Evolutionary change. Evolution and Diversity. Two British naturalists, one revolutionary idea. Darwin observed organisms in many environments

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1 Evolutionary change Evolution and Diversity Ch 13 How populations evolve Organisms change over time In baby steps Species (including humans) are descended from other species Two British naturalists, one revolutionary idea Darwin observed organisms in many environments A remarkable and elegant explanation of how things evolve Evolution by Charles Darwin Alfred Russell Wallace Saw an amazing diversity of plants and animals Noticed that organisms adapt to their environments For example, marine iguanas have webbed feet and flattened tail that aid in swimming A priceless legacy Descent with Modification Darwin and Wallace left the world two key ideas: Descent with modification All organisms are descended with changes from common ancestors Evolution Individuals better adapted to the environment survive and reproduce This is the mechanism of evolution Organisms change over time or evolve This evolutionary tree of the horse family is based on fossil evidence

2 Evolution by natural selection 3 insects well adapted to their environment Camouflage an example of adaptation Survival of the fittest Individuals best suited for their environment are more likely to survive and reproduce 3 closely related insects that evolved in different environments Natural Selection adaptations to different food sources Evolution by natural selection Long narrow beak for holding tools Large beak specialized for cracking seeds Individual variation Small beak for catching insects Darwin s finches an excellent example of natural selection and adaptive evolution. This causes individuals to compete with each other for limited resources An example of natural selection Those individuals with traits best suited for the local environment leave more offspring. Populations produce more offspring than the environment can support Galapagos finches have beaks adapted for specific diets Key insight Individuals in a population vary in many ways Inherited traits affect evolution Differential reproductive success This, of course, is natural selection Insects develop resistance to pesticides.

3 Insecticide application The Evidence for Evolution Survivors Reproduction Chromosome with gene conferring resistance to pesticide Fossil record Many fossils link ancestral and living species Comparative anatomy Similarities in structures among organisms Closely related species have similar stages in their embryonic development Molecular biology All species share the genetic code, suggesting that all forms of life are related Figure The Fossil Record Evidence for evolution: Transitional forms in the fossil record Fish fossil Fossils are the preserved remains or tracks of once-living organisms They are created when organisms are buried in sediment and their bones become mineralized You can tell how old fossils are by the layer of rocks where they are found. Did the ancestors of whales live on land? This is a picture of fossilized leg bones of Basilosaurus, an ancient whale connects present day whales to four-legged ancestors Evidence for evolution: Comparative anatomy Homology among vertebrate limbs the same basic bones are present in each forelimb Compare body structures among different species The forelimbs of all mammals are constructed from the same skeletal elements Confirms that evolution is a remodeling process Evidence for evolution: Molecular Biology Evolutionary relationships are determined by comparing Genomes Genes Proteins Organisms that are more distantly related show more differences in their DNA

4 Molecular biology: Comparing protein structure Comparison of the amino acid sequence of hemoglobin in different vertebrates The greater the number of differences in the amino acids in hemoglobin, the greater the evolutionary distance from humans Molecular biology: A new tree of life Ribosomes are found in nearly all organisms Comparing the sequence of RNA in ribosomes better understanding of evolutionary relationships How do organisms evolve? Variation in Populations 4 mechanisms of evolution Mutations Genetic drift Gene migration (or gene flow) These forces act on populations to change their DNA *a population is a group of individuals of the same species living in the same area Starting material: variation in populations of plants and animals Not all variation is genetic and can be inherited. Only the genetic component of variation is relevant to natural selection. Polymorphism in garter snakes These 4 snakes belong to the same species Mechanisms of evolution #1 Mutation Mutations changes in DNA are the source of genetic variation Create variation in a population Can be inherited IF they are in a germ-line cell Mutations can take many forms Changes in blocks of DNA Changes in a single nucleotide (or base) Mutations Mutations are random Mutations can be beneficial, neutral, or harmful for the organism Not all mutations matter to evolution Mutations that occur in somatic cells won t be passed onto offspring Only mutations that occur in reproductive cells (eggs and sperm) matter

5 Mechanisms of evolution: #2 Natural Selection occurs when some individuals leave more offspring than others Of all mechanisms of evolution, only natural selection leads to adaptation. : a closer look Reproductive success subtle For example, some plants attract more pollinators More successful in reproducing It s not always survival of the fittest Natural Selection Certain moths survive and produce more offspring because they are better camouflaged A frog may produce more eggs than her neighbors because she is better at catching insects Two forms of the peppered moth Mechanisms of evolution #3 Genetic drift Chance events that cause the frequency of an allele to change Another example of genetic drift: the bottleneck effect The effect of genetic drift becomes increasingly important as the size of the population decreases. A bottleneck occurs when a disaster (flood, fire, etc) kills large numbers of individuals Reduces genetic variation Due to chance

6 Genetic Drift in Human Populations Mechanisms of evolution: #4 Gene migration A few individuals colonize a new place Gene pool differs from the parent population relatively high rate of inherited disorders Called the founder effect Residents of an isolated island in the Atlantic Ocean have a high rate of hereditary blindness Movement of individuals between populations migration Introduces new genes Here the genes for brown beetles become more frequent in the green beetle population Gene migration Evolution by natural selection Computer-generated image blending features from several races Genetic exchange with another population Tends to reduce genetic differences between populations. The migration of people throughout the world introduces new alleles An important agent of evolutionary change Individuals best suited for their environment are more likely to survive and reproduce Another type of selection Artificial selection farmers breed domestic animals and plants for certain traits Size, speed, milk production Examples: race horses, dairy cows, corn, wheat, etc etc Sexual selection Sexual selection choosing a mate based on physical characteristics A type of nonrandom mating individuals choose their mates based on certain traits Vol. 3 Why sex? (episode 2/6 and 3/6 - end at 4:05 after elephant seals) The peacock s tail - starts at 7:37 (5 min)