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Evolution: How species have changed over time First a Perspective of Time 18th/19th century thoughts Focus on variability Paleontology emerges as a science study of prehistoric fossils and rocks Stratification of rock Variation in species (prehistoric and current) Uniformitarianism Catatastrophism Evolution at its most basic Change over time In reality: change in allele frequencies change in phenotypes Those who influenced Darwin http://www.pbs.org/wgbh/evolution/library/02/5/l_025_01.html Charles Darwin a Naturalist – mostly observed organisms in their natural habitats rather than conducting experiments. Was Made most of his observations on the Galapagos Islands Charles Darwin Did much of his work in the Mid1800’s ** Keep in mind this is BEFORE Mendel, Watson and Crick*** Charles Darwin Introduced the idea of Natural Selection as a way for new species to form (speciation). Published The origin of Species in 1859 http://www.youtube.com/watch?v=F_IhC_5FfbE http://www.youtube.com/watch?v=oMYSMdqzFEA http://www.youtube.com/watch?v=hJu2gZQRmTM THE GENIUS OF CHARLES DARWIN Note: These videos have great information about the theory of evolution. But keep in mind… he is trying to disprove another theory. The Theory of Natural Selection Assumptions: There are not enough resources for all to survive genetic variation exits in all populations. Results: 1. Competition 2. Survival of the fittest 3. Descent with modification Assumption 1: Not enough resources What resources are we talking about? Food Are Shelter Suitable Mates there enough for everyone? Assumption 2: Genetic variation exists Where do these differences come from? Mutations Genetic Recombination Sexual reproduction Migration Remember it doesn’t have to be a NEW gene, just a new combination of genes Result 1. Competition What Who are we competing over? What wins? What is the prize? happens to those that don’t win? Result 2. Survival of the Fittest In nature are we all really equal? What Is do we mean by “fittest”? it enough to survive? Result 3. Descent with Modification Break it up, what does it mean? What happens to the frequency of fit genes and unfit genes? What do we see in future generations? 3. Descent with Modification New generations will resemble previous generations (descent) BUT more individuals will have the “best” variation PLUS new mutations and combinations (with modification) An Example Example: What is the genetic variation? What is the selective pressure? Who has the advantage? What would we predict for the next generation? Why might the “unfit” phenotype stick around? Types of Selection – some examples of natural and artificial Natural Selection What determines which variation gets passed on? What is the outcome? Artificial Selection (a.k.a. selective breeding) What determines which variation gets passed on? What is the outcome? Types of Selection Directional Selection: One extreme or the other is “favored” and increases in frequency while midrange and other extreme decrease Types of Selection Stabilizing Selection: Midrange is favored and increases in frequency while both extremes decrease. Types of Selection Diversifying/disruptive Selection: Both extremes are favored and increase while midrange decreases. At what point is a new species formed? Evolution – change in allele frequency – such change that new population is a different species Speciation – two organisms that can successfully reproduce and produce viable, fertile offspring Examples: Cross between a Pug and a Beagle - different breeds but SAME species Examples: Offspring: Puggle! Both viable (obviously) and fertile Examples: Cross between a Horse and Donkey - different species Examples: Offspring: Mule! Viable but infertile Gene Pool Isolation Two populations become separated so their genes are no longer mixed Mutations appear independently in each population Selection happens independently in each population Mechanisms of Isolation Geographic – Physical barrier separates two populations Behavioral – mating behaviors of some are not attractive to others. Temporal – fertility occurs at different times Mechanical – different physical means of reproduction Principle of a Common Ancestor with Modification – over generations descendents can look quite different from ancestors. Descent Thus, organisms that seem very different might share a common ancestor Suggests if you go far enough back, we are all related! Phylogenetic tree: Family Tree of Life Common ancestor Humans and chimps have a common ancestor. THAT IS NOT THE SAME AS SAYING WE WERE ONCE CHIMPS!!! Think about it: Do you and your cousin share a common ancestor? Does that mean you are your cousin? Does that mean that either of you are that ancestor? Evidence of Common ancestry Comparative Anatomy Comparative Embryology Comparative Biochemistry See Determining evolutionary relationships assignment Rules of Evolution Mutations and their phenotypes are random. Meaning? Variation must exist in the population BEFORE selective pressure occurs Rules of Evolution Individuals can not evolve, only species A fit trait in one environment might be eliminated as a weakness in another Evidence of a Universal Common Ancestor What do we ALL have in common Additional Evidence of Evolution (but not necessarily common ancestry) Fossil Record Preserved remains of ancient life in sedimentary rock Even of species no longer in existence (most!) Fossils Fossils are often found in the layers of sedimentary rock. See changes in fossils over time Dating Fossils Absolute Dating: Using radioactive organic material in a sample we can get a more accurate age of a fossil Dating Fossils Relative Dating: Fossils found in lower levels are older than upper levels. Can’t provide exact age, just which is older Dating Fossils Absolute Dating: Radioactive organic material is used to get a more accurate age of a specimen. Radioactive material decays into a non-radioactive decay product at a steady rate. Half life = time it takes half a sample to decay. Example: Some carbon is naturally radioactive – C14. Half life of C14 – 5,730 years Decay product is N14 If we look at the sample and determine the ratio of C14 to N14 we can get an idea of how much time has passed Assume we start with a sample that is 100g of C14 C-14 remaining C14:N14 Years from start date 100g 1:0 0 50g 1:1 5,730 25g 1:3 11,460 12.5 1:7 17,190 Geographic Distribution Biogeography and Convergent Evolution: See Determining evolutionary relationships assignment Vestigial Organs Structures that serve little to no purpose NOW Snake skeletons with leg bones and pelvis Blind, cave-dwelling fish have eyesockets but no eyes. Vestigial Organs Gives insight into PAST needs of organism as well as where this organism has come from What happens first: Need Or for organ disappears? mutated organ appears? Evidence in support of evolution Vestigial structures See handout Evidence in support of Evolution Comparative Biochemistry See handout Genetics in Evolution Darwin did his work before Mendel and didn’t understand genes or how inheritance worked. Thanks to Mendel we know how/why traits get passed from parent to offspring Gene Pool The set of all genes (and their alleles) within a population or species. The set of all genes within interbreeding populations The frequency of alleles, and even genes can change. Phenotypes NOT genotypes Natural selection acts on phenotypes NOT genotypes But in turn will influence allele frequency. Alleles and allele frequencies determine the gene pool Why aren’t all bad alleles eliminated?? Mechanisms of Evolution Things that cause the allele frequencies to change Remember, it is variation that proposes and selection that disposes Sources of Genetic Variation Mutations Crossing over Independent assortment Sexual reproduction External factors impacting allele frequency What happens that results in changes in phenotype (remember selection acts on phenotypes, which impact genotypes Genetic Drift (founder’s effect is one type) Endosymbiosis Mass extinction Adaptive radiation Mechanisms of Evolution Genetic Drift Evolution without natural selection Chance occurrences change allele frequency More common in small populations What if more of the “unfit” survive? Genetic Drift Founder Effect Sample of Original Population Descendants Founding Population B Mechanisms of Evolution Endosymbiotic theory • Mitochondria and chloroplasts evolved from free living prokaryotic organisms • A larger cell engulfed them • A symbiotic relationship formed Endosymbiotic theory Evidence of endosymbiosis Both have their own DNA and produce their own proteins Both reproduce independently from the cell through a process like binary fission (bacterial reproduction) Double membranes of both are similar to prokaryotic membranes Patterns of Evolution Mass Extinction Periodic large-scale extinction events Dramatically changes landscape eliminating or creating selective pressures Patterns of Evolution Adaptive Radiation Single species evolves into several different species that live in different ways (adaptations) Patterns of Evolution Co-evolution Due to close relationship two species share with each other, change in one organism results in a change with the other. Patterns of Evolution Gradualism What Darwin subscribed to Tiny changes accumulate over huge period of time to yield large changes. Think Grand Canyon only organisms Patterns of Evolution Punctuated Equilibrium More modern theory proposed by Gould and Eldridge Proposed change occurs in spurts followed by periods of stasis More support in fossils! Are organisms always evolving? Hardy Weinberg Equilibrium – suggests no! Under certain conditions, populations won’t evolve Conditions: 1. Large population 2. No migration in or out 3. No natural selection 4. Random Mating 5. No net mutations How do we tell? Determine allele frequencies over different generations and see if they change p = frequency of dominant allele q = frequency of recessive allele p+q=1 p2 + 2pq + q2 = 1 p2 = frequency of homozygous dominant q2 = frequency of recessive genotype 2pq = frequency of heterozygote Example problem: A population of aphids can either be brown or green. Green is recessive. In a population of 1000 aphids 250 are green. What are the allele frequencies for the green and brown alleles? Then figure out the homozygous dominant and heterozygote populations too.