【Biology】 Evolution and Phylogeny: A Journey to Understand the Connections of Life
Hello! In this chapter, we will explore the "magnificent history" of how the many living things on Earth were born and changed over the course of hundreds of millions of years.
"Evolution" might sound difficult, but the truth is that the evidence of this history is carved right into our own bodies. At first, there might seem to be many terms to memorize, but don't worry. If you organize them one by one, you'll be able to understand them in a fun way, like fitting pieces of a puzzle together!
1. The Birth of Life and the Path of Evolution
Earth was born about 4.6 billion years ago. How did life emerge from that?
(1) Chemical Evolution and the First Life
The process where inorganic matter in the primitive Earth formed organic matter, which then gathered to create life, is called chemical evolution.
Key Point: The first organisms were prokaryotes (cells without a nucleus) born in the sea, which gained energy without using oxygen.
(2) Photosynthesis and the Emergence of Eukaryotes
Eventually, "cyanobacteria" that perform photosynthesis appeared, and oxygen increased on Earth. Organisms that utilized this oxygen and eukaryotes (cells with a nucleus), which have more complex structures, emerged.
What is important here is the endosymbiotic theory.
【Analogy: The Roommate Theory】
This is the idea that other organisms (aerobic bacteria or cyanobacteria) entered the cells of eukaryotes and just moved in to stay.
・Aerobic bacteria ➡ became mitochondria!
・Cyanobacteria ➡ became chloroplasts!
*Evidence: Mitochondria and chloroplasts have their own DNA.
★Summary: Remember this!
・Prokaryotes appeared first, followed by eukaryotes.
・Mitochondria and chloroplasts were originally different organisms (endosymbiotic theory).
2. Evidence and Mechanisms of Evolution
How can we say for sure that "evolution" happened? Let's learn the evidence and the rules by which evolution occurs.
(1) Evidence from Body Structures
Even if shapes differ, there is evidence that they were originally the same.
・Homologous organs: Parts that have different shapes or functions but the same origin. (Example: Human arm and bat wing)
・Analogous organs: Parts that have the same function but different origins. (Example: Bat wing and insect wing)
【Common Mistake】
A common mistake is to assume "They have the same function, so they must be related!" that is the definition of analogous organs. Insects and bats are entirely different groups; they just happened to evolve similar shapes to fly. Homologous organs are what provide the real proof of "evolutionary connections."
(2) The Engine of Evolution: Natural Selection and Mutation
How does evolution progress? Let's organize this based on Darwin's ideas.
1. Mutation: Offspring with different characteristics than their parents are born due to DNA copying errors, etc.
2. Natural Selection: Individuals with characteristics suited to the environment survive and are more likely to leave offspring (survival of the fittest).
3. Genetic Drift: Changes in the ratio of genes in a population due to chance (e.g., disasters).
Trivia: Why does a giraffe have a long neck?
The idea that "it stretched its neck trying to eat leaves from high up" is wrong (Lamarck's theory of use and disuse). The correct answer is: "Individuals with accidentally longer necks were born, and because they were more likely to survive, that trait was passed on."
★Summary: Remember this!
・Homologous organs represent evolutionary kinship.
・Evolution is driven by mutation and natural selection.
3. Speciation and Population Genetics
Let's look at the mechanism by which one species splits to form a new species.
(1) Geographical Isolation and Reproductive Isolation
・Geographical isolation: A population being separated by mountain ranges or oceans.
・Reproductive isolation: The result of living separately for a long time, eventually becoming unable to mate. This is the birth of a new "species"!
(2) Hardy-Weinberg Equilibrium
This is the law that if certain conditions are met, the ratio of genes (gene frequency) will not change even across generations.
Formula: \( p^2 + 2pq + q^2 = 1 \)
(\( p \) is the frequency of the dominant allele, \( q \) is the frequency of the recessive allele)
*In the real natural world, evolution is always occurring, so the conditions for this law are often broken.
4. Phylogeny and Classification of Organisms
If we group all the organisms on Earth, what happens?
(1) Three-Domain System
The current mainstream approach is to divide all life into three large groups (domains).
1. Bacteria: E. coli, etc.
2. Archaea: Many live in extreme environments.
3. Eukarya: Plants, animals, fungi (mushrooms, etc.), and us—this is the group we belong to.
Point: Even though "Archaea" has "arch" (ancient) in the name, they actually have more characteristics similar to Eukarya than to Bacteria!
(2) How to Build a Phylogenetic Tree
A diagram representing the path of biological evolution like a tree is called a phylogenetic tree. Recently, instead of just appearances, we compare base sequences (DNA) and amino acid sequences to investigate connections more accurately.
Molecular Clock: A method that uses the fact that DNA changes (mutations) occur at a constant pace to estimate "when two species diverged."
★Overall Summary (Key Takeaways)
1. History of Life: Chemical evolution ➡ prokaryotes ➡ endosymbiosis ➡ eukaryotes.
2. Mechanism of Evolution: Mutation and natural selection are the main drivers.
3. Classification: Divided into large units called domains, and we use DNA (molecular clocks) to study phylogeny.
Finally...
Great job learning about evolution! Doesn't it feel exciting to think that "we humans also originally started from one tiny little cell"? For the difficult terms, start by trying to link them to the living things around you. Let's take it one step at a time!