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What is Evolution? A change in the genetic make-up of a population over time.
How Does Evolution Occur? There were two hypotheses
Do we acquire traits? Climbing Eating Show affection Defend yourself Act nutty
How Does Evolution Occur? • NATURAL SELECTION !!! (major mechanism of evolution) • Darwin’s hypothesis (now a theory): • Survival of the fittest…..how is fitness measured? • By reproductive success • Inheritable variations occur in individuals in a population. Due to competition for limited resources, individuals with more favorable variations or phenotypes are more likely to survive & produce more offspring, thus passing traits to future generations.
Darwin’s View of History • Like a Tree with multiple branches. • Common trunk • Tips of twigs = diversity of organisms living in the present. • Forks = most recent common ancestor
Natural Selection Summary • A process in which individuals that have certain heritable traits survive & reproduce at a higher rate than others because of those traits. • Over time, natural selection can increase the match between organisms & their environment. • If an environment changes, or if individuals move to a new environment, natural selection may result in adaptation to these new conditions, sometimes giving rise to new species.
Natural Selection acts on phenotypic variations in populations • Environments change and act as selective mechanism on populations.
Phenotypic variations are not directed by the environment but occur through random changes in the DNA and through new gene combinations.
Some phenotypic variations significantly increase or decrease fitness of the organism and the population.
Examples of Natural Selection • Antibiotic resistance in bacteria • Develops in several steps: • Person is sick from a bacterial infection • Takes antibiotics • Gets better • One bacteria is not killed by the antibiotics, due to genetic modification & reproduces • Person gets sick again & goes to the doctor again • Same antibiotic is prescribed but there’s no effect • Doctor prescribes a different antibiotic which hopefully works. • If bacterium continues to change it could become resistant to more antibiotics
Examples of Natural Selection • Pesticide resistance in rats • Apply pesticide…most rats killed • Due to natural variations, a few rats are not affected by the poison. • They reproduce passing on the trait to some if not all of their offspring • Seeing the rats, you apply the pesticide again with worse results. • A new pesticide must be used.
Artificial Selection Darwin used these examples to help him derive another piece of his theory. Humans impact the gene variation.
What Evidence is there that Evolution by Natural Selection occurs? • Direct observations • Peppered moths • Drug resistant bacteria • Analysis of similarities among different organisms. • Homologous structures • Vestigial structures • Fossil Record • Documents the pattern of evolution • Shed light on origin of new groups of organisms • Biogeography • Geographical distribution of species • Continental drift (slow movement of the continents over time) • Help make predictions of where fossils of groups of organisms can be found
Analysis of similarities among different organisms Homologous structures: represent variations on a structural theme that was present in their common ancestor. Vestigial structures: remnants of features that served important functions in the organism’s ancestors.
Convergent Evolution: independent evolution of similar features in different lineages. Species that share features because of convergent evolution are said to be analogous. Analogous features share similar function but not common ancestry.
Fossil Record Fossils can be dated, this helps us provide evidence of evolution by dating rocks where fossils are found, it helps indicate the relationships within phylogenetic trees, as well as chemical properties &/or geographical data. Allow us to take a look at how new groups came about & when characteristics showed up. EX: Cetaceans are closely related to even-toed ungulates.
Biogeography We can use our understanding of evolution and continental drift to predict where fossils of different groups of organisms might be found. EX: Horses Fossil dating indicates horses originated in North America 5 million years ago. Since North & South America were not yet connected it was predicted that the oldest horse fossils would be located in North America. So far the prediction has been upheld
The environment does not direct the changes in DNA, but acts upon phenotypes that occur through random changes in DNA These changes can involve alterations in DNA sequences, changes in gene combinations, and/or the formation of new gene combinations.
Environments are always changing • No “perfect” genome(entirety of an organism's hereditary information). • Diverse gene pool = long term survival in a changing environment. • Mutations contribute to diversity • Some are silent • Some result in phenotypic difference • Interaction of the environment & phenotype determines fitness.
What does “survival of the fittest” mean? The species that are most adapted in their environment by reproducing more successfully will survive.
Mutation • Change in the nucleotide sequence (are rare; change from gen. to gen. is very small. • In multicellular organisms, only mutations occurring in gametes will be passed on to offspring • Most occur in somatic cells. • Some are “silent” changes Point mutation
Genes duplicating • Due to errors in meiosis • During DNA replication • Activities of transposable elements (wandering DNA segments) • Large chromosome segment duplications are usually harmful • Smaller pieces of DNA may not be. • Gene duplications that don’t have severe effects can persist & accumulate = potential new genes with new functions.
Example of Beneficial Increases in Gene Numbers A remote ancestor had a single gene for detecting odors & have duplicated over time. Today we have about 1,000 olfactory receptor genes.
Evolving Populations What is a population? A group of individuals of the same species that live in the same area & interbreed, producing fertile offspring.
We can characterize a population’s genetic make-up by describing its gene pool. What is a gene pool? • The total number of genes of every individual in an interbreeding population.
Why is it vital to have diversity in the gene pool? • Environmental conditions change • What kills one would kill them all.
Clearing Potential Misunderstandings • Natural selection acts on ___________ • Ex: pepper moths • _____________not individuals evolve. • Organisms that are _______________ fit will pass on their _________. PHENOTYPE POPULATIONS REPRODUCTIVELY GENES
G.H. Hardy & W. Weinberg (1908) • Both men independently suggested a scheme whereby evolution could be viewed as changes in the frequency of alleles in a population of organisms.
Calculating changes in allele frequency • Hardy-Weinberg • For populations to be at equilibrium (remain constant) the following conditions must be met. • Population must be large • Absence of migration • No net mutations • Random mating • Absence of selection • These conditions are seldom if ever met.
Of what value is this? Provides a yardstick by which changes in allele frequency, & therefore evolution, can be measured.
The Hardy-Weinberg Principle • The equation determines what the genetic make-up of a population would be if it were not evolving at that locus. • Those results are compared to data from an actual population • If there are no differences, it can be concluded that the “real” population is not evolving • It is also used in medical applications: • Est. the % of a population that carries the allele for an inherited disease.
Knowing the frequency of the phenotype, you can calculate the frequency of the genotypes and alleles in the population. • Because there are only two alleles, the sum of p and q must always equal 1. This equation finds the allele frequency p + q = 1 = dominant allele = recessive allele
Number of alleles present after several generations Number of offspring with AA _____X 2= _________ A alleles Number of offspring with Aa _____X 1= __________A alleles Total= __________A alleles TOTAL number of A alleles p = TOTAL number of alleles in the population (number of students X 2)
The Hardy-Weinberg equilibrium can be written as an equation p2 q2 1 2pq + = + % of Individuals homozygous for allele bb written as a decimal % of Individuals homozygous for allele BB written as a decimal % of Individuals heterozygous for alleles Bb written as a decimal • By convention • The more common allele (B) is designated p • The less common allele (b) is designated q This equation lets us calculate genotypic/phenotypic frequencies in a very simple way.
Let’s say we have a population of 546 frogs. 142 of these frogs have dark spots on them. The plain green frogs are completely dominant to the spotted frogs. Determine the genotypic frequencies within this population. 142/546 = .26 which represents q2 or gg In order to get the homozygous dominant & heterozygous we need to use the p + q = 1 equation. q2 = .26 take the square root of each side to get q which is .51 p + q = 1 p= 1- q p= 1- .51 = .49 p2 = .24 2pq = 2(.51 x .49) = .50 So, 26% are recessive; 50% are heterozygous; 24% are homozygous dominant.
To determine the frequency of gametes. Meaning the percent of the population carrying the G allele: You would take half of the 2pq frequency and add it to the p2 frequency. .25 + .24 = .49 Which means, 49% of the population carry at least one G allele. To determine g frequency take half of the 2pq & add to q2: .25 + .26 = .51 Which means 51% of the population carries at least one g allele.
√ 0.00048 CALCULATING THE FREQUENCY OF CYSTIC FIBROSIS Cystic fibrosis is caused by the recessive allele b. Calculate the frequency of the recessive allele If q2, the frequency of recessive homozygotes, is 0.00048, then q is , or 0.022. Calculate the frequency for the dominant allele B: p + q = 1 , p = 1-q SO p = 1 – 0.022, OR 0.978 Determine the frequency of heterozygotes 2pq = 2 x 0.978 x 0.022 = 0.043 Meaning: 43 of every 1,000 Caucasian North Americans are predicted to carry the cystic fibrosis allele unexpressed.
Time for a simulation!!! Let’s Get It On!!!!
TONGUE CURLING!!! What percentage of the class are carriers for the tongue curling trait? (Tongue curling is a dominant trait.) Try to curl your tongue upwards. How would we determine q2? Divide the total number of non-curler students by the total number of students. How do you determine q? Calculate the square root of q2. Since p + q= 1; determine p. Now plug in numbers for 2pq.
Altering Allele Frequencies in Populations Directly & cause most Evolutionary Change • Natural Selection • Genetic drift • Gene Flow
Genetic Drift Random fluctuation of alleles due to chance events • More likely to occur in smaller population • Founder effect • Small group of individuals establishes a population in a new location • Bottleneck effect • A sudden decrease in population size to natural forces
Case Study: Genetic Drift The land was being converted to farmland and other uses. Led to significant loss of genetic variation & an increase in the frequency of harmful alleles. A reduction of genetic variation within a given population can increase the differences between populations of the same species.
Gene Flow The transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes. If there is gene flow between two populations there is a tendency for the amount of genetic variation between the populations to decrease.
Natural Selection is the only mechanism that consistently causes adaptive evolution
Fig. 13.13 Stabilizing Selection Increase in the frequency of the intermediate phenotype • In humans, infants with intermediate weight at birth have the highest survival rate • In chicken, eggs of intermediate weight have the highest hatching success
Disruptive Selection • In the African seed-cracker finch, large- and small-beaked birds predominate Can open tough shells of large seeds • Intermediate-beaked birds are at a disadvantage • Unable to open large seeds • Too clumsy to open small seeds More adept at handling small seeds
Directional Selection • Drosophila flies that flew toward light were eliminated from the population • The remaining flies were mated and the experiment repeated for 20 generations • Common when environment changes or members of a population migrate out. Phototropic flies are far less frequent in the population