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Darwin and Evolution Chapter 15
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, now-
extinct 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.
