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Chapter 15 Notes, Natural Selection and the Theory of Evolution. Darwin's Voyage. In 1831, a young amateur naturalist by the name of Charles Darwin traveled on a ship called the HMS Beagle . On his voyage, he collected specimens of rocks, fossils, animals, and plants .
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Chapter 15 Notes, Natural Selection and the Theory of Evolution
Darwin's Voyage • In 1831, a young amateur naturalist by the name of Charles Darwin traveled on a ship called the HMS Beagle. • On his voyage, he collected specimens of rocks, fossils, animals, and plants. • Along the voyage, Darwin stopped at an island archipelago (a chain of islands) called the Galapagos Islands. • On the Galapagos Islands, Darwin observed strange creatures like giant tortoises, marine iguanas, and flightless cormorants.
Darwin's Discovery • As fascinating as these creatures were, it was not these creatures that made Darwin famous. • It was actually the discovery of some finches. • When Darwin discovered the finches, he thought that each bird was an completely unrelated kind (species) of bird. • But, after returning to England, Darwin showed his specimens to an Ornithologist who confirmed that the birds were not different kinds of birds, but different varieties of the same kind (species) of bird.
Artificial and Natural Selection • At the time of his discovery, selective breedingwas very common with domesticated breeds of animals. • Darwin noticed similarities between the practice of selective breeding of domestic plants and animals and the different varieties of finches that he found. • The selective breedingof plants and animals that Darwin observed is often referred to as artificial selection. • Darwin made a hypothesis that something was happening in naturethat was similar to artificial selection, but it was occurring naturally rather than by the direction of man. He called this phenomenon natural selection.
Natural Selection • The only thing Darwin needed was a mechanism to explain what causes natural selection to occur. • Then Darwin noticed that each finch had a unique beak sizeand shape that assisted them in eating a particular type of food that was most abundant on each particular island. • After observing the different beak shapes and sizes of the finches, then Darwin discovered the mechanism he needed to explain natural selection.
Mechanism for Natural Selection • These are the four principlesDarwin used to explain what causes natural selection. • 1) All living things have variety within species. • 2) Traits are inherited from parents to offspring. • 3) Species compete with one another for limited resources (food, shelter, water, nutrients etc.). • 4) Those individuals that inherit an advantageous trait from their parents will be more fit to survive and therefore more successful at reproducing and passing on their genes to the next generation.
Natural Selection and Evolution • Darwin published his book On the Origin of Species by Means of Natural Selection in 1859. • In his book Darwin introduced a new controversial idea he called the theory of evolution. • Darwin’s proposedthat if natural selectioncould produce small changes in species by natural selection, then in perhaps in theory, these small changes could gradually produce much larger changesif given a very large quantity of time. • In other words, he introduced atheory that natural selection might cause evolution, if given enough time.
Natural Selection and Evolution • Natural selectionand Evolution arenot the same thing. • Natural selection produces small changes in populations to produce new species or new varieties of the same species (subspecies). • Natural selection is generally accepted as a fact and can be directly observed, repeated, and tested. • Natural selection can often occur in a few generations.
Natural Selection and Evolution • Evolution is a theory, that offers an explanation for the origin of all living things. • It suggests that all living organisms share a common ancestor in the past. • Because this theory is based on something that may have occurred in the past, we can not directly observe it, repeat it, or test it.
Examples of Natural Selection(Bergman’s and Allen’s Rule) Snowshoe Hare Desert Jackrabbit Cottontail Rabbit
Examples of Natural Selection (Bacterial Resistance to Antibiotics)
Examples of Evolution Black-Capped Chickadee Tyrannosaurus Rex
Similarities and Differences Between Birds and Reptiles Birds Reptiles Cold-blooded (ectotherms) Skin with scales made of keratin Bones more similar to mammals Leathery Amniotic Eggs Respiratory System with fully functioning lungs 3-Chambered Heart (in most) Mostly Quadruped Teeth • Warm-blooded (endotherms) • Feathers made of keratin • Hollow bones (air-filled cavities) • Brittle Amniotic Eggs • Air-Sac Respiratory System (No lungs) • 4-Chambered Heart • Bipedal with Wings • Muscular gizzard (no teeth)
The Fossil Record • During a period of time that is often referred to as the “Cambrian Explosion” almost all representatives of every animal phylum alive today suddenly appear.
What is a Species? • The definition of a species has historically been a topic of mixed opinions and varying interpretations. • There is the Typological species concept or morphological species concept, which is based on similarities and differences of the physical characteristics of organisms. • There is also the Biological species concept which is based on whether or not organisms can interbreed and produce viable and fertile offspring.
Why use the Biological Species Concept? • The western meadowlark and eastern meadowlark may look alike and live in overlapping regions, but they have different songs and do not breed with one another. Are they the same species? Most biologists say no. Why? Because they do not breed.
Biological Species Concept • Some species can interbreed and have offspring, but their offspring are typically unviable (unable to survive) or sterile. • Some examples of these hybrids between species are: mules, zonkeys, ligers, pizzly or grolar bears, beefalo.
Speciation • Natural selection can cause speciation. • Speciation is the separation of one species into two or more species. • Speciation usually begins with genetic drift. • Genetic drift occurs when the frequencies of alleles in a population change. • A form of genetic drift that can occur when a small population branches off from a large population is called the founder effect.
Speciation • There are two common ways that speciation often occurs; allopatric and sympatric speciation. • Allopatric speciation occurs when a population of organisms becomes separated and isolated by a geographical barrier. • A geographical barrier could be a mountain range, a large river, a canyon, a desert etc.
Allopatric Speciation • The Abert's squirrel (found on the south rim) and the Kaibab squirrel (found on the north rim) are separated by the Grand Canyon.
Sympatric Speciation • Sympatric speciation can occur when populations become isolated by ecological factors, instead of geographic barriers. • For example different habitats may be occupied, a different niche(job or role in the ecosystem) may be established, or a different resource may be used. • Any of these factors could cause genetic drift to occur within a population of organisms living in the same area. • Cichlid fish species in Africa's Lake Victoria is a good example of sympatric speciation.
Reproductive Isolation • After speciation occurs, reproductive isolation must occur for a new population of organisms to become a separate species. • In other words, successful reproduction between two different populations must be prevented. • Prezygotic isolation occurs when allele frequencies change dramatically enough that reproduction between two populations becomes unlikely due to reproductive timing, behavior etc. • A good example of this is with Pacific Salmon.
Reproductive Isolation • Another way that populations can become reproductively isolated is by postzygotic isolation. • Postzygotic isolation occurs when two organisms can reproduce, but the offspring are sterile or unviable. • An example of this is a mule. A donkey's diploid number is 62. A horse's diploid number is 64. • A hybrid mule has a diploid number 63, which makes the mule sterile or unable to reproduce. • Offspring can also become unviable, or unable to survive or develop normally to reproduce.
The Hardy-Weinberg Principle • In 1908, English mathematician Godfrey Hardy and German physician Wilhelm Weinberg developed an equation that demonstrates how allele frequencieswill most often remain stable unless acted upon by certain outside forces or circumstances. • In other words, they mathematically demonstrated how and when natural selection takes place. • The Hardy-Weinberg principle can be written algebraically by the following equation p2 + 2pq + q2 = 1 Or p + q = 1
So what does the Hardy-Weinberg Principle Mean? • Natural selection will not occur unless the following situations occur. • A large population becomes small. • Immigration or emigration occurs. • Mating becomes nonrandom. • A specific trait is selected. • A mutation occurs.
What are the four different forces of Natural Selection? • The four forces of natural selection that can push or drive natural selection in a specific direction are referred to as stabilizing, directional, disruptive, or sexual. • Stabilizing selection occurs when the intermediate or average form of a trait is selected. Stabilizing selection operates to eliminate extreme traits.
Forces of Selection • Directional selection is when one extreme form of a trait is favored or selected.
Forces of Selection • Disruptive Selection is a form of selection that selects for the two different extreme forms of a trait and selects against the average form of a trait. • In this example, both extremes use mimicry as a camouflage strategy
Forces of Selection • Some forces of selection do not enhance an organism's fitness or ability to survive, but rather advertise an organism's health or dominance. • This type of selection is called sexual selection.
