Presentation on theme: "Chapter 3: The Emergence of Life. Darwin’s Proposal In The Origin of Species (1859) Darwin rarely used the term “evolution,” though Herbert Spencer often."— Presentation transcript:
Chapter 3: The Emergence of Life
Darwin’s Proposal In The Origin of Species (1859) Darwin rarely used the term “evolution,” though Herbert Spencer often described the theory in those terms. Perhaps, Darwin wanted to make it clear that his theory was not about “progress” (unlike Spencer); it was about change. Darwin (and pretty much everybody else) knew that breeds of plants and animals could be deliberately shaped by human intervention (what we might call “artificial selection”). Darwin also knew that in any given population only a small minority survives long enough to produce offspring. The future of a species can only be shaped by those who live long enough to reproduce. So future generations are produced only by those survivors. Those members of a species most likely to survive to reproduce would be those lucky enough to have inherited features that made them more likely to reproduce.
When we speak of “adaptation” we are referring to those features of a species that seem to make them “ideally” suited to their environment.
In the 18 th -century Carl Linnaeus established the modern system of taxonomy. Most people, including scientist, believed that God had created the world about 6000 years ago, and created all of the species at that time, placing them in different environments. (Most also believed that representatives of all the earth’s species had, at one time, been able to fit on a single boat.) But there were problems with this idea: Fossils had been found of strange creatures not mentioned in the Bible or historical records. Some of these were sea creatures found on top of mountains, buried deep inside rocks. As European exploration began to encompass the globe, people were aware that there were far more species than they could have imagined.
In the early 19 th century, Erasmus Darwin, Charles Darwin’s uncle, had already suggested that species had evolved to adapt to their environment, but no one understand how they could have done this. In 1809 Jean-Baptiste Lamarck suggested that perhaps parents acquired certain features that allowed them to better survive in a specific environment, and they then passed those on to their offspring. In 1831 Charles Darwin got the chance to take an around the world voyage on The Beagle.
The textbook summarizes it this way: 1.Tiny random variations among individuals mean that some do better than others. 2.Those lucky enough to be well adapted to conditions on a particular island flourish and have more descendants. 3.Gradually populations change and diverge on different islands. 4.Eventually the difference between birds on different islands became great enough that individuals can no longer interbreed, which is the usual definition of a new species.
In 1838, Darwin read the work of Thomas Malthus. He also reflected on the ways in which animal breeding took place. In The Origin of Species he wrote: Merino Rams
So, Darwin asked what would drive selection in the natural world? His answer: “fitness.” So it wasn’t so much that species adapted to their environments as those individuals that were (even slightly) better adapted to their environment would more likely survive to reproduce and thus pass on those characteristics to future generations. As Darwin explained it: See p. 59 in the textbook.
Though there were many fossils that demonstrated species had gone extinct, Darwin needed a transitional species, and in 1862 one was found.
Geographical distribution also helped his theory. Why didn’t all deserts have camels? Why aren’t there giraffes in North America? What about all those marsupials in Australia? Darwin’s answer was that species evolved to adapt to specific environments. Species have a history located in a specific geography.
But there were problems with and objections to Darwin’s theory: At the time most geologist believed the earth was 100 million years old (Darwin thought 300 million), and that was just not enough time for evolution to take place on such a scale. Also, 19 th -century biologist had no real idea of how inheritance worked. How could it be stable enough to reproduce a species, but not so stable that no changes were possible? Now we know that the earth is much older than 100 million years. The fossil record is much more extensive than in Darwin’s day. We have also had the chance to observe natural selection at work (e.g. in the response of bacteria to antibiotics and plants and insects in response to pesticides). Gregor Mendel had understood the basic workings of heredity, but his work was not widely known until the 20 th -century. And then we discovered DNA...
“A star is simpler than an insect.” –Martin Rees. Systems that challenging entropy aggressively tend to last less longer than those that don’t.
How do we explain the origin of life? The textbook defines life as: 1.Uses energy from the environment by eating or breathing or photosynthesizing (metabolism). 2.Makes copies of itself (reproduction). 3.Over many generations it can change characteristics to adapt to its changing environment (adaptation). There is continuum between life and non life (think of viruses).