2. Charles Darwin & Natural Selection

Charles Darwin (1809-1882) wrote On the origin of species by means of natural selection or the preservation of favoured races in the struggle for life, which was published in 1859. This book caused fundamental changes in thinking about the nature of man. The intellectual revolution it spawned was unprecedented – it has been called “the book that shook the world.” The effect was immediate: the first edition sold out the day it appeared, with 3800 copies sold in the first year. In Darwin’s lifetime, the British printings alone sold more than 27,000 copies. The immediate success of the “Origin” indicates that the world was ready for the theory of evolution. Several basic ideas fundamental to evolutionary thinking were already present in the writings of the time.

These ideas included:

• That the earth was very ancient

• That fossils were the remains of extinct species

• That many species showed close similarities to one another.

• That organisms descended from previously existing organisms.

In other words…..they were NOT spontaneously generated from inanimate structures as had once been thought. While many people were writing about the subject of evolution at the time, there was no unifying explanation as to HOW new species were formed. As a result, at the time of the “Origin’s” publication, most anatomists (scientists that study body structure), zoologists (scientists who study animals) and botanists (scientists who study plants) strongly upheld the dogma of creation. Darwin alone deserves the credit for changing this situation overnight. He was the first to propose a credible “mechanism” for evolutionary change. This was a tremendous accomplishment considering that very little was known then about the scientific evidence we now take for granted:

• The number of known fossils was small

• No connecting links between major groups had yet been discovered

• The method of inheritance was unknown

• Cytology (study of cells) did not exist

• Embryology (study of embryonic development) was still in its infancy

Darwin marshalled his evidence with supreme skill, and eventually convinced the majority of his biological colleagues that descent thru natural selection was the mechanism for new species formation. He had spent more than 20 years working on his book. Despite having been published more than 140 years ago the “Origin” is amazingly NOT obsolete.

2.1 Darwinian Premise
What is Darwinian evolution? In Darwin’s words, evolution is “descent with modification.” In other words, closely related species resemble one another because of their common inheritance and differ from one another because of hereditary differences they have accumulated since the separation of their ancestors. How did Darwin reach his conclusions regarding the nature of inheritance? Darwin based his theory on three observations about the natural word:

2.1.1 Variation occurs among individuals
Darwin noted that any two individuals of a species generally exhibit easily recognizable differences. Just look around the room - every one of us is different. This fact is exploited by animal breeders all the time. Darwin was a pigeon fancier and breeder and hence understood artificial selection quite well. Artificial selection is the mating of specific individuals by plant and animal breeders to increase the frequency of desirable traits in their stocks. If you want a fast racehorse you mate two Kentucky Derby winners!

2.1.2 Organisms reproduce in excess

We know that organisms reproduce in excess, yet adult populations remain relatively constant (barring human intervention). To demonstrate this principle in his book, Darwin considered one of the slowest reproducing organisms: the elephant. Elephants have an average lifespan of approximately 100 years. During this lifetime, they are capable of reproducing for an average of 60 years. A single adult female will produce roughly 6 offspring (babies) during this time. If all of her offspring survive and reproduce at the same rate, then in 750 years there would be 19 million descendents from this single female. Other examples of organisms reproducing in excess include the Codfish (a single female produces 10 million eggs in a single spawning season), the Oyster (114 million eggs in a single spawning season), and the Leatherback sea turtle (595 eggs in a single nesting season). If organisms are producing all of these offspring each year, then why isn’t the earth overpopulated with elephants, cod, oysters and sea turtles?

2.1.3. Differential Survival or the Struggle for existence
Darwin observed the struggle for existence all around him. Organisms perish due to many factors including: adverse environmental conditions (drought, floods, and severe winters), predation (one organism eats another), and limited resources (water, food, nesting sites). Plant populations or animals with limited dispersal ability (ability to move to a new location) are especially prone to resource limitations. This struggle for existence among organisms results in differential survival, i.e., some die and some live. Organisms that are more tolerant of adverse conditions, are better at avoiding predation, or can subsist on fewer resources then other members of their species tend to survive. Those that are less tolerant - perish.

2.2 Natural Selection
Darwin’s observations concerning individual variation, reproductive excess, and a struggle for existence among organisms, along with his knowledge of pigeon breeding all helped him to develop his “biological mechanism” for evolutionary change – the process of natural selection. What is natural selection? Natural Selection is the process in nature that causes evolution through differential reproductive success among members of a population. Differential reproductive success means that some individuals will produce more offspring than other individuals. As mentioned above, individuals that are more tolerant of adverse environments, are better at avoiding predation, or can subsist on fewer resources tend to survive to have babies. Those that are less tolerant, tend to perish and hence do not have babies or have fewer babies because of a shorter lifespan. Darwin proposed three postulates that are key to his mechanism of natural selection.

Natural selection occurs if:

Postulate 1. There is variation among individuals

Variation refers to differences in physical characteristics

Postulate 2. There is inheritance of these variations

This simply means that the physical characteristics can be passed from parent to offspring, i.e., tall parents tend to have tall children.

Postulate 3. There are fitness differences among individuals

Fitness means the number of offspring that survive to reproduce. This is tied to the above discussion regarding the struggle for existence. Some individuals will have more offspring than others.

If ALL three of these occur then individuals surviving in a population will differ predictably from those that do not.

This means that certain physical characteristics will be passed on more frequently than others, resulting in a shift in the features of a group of organisms (population). For example, if only tall people survive to reproduce, then over time the population or group will become taller. This was the simplicity and beauty of Darwin’s mechanism of natural selection. Darwin’s theory was so intuitive that it led Charles Lyell to exclaim upon reading the Origin “how simple, I should have thought of that myself.”

2.3 Where does the variation which provides material for natural selection come from?

4. The Role of Mutations (reprinted from unit 5)

We have seen so far in this section how genotype and phenotype can change from generation to generation in predictable ways. Some changes are explained by simple Mendelian inheritance while others are more complicated because they are caused by gene interactions or by multiple genes. But all of these mechanisms for change can be traced back to predictable genetic events. Once we learn how many steps are involved in the production of gametes and in the development of embryos, it is amazing to consider how few errors occur. Yet occasionally we see or read about the really odd offspring that can occur such as the frog with a missing leg or a chicken with a second head. This type of sudden change is rarely due to predictable genetic events but rather to a change in DNA. We call such changes in DNA mutations and most mutations do not result in an improvement in the next generation. The geneticist David Suzuki once compared our genome to a fine Swiss watch that has been tinkered with and developed over many generations by master watchmakers, resulting in an incredibly accurate timepiece. What are the chances that if we were to open up that watch and plunge a probe into it that we would improve the performance of the watch? Really slim, I would say. Such is the chance that changing our DNA even slightly would be expected to improve our offspring in any way.

Mutations can occur in our somatic cells as well as in our gametes. If they occur in our somatic cells, then we don’t pass these changes on to our offspring. For example, if we smoke enough cigarettes, we are likely to damage the DNA in the cells of our lungs and cause mutations that lead to lung cancer. If a woman with lung cancer were to become pregnant, she wouldn’t pass this mutation on to her infant since her ova or eggs weren’t damaged. On the other hand, if a young woman is exposed to too much radiation in her life, perhaps because she grew up near Chernobyl in Russia where a nuclear power plant released excessive radiation, it is possible that the DNA in her eggs would be damaged and her children would be stillborn or be severely handicapped. Similarly, damage to frog eggs by chemical pollutants in water has been shown to result in those frogs that are missing a leg.

Can random changes in DNA of our gametes ever lead to an improvement in offspring? The answer must be yes because this is how evolution occurs. Mutation is the driving force for evolutionary change. We have to conclude that mutations that lead to improvement within a population must cause subtle changes. For instance, we know that in the tropical regions of the world, malaria is a serious and often fatal infection. We also know that humans who have lived in the tropical region of Africa for hundreds of generations have developed a resistance to that disease. We now know that African peoples who are resistant to malaria have a mutation in one of their genes that codes for hemoglobin. That mutation results in a single error in the hemoglobin molecule that causes one amino acid to be replaced by another. One wrong amino acid out of 146 amino acids in that molecule causes the red blood cells to be misshapen. A person who is homozygous for this trait has sickle cell disease, which is a debilitating and fatal disease. However, a person who is heterozygous for this trait does not have the disease and is protected from malaria. It turns out that the odd shape of some of the blood cells in a person who is heterozygous prevents the parasite that causes malaria from attaching to their cells.

Thus in this example we see that a mutation that occurred in the gametes of an individual who lived in Africa perhaps hundreds of generations ago caused that person to bear children that were protected from malaria. Those children would live longer, healthier lives, and be able to have more healthy children than people who didn’t have this “error” in one of their alleles that coded for hemoglobin. Since the mutation was in the gametes, subsequent generations would have this trait. This apparently random error in DNA led to a more “fit” offspring.