Lecture#4—Darwin’s Evidence I
This is the first of two lectures on the evidence that Darwin amassed and presented in his book On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. The Origin of Species wasa key turning point in the way people viewed their place in the universe. Darwin in his various books clearly established that humans were to all related to all other animals.
In this lecture we follow the key arguments Darwin presented to the public in the format of the hypothetico-deductive method; what are the questions, hypotheses, predictions, evidence, and conclusions we can draw?
• The first part of the lecture recalls how science became a distinct way of viewing the world. Then we cover the exact predictions of the argument of Darwin’s Hypothesis of Evolution by Natural Selection. That is, if the hypothesis of evolution by natural selection is correct, then……
•Individuals vary
•Populations tend to over-breed
•Struggle for existence
•Survival of the fittest
•Variations are inherited
•New species develop
So organisms on earth change through time.
Darwin brought these arguments to the table.
•Prediction # 1:
•If the hypothesis of evolution by natural selection is correct then variations should exist among organisms.
•Data: It was easy for Darwin to provide evidence of this because the public was well aware that not all roses, or cats, or cattle were alike and it was easy to accept that the same thing occurred in nature.
•Prediction #2:
•If the hypothesis of evolution by natural selection is correct
thenmore offspring are born than will survive to reproduce.
- Data: This is the tendency to overbreed argument made by Reverend Thomas Mathus for humans This too was easy to establish because most people knew that fish, orchids, elephants and humans all had the capacity to overbreed, yet the world was not overrun by fish, orchids, elephants and humans. The populations tended to remain stable.
- Prediction #3
- If the hypothesis of evolution by natural selection is correct
thendifferences should exist among offspring that survive & breed & those that don’t.
- Data: This is the central argument of natural selection. Data were difficult to come by in Darwin’s time, but we have lots of evidence now. To make this argument in his time, Darwin had to argue by the analogy of artificial selection. He said that farmers were constantly selecting out which animals to breed and which to slaughter, keeping only the best ones for breeding. He argued that this is the same thing that nature does—the best ones survive and produce the next generation. Hence, he called it Natural Selection
Flemming Jenkin had a powerful counter argument. He said that even if the best animals of a species did have the highest survival rate, they still had to breed with the more numerous average animals, and thus the good trait effect would be progressively diluted as their traits would be swamped out.
- Prediction #4
- If the hypothesis of evolution by natural selection is correct,
thenvariations important to survival must be inherited.
- Data: This was a significant weakness in Darwin’s argument since no one understood just how inheritance occurred. Only after Gregor Mendel’s work did we began to understand genetics.
- Prediction #5
•If the hypothesis of evolution by natural selection is correct
thenthe Earth must be old so that evolution had time to occur.
Data: In Darwin’s time the Biblical chronology was still accepted; i.e. that the earth was only about 6,000 years old. But the geologists said the earth must be much older because of the time it takes for glaciers to carve valleys, rivers to excavate canyons, and miles of sediments to build up. The real date for earth had to wait until radioactive dating was discovered in the 1950’s.
The predictions will be continued the next lecture.
Define these Terms/Concepts:
Struggle for existence
Survival of the fittest
Artificial Selection
Natural Selection
Flemming Jenkin
Can you answer the following questions?
1)What is the difference between artificial selection and natural selection?
2)What was Flemming Jenkin’s argument and why did Darwin have such difficulty countering it?
3)Why was it essential to Darwin’s argument that the earth must be millions of years old rather than a few thousand?
4)Darwin said that in order for his hypothesis to be correct then traits important to survival must be inherited. Why was this critical?