Presentation on theme: "Population Evolution Biology Chapter 16"— Presentation transcript:
1 Population Evolution Biology Chapter 16 Individuals within a population show variation in traits between themselves.Biology Chapter 16
2 Genetic VariationPopulations always show variation in traits – no two individuals are EXACTLY the sameMost genes have at least two alleles (some have many more). Lots of genetic “variation” is unseen. Variation is very common but not always “seen”.Variations are caused by two main sources:MutationsGene shuffling during sexual reproduction
3 Gene PoolsGene pools consist of all genes (AND all alleles) within a populationGene frequency is the number of times that an allele occurs in a gene poolEXAMPLE of gene pools - A homozygous black mouse has two alleles for black fur. A heterozygous black mouse has one allele for black fur and one allele for brown fur. A homozygous brown mouse has two alleles for brown fur.Gene Pool:Homozygous Black =Homozygous Brown =Heterozygous Black =BBbbBb12 Heterozygous BlackAny change in allele frequency means that evolution (allele changes) is taking placeHow many total genes?How many total alleles?How many B alleles?How many b alleles?255020304 Homozygous Black9 Homozygous BrownAllele B frequency = 20/50 = 40% = 0.4Allele b frequency = 30/50 = 60% = 0.6
4 Sources of Genetic Variation Changes in allele frequency indicate that a change in the allele populations has occurred through evolutionSources of Genetic VariationMutationsANY change in the DNA sequence due to mistakes, environmental sources (radiation, etc.)Can have no effect or drastic effectsGene shuffling as a result of sexual reproductionCrossing over during meiosis IRandom contributions from mother & fatherMost genetic variation results in little or no observable changes in organisms
5 Trait distribution in populations Polygenic traitsSingle-gene traitsPolygenic = more than one gene influencing a trait. Know the difference between polygenic and single-gene trait graphs
6 Natural Selection effects on single- gene traits CAN lead to allele frequency changes = evolution!What happened?Environmental changes allowed black lizards to be less conspicuous to predators than the previous environment. Therefore, red and light colored lizards stood out more and were more easily eaten by predators than black lizards causing a change in the allele frequency.
7 Natural Selection effects on polygenic traits More complex than single-gene trait resultsDirectional selectionDisruptive selectionStabilizing selectionThese natural selection strategies occur as a result from a shift in the environmental conditions. Be able to identify the type of selection based on the graph
8 Directional Selection Example BEFORE Industrial Revolution (and soot)AFTER Industrial Revolution (and soot)
9 Stabilizing Selection Example Healthy birth weight leads to better survival rates
11 Genetic DriftIn a population, an allele can become more or less common solely due to chance = genetic “drift”. (Individuals can immigrate., emigrate, etc.)Greatest effect in small populationsFounder Effect –Individuals leave a population to initiate a new population, or change another population
12 Genetic Equilibrium = Genetic Equilibrium No change in allele frequency = genetic equilibriumHardy-Weinberg Principle = a mathematical way to quantitatively determine whether a population is showing evolution by studying allele frequenciesThe determination only works when there is no evolution (genetic equilibrium). Need to have these conditions:Random matingVery large populationNo immigration nor emigrationNo mutationsNo natural selectionCan ALL of these conditions actually be met in nature? Is there ever genetic equilibrium in nature, or are there ALWAYS changes going on?= GeneticEquilibrium
13 Speciation The process of forming new species If an allele mutation allows better “fitness” for a population, then that allele will become more commonWHAT causes a species to form two (or more) new species?Isolating Mechanisms – causing “reproductive isolation”BehavioralGeographicTemporal (time)
14 Behavioral IsolationA form of reproductive isolation in which two populations that may be able to mate have differences in courtship rituals or other types of behavior that prevent them from interbreeding
15 Geographic IsolationA form of reproductive isolation where two populations are separated by a geographical border – keeping them from interbreeding.Northern Spotted owl videoMexican Spotted owl video
16 Temporal IsolationA form of reproductive isolation in which the two species reproduce at different times of the day, season, or year. Wood and leopard frogs are an example of two similar species whose ranges overlap.
17 Darwin’s Finches Speciation Example Gene pool changesFounders arriveGeographic isolationSpeciation – the formation of new species – takes place over many years and is influenced by many factors.Eventually, changes in the gene pool happens as a result of the birds adapting to their local environment. If some birds from island B fly back to island A, they may have changed enough that they can no longer interbreed (reproductive isolation). If they both exist in island A, they will compete for local resources, perhaps causing further evolutionary changes.Be able to use the Galapagos finches example to describe how speciation works
18 Unanswered QuestionsHow does evolution add information? Mutation explains how existing genetic information is changed, but it doesn't explain where new genetic information comes from.How can evolution be so quick? Millions of years can seem like a long time to the average person, but in terms of evolution, it's quite quick. Current evolutionary theory hasn't explained how a one-celled organism can evolve into an organism as complex as a human being in the time available in Earth's history.Where did the first living cell come from? In order for mutation and natural selection, processes essential to evolution, to operate, life must already exist.The current hypothesis is that life formedspontaneously from chemical reactionsabout 4 billion years ago, but is this reallyPossible?Do you have other questions?
19 What’s next?Click on “the Wild Future” to see an introduction of the series created by scientists about organisms of the future without humans.
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