cut off from the main population they will go on to build a population of their own. That new population will reflect the genetics of the founders rather than the genetics of the original population.
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