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Next, we will go over the basics of the biological sciences and get ready for more in-depth reviews into all of the major disciplines of science. Our first topics will deal with how scientists organize the diversity of living organisms and how everything in biology can be best conceptualized in the context of evolution.
In the last lesson, we reviewed some of the major topics of physics, including the four of its fundamental fields: mechanics, energy, waves, and the electromagnetic spectrum.
When we observe the natural world, one of our innate abilities is to classify things and organize them into groups. It is how our brains work as hunters and gatherers. In order to survive as long as we have, natural selection has shaped our abilities and behaviors. One of our most remarkable abilities is to classify things into meaningful categories. This is one of the few things that humans can do better, more effectively and more efficiently, than computers.
Biologists use a classification system to organize all living things into groups based on the organisms’ evolutionary relationships. This also makes them easier to study and understand. Biologists use a hierarchical system of roughly eight groups to classify organisms.
| Group | Effective size | Example |
|---|---|---|
| Domain | Largest – there are only three known Domains of life | Eukarya |
| Kingdom | Used to be the largest with at least seven categories | Animalia |
| Phylum | Extremely broad | Chordata |
| Class | Recognizably related | Mammalia |
| Order | Small and familiar | Primates |
| Family | Most understood | Hominidae |
| Genus | Tightly related | Homo |
| Species | Smallest groups of a single kind | sapiens |
A three-domain system of classification is the most commonly used system in modern biology and is based on nucleotide sequencing in RNA molecules that may have the most information about how life evolved on Earth. The three domains are bacteria, archaea, and eukarya. All known organisms fall into one of these categories.
What is the proper order of the classifications of life beginning with domain?
The correct answer is B.
What are the three domains in which all organisms are classified?
The correct answer is D. Bacteria, archaea, and eukarya are the three domains that encompass all know life.
In all of human history, one of the most important and powerful scientific theories is that of evolution. Evolution is the process by which organisms acquire and pass on traits that were not present previously. This change-through-time is generated by inherent variation and mediated by natural selection. When Charles Darwin and Alfred Russel Wallace co-founded the theory of evolution through natural selection, they were well aware of the controversy. But the evidence was overwhelming, even back in 1858. Both men had spent large amounts of time exploring the tropical rainforests and islands of the world. Darwin mostly explored South America and the Galapagos Islands from aboard the Beagle, while Wallace actually lived and worked for extensive time periods across South America and the Indo-Malaysian archipelago. Between the amazing field observations and data collected by these two outstanding natural historians, there was ample evidence to form the ideas that have led to our current understanding of evolution.
In the mid 1800’s, Charles Darwin and Alfred Russel Wallace were exploring some of the most diverse ecosystems on the planet. Both scientists had uncanny abilities to categorize and theorize about the origin and maintenance of the tremendous diversity they were witnessing. Both men came up with ideas about natural selection, speciation, and the fundamentals of evolution. Observations included the sheer diversity of living things, the remains of ancient organisms, and the characteristics of organisms on islands versus their mainland counterparts (the Galapagos Islands for Darwin and the Indo-Malaysian islands for Wallace). These observations led them both to separately, and then together; develop one of the most important scientific theories of all time — the theory of evolution.
After returning to England, Darwin compared Galapagos organisms to organisms that lived elsewhere, and found many similarities and differences. He found that plants and animals were almost identical in the Galapagos as they were in South America. However, as he studied his observations more closely, he found important differences, too. He theorized that some of the organisms in the islands must have come from the mainland, perhaps blown out to sea during a storm or set adrift on a fallen log. Once they reached the islands, they reproduced for many years and the offspring, while very similar, developed differences from their mainland relatives.
The most obvious differences Darwin noticed were the sizes and shapes of bird’s beaks on the various islands of the Galapagos. An examination of birds from different islands and the South American mainland revealed that the birds had slightly different beak sizes and shapes. The differences in the beaks of birds in different locations seemed to show interesting patterns: there were classes of beak size and shape that each island seemed to share. Moreover, each island seemed to not only have similar bird beak adaptations in different species, but each island’s species were slightly different from every other island’s and the mainland’s species. Darwin hypothesized that these differences in beak shape and size were adapted to best obtain food that was available in various environments and that the differences were variations on a theme, all coming from a common ancestor.
Figure 1. Darwin made observations of different bird beaks like these, each adapted to food availability for the different species.
Darwin reasoned that plants and animals that arrived on the Galapagos Islands encountered conditions that were different from those they experienced on the mainland, and must have evolved over many generations to become better adapted to their new surroundings. While Darwin was realizing the implications of these findings, another naturalist, Alfred Russel Wallace, was in the midst of one of the most important field expeditions of all time. Unlike Darwin, whose careful and meticulous fieldwork was synergized by months of contemplative research and theorizing back home in England, Wallace was more cavalier with his thinking while in the field. For many years after his trip around the world, Darwin continued to think about what he observed and was shocked to learn that Wallace had come to the very same conclusions, while in the Indo-Malay archipelago. While Darwin refined his theory based on countless observations made 15 years earlier, Wallace was publishing the first papers on the idea of natural selection. Finally, after many letters back and forth, the pair decided to jointly publish a paper that demonstrated how evolution, whose primary mechanism was natural selection, was a gradual change in a species over long periods of time. Although Darwin gets most credit for the theory of evolution through natural selection, both he and Wallace came to the same conclusions based on tropical fieldwork.
What is the building block of change that helps a species survive and reproduce in its environment?
The correct answer is C. While the other choices are viable parts of the evolutionary process, variation is the primary building block that allows natural selection and adaptation to affect change over time. If there were no variation in the first place, natural selection would have nothing to work with – and evolution wouldn’t occur.
Darwin and Wallace proposed an explanation of how evolution occurs in nature. In 1858, they proposed that evolution occurs by a process of natural selection. Natural selection limits unsuitable variations that can be passed on to the next and future generations. Organisms that have successful variations are more able to pass on their genetic material (for whatever reason) and become “selected” naturally.
Wallace and Darwin identified many factors that can affect natural selection. They knew that most organisms produced many more offspring than could survive on the available resources. Among these numerous offspring will be variation — they are not only different from each other, but also different from their parents. Any variation that can enable certain organisms to out-breed others will be advantageous in passing on those traits. For example, if an organism is able find food faster in their environment, that may enable an organism to be more able to mate successfully, so that trait (ability to find food faster) will be passed on. Another example: one organism might have a color that protects it from predators better than the coloration of another organism. Competition is common among members of the same species when food and other resources are limited. The important factors that must accompany a biological system in order for natural selection to act, are inherent variation, some sort of competition, and differential mating success based on whatever factors are acted upon by natural selection.