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Natural Selection Charles Darwin Theory of Natural Selection Species that are better adapted to their environment will live longer and reproduce creating more offspring like them “Survival of the fittest” How did Darwin form his theory? 1831: naturalist on the H.M.S. Beagle for a 5 year voyage around the world Galapagos Islands Finches 13 species of finches Beak size and shape varied, most likely due to their island locations and available food Finches had adapted to best obtain food in their environment Darwin’s Finches Natural Selection Darwin’s Breakthrough Theory The Struggle for Existence Darwin believed that there is a constant struggle for survival in the natural world The faster or “smarter” the predator the more likely it is to capture food The better camouflaged, protected, or cunning the prey the more likely it would not be eaten Survival of the Fittest Fitness – how well an individual is able to survive and reproduce in a specific environment Adaptation – an inherited trait which makes an individual more fit in its environment Darwin called this natural selection because of its similarity to artificial selection How Natural Selection Works All populations have genetic diversity (they are not 100% identical) If an individual is born/produced that has trait which make it more fit it then is more likely to survive and reproduce When it reproduces there is a higher chance that the beneficial traits will be passed on to its offspring Natural Selection Over very long periods of time this selection can lead to the entire population having the similar beneficial traits which then makes the entire population more fit Peppered moth activity Survival of the Sneakiest What is variation? Variation: differences between members of the same species Variation is important to survival! Imagine that you go to a corn field and sample beetles. 80% of the genes in the population are for green coloration 20%of them are for brown coloration You go back the next year, repeat the procedure, and find a new ratio: 60% green 40% brown genes. A change in gene frequency over time means that the population has evolved. Genetic Flow Gene migration The transfer of alleles of genes from one population to another gene flow: beetles with brown genes immigrated from another population, or some beetles carrying green genes emigrated. Genetic drift Random fluctuations in the frequency of the appearance of a gene in a small isolated population Leaving gene frequency up to chance Genetic Drift In small populations if an individual that carries a particular trait and by chance has more offspring, then the trait or allele can become more common (by chance, not selection) If this happens when a small group of individuals break away from the main group and colonize a new habitat it is called the founder effect Genetic drift: when the beetles reproduced, by luck more brown genes ended up in the offspring. Natural Selection: beetles with brown genes escaped predation and survived to reproduce more than beetles with green genes, so more brown genes got into the next generation. Also known as: survival of the fittest or only the strong survive Mutation Genes randomly mutate. This changes the genetic makeup of certain organisms phenotype allowing for changes that can be harmful or beneficial MUTATIONS Some “green genes” randomly mutated to “brown genes” and were passed on *organisms do not mutate because it will benefit them. MUTATIONS CAUSE VARIATION! Natural selection happens because of: Mutations Variation Environmental influence Let’s look at an example Example: In the U.S., where people use shampoos with particular chemicals in order to kill lice, we have a lot of lice that are resistant to the chemicals in those shampoos. Resistant means unaffected Hypothesis A: Exposure to lice shampoo actually caused mutations for resistance to the shampoo. Hypothesis B: Resistant strains of lice were always there—and are just more frequent now because all the non-resistant lice died a sudsy death. Which of these hypotheses is correct? Hypothesis B Why? Variation is there already Shampoo killed the “weaker” lice Other lice live and reproduce to make more resistant lice MUTATION ARE NOT CAUSED! Lamarck was an evolutionist who believed in hypothesis A. He was proven wrong. He called his theory “Acquired Characteristics”. DARWIN Hypothesis B is what Darwin discovered. He called these mutations or changes ADAPTATIONS Evolution What is evolution? Process by which organisms pass on traits from generation to generation Explains the origin of new species and diversity The living species of today are related to each other through common traits Evidences of Evolution Fossil Records Homologous Structures Analogous Structures Vestigial Structures Embryology DNA / Amino Acid Homology Fossil Records Fossil- A piece of organism or imprint left behind. Fossils show that species have changed over time. Homologous structures a structure like a bone, that is shared by a group of different species because it is inherited from a common ancestor Analogous Structures two structures are said to be analogous if they perform the same or similar function by a similar mechanism but evolved separately. Did you know that whales have feet? Well, sort of…. Fossils help us determine whale evolution 60 million years ago 50 million years ago 40 million years ago Yesterday Vestigal Organs “left over” structures from a common ancestor Bat eyes Similarities in Embryology The early stages of development of animals with backbones are very similar (Means the same!) DNA/Amino Acid Homology Human 5’A-T-C-T-T-A-C-G-A-A-T-C-A-T-G-C-C-C-T-A-AC-T-T-C-G-G-C-A-T-T-A-C-G-C-T-A-G-C-3’ Whale 5’A-C-C-T-T-A-C-C-A-A-T-C-A-T-T-C-C-C-T-A-AC-T-T-A-G-G-C-A-T-T-A-C-G-C-T-A-G-C-3’ Frog 5’A-C-C-A-T-G-C-G-T-G-T-C-A-T-C-C-C-C-G-AG-C-T-T-C-G-G-T-A-T-T-A-C-G-C-A-A-G-C-3’ Gorilla 5’A-T-C-T-T-A-C-G-A-A-T-C-A-T-G-C-C-C-T-A-AC-A-T-C-G-G-C-A-T-T-A-C-G-C-T-A-G-C-3’ Defining a Species These spiders are from the same species * They look different because all species show VARIATION Speciation Species: a population of individuals that can interbreed and produce FERTILE offspring Speciation: The process in which new species are formed Isolating Mechanisms As new species evolve, populations become reproductively isolated from each other When individuals of two populations cannot produce fertile offspring, then reproductive isolation has occurred Three ways this can happen… Geographic Isolation Physical separation of members of a population Geographic Isolation Populations are separated by geographic barriers such as rivers, mountain ranges, ect… The Abert squirrel population was split into two groups about 10,000 years ago by the Colorado River. A new species formed from on one side of the river, the Kaibab, which are significantly different from the original population. Reproductive Isolation Prezygotic Inability to breed because of being geographically isolated from the other species, breeding times being contradictory, or the animals behavior not being consistent with the other animals. Reproductive Isolation Postzygotic Mishaps after the egg is fertilized. This includes the zygote not surviving because it is rejected by the mothers body, hybrid sterility, gametic isolation, and hybrid inviability. Behavioral Isolation Two populations are capable of interbreeding but do not because of differences in courtship rituals or reproductive strategies The eastern meadowlark and western meadowlark are capable of interbreeding but do not due to differences in mating songs Temporal Isolation When populations reproduce at different TIMES Three species of orchids all live in the same rainforest. Each one only releases pollen for one day, and each species does this at different times throughout the year, so they do not interbreed Evolution of populations Natural Selection on Polygenic Traits When a trait is controlled by two or more genes natural selection can affect a population in one of three ways. Directional Selection Stabilizing Selection Disruptive Selection Directional Selection When individuals at one end of the curve (one extreme OR the other) have the advantage and the population shifts in one direction. Stabilizing Selection When individuals near the center of the curve are selected for (average), keeps most individuals in a population near the average Disruptive Selection When individuals near the middle are selected against, and the population splits so that each extreme grows Founder effect This effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. Bottleneck effect This effect happens when a significant percentage of a population or species is killed or otherwise prevented from reproducing Evolution vs. Genetic Equilibrium The Hardy-Weinberg Principle states that allele frequencies will remain constant unless one or more factors cause those frequencies to change When the allele frequencies remain constant it is called genetic equilibrium and the population will not evolve Genetic Equilibrium In order for a population to be in genetic equilibrium five conditions have to be met Random mating, very large population, no movement in or out, no mutations, no natural selection If one of these conditions are not met the pop. will evolve Cladogenesis Cladogenesis The process by which species split into two distinct species Cladogram - diagram used in cladistics which shows relations among organisms Punctuated Equilibrium • Equilibrium is broken by rare events of large change away from the norm phenotype, characterized by rapid events of branching speciation called cladogenesis. Gradualism Gradualism- Homeostasis is broken by small changes that occur, or ought to occur, slowly in the form of gradual steps over a long time Divergent Evolution The accumulation of differences between groups which can lead to the formation of new species. Ex: Homologous Species Convergent Evolution The acquisition of the same biological trait in unrelated lineages. Ex: Analogous Species