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McMillan preAP Biology Ch. 16 and 17
Transcript of McMillan preAP Biology Ch. 16 and 17
Genetic variation is studied in populations (group of same species that interbreed)
Gene pool – all genes that are present in a population
Relative frequency – number of times an allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur
Evolution is change in the relative frequency of alleles in a population Chapter 16 – Evolution of Populations Two main sources of genetic variation
Change in a sequence of DNA
Independent movement of chromosomes during meiosis and Crossing-Over Genetic Variation The number of phenotypes produced for a trait depends on how many genes control the trait
Single gene with two alleles
Fewer phenotypes than polygenic traits
Ex: widow’s peak
Controlled by two or more genes (each with more than 2 alleles)
Many different genotypes and phenotypes
Ex: height Single-Gene and Polygenic Traits High mortality, low fitness Low mortality, high fitness Food becomes scarce. Directional Selection Directional Selection
Individuals at one end of the curve have higher fitness than those in the middle or the other end Distribution of Phenotypes Polygenic Traits Stabilizing Selection High mortality, low fitness Low mortality, high fitness Selection against both extremes keep curve narrow and in same place. Birth Weight Percentage of Population Key Stabilizing Selection
Individuals near the center have higher fitness than those at either end of the curve. Stabilizing selection favors the norm, the common, average traits in a population. Look at the Siberian Husky, a dog bred for working in the snow. The Siberian Husky is a medium dog, males weighing 16-27kg (35-60lbs). These dogs have strong pectoral and leg muscles, allowing it to move through dense snow. The Siberian Husky is well designed for working in the snow. If the Siberian Husky had heavier muscles, it would sink deeper into the snow, so they would move slower or would sink and get stuck in the snow. Yet if the Siberian Husky had lighter muscles, it would not be strong enough to pull sleds and equipment, so the dog would have little value as a working dog. So stabilizing selection has chosen a norm for the the size of the Siberian Husky. High mortality, low fitness Low mortality, high fitness Number of Birds in Population Beak Size Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Largest and smallest seeds become more common. Disruptive Selection Disruptive Selection
Individuals at the upper and lower ends of the curve have higher fitness than those in the middle. Suppose there is a population of rabbits. The color of the rabbits is governed by two incompletely dominant traits: black fur represented by “B” and white fur represented by “b”. A rabbit with the genotype of “BB” would have a phenotype of black fur, a genotype of “Bb” would have gray fur (a display of both black and white) and a genotype of “bb” would have a phenotype of white fur.
If this population of rabbits were put into an area that had very dark black rocks as well as very white colored stone, the rabbits with black fur would be able to hide from predators amongst the black rocks and the white furred rabbits would be able to hide in the white rocks, but the gray furred rabbits would stand out in both of the habitats and would suffer greater predation.
As a consequence of the selective pressures of their environment, our hypothetical rabbit population would be disruptively selected for extreme values of the fur color trait: white or black, but not gray. If a population did not evolve, or change, the population would reach equilibrium – when allele frequencies stay the same
Conditions required to reach equilibrium:
No movement in or out of population
No natural selection Genetic Equilibrium
Hardy-Weinberg principle Isolation prevents interbreeding between populations
Different courtship rituals or reproductive strategies
Example: remember the Drosophila fly songs?
Two populations are separated by geographic barriers (rivers, mountains, oceans, etc..)
Species reproduce at different times (winter –vs- spring) Speciation Genetic Drift
Random changes in allele frequencies that occur in a small population because of possible reproductive abilities of some and not others -Could result from a bottleneck or founder effect
Example: Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000 — but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted. Bottleneck: an intense pressure or calamity reduces the numbers in a population
Changes in allele frequencies as a result of migration of a small subgroup
Example: the Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington's disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency. This effect is easy to recognize in genetic diseases, but of course, the frequencies of all sorts of genes are affected by founder events Paleontologists- scientists who study fossils
Provides evidence about the history of life
Shows how different groups of organisms have changed over time
99% of species ever to have lived on earth are extinct (species died out) Ch 17.1 – The Fossil Record Fossils can be:
Entire preserved animals
Small pieces like fragments of bone or leaves
Eggs, footprints, animal droppings, pollen, shells, wood
1. Particles of sand and rock are brought to water and settle to the bottom
2. Dead organisms are buried by layers of sediment, which forms new rock
3. Over time, the layers are compressed and the remains are left to be found many years later. How fossils form Shells
Bones Natural forces cause the Earth to lift rocks up in a mountain range, or erode rocks down by wind or water.
To determine the age of fossils paleontologists use:
Relative dating – age of a fossil compared
with that of other fossils in other layers of
rock (oldest layer on the bottom)
Index Fossils- easily recognized organisms who
existed for a short period of time but had a wide
geographic distribution Interpreting Fossil Evidence To determine the age of fossils, scientists also use:
Radioactive dating – some fossils have radioactive elements in the rocks
Half life – length of time required for half of the radioactive atoms in a sample to decay
Scientists use half-life to determine age of fossil by determining how much of the radioactive isotope remains
Common elements: Carbon-14 =5730 years Earth’s early atmosphere probably made up of hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water
About 3.8 billion years ago, Earth cooled enough for water to remain liquid. The earliest sedimentary rocks have been dated to this period. Earth’s Early History In the 1950’s Stanley Miller and Harold Urey tried to determine whether organic compounds could have formed under the conditions of early Earth.
They passed electric sparks through a mixture of hydrogen, methane, ammonia, and water
After a few days, several amino acids (the building blocks of proteins) were formed
We now know that their ideas about Earth’s early atmosphere were not accurate, but experiments based on current knowledge have produced organic compounds. Miller and Urey’s Experiments Microfossils of single-celled prokaryotes have been found in rocks more than 3.5 billion years old
By 2.2 billion years ago fossil evidence indicates that photosynthetic bacteria became common, producing oxygen
Rise of oxygen in the atmosphere drove some life forms to extinction, while others evolved new, more efficient metabolic pathways that used oxygen for respiration. Rise of oxygen According to the Endosymbiotic Theory eukaryotic cells formed from a symbiosis among several different prokaryotic organisms.
Shared traits by Mitochondria, Chloroplasts, and Bacteria: similar type of DNA, similar ribosomes, and reproduction by binary fission. Endosymbiotic theory Convergent Evolution
Process when unrelated organisms come to resemble one another
Ex: shark and dolphin
Process by which two species evolve in response to changes in each other over time Patterns of Evolution Extinction – disappearance of a species
Adaptive Radiation – process where a single species evolves into several different forms that live in different ways through natural selection
Punctuated equilibrium- long stable periods of time interrupted by brief periods of more rapid change