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A Darwin Day at the Zoo
2009 marks Charles Darwin’s 200th birthday and the 150th anniversary of his defining work, On the Origin of Species. Today’s biologists have great respect and admiration for Darwin and his carefully constructed ideas. His logical presentation of evolution through natural selection is the central organizing principle for theory and application in modern biology.
by Don Moore
Darwin recognized that most species had descended, like varieties of domestic animals, from common ancestors. He noted that species were changeable over time. Darwin also wondered about natural selection before scientists even knew about genetics.
The basis for evolution through natural selection is simple to grasp: First, we know that individual animals and plants show variable differences, and this variability is inherited by their offspring. Second, some animals and plants have particular variations that allow them to survive and reproduce, producing more offspring with similarly winning traits. Each surviving individual produces more offspring than can possibly survive, so the competition among these allows for only “survival of the fittest.”
Since the best clue to evolution through natural selection was probably in the artificial selection of varieties of domesticated animals, Darwin focused his work on this subject, and found a clue to natural selection in domestic pigeons, which are descendants of rock doves. Darwin was a recognized expert in pigeon care and breeding, and had breeds as diverse as fantails, carrier pigeons, and pouters. Darwin used the huge diversity of fancy pigeon breeds to represent the artificial selection comparison—arguing this was a domestic model of wild species descending from common ancestors in nature.
|A Linnus two-toed sloth lives in the National Zoo's Amazonia building. (Mehgan Murphy/NZP)|
If Darwin were to visit the National Zoo and give a tour to guests, he would first visit the domestic animals at Kids’ Farm, observing the variety in the individual breeds of goats and cows. Darwin said in his book, “Man can and does select the variations (under domestication)…He thus adapts animals and plants for his own benefit.” Different cultures through the ages have developed different domestic breeds to adapt to local environments and cultural uses. At Kids’ Farm, milk-giving Holsteins and Hereford cows bred for meat production are thought to have been developed from the single primitive species Bos primigenius, and the various goat, rabbit, and pig breeds were selected over time from their own wild ancestors.
The variations we see in captive conditions result from a lack of natural selection, which tends to eliminate departures from normal conditions in nature; but humans have selected those variations beneficial to us—for beautiful coloration, and for milk, protein, and fiber production. In contrast to the highly diverse species of goats and cattle at Kids’ Farm, Darwin remarked on the lack of diversity in species like donkeys by acknowledging the lack of artificial selection when he said, “In donkeys, from only a few being kept by poor people, and little attention is paid to their breeding.”Visitors can see the different colors in the donkeys living at Kids’ Farm, but unlike cow breeds developed for other uses, the donkeys are similarly sized and shaped, and are more similar to one another and to their wild ancestor.
On to Amazonia — Identifying Frogs
|Spot a strawberry dart frog in the Reptile Discovery Center. (Jessie Cohen/NZP)|
Darwin noted that individual animals in nature differ slightly from their parents, just as humans do. Take the example of the poison dart frogs, which can be identified by the different patterns on their backs. Darwin would most certainly ask the Amazonia staff to show dart frogs on the tour, so visitors could see the individuality— such as varying skin patterns of young dart frogs changing into adults —to compare these to the adult parents. Darwin recognized that the variety a select group of similar individuals might develop could become the intermediate varietal from which a new species might develop. He said, “Thus, the forms of life throughout the universe become divided into groups subordinate to groups.”
Next Stop — Reptiles and Inverts
Darwin’s next stop in his tour of the Zoo would most likely be the Reptile Discovery Center and the Invertebrate Exhibit. Species housed in these exhibits illustrate the concept he learned from Malthus’ 1838 Essay on the Principle of Population, which emphasized one general principle of nature—that living creatures produce more offspring than can possibly survive to be mature, to become reproducing organisms themselves.
Before he entered the Reptile Discovery Center, however, Darwin would probably stop at an oak tree and pick up some acorns from the ground to show us that the strategy of producing many offspring applies to plants as well as animals. Outside the reptile house, he would show visitors the huge, natural school of tadpoles found in the pond each spring. Of these hundreds of tadpoles, only a few will survive to the age when they can mate and produce other tadpoles that will grow into adult amphibians. Amphibian survival, of course, depends upon the amount of available food, available habitat, and diseases like the chytrid fungus, currently devastating populations around the world. Sometimes, Darwin would suggest, even the “fittest” will not survive this type of great environmental change.
Huge reproductive potential is also observed in the millions of fertilized eggs produced by corals in the Invertebrate Exhibit. Only a few of these will survive to settle and grow into polyps. In the wild, the rest die due to current environmental conditions, or may be eaten by voracious predators.
Adaptations in Small Mammals
Darwin would next lead us to the Small Mammal House to examine the concept of adaptations. Small mammals have an incredible diversity of form and function, and the Zoo has small mammal examples from around the world. The two-toed sloth(Choloepus didactylus) is a great example from South America, with its adaptations of strongly recurved claws for moving around trees and slow movement that assists with camouflage against predators. Black and rufous giant elephant-shrews (Rhynchocyon petersi), with their strong back legs used to escape predators, and long, mobile noses for finding insect food, are cartoon-like examples from Africa. And Prevost’s squirrels (Calloscinous prevosti), with their adaptive camouflage coloring and balancing tails, are great examples from Asia.
Darwin recognized potential criticisms of evolution based on natural selection alone, and developed an extension of his theory to “sexual selection.” Acknowledging that there could be differences between the sexes in behavior or in physical structure, he said, “This form of selection depends not on a struggle for existence in relation to other organic beings or external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession of the other sex.”
Darwin’s tour has included his main theory about evolution, which explains the diversity of life on Earth. How would this great writer end his Zoo visit? He might just fade away, leaving visitors to observe the adaptations of different animals, to think about what adaptations their long ago ancestors had, and to speculate about what their descendents might look like far in the future.
|A pair of black-and-rufous giant elephant shrews plays in the Small Mammal House.(Mehgan Murphy/NZP)|
However, he would be especially pleased if the tour continued to include the animals he found while on the H.M.S. Beagle’s voyage around the world—giant tortoises and other island giants, flightless rheas, long-legged maned wolves (Chrysocyon brachyurus), armadillos (Tolypeutes matacus) and giant anteaters (Myrmecophaga tridactyla)—all found at the National Zoo.
Darwin had keen insight, and was a thoughtful writer who was able to put forward his theory of evolution in a way that captured the imagination of the world. His great contribution to the view of life still drives discoveries in biology and medicine. He would have loved visiting the National Zoo to observe the many animals he used as examples for his revolutionary theory of evolution.
— DON MOORE is the associate director of animal care at the National Zoo. He is also an accomplished writer and author of a successful children’s book.
Did you know?
“Theory” in everyday language means a hunch or speculation. A “Theory” in science, like the Theory of Gravity, the Theory of Relativity, and the Theory of Evolution, in contrast, has so much supporting evidence from observation and experimentation that it becomes an established scientific explanation.
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Smithsonian Zoogoer 38(6) 2009. Copyright 2009 Friends of the National Zoo. All rights reserved.