Darwin and Evolution Chapter 15
In the time before Darwin Evolution has been called one of the great unifying theories of biology due to the amount of time it has been tested, and lack of any exception to its rules throughout the world. Not every part has been proven, but nothing has been disproven Although deservedly receiving credit for most of the work on evolution, the ideas that helped form Charles Darwin s theory began to arise almost 100 years earlier. One of the earliest scientists to publish on the subject was Darwin Erasmus Darwin (Charles grandfather). Erasmus was troubled by the numerous vestigial structures he continually found in organisms. Vestigial structures are anatomical structures which do not have any current function but seemed to have been important in the past He began to propose ideas about species evolving, but he never came up with any mechanism for it.
Descent with Modifications Around the same time, some extraordinary fossil records were being uncovered. Sir Richard Owen first coined the term dinosaur in 1846, but 1815 was the first time anyone suggested the possibility of an ancient, nowextinct species of animal Dinosaur bones had been collected since the medieval times, but were mostly attributed to Biblical giants or mythical dragons Animals such as the mastodon and brachiosaur seemed to have similarities to modern day elephant and giraffe Could these animals have been descended from the ancient dinosaurs, with slight modifications?
Cuvier and Catastrophism Georges Cuvier was the first to use comparative anatomy to classify animal species and was the founder of paleontology, the study of fossils Cuvier did not believe that species could change on their own, but he admitted that the fossil record clearly showed sudden and dramatic changes in the shape and size of similar species. He hypothesized that the cause of these changes were major catastrophes that struck the earth and artificially forced species to evolve. He even proposed the radical notion of mass extinctions After each extinction, he reasoned that a small number of species survived (or were created again) and repopulated the earth. This thinking explained the appearance of new, slightly altered species and was called catastrophism.
Lamarck and Acquired Characteristics One of the first scientists to offer a scientific, testable study on evolution was Jean-Baptiste de Lamarck. Lamarck supported the idea of acquired characteristics, or that the environment produces physical changes in a species that can be inherited by offspring His most famous example: giraffe s necks grew over short generations in order to reach the leaves on tree tops. The genes that changed were then passed on to offspring. Although he was wrong about the mechanism for change, he was correct in his belief that changes in species aren t limited to one generation they are passed to the next one
Hutton & Lyell and Uniformitarianism Although a generation apart, James Hutton and Charles Lyell worked together to study geologic patterns in the Earth. Their first conclusion was that the Earth wasn t shaped by sudden, major catastrophes (although they did occur). In between each catastrophe the Earth experienced slow, gradual changes as well. If the Earth was slowly changing, and Cuvier said changes in the Earth resulted in changes in species, then species must be slowly changing as well. They eventually developed Uniformitarianism, which says that the changes the Earth is going through today (and thus species) are the same as the changes the Earth went through in the past The most dramatic implication of this theory is that in order to go through these slow, gradual changes, the Earth must be old much, much older than was previously thought.
Thomas Malthus and Population Another important contributor to evolution, Thomas Malthus, wasn t a scientist but an economist. Malthus studied populations and discovered that the biggest stresses on populations (famine, disease, war) always occurred when populations stretched beyond what he considered was their limit. Strangely the most successful cities in the world hadn t avoided these stresses but experienced them frequently. These stresses, while tragic, had a great effect on the survivors. They were able to learn from past mistakes and were stronger for having overcome them A young college student named Charles Darwin read this paper and wondered if this economic concept applied to biology as well
The HMS Beagle Charles Darwin was 22 years old when he joined the HMS Beagle as the ship s naturalist. The role of the naturalist is Search for valuable plants, spices, minerals, etc. Collect specimens for research Serve as a backup to the ship s doctor, navigator The Beagle spent five years sailing along the southern hemisphere, visiting Australia, Africa, South America and the West Indies Everywhere they went, Darwin had the chance to explore In attempting to build a new collection of specimens, Darwin inadvertently made some observations Example: fossils of sea animals on mountains, similar animals on totally different continents
The HMS Beagle Were the similarities between animals on different continents due to random chance? The environments? Common ancestors? Migrations? Darwin kept logs of his records, but his work was hardly world changing until the Beagle made a stop on the Galapagos Islands. The Galapagos were a small group of islands, flat and mountainous, sandy and rocky, and 500 miles from South America (which meant migration would be difficult). To Darwin, the Galapagos were like a set of miniature continents and could serve as a model for the earth.
The Galapagos Animals Tortoises Each of the islands on the Galapagos had their own species of tortoises Darwin noticed on the barren islands, all the tortoises had long necks which helped to pick fruit off the cacti On the lush islands, the tortoises all had short necks. Did the islands cause the necks in the tortoises? Or did the tortoises live on the islands that were best suited for them?
Harriet: Born: 1835; Died, 2006
The Galapagos Animals Finches Today there are only 13 species of finches on the islands, but Darwin found many more. He noticed that although the finch s appearances differed greatly, the one similarity was the relationship between food and beak size. Finches with large beaks broke seeds or nuts and used a long tongue to retrieve food inside Finches with pointed beaks act like woodpeckers, but once they create a hole they use tools to force insects out. Did food choices cause beak sizes? Or did beak sizes dictate food choices?
Return to England After Darwin s return to England in 1836, he waited 20 years before publishing his findings Partly because his ideas hadn t formed yet, partly because he wanted to experiment, and partly because it was such a radical idea. Darwin s basic theory was that if organisms could change slightly from generation to generation, why couldn t they change dramatically over time? The main problem to tackle: what was the motivation for change?
Published Darwin finally published his works when, after reading an article by a Alfred Wallace (who had similar ideas), he became worried that he d finish in second place. On November 24, 1859, in the middle of the Industrial Revolution of Europe, Darwin published his manuscript titled On the Origin of Species. Europe had fallen in love with a mechanized economy. Darwin s opinions about a mechanized system of life fit perfectly with the times. In the manuscript, Darwin promoted descent with modifications, a common ancestor, and natural selection
Natural Selection Natural Selection contains the following tenets 1) Organisms in a population have heritable variations Darwin believed and we have since proven correct that variations are random. Variations are just as likely to be harmful as helpful (Keep in mind, Darwin is publishing the same time Mendel is experimenting. We still don t know about genes or even what nucleic acids are.) 2) Eventually, more individuals are produced in a population than the environment can support Remember the carrying capacity? Births are usually much higher than deaths in a population too
Natural Selection 3) Some individuals have characteristics that enable them to survive AND reproduce better than other individuals Fitness is described as reproductive success relative to other members of a population. If you re able to survive, you re more likely to reproduce and, thus, more likely to pass on the genes that enabled you to survive. If you re less likely to reproduce, your unhelpful genes die with you 4) As generations progress, a higher percentage of individuals will have these traits than previous generations Adaptations are the genes that help organisms to survive in their environments 5) The result are populations built for their local environment
Natural Selection and the Galapagos Tortoises The tortoises on the lush islands had no need for longer necks The tortoises on the dry lands benefitted from the longer necks though The tortoises with the longer necks on these islands were the most likely to reproduce and pass on longer-necked genes Eventually, only long-necked tortoises inhabited the island Finches Similarly, the finch s beaks did not grow to meet the needs of the food. The parents with the best beak genes for their specific chosen food sources passed along those specific genes only.