THE EVOLUTION OF DARWIN S THEORY PT 1. Chapter 16-17

Transcription

1 THE EVOLUTION OF DARWIN S THEORY PT 1 Chapter 16-17

2 From Darwin to Today Darwin provided compelling evidence that species and populations change. What he didn t know (and neither did anyone else at the time) was the how In these chapters we will talk about all the evidence Darwin used to explain his theory We will then look at how his theory has changed over time as our understanding of genetics, mutations, populations, ecology, geology, and many of fields of science have also changed

3 Evidence The fossil record The fossil record is the history of life told through remains from the past (plant, animal, bacterial, geologic) The fossil record shows plants and animals from the past that are currently extinct but similar to today s plants and animals The ancient mesohippus and the modern-day horse Bones of animals millions of years old and 5-6 times the size of modern day animals were remarkably similar Some even show hybrids, like the archaeopteryx, a reptilelike creature with avian-like feathers

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7 Evidence Biogeography Biogeography is the study of the distribution of plants and animals throughout the world Why does South America have no rabbits? No cacti in northern Africa? The Galapagos tortoises and finches only in the Galapagos? These species would thrive if they were found there. Darwin concluded that species don t simply appear randomly Organisms do not randomly emerge. Therefore, their ancestors migrated and carried various traits with them until we get the diversity we see today

8 Evidence Common Anatomy An enormous number of species all show common homologous structures (anatomically similar in different species) Forelimbs of humans, horse, whale, cat, bird This should not be confused with analogous structures, or structures which serve a similar purpose but likely are not ancestrally related Wings of a bird vs an insect

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11 Evidence Common Anatomy Vestigial structures are anatomical features that are functional in some species but not in others Wings of an ostrich Eyes of cave-dwelling salamanders Spending time building structures that serve no purpose seems like a waste. Most likely they either USED to serve a purpose, or eventually WILL serve a purpose.

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14 Evidence Biochemical evidence All life is built on DNA, which contains the same 4 nucleotides All life is built on proteins containing the same 20 amino acids The same codons code for the same amino acids The same basic proteins exist in single and multicellular organisms

15 Evidence Biochemical (Example) One important protein involved in cell reproduction (and so far found in every species on the planet) is 16s rrna. By taking the protein and DNA sequences of 16s rrna using electrophoresis and PCR, geneticists have been able to compare how closely organisms are related to each other. The more similar the DNA sequence, the closer the relation. Using this technique, we ve discovered some interesting relationships (related does not necessarily mean descended) Roses are in the same family as peaches and apples, but nowhere close to oranges or bananas Humans are more closely related to orangutans then chimps or gorillas.

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18 The Theory of Evolution Evolution during Darwin s age was a hypothesis. It is now widely accepted as a theory. A theory is a concept that has been supported by multiple rounds of evidence and observations Theories are never proven. They are disproven or failed to disprove yet. It s impossible to prove. As with all theories, they constantly change as we learn new evidence.

19 Microevolution In a population, all of the alleles for a specific gene within the population are called a gene pool Gene pools are described using gene frequencies, or the frequency that the allele appears in a population To determine frequencies of alleles, calculate the number of that specific allele divided by the total number of alleles If done right, all the allele frequencies in a population will equal 1.0

20 Hardy-Weinberg In 1908, G. H. Hardy and W. Weinberg wrote a series of rules and calculations to describe the genotype and allele frequencies in the population. While the equations can be altered to account for multiple allele traits, the simplest version concerns simple Mendelian traits and only two alleles Dominant allele = p; Recesive allele = q. p 2 + 2pq + q 2 p + q = 1.0 P 2, q 2 = homozygous genotypes; pq = heterozygous genotype

21 Hardy-Weinberg The Hardy-Weinberg equation shows 1) How genotype and phenotype frequencies can fluctuate but allele frequencies stay the same 2) How genotype frequency fluctuation leads to changes in phenotypes 3) How genotype and allele frequencies can be determined by calculating phenotype frequencies The Hardy-Weinberg equation only works if the allele frequencies don t change in sexually reproducing populations Therefore, the Hardy-Weinberg equations have five conditions that must be met:

22 Hardy-Weinberg 1. No mutations. Either mutations don t occur or two mutations cancel each other out 2. No gene flow, either immigration or emigration 3. Random mating. Individuals pair by chance, not according to genotypes or phenotype 4. No genetic drift. The population is large enough that chance alone does not affect frequencies 5. No selection. No selective agent in the environment favors one genotype over another. If any of these conditions aren t met, it means the allele frequencies are changing and evolution is happening

23 Genetic Mutations Without mutations, there can t be any new alleles in a population Many mutations are not immediately detected because they do not affect phenotypes Once a mutation has occurred, If the mutation causes a less favorable genotype then it will be wiped out of the population. If the mutation causes a more favorable genotype then it will be passed on more and more over subsequent generations until it becomes the only trait If both genotypes are equally favorable, the result is a new phenotype for the population (red hair, blue eyes, etc.)

24 Gene Flow (or Gene Migration) Gene flow is the movement of alleles between populations by migration of the organisms Large migrations of organisms, into or out of a population, will carry their alleles with them This inevitably affects the frequencies of the population It is possible that one allele s arrival or departure may be so dramatic that the result is only one allele exists in the population within future generations

25 Nonrandom Mating While we d like to think everyone s equal, almost all species on the planet are picky about their mates If an organism has features that are undesirable or unhealthy, they will mate fewer times and their alleles will have a smaller chance of being passed along Therefore, their allele frequencies will decrease in future generations Assortative mating: individuals mate with those with specific phenotypes that they personally desire Sexual selection: males compete over the right to mate, but the winner gets as many mates as he chooses

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28 Genetic Drift Genetic drift is changes in allele frequencies due to chance events in the population Larger populations are less likely to have a natural disaster, disease, or other issue dramatically affect the frequencies Drift is random, so a change in one population may not have any effect whatsoever on a neighboring population A drift that occurs in elm trees in northern California will probably not be seen in a similar population in northern Idaho

29 Genetic Drift: Bottleneck effect Natural disaster, overharvesting or habitat loss can drive a species to near extinction at times Only a few will make it through the bottleneck The few remaining survivors alleles then make up the new allele frequencies Cheetahs are an example of this. All cheetahs, no matter their locations, have a dramatically similar genome It is believed sometime between 4,000 and 10,000 years ago they suffered a near-extinction, the result being low variations in genotypes and alleles

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32 Genetic Drift: Founder effect The founder effect is when a rare allele or combination of alleles is unusually high in a population of a species that is isolated from the other populations The founding members of this population happen to carry these rare alleles. Thus, all their offspring over the generations contain the same rare combination Since the population doesn t interbreed with other populations the frequency is much higher than usual. Example: the dwarfism fingers in the Amish population of Pennsylvania

33 Natural Selection Natural Selection is the process of allele frequencies adapting to the needs in a particular environment Darwin hypothesized a form of natural selection, which has been altered in the modern day to meet our new knowledge of genes to the following: Evolution requires inheritance. Traits are able to be passed from parents to offspring Evolution requires variation. At least two phenotypes exist Evolution requires differential adaptiveness. One allele is better suited for an environment than the other Evolution requires differential reproduction. Offspring of parents with the better allele will also have the better allele.

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