Natural Selection – Crash Course Biology #14

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Hank guides us through the process of natural selection, the key mechanism of evolution.

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Table of Contents:
1) Natural Selection 1:27
2) Adaptation 2:56
3) Fitness 3:36
4) Four Principals 3:54
a. Variations 4:01
b. Heritability 4:17
c. “The Struggle for Existence” 4:25
d. Survival and Reproductive Rates 5:00
5) Biolography 5:59
6) Modes of Selection 7:40
a. Directional Selection 8:17
b. Stabilizing Selection 8:56
c. Disruptive Selection 9:27
7) Sexual Selection 10:22
8) Artificial Selection 11:24

References for this episode can be found in the Google document here:

This video uses the following sounds from
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“ForestBirds.wav” by HerbertBoland

crashcourse, science, biology, natural selection, genetics, peppered moth, inherited traits, population genetics, charles darwin, darwin, on the origin of species, evolution, adaptation, galapagos finches, fitness, variation, phenotype, heritable, malthus, survival rate, reproductive rate, crossbreeding, inbreeding, genotype, directional selection, stabilizing selection, disruptive selection, selective pressures, sexual selection, artificial selection, selective breeding Support CrashCourse on Patreon:


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  1. I'd actually argue that peacock tails would be a great example of stabilizing selection. They need a big enough tail to attract the ladies, but not so huge that they can't move to find food or escape predators before they've had a chance to pass on their genes. Thoughts? Love this channel : ) -BOGO

  2. WALKING WHALES LIE; Pakicetus was a wolf like mammal. It had a bone in it's inner ear called an "involucrum" that resembled something found in whales. It was fully terrestrial (lived it's entire life on land) and looked nothing like a whale but neo-Darwinist, desperate for a whale ancestor, tagged it Pakicetus or "Pakistani whale". Ambulocetus, Maiacetus, Kuchicetus and Rodhocetus were extinct variations of sea lions or otters. They were amphibious (lived part of their lives on land and part in the water). They used their hind legs and tail for swimming and their four limbs for walking on land.
    Basilosaurus and Dorudon were extinct whales that were fully aquatic ( lived their entire lives in the water).
    Land and amphibious mammals have nostrils for breathing on the front of their heads. Whales have a blowhole on the top of their bodies to breath.
    The blowhole of a whale is surrounded by thick muscular “lips” that keep the hole tightly closed except when the animal makes a deliberate effort to open it at the surface. Total submersion thus takes less effort for whales than for animals that must actively exclude water from their air passages. Coming onto land is not a natural act for a whale; beached whales die if they're not quickly helped back in the water.
    Land and amphibious mammals use four legs—tail assisted for amphibians while in water—to move around in their environment. Whales use a fluke to move around in their environment. Land and amphibian mammals can survive without the tip of their tail. Whales can't survive without it.
    Flukes are flat horizontal lobes at the ends of their tails. Fluke movements are coordinated by a complex system of long, powerful tendons connecting them to specialized muscles in the tail.
    These are a few of the myriad of changes they would have to undergo. For neo-Darwinist it would take hundreds of millions of random mutations (random variations for traditional Darwinist), to compel a fully terrestrial mammal to turn into a fully aquatic mammal.
    The biggest problem for the "walking whales" is the empirical evidence–Darwinist refuse to acknowledge–not found in the fossil record. It's all but complete. Paleontologist have not discovered the fossils of innumerable transitional intermediate mammals, falling between and linking Pakicetus and whales. All the natural history museums in the world should be overflowing with them.
    With enough imagination anyone can invent a story about how land mammals evolved into whales. But an imaginative story isn't empirical science.

  3. I've got an entrance exam on the same level as A-level, is this video series of A-level crash course helpful to at least get a 50? I NEED TO PASS!! T-T

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