Campbell Bio
EVOLUTION STUDY GUIDE
TOPIC 1: The Origin of Life on Earth
All change occurs over time. We need to understand that. Change in species of organisms is related to evolution, the patterns of change and adaptation within a population of organisms.
The Earth houses many organisms today - a variety of mammals, insects, reptiles, and more. It came together about 4.5 billion years ago, and life evolved around 3.7 billion years ago. We know that most of Earth’s existence consisted of its early conditions.
However, when Earth formed, it was basically a huge blob of rock spewing lava all over the place. Life was extremely unlikely to form from this mixture, as scientists thought. These collections of very hot and unsafe, extreme conditions formed something called the “primordial soup”, a theory established by two scientists. Essentially, under the primordial soup, life was very rare. Extreme heat killed everything and vaporized vast oceans. As we move on, it will become very clear that water played a large role in the formation of complex organic molecules from simple inorganic/organic molecules. Additionally, the atmospheric conditions were much more different; the early Earth atmosphere consisted of gases like methane, ammonia, water vapor, and hydrogen, while earth’s atmosphere now consists of carbon dioxide, oxygen, and nitrogen. Additionally, it was concluded that under these primitive conditions, that many energy inputs were present: lightning, uv light, visible light, x rays, and more. These energy inputs may have caused organic molecules to form, leading to the evolution of the first cells and organisms once the Earth cooled and assumed the form we know in the present.
Two scientists, Miller and Urey, conducted an experiment to settle once and for all - was water and the cooling of the atmosphere responsible for the development of life? First, they set up an apparatus consisting of one main chamber and two side chambers. The main chamber simulated the early conditions - atmospheric gases and heat. Additionally, they added electrodes that emitted electricity to simulate lightning that provided ample energy for chemical reactions. The second chamber, below that, condensed the products formed for analysis. This simulated the cooling of the Earth after primitive conditions. The third chamber was on the side - it was water being vaporized, traveling into the atmosphere, which provided a catalyst/reactant for these processes to take place. After many cycles of this experiment, they analyzed the results - they found simple amino acids - the building blocks of life - that had formed! This experiment was groundbreaking - it demonstrated that life formed under early conditions and was able to flourish with the cooling of the Earth. It also reinforced the idea that water was extremely vital for all life - the universal solvent had been the key to forming organic compounds.
After this, many processes occurred, and life rapidly evolved through natural selection, genetic drift, and gene flow, following stabilizing, disruptive, or directional patterns, leading to the incredible diversity present today.
First, the organic compounds began to diversify into other compounds - lipids, proteins, nucleic acids. This led to the formation of the first simple cells - lipid membranes assembled around early RNA. The RNA world hypothesis states that RNA was the first genetic material that the first protocells assembled around, capable of replicating themselves. Eventually, these cells evolved into the first modern cells, capable of using various organelles to sustain themselves through cellular respiration. This marked the true beginning of “life”, as we define it, on Earth, where cells would grow and divide, forming the first anaerobic heterotrophs! These organisms lacked oxygen in the atmosphere, so relied on anaerobic processes. They were simple, as large/multicellular organisms cannot sustain themselves solely on anaerobic respiration. Then, as carbon dioxide started to fill the atmosphere, organisms began to evolve and adopt ways of creating their own foods - taking in the CO2 and releasing oxygen into the atmosphere - finally! These used chemicals, or energy, through chemosynthesis to make their own foods. This is also because organic molecules became scarce, so heterotrophs (anaerobic) were at a disadvantage. This allowed the evolution of the first aerobic, complex, heterotrophs! They finally had oxygen and started a cycle of exchange that still lasts to this day, where heterotrophs and autotrophs (photo) support each other. Life diversified from there, and here we are!
This is a bit random in the notes, but let’s talk about extinction events!
Every 70 million years or so, a mass extinction event occurs, in which a large majority of Earth’s organisms are wiped out due to a natural disaster, asteroid, disease, or competition, etc, with only some left. Almost 99% of species that have ever lived are extinct!
Done
Topic 2: Darwin, Lyell and Hutton, Malthus, and Natural Selection.
All organisms have evolved. But what is evolution? What is the primary mechanism that drives evolution?
First, we must understand that in the field of biology, and the universe in general, change occurs over time. Things are constantly shifting, and even if these shifts may not be noticeable, they culminate over time. This principle describes evolution, essentially.
Evolution is also random. It does not follow a pattern or a linear sequence. Organisms branch off from one another, creating two new species from one. This is how life has diversified over time. One common ancestor, the first-ever organism, did not evolve linearly into different twwgshings. It evolved into different species, those species evolved into more species, and the branching continued. This led to the organisms we have today.
At first, Aristotle thought that organisms were fixed creations that didn’t change at all with relation to each other. These ideas held for about 2000 years. However, a religious view dominated the early modern and middle ages, Creationism, became prevalent. This was the view that all organisms and everything in the universe was created by god or a supernatural force, and suggested that only new species could arrive from god’s creations.
However, two scientists, named James Hutton and Charles Lyell, were the first to discover and suggest that all of the operations that were performed on earth and geological processes continued today, and that this caused change to occur over time. These processes changed Earth then and now. Then, a scientist named Thomas Malthus was the first to suggest that populations of species changed - that there was a limit, or carrying capacity of the environment that the population would fluctuate around. He was the first scientist to suggest that actual groups of organisms varied and changed.
Now, we move to the first person to suggest that individual organisms changed over time - Jean - Baptise Lamarck. He was wrong on many levels about his theories of evolution, as genetics weren’t fully understood yet, but he was extremely important in determining that the environment shaped the organisms around it. His theory was as followed - organisms acquired traits throughout their lifetimes. Then, they passed these traits to their already living offspring. These traits happened through his law of use and disuse. This states that as an organism uses its body parts, they become stronger or larger, leading to evolution. He called these changes adaptations. He stated that as the environment changes, the organisms adapt to survive. However, Lamark did not know that changes only arise through random, spontaneous mutations, not through use and disuse. He also was wrong that traits were acquired. Activities don’t change the genome. Finally, he didn’t know that only changes in cells producing gametes or the actual gametes themselves can be passed on. It must occur before the organism is born. So, the new organism is simply born with a different trait.
But then, Charles Darwin, a very aspiring naturalist that studied at Cambridge University. He developed an interest, and to study organisms and their evolution, he went on a 5-year voyage on the HMS Beagle. The most famous place that he sailed was the Galapagos Islands, where mainland animals were different from island animals because of their environments. With new genetic knowledge in mind, he was able to form the accurate and now accepted theory of natural selection. He also published the book, “On the Origin of Species” that explained this theory.
The animals on each island were unique - especially tortoises and finches with relation to their environments and conditions for survival. He noticed that tortoises on different islands were different - on land islands with food in holes, the tortoises had large legs for moving and sharp mouths. On ocean islands, the tortoises had fins and flippers, moving through the water effectively. They had resembled a mainland tortoise, but had acquired these adaptations to survive in their environments separately. However, it was really with the finches that he had his aha moment. He noticed that they had all descended and branched off of a mainland finch. These had been separated into different islands with different conditions, and each finch species on the island had over time, developed adaptations to that environment that helped them survive. The most common adaptation was with beaks for getting food to eat and survive! He realized that evolution occurred through a primary mechanism, along with some other stuff I’ll explore later. This primary mechanism is called natural selection. In the context of the finches, some had developed mutations in the gametes that they were born with. Their environment changed, and by chance, the environment selected them to be at an advantage. Organisms do not adapt. The environment chooses who is adapted. Those who are selected are more likely to survive the competition and scarcity with their mutation, leading to the passing on of this trait once they reproduce. This is natural selection. Then, the trait becomes more and more prevalent in the community. As these populations are separated, new and different environments form different species through speciation. This is essentially the survival of the fittest.
His main observations included: 1. Variation occurs within populations. Genetic variation is good because when the environment changes, there is a higher chance of natural selection of potential adaptations, leading to evolution in that species. 2. Left unchecked, a population will increase in number exponentially, but this rarely occurs in nature. 3. He collected fossils. 4. When too many organisms are produced, it leads to competition for resources, leading to the survival of the fittest. These mechanisms are what make evolution occur. Without overproduction or competition, evolution would not occur.
The six mechanisms that cause natural selection are:
Selecting Agent = The part of the environment that determines which individuals are fit.
Survival of the fittest = in competition, those best adapted survive and reproduce.
Variation = Already talked about
Adaptation = Already Talked about
Competition = Already Talked about
Overproduction = Already Talked about
But how do changes occur in a population? Well, mutations, crossing over, or recombination of alleles during meiosis or randomly can change an organism and possibly produce new allele frequencies, the ratio of different alleles in a population that varies if evolution occurs. Take a tall plant pop. t is recessive for tall, T is dominant for short. Most organisms are tt to grow toward the sunlight. However, if an avalanche occurs, the population will adapt and gain Tt or TT phenotypes to be short and protected. This changes the frequencies of the t and T alleles with relation to each other. One more thing - mutations can occur from radiation, chemicals, or just randomly.
Finally, the formal Theory of Natural Selection States: Nature selects which organisms are best able to survive. Organisms with certain adaptations are more likely to survive and reproduce thus leaving more offspring than other less fit individuals. This leads to the accumulation of favorable traits in the population over generations. If an environment changes over time, natural selection may result in adaptation to these new conditions and may give rise to new species!