Welcome to Unit 7: Natural Selection!

Welcome to one of the most exciting parts of biology! If Unit 1 through 6 were about how the "machinery" of life works (DNA, cells, energy), Unit 7 is about how that machinery changes over millions of years. Don't worry if you've heard that "evolution is hard"—we’re going to break it down into simple, logical steps. By the end of these notes, you'll see how every living thing on Earth is connected.

7.1 & 7.2: The Core of Natural Selection

Natural Selection is the process where organisms that are better suited to their environment tend to survive and produce more offspring. It’s not about being the strongest or fastest; it’s about Fitness.

What is "Fitness"?

In biology, Fitness doesn't mean how many pull-ups you can do. It means reproductive success. If an organism lives a long life but never has babies, its biological fitness is zero. To have high fitness, you must survive AND reproduce.

The Four Ingredients of Natural Selection

For natural selection to happen, you need these four things:

  1. Variation: Individuals in a population must be different from each other (due to mutations and genetic shuffling).
  2. Overproduction: Populations produce more offspring than the environment can support.
  3. Competition: Because resources are limited, there is a "struggle for existence."
  4. Differential Survival and Reproduction: Those with favorable traits (adaptations) live longer and have more babies, passing those traits to the next generation.

Quick Review: Natural selection acts on phenotypes (physical traits), but it changes the genotype (DNA) of the population over time. Also, remember: Individuals do not evolve; populations do!

Example: Imagine a group of mice living on dark rocks. Some are light tan, and some are black. Owls eat the tan ones because they are easier to see. Over time, the population will have more black mice. The individual tan mouse didn't "turn" black; the population shifted because the black ones survived more often.

7.3: Artificial Selection

Artificial Selection is when humans do the choosing instead of nature. We pick the traits we like and breed plants or animals to have those traits.

Examples: All dog breeds (from Chihuahuas to Great Danes) came from a wolf ancestor because humans chose specific traits. Most of our food, like corn and broccoli, was created this way too!

Key Takeaway: Artificial selection proves that species can change significantly over time when specific traits are favored.

7.4 & 7.5: Population Genetics and Hardy-Weinberg

This is where we add a little bit of math. Don't panic! It's just a way to measure if a population is evolving.

Hardy-Weinberg Equilibrium

If a population is in Hardy-Weinberg Equilibrium, it is NOT evolving. The allele frequencies stay exactly the same. For this to happen, five strict rules must be met:

  1. No Mutations (no new alleles).
  2. Random Mating (no choosing mates based on traits).
  3. No Natural Selection (everyone has the same chance of survival).
  4. Extremely Large Population (no accidental changes).
  5. No Gene Flow (no one enters or leaves the population).

Memory Aid: Think of "Large M&M" to remember the rules: Large population, Mating is random, no Mutations, no Migration (gene flow), no Matural selection.

The Formulas

There are two main equations you need to know:

\( p + q = 1 \)

\( p^2 + 2pq + q^2 = 1 \)

  • \( p \): Frequency of the dominant allele (e.g., 'A').
  • \( q \): Frequency of the recessive allele (e.g., 'a').
  • \( p^2 \): Frequency of homozygous dominant individuals (AA).
  • \( q^2 \): Frequency of homozygous recessive individuals (aa).
  • \( 2pq \): Frequency of heterozygous individuals (Aa).

Common Mistake: Students often confuse "allele frequency" (\( p \) or \( q \)) with "genotype frequency" (\( p^2 \), \( 2pq \), or \( q^2 \)). Always read the question carefully to see which one they are asking for!

7.6 & 7.7: Evidence of Evolution and Common Ancestry

How do we know evolution actually happened? We look at the "receipts" left behind by nature.

Types of Evidence

  1. Geographical: Where organisms live (Biogeography).
  2. Geological: The fossil record shows transition species.
  3. Physical:
    • Homologous Structures: Similar structures in different species because they shared a common ancestor (e.g., the arm bones in a human, whale, and bat).
    • Vestigial Structures: Remnants of features that served a purpose in an ancestor but are now useless (e.g., tailbones in humans).
  4. Biochemical/Molecular: Comparing DNA and amino acid sequences. This is the most accurate way to show how closely related two species are.

Did you know? You share about 98% of your DNA with a chimpanzee, but you also share about 50% of your DNA with a banana! This shows all life is connected at a molecular level.

7.8 & 7.9: Continuing Evolution and Phylogeny

Evolution isn't just something that happened in the past; it’s happening right now! We see this in antibiotic-resistant bacteria and pesticide resistance in insects.

Phylogenetic Trees and Cladograms

These are diagrams that show evolutionary relationships. Think of them like a family tree for species.

  • Nodes: The points where branches split. This represents a common ancestor.
  • Outgroup: The lineage that is the least closely related to the others (it branches off first).
  • Shared Derived Characters: Traits that evolved in a lineage and are shared by its descendants.

Key Takeaway: The more recently two species shared a common ancestor (the closer the "node" is to the tips of the branches), the more closely related they are.

7.10: Speciation

How do we get a brand new species? Speciation occurs when two populations become so different that they can no longer interbreed to produce fertile offspring.

Two Main Types

  1. Allopatric Speciation: A physical barrier (like a mountain or river) divides a population. "Allo" means "other," "patric" means "homeland."
  2. Sympatric Speciation: Evolution of a new species while living in the same geographic area (often due to polyploidy in plants or habitat differentiation).

Reproductive Isolation

To keep species separate, there are barriers:

  • Prezygotic Barriers: Prevent the egg and sperm from ever meeting (e.g., different mating seasons, different mating calls, or physical inability to mate).
  • Postzygotic Barriers: The egg and sperm meet, but the offspring is either weak, dies early, or is sterile (like a mule, which is the sterile offspring of a horse and a donkey).

7.11: Extinction

Extinction is the end of a lineage. While it sounds sad, it's a natural part of Earth's history.

Mass Extinctions: These wipe out a large percentage of species quickly. However, they provide an "opportunity" for the survivors. After a mass extinction, we often see Adaptive Radiation, where new species evolve rapidly to fill the empty roles (niches) in the environment.

7.12: Variations in Populations

Diversity is the "safety net" of a population. If every individual is genetically identical and a new disease arrives, everyone dies. If there is genetic diversity, some individuals might have a mutation that makes them resistant, allowing the population to survive.

Quick Review: Genetic diversity comes from mutation (the ultimate source of new alleles), crossing over during meiosis, and independent assortment.

7.13: Origin of Life on Earth

How did it all start? Scientists believe life evolved from non-living matter in several steps:

  1. Small organic molecules (like amino acids) formed.
  2. These joined into larger molecules (polymers).
  3. Molecules were packaged into "protocells."
  4. Origin of self-replicating molecules (RNA).

The RNA World Hypothesis

Most scientists believe RNA came before DNA. Why? Because RNA can both store genetic information AND act as an enzyme (ribozyme) to catalyze reactions. DNA can't do its job without enzymes, but RNA can be its own boss!

Key Takeaway: The Miller-Urey experiment showed that organic molecules (the building blocks of life) could form spontaneously in the conditions of early Earth. It didn't "create life," but it showed the "ingredients" were possible!

Don't worry if this seems like a lot to memorize! Just remember the big picture: Evolution is simply the change in the genetic makeup of a population over time, driven by who survives and reproduces in a changing world.