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Evolution Chapter 15 Darwin ’ s Theory of Evolution
15-1 The Puzzle of Life ’ s Diversity Biological Diversity- variety of living things. Evolutionary Theory- a collection of scientific facts, observations & hypotheses; it can explain the diversity of life on Earth. Evolution- change over time; the process by which modern organisms have descended from ancient organisms. Scientific Theory- a well supported testable explanation of natural events; supported by evidence & can be tested with new evidence. Theory- is a well supported TESTABLE explanation of phenomena that have occurred in the natural world.
Charles Darwin - Born in England He contributed more than any other scientist to our understanding of evolution. Voyage of the Beagle- sailed around the world stopping at the Galapagos Islands. He made observations & collected evidence. He noticed many plants & animals were well suited to their environment. He collected fossils- preserved remains of ancient organisms. He collected many different species he had never seen before.
Galapagos Islands Islands west of South America that are close together but have very different climates. Darwin studied shell shape of the many different tortoises. He discovered many similar but different birds that turned out to be finches. Each species of finch had a different shaped beak which was a structural adaptation. Darwin felt the animals on all the different islands once came from the same species or common ancestry.
15-2 Ideas that shaped Darwin ’ s Thinking He felt that many would find his ideas too shocking to accept. He began to put all of his work on paper. Several scientists influenced Darwin: Hutton- earth is shaped by geological forces Lyell- earth is shaped by geological forces Lamarck- in inherited acquired traits; organisms change over time. Malthus-predicted that human pop. Would grow faster than space and food would provide.
15-3 Darwin ’ s Case He was reluctant to publish his ideas because they were so radical. He was disturbed by his own findings which challenged fundamental scientific beliefs. When he heard a scientist name Wallace had the same idea as his, Darwin published his work.
Darwin Published his Book On the Origin of Species – 1859. The book provided evidence that evolution has occurred. His theory on how evolution comes about was based on artificial selection. He believed that natural selection occurs in nature.
Artificial & Natural Selection Artificial Selection Animal breeders select for breeding only animals with the desired traits. Ex. Crops, livestock & horses. Natural Selection Individuals differ & some of the differences can be passed on to their offspring. More offspring are produced than can survive or reproduce. Individuals best suited to their environment survive & reproduce most successfully. Ex. run fast, outsmart with behavioral tactics.
Artificial Selection Animal breeders select for breeding only animals with the desired traits. Ex. Crops, livestock & horses.
Banana Ancestor Banana after many years of the artificial selection of traits by humans.
Natural Selection- is the process by which individuals that are better suited to their environment survive and reproduce. Individuals differ & some of the differences can be passed on to their offspring. More offspring are produced than can survive or reproduce. Ex. run fast, outsmart with behavioral tactics. The giraffes are also an example: Long neck giraffes survived because of their abilities to reach their food source.
Finches The warbler finch (top) boasts a thin, sharp beak best suited for spearing insects. Ground finches' shorter, more robust beaks (center) are adapted for eating seeds found on the ground. Those of cactus finches (bottom) are shaped for getting seeds from cacti.
Does something like artificial selection work in nature? Yes - Darwin called it Natural Selection!
Charles Darwin Overview Darwin noticed that animals had different variations which enabled them to survive in their environment. Natural selection (survival of the fittest)- he believed that the organisms best equipped to survive in an environment would reproduce and become the dominant species.
Survival of the Fittest Animals prey on other animals. Some die and some survive. Fitness- the ability to survive & reproduce in a specific environment. Only the fittest organisms pass on their traits, thus the species change over time. Adaptation- an inherited characteristic that can be physical or behavioral. Speciation- is the formation of a new species. Speciation can occur because adaptations which were obtained, varied to a degree where the species with different variations do not mate anymore.
Organisms not equipped to survive may become extinct. Living in an unstable environment may contribute to variations among organisms as they compete to survive. Living in a stable environment slows an organism’s need to evolve or to go through speciation. Biodiversity- is the sum of the variety of organisms in the biosphere.
Say a population of rabbits lives in Pennsylvania. Most rabbits are a shade of brown, but every so often due to natural variation a white rabbit is born. These white rabbits tend to stand out in the dark forest and are eaten at a higher rate by predators. Thus they rarely survive to reproduce and pass on their white fur coat to a new generation of rabbits. In the forest, brown coated rabbits are the most fit to survive.
As the rabbit population grows they start to inhabit Canada and the far north. Here snow covers the ground 75% of the year. The white furred rabbits are still occasionally born, but here they tend to survive better than their brown coated relatives. Here the white fur is an adaptation and the white rabbits survive longer and produce offspring. Eventually all the rabbits of the north are white and brown furred rabbits are rarely seen. White rabbits are the most fit to survive there.
A trait, that exists only by chance through the natural variation of life is naturally selected as more fit and therefore to survive and reproduce more offspring than others of the population. This is known as Darwin’s Theory of Natural Selection or Survival of the Fittest.
Lamarck Theory of Evolution Lamarck’s theory- he believed that organisms had the ability to obtain or even lose adaptations depending on their use or non-use of the organ. (Vestigial organ is an useless organ.) He believed that the crab’s claw would increase in size because of extensive use and that the new adaptation would pass on to their offspring.
Darwin –vs- Lamarck (Long neck vs Short neck Giraffes) Darwin believe that 2 types of giraffes existed, but the short neck giraffes died off because they could not reach their food source. Only the long neck giraffes were able to survive and reproduce successfully. Lamarck believe that the giraffes continued to attempt to reach for the tree leaves and in time their necks increased in length creating the giraffes that we see today.
Principle of Common Descent All species come from common ancestors. It links all organisms on Earth into a single tree of life. Ex. Tigers-panthers-cheetahs, felines-horses-dogs-bats, mammals-birds- alligators-fish.
Evidence for Evolution 1.Evidence from the fossil record- comparing fossils from older & younger rock layers documents the fact that evolution has occurred; it is a detailed record of evolution.Pg.382,15-13 2. The geographical distribution of species-the presence of similar environments suggests natural selection. Ex.-the finches could have descended from common ancestors on the mainland. Pg. 383, 15-14 3. Homologous structures- they have different mature forms but develop from the same embryonic tissues. Bone structure is the same but they look different. All are adapted to survive in different environments. Ex. Legs, wings, fins. Pg 384, 15-15 Vestigial organs- some homologous structures no longer serve important functions in descendants. Their size is reduces. 4. Similarities in embryology- early stages, or embryos, of many animals are very similar. Pg. 385, 15-17 5. DNA evidence
Example: the wing of a bat, the flipper of a dolphin and the hand of a human all have five ‘fingers.’ These structures despite having completely different functions are similar because they all come from the common ancestor of all mammals.
Homologous structures are structures that have different mature forms in different organisms but developed from the same embryonic tissue. COMPARE THE BONES OF THESE DIFFERENT ORGANISM. Scientist believe that they are similar because of similar embryonic tissue.
Analogous Structures Structures that have similar functions, but no common evolutionary origin.
Analogous Structures Example: the wing of a bat and the wing of an insect both serve the same function (to allow the organism to fly) but they come from very different evolutionary origins. Bat wings are made of bones and skin, insect wings have neither.
Vestigial Structures Example: Your appendix. Once upon a time it was used to store the inedible things people once ate like bark, fruit peelings and even rocks. This is why a surgeon can remove it without causing any harm.
Vestigial Structures Another Example: A whale has thigh bones left over from its ancestors that were cow-like land animals!
Analyzing Geological and Environmental Changes On standardized exam you may be asked to analyze changes over time. Analyze the fossil layers to the right and explain what happen to the area. NOTE: Play close attention to the arrow to the right. (Shows the direction of the time line)
Analyze the evolution of the horse. List various changes that took place in each column.
Analyze the diagram below of a species A thru G. What does the diagram tell you about the 7 species? Genetically, which species are probably the most similar?
Primates- is a mammal such as an ape, monkey lemur or human. Most primates have an opposable thumb. (a thumb that can be brought around opposite of the fore-fingers) Some primates have a prehensile tail. ( a muscular tail that can grasps or wrap around branches.) Humans are bipedal. (Can walk on two legs) Hominids- are human like, bipedal primates. Stereoscopic vision- eyes are centered in the front of the head giving primates the ability to perceive distance. (Need to throw things like a ball)
16-1 Genes & Variations What we already know: Traits are controlled by genes. Many genes have at least 2 forms or alleles. Individuals of all species are heterozygous for many genes.
Genetic Variation Studied in populations Population- a group of individuals of the same species that interbreed; share a common group of genes. Gene Pool- results from interbreeding in populations; it ’ s all the genes that are present in a population; contains 2 or more alleles for each inheritable trait. Relative Frequency- the # of times the allele occurs in a gene pool, compared with the # of times other alleles for the same gene occur; it ’ s often a %. Pg. 394, 16-2 Ex. An allele makes up ¼ of population ’ s allele for a given trait. The relative frequency is 25%. In genetic terms, evolution is any change in the relative frequency of alleles in a population.
2 Main Sources of Genetic Variation Mutations Gene Shuffling Occurs during gamete production; results in many different combinations. Single gene trait controlled by 1 gene. 2-3 different genotypes are possible. Compare populations gene pool to a deck of cards. Shuffle & you get a different hand each time but it never changes the # of kings, aces, etc.
Polygenic Trait Can have many possible genotypes, producing many possible phenotypes. Controlled by 2 or more genes, each may have more than one allele. Ex. Height. Bell Shaped Curve Pg. 396, 16-4 How phenotypes are expressed. Most people are in the middle of the curve
16-2 Evolution as Genetic Change Natural Selection acts directly on phenotype. Ex. Lizard color. Pg. 397, 16-5 Natural selection on Polygenic Traits are more complicated. Directional Stabilizing Disruptive
Directional Selection When individuals at one end of the bell-shaped curve have higher fitness than individuals near the middle or the other end of the curve. The curve shifts toward the higher fitness end. Ex.- birds with larger beaks will have higher fitness if a small-medium seed shortage occurs. Pg. 398, 16-6
Stabilizing Selection When individuals near the middle of the curve have higher fitness than those on either end of the curve. This is where the average form of trait is. The curve narrows around the middle Ex.- a babies birth weight. Less wt., lower survival rate. Higher wt., difficult birth. Pg. 399, 16-7
Disruptive Selection When individuals at the upper and lower ends of the curve have higher fitness than the individuals near the middle. The curve develops a peak at each end and a low point in the middle. Ex.- Medium bird seed has a shortage. Small & large bird seed are abundant. Birds with small & large beaks will have greater fitness. Pg. 399, 16-8
Genetic Drift Allele frequencies change. Occurs in small populations. Individuals with a particular allele leave more descendants than other individuals. Alleles become more or less common by chance. The only similarity to natural selection is that they involve a change in a population ’ s allele frequencies.
Genetic Equilibrium A situation in which allele frequencies of a population remain constant. 5 conditions required to maintain genetic equilibrium: –Random mating. –Population must be very large. –There can be no movement into or out of the population (migration). –No mutations. –No natural selection When all 5 are met, evolution will not occur.
16-3 Process of Speciation Speciation- formation of new species –For 1 species to evolve into 2 new species: Gene pools of 2 populations must become separated. They must be reproductively isolated. Ex. – finches on the Galapagos islands.
Reproductive Isolation When members of 2 populations cannot interbreed & produce fertile offspring. –Behavioral- different courtship rituals. Ex. Songs. –Geographical- populations are separated by geographic barriers. Ex. Mountains, rivers. –Temporal- populations reproduce at different times. Ex. Pollination.
Speciation of Darwin ’ s Finches 1.The species evolved on the Islands from a single species of founding birds. 2.They traveled from mainland South America to one island to found a new population. 3.They survived & reproduced then crossed to a 2 nd island. 4.The became geographically isolated & gene pool was no longer shared. 5.Seed size on the island favored large beak birds. 6.Large beak birds reproductively isolated & evolved into a new species.