Lecture notes · Evolution & Ecology · Lecture 1

The Nature of Evolution: Selection, Inheritance, and History

Yale EEB 122 Principles of Evolution, Ecology and Behavior, lecture 1, explained in plain English

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Part 1 · 0:00–10:00

This part introduces the two big parts of evolution, shows how thinking about it changed from the 1790s to Darwin, and explains how Darwin came to his ideas.

Key ideas

Evolution has two big parts. Microevolution is change happening now, and macroevolution is the long history of life that has built up over time. [0:00] 0:00
Past events in the history of life limit what can change today. So the history of life affects the process of microevolution. [0:50] 0:50
In the late 1700s and early 1800s, most educated people believed the world was only thousands of years old, that species had never changed, and that adaptations came from divine intervention. [5:03] 5:03
Darwin's key idea was that adaptations are produced by natural selection. He developed it between 1838 and 1859. [7:17] 7:17

Words to know

microevolution: Small changes in living things that happen over short periods, like the changes going on in bacteria inside your body right now.
macroevolution: The large, long-term patterns and history of life on Earth, built up over millions and billions of years.
homology: Body parts that look alike in different animals, such as a hand, a bat's wing, and a porpoise's fin, because they came from a shared ancestor.
natural selection: The process where some living things survive and reproduce better than others because of their traits, so those traits become more common over time.

Example

Think of a family photo album. Old photos show ancestors who look a lot like their great-grandchildren, but not exactly the same. Each generation is a little different, and the changes add up over a long time.

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Part 2 · 10:00–20:00

This part explains how Darwin came up with natural selection, and the simple conditions a trait must meet for natural selection to happen.

Key ideas

Darwin's trip to the Galapagos showed him that animals of the same kind, like mockingbirds, looked a bit different from island to island. He began to see that living things change over time. 10:23
Darwin read Malthus, who argued that populations grow faster than their food supply. Darwin concluded that living things must always be competing for resources. 12:15
Darwin wrote down the idea of natural selection in 1838. Later, he and Alfred Russel Wallace published the idea together in 1858. 13:00
Natural selection needs variation: different individuals must have different numbers of offspring, and they must differ in some trait. 16:17
Natural selection needs three more things: a link between the trait and reproductive success, and a trait that can be passed on to offspring. If any of these is missing, natural selection cannot happen. 18:12

Words to know

archipelago: A group of islands close together, like the Galapagos.
correlation: A link between two things, so that when one changes, the other tends to change too. A non-zero correlation means there is some link.
heritability: How much of a trait is passed on from parents to children through genes. A trait with no genetic part is not heritable.
neutral evolution: Change that happens by chance, not because a trait helps or hurts survival and reproduction. Things just drift over time.

Example

Imagine a field of bean plants where some plants have bigger beans and grow more seeds for the next season. If the bigger beans are passed on to the seeds, the next field will have more big-bean plants. This is natural selection, because all four conditions are met.

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Part 3 · 20:00–30:00

This part explains the two ways variation in traits and reproductive success can change a population: natural selection, which builds adaptations, and random drift, which causes chance changes.

Key ideas

Natural selection happens when a trait varies, reproductive success varies, and the two are correlated. Here the trait gets pushed in one direction over time. 20:00
Random drift happens when the trait and reproductive success both vary but are not correlated. The changes come from chance, not from the trait helping or hurting. 20:04
Selection produces adaptation, such as the eye and the brain. Drift produces the patterns that let scientists use DNA differences to study history. 20:24
Drift gives patterns that are random at first. Each population starts at a gene frequency of 0.5, and some drift toward 1 or 0 and then stop changing. 28:01
A whole-body trait is usually shaped by selection, while many DNA sequences are shaped mostly by drift. So we see design in the body and more noise in the genome, with many exceptions. 29:10

Words to know

natural selection: The process where traits that go with more successful reproduction become more common over generations, because the trait and reproductive success are linked.
random drift: Changes in how common a gene or trait is that happen by chance, when the trait has no link to reproductive success.
absorbing state: A point where the process stops changing. For drift, this is a gene frequency of 1 (everyone has it) or 0 (no one has it).
reproductive success: How many offspring an individual leaves behind that survive and reproduce themselves.

Example

Imagine a bag of marbles where each marble is a copy of a gene, and you pull out marbles at random to make the next generation. Pulling by chance makes the mix drift up and down, and once one color fills the whole bag, it cannot change again.

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Part 4 · 30:00–43:06

This part explains how evolution can produce traits without choosing them, how scientists map the tree of life, and how the history of life connects all living things.

Key ideas

Some traits are side effects, not adaptations. The chin appeared because evolution changed the face's development, which pushed back the front of the face, and the chin stuck out. 30:19
Natural selection causes steady change only when a trait's variation is linked to reproductive success. Without that link, traits drift without a clear direction. 30:40
All life is part of one tree of life with a common origin. The branch points on the tree are speciation events, when new species form. 31:27
Scientists cannot read relationships directly from organisms. They build hypotheses about history, test them against each other, and keep the one that best fits the data. 35:05
If you trace your ancestors back, each step has a parent until the origin of life, where there is no parent. This links living things to nonliving matter. 41:59

Words to know

macroevolution: Evolution on the very large scale, the big picture of how life changed over huge stretches of time.
speciation: The process by which a new species forms.
phylogenetics: The study of how living things are related, used to work out the history of life.
meiosis: The special cell division that makes sex cells. The lecture describes it as organized sex.

Example

Think of a sweater that shrinks in the wash and leaves a loop of yarn sticking out. Nobody knitted the loop on purpose; it showed up because something else changed. The lecture uses this kind of idea to explain why the chin may be a side effect rather than a designed feature.

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Notes written by AI (Claude) from the lecture's official transcript, so check anything important against the lecture. Open Yale Courses · CC BY-NC-SA 3.0 · not affiliated with or endorsed by Yale. These notes are shared under the same license.

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