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MICROEVOLUTION

MICROEVOLUTION. HARDY WEINBERG EQUATIONS Ms.Kim H. Biology. The Smallest Unit of Evolution. Darwin difficulty = what is the mechanism of natural selection?? How does it occur?? Genetic basis

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MICROEVOLUTION

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  1. MICROEVOLUTION HARDY WEINBERG EQUATIONS Ms.Kim H. Biology

  2. The Smallest Unit of Evolution • Darwin difficulty = what is the mechanism of natural selection?? How does it occur?? • Genetic basis • After Darwin’s paper, Mendel inheritance was published (discrete, heritable characteristics “genes”) • By 1940’s modern synthesis combined ideas from different fields • Population is the unit of evolution • Natural selection is the main mechanism • Gradualism explained accumulation of small changes led to large ones over time

  3. Population genetics • Population genetics is the study of how populations change genetically over time • Population: • Group of the same species living in the same area that can interbred • Species: • a group of populations whose individuals have the potential to interbreed and produce fertile offspring • Gene pool: • the total combination of genes (alleles) in a population at any one time

  4. “Individuals are selected, but populations evolve.”

  5. What is an Allele? Microevolution • The change in the frequency of ALLELES (or how often a certain allele appears)in a population over time • The change in a population’s gene pool over time • A complete gene contains 2 parts called alleles • An allele = a alternative form of a gene • You get 1 allele from “mom” and 1 from “dad” • Alleles are made up of DNA • Genotype = the genetic combination of 2 alleles

  6. Allele Frequencies • Each allele has a frequency in a population’s gene pool  a # of times it appears in a population a A A a a A

  7. Allele Frequencies- EXAMPLE • In a population of wildflowers, the Red allele is A and white allele is a. • 500 total plants in the population, 20 are white (aa), 320 are red (AA), 160 are pink (Aa) • How many total alleles for flower color are there in this population? • 1000 (500 plants with 2 alleles EACH) • a= 20 + 20 + 160 = 200 alleles • A = 320 + 320 + 160 = 800 alleles

  8. Allele Frequencies- EXAMPLE (Con’t) What is the frequency of “A” allele and “a” allele? Always use a decimal for frequency! • 800/1000  A = 0.8 • 200/1000  a = 0.2

  9. How do we KNOW if a population is evolving? • Use the Hardy-Weinberg theorem (or equilibrium) • It is used to describe a population that is NOT evolving • Conditions in the population are completely RANDOM

  10. Hardy Weinberg Theorem • Conditions for non-evolving (NOT CHANGING) population: • Very large population size • No migration • No mutations • Random mating • No natural selection Since all is RANDOM, the null hypothesis is that the population is not evolving. RARELY MET IN NATURE

  11. 2 equations are used in the Hardy Weinberg Theorem p + q = 1 (1 means 100% of all alleles) • This means that there are only 2 possible alleles p and q • p = dominant allele frequency • q = recessive allele frequency • The equation that corresponds to the frequency of individuals regarding these 2 alleles: p2 + 2pq + q2 = 1

  12. p2 + 2pq + q2 = 1 • p2 = frequency of homozygous dominant individuals • 2pq = frequency of heterozygous individuals (frequency of Aa plus aA genotype) • q2 = frequency of homozygous recessive individuals EXAMPLE • Round head is dominant to cone heads, 51% of the individuals in the population have round heads. What portion of this 51% are homozygous? • 0.49 = q2 (recessive), Therefore q = 0.7, so p = 0.3 • p2 is the frequency of homozygous dominant individuals = 0.09 or 9%

  13. Hardy-Weinberg Theorem States that… • the frequencies of alleles and genotypes will stay CONSTANT over generations UNLESS acted upon by agents or • It describes a population where allele frequencies do NOT change • p and q do NOT change over generations

  14. Hardy-Weinberg Equation When using this equation, we assume fertilization is RANDOM and all male/female combinations are likely

  15. Conditions for Hardy-Weinberg Equilibrium • The Hardy-Weinberg theorem describes a hypothetical population • In real populations, allele and genotype frequencies DO change over time • http://www.hippocampus.org/Biology;jsessionid=6E6D9D7721EBDBFE9BD00616517846DD

  16. Causes of Microevolution A change in the gene pool of a population over a succession of generations • We use 5 criteria for non-evolution to determine causes of microevolution • The Hardy Weinberg equations are used to determine the degree of microevolution that is occurring for a given allele

  17. Cause #1: Small Populations How can this happen? Genetic Drift • Genetic drift: changes in the gene pool of a SMALL population due to chance (usually reduces genetic variability) • http://highered.mcgraw-hill.com/sites/0072835125/student_view0/animations.html# • 2nd to last video

  18. Genetic Drift Example #1 The Bottleneck Effect • Results from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population Reduced genetic variation means that the population may not be able to adapt to new selection pressures, such as climatic change or a shift in available resources, because that genetic variation that selection would act on may have already drifted out of the population.

  19. 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. Ex: NORTHERN ELEPHANT SEAL

  20. Genetic Drift Example #2 Founder Effect:a cause of genetic drift due to the colonization by a limited number of individuals from a parent population http://bcs.whfreeman.com/thelifewire/content/chp24/2402002.html

  21. Founder Effect in Amish Causes dwarfism and polydactyly http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m2/s4/evm2s4_7.htm

  22. Cause #2: Migration of Alleles Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)

  23. Cause #3: Mutations Mutations: • a change in an organism’s DNA • original source of genetic variation (raw material for natural selection) • Mutations can immediately alter p and q in a population • Individual mutations are rare in a population, but there may be cumulative mutations that have an effect  cause evolution

  24. Cause #4: “Picky” Mating Nonrandom mating • “picky” mating • Unequal chances of each egg getting fertilized • Assortative mating - choosing individuals more like self

  25. Sexual Selection • Sexual selection is natural selection for mating success • It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

  26. Intrasexual selection • competition among individuals of one sex for mates of opposite sex • Ex: Males competing for female attention • Intersexual selection • when individuals of one sex (usually females) are choosy in selecting their mates of the other sex • Selection may depend on the showiness of the male’s appearance

  27. Males are usually more colorful and larger Does NOT help cope with environment DOES lead to reproductive success Can be termed “Female Selection” Sexual Dimorphism

  28. Cause #5: Natural Selection Natural Selection • differential reproductive success in reproduction due to variation • only form of microevolution that adapts a population to its environment • Phenotypes are selected FOR or AGAINST (not genotypes) • In a population at equilibrium, no phenotypes (therefore, alleles) are selected over other alleles • http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m2/s1/evm2s1_6.htm

  29. 3 Modes of Natural selection • Directional selection favors individuals at one end of the phenotypic range • Disruptive selection favors individuals at both extremes of the phenotypic range • Stabilizing selection favors intermediate variants and acts against extreme phenotypes • http://wps.pearsoncustom.com/wps/media/objects/3014/3087289/Web_Tutorials/17_A02.swf

  30. Directional Selection • This form of selection increases the expression of the extreme versions of a trait in a population. • For example: most of the moths had light-colored wings, but dark moths started to appear. Because moths live against tree barks, years later, most of the moths were dark. Thus, more dark moths survived, adding more genes for dark color to the population.

  31. Disruptive Selection • Disruptive selection: a process that splits a population into two groups. • it tends to remove individuals with average traits but retain individuals expressing extreme traits at both ends of a continuum. • For Example, snakes that live on rocks and are grey will survive. Snakes that live on grass and are green will survive. Snakes that have an intermediate coloring would be disadvantaged because it would be more visible to predators.

  32. Stabilizing Selection • It operates to eliminate extreme expressions of a trait when the average expression leads to higher fitness. • For example: human babies born with below-normal and above-normal birth weights have lower chances of survival than babies born with average weights. • Therefore, birth weight varies little in human populations.

  33. LIGHT/DARK ROCK HABITAT DARK ROCK HABITAT MEDIUM COLOR ROCK HABITAT

  34. Sources of Variations • Mutations • At first the mutation may not be beneficial to the organism (resistance to antibiotics) • Once antibiotics are introduced the mutation is beneficial 2. Sexual recombinationincreases variation

  35. Evolutionary Fitness • The phrases “struggle for existence”/“survival of the fittest” are commonly used to describe natural selection but can be misleading • Reproductive success is more subtle and depends on many factors • Fitness is determined by REPRODUCTIVE AND SURVIVAL success of individual • Variations & natural selection affect fitness • Sterile individuals even with a relatively fit allele will not be considered fit overall (=0) • Fitness is not determined by one trait but by the totality of traits in the organism

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