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Evolution 19.1, 16.1-16.4, 17.1-17.4, 19.2 Fossils provide evidence about extinct species Earth is more than 4.6 billion years old. Relative dating - Uses index fossils to determine the relative dates of rock layers. Radiometric (absolute) dating – Uses the proportion of radioactive : stable isotopes to calculate age. Half-life – time required for half of the radioactive atoms in a sample to decay, or break down into stable isotopes. • Each radioactive element has a different half-life. • This provides natural “clocks” that tick at different rates useful for dating rocks or fossils. • carbon-14 – produced at a steady rate in the atmosphere, decays into carbon-12 in 5730 years, used in absolute dating. Theory of evolution Theory – a well supported testable explanation of phenomenon occurring in the natural world. Evolution – the process by which modern organisms changed over time from ancient common ancestors. Microevolution – change in allele frequency in populations over generations. Macroevolution – large scale change, such as the formation of new species. 3 patterns of biological diversity Species – group of similar organisms that can breed and produce fertile offspring. 1. Species vary globally – different yet ecologically similar animals are found in different yet similar environments. 2. Species vary locally – different yet related species occupy different habitats in one area. 3. Species vary over time – fossils of extinct species are similar to current species. Remember… The Earth is old and the process of change exists today. Traits acquired during an organism’s lifetime are NOT passed to it’s offspring! Most organisms don’t survive to reproduce! Examples: sea turtles, insects, etc. Artificial selection Artificial selection – nature provides variation (variety) in organisms’ traits, but humans choose to breed those organisms that have the most useful traits. Example: humans breed cows that produce the most milk. Example: humans breed trees that create the most fruit. Summary: Theory of evolution • • Species are different due to variation in their genes (variation results from random mutation). Some individuals are better suited for survival, and will leave more offspring (natural selection or survival of the fittest). Summary: Theory of evolution • • Over time, change within species leads to the replacement of old species by new species as less successful species becomes extinct. There is clear evidence from fossils, anatomy, physiology, DNA, and embryology that the species now on Earth have evolved from ancestors that are now extinct. Natural selection Darwin proposed a mechanism for evolution that he called natural selection. Individuals whose characteristics are well-suited to their environment survive and reproduce. • By surviving, these attributes can be passed onto their children, causing an increase of these traits in the species population, thus causing a gradual change in the characteristics of the population. Individuals whose characteristics are not well-suited to their environment die or leave few offspring. Because natural selection favors a certain trait over others, more individuals in the population carry the genes for that trait. AKA: survival of the fittest. Parameters of evolution by NS 1. Struggle for existence – more organisms are produced than can survive. Competition – individuals or groups of organisms compete for similar resources (territory, mates, food, water, etc.) in the same environment. 2. Variation and adaptation – some variations are better suited. Adaptation – heritable characteristic that increases an organism’s ability to survive and reproduce. 3. Survival of the fittest – individuals with adaptations that are well suited to their environment survive and reproduce. Fitness – measure of how well an organism survives/reproduces. Common descent Common descent – all species (living and extinct) descended from a common ancestor. Over many generations, adaptations caused a successful species to evolve into a new species. The fossil record provides evidence for this descent with modification. Evidence for evolution includes: 1. 2. 3. 4. 5. 6. 7. Geographic distribution of species Fossils Anatomy (homologous structures) Physiology (analogous structures) Embryology Universal genetic code Biochemical homology Geographic distribution of species Species of animals on different continents had similar structures and behaviors. Darwin theorized that animals on each continent were living under similar ecological conditions and were exposed to natural selection in a similar way. Similar selection pressures caused animals to evolve common features. Fossils A fossil is the preserved or mineralized remains (bone, tooth, shell) or trace of an organism that lived long ago. Fossils show evidence that support the ancestry between species. Fossils trace the evolution of modern species from extinct ancestors. • Example: Fossils have shown that whales evolved from four legged land mammals; 1990’s transitional forms of whales found. Anatomy Evolutionary relationships can be viewed by studying and comparing anatomy. Scientists view the anatomy of limbs and see common similarities. Over course of evolution, vertebrates moved into environments causing different survival needs. Anatomy Homologous structures – parts of different organisms (that are often quite dissimilar) that developed from the same ancestral body parts. Forelimbs of whales, bats, crocodiles, and chickens have similar anatomy but are modified for different functions – common ancestry. Are similar in structure but differ in function! Physiology Analogous structures -- structures that are similar in appearance and function but have different origins and usually different internal structures. Examples: a bat’s wing and a moth’s wing-both are wings and both are used for flight, but a bat has bones and a moth does not. Physiology Vestigial structures – any body structure that has reduced or no body function. Examples: A human’s appendix; a whale’s pelvis. As species adapt to environments the change in form and behavior and continue to inherit these structures as part of the body even though they have no function. Embryology Embryology shows links between different species. Embryos of different species in early development are indistinguishable from each other. Common cells and tissues develop in similar patterns in all vertebrates. Illustrates descent from a common ancestor. Universal genetic code Genetic code – mRNA codons specify particular amino acids. The genetic code of all organisms on Earth (bacteria, yeast, fruit flies, humans) is the same! Example: the AUG codon always codes for the amino acid methionine. Biochemical homology Similar DNA, RNA, and amino acid sequences amongst species in same taxonomic group. Remember comparing your insulin gene DNA and amino acid sequences to that of a cow? Hox genes determine embryonic head-to-tail patterning and are conserved in almost all multicellular animals. Allele frequencies Population – mating group of organisms of the same species. Gene pool – all genes (and their alleles) present in a population. Allele frequency - # of times allele occurs in a population. Changes as population evolves over time. Natural selection operates on individuals, but causes a change in the allele frequency. Sources of genetic variation The main source of genetic variations in populations is mutations!!!! These mutations occur randomly. Not all mutations affect an organism’s fitness. Only heritable mutations matter for evolution. Neutral mutations don’t change phenotypes. Other sources of variation include: 1. 2. Genetic recombination during crossing over and independent assortment in meiosis. Lateral gene transfer (bacteria only) • Bacteria swap plasmids between members of the same generation, then pass them to their offspring. NS and phenotype An organism’s genotype and environmental conditions makes up its phenotype. Natural selection operates on variation in organisms’ phenotypes. Higher fitness = phenotypes better suited for the environment. Phenotypes for traits Number of phenotypes for a trait depends on how many genes control the trait. • Single-gene trait – trait controlled by one gene. • Ex. Banded or un-banded shell. • NS on these traits leads to changes in allele and pheno. frequencies. • Polygenic trait – trait controlled by two or more genes. • Ex. Height in humans. • NS on these traits affects fitness of phenotypes. NS on polygenic traits leads to selection in populations When NS on polygenic traits affects the fitness of phenotypes, it leads to selection: 1. 2. 3. Directional selection Stabilizing selection Disruptive selection Types of selection in populations Directional selection – organisms at one end of the curve have a higher fitness than those in the middle or at the other end. Stabilizing selection – organisms in the center have highest fitness. Disruptive selection – organisms at the ends of curve have highest fitness. NS is not the only source of changes in allele frequencies Genetic drift – change in allele frequency that occurs in small populations due to random chance. Genetic bottleneck – change in allele frequency following a dramatic reduction in population size. Founder effect – change in allele frequency following migration of a small subgroup out of the population to start a new population. Genetic equilibrium Genetic equilibrium – a MODEL to explain what would happen to a hypothetical, nonevolving population. Allele frequencies NOT changing. Hardy-Weinberg principle – states that allele frequencies in a population should remain constant unless something causes them to change. Hardy-Weinberg P = frequency of dominant allele. Q = frequency of recessive allele. Equation: p2 + 2pq + q2 = 1 AND p + q =1 Equation in words: (frequency of AA) + (frequency of Aa) + (frequency of aa) = 100% AND (frequency of A) + (frequency of a) = 100% Hardy-Weinberg Remember genetic equilibrium occurs in large populations. HW predicts that 5 conditions can disrupt genetic equilibrium and cause evolution to occur: 1. 2. 3. 4. 5. Nonrandom mating (sexual selection) Small population size – leads to of genetic drift. Migration (immigration or emmigration) – aka gene flow into or out of a population. Mutations *** Natural selection – different fitness exists for different alleles. Macroevolution Speciation – evolutionary process by which new species arise. Extinction – the end of a species. When environments change, the process of evolution enables some species to adapt to new conditions and thrive while some species fail to adapt and become extinct. Speciation Species – population of organisms that can interbreed. Speciation – evolution/formation of a new species. Niche - combination of an organism’s profession and place where it lives. No 2 species can occupy the same niche in the same location for a long period of time! The more efficient species will survive and reproduce driving the other to extinction. Isolating mechanisms of speciation Reproductive isolation occurs when 2 populations can’t interbreed - causes speciation! Once reproductive isolation occurs, natural selection increases the differences between the separated populations. 1. Behavioral isolation – different courtship. 2. Ecological/habitat isolation – can only mate in specific or preferred habitats. 3. Mechanical isolation – no sperm is transferred. 4. Gametic isolation – no fertilization of egg occurs. 5. Temporal isolation – reproduce at different times. Geographic isolation – population becomes divided (isolated) by a physical barrier. Rates of speciation Gradualism – slow, steady change leading to new species. Punctuated equilibrium – brief periods of rapid change leads to the formation of new species. • Rapid change occurs when a small population is isolated from the rest of the population or migrates. Molecular evolution Molecular clock – uses mutation rates in DNA to estimate the time 2 species have been evolving independently. Based on neutral mutations – those not under selection. Genes evolve through: Modification of existing genes. Duplication of existing genes. • Crossing over – during meiosis. • Gene duplications – extra copies undergo mutations. • Gene families – multiple copies of duplicated gene make similar yet different proteins. Patterns of evolution Divergent evolution – single species or group of species evolve over a short period of time into different forms living in different ways due to a change in environment that makes new resources available. Aka adaptive radiations Ex. Dinosaurs, Darwin’s finches. Convergent evolution – similar structures are produced in distantly related organisms. Ex. Mammals that feed on ants/termites evolved independently 5 times. Coevolution – 2 species respond to changes in each other over time. Neither can survive without the other. Summary… 1. 2. 3. 4. Individual organisms differ from one another and differences are inherited. Organisms produce many offspring and some do not survive. Organisms compete for limited resources. Each organism is unique and has different advantages and disadvantages in its struggle to survive. Summary… 5. 6. 7. 8. Adaptations that are best suited for an environment allow organisms to survive and therefore reproduce. Species change over time. Species alive today have modifications from ancient ancestors. All organisms on Earth are united by common descent.