Darwin observed organisms in diverse environments. Evolution and Diversity. Descent with Modification. Descent with Modification

Transcription

1 Darwin observed organisms in diverse environments Evolution and Diversity Ch 13 How populations evolve Darwin observed how organisms adapted to their environments The animals living on the Galápagos Islands were unique but resembled those in South America. Descent with Modification Descent with Modification Snail fossil Darwin found marine fossils on mountaintops in the Andes observed the amazing diversity of plants and animals His observations lead him to believe that organisms change over time, or evolve This evolutionary tree of the elephant family is based mainly on fossil evidence Charles Darwin wrote The Origin of Species Darwin established the ideas of evolution and natural selection in 1859 He proposed that: Modern species descended from ancestral species descent with modification Natural selection is the mechanism of evolution Darwin s theory: Evolution by natural selection Survival of the fittest Individuals best suited for their environment are more likely to survive and reproduce 1

2 Natural selection results in adaptation to the environment Natural selection The basic idea: Organisms can change over generations. Individuals with certain inherited traits leave more offspring than others. Camouflage an example of evolutionary adaptation Darwin s Theory of Natural Selection Natural Selection Long narrow beak for holding tools Large beak specialized for cracking seeds Observation 1: Overproduction of offspring Small beak for catching insects Darwin s finches are an excellent example of natural selection and adaptive evolution. Cloud of spores exploding from a puffball fungus Galapagos finches have beaks adapted for specific diets Darwin s Theory of Natural Selection All species tend to produce more offspring than the environment can support. This leads to a struggle for existence. Observation 2: Individual variation Darwin s insight Differential reproductive success Variation exists among individuals in a population. Much of this variation is heritable. Those individuals with traits best suited for the local environment leave more offspring. This, of course, is natural selection Asian lady beetles 2

3 The Evidence for Evolution Fossil record the most direct evidence 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 The Fossil Record Fish fossil Fossils are the preserved remains or tracks of onceliving organisms They are created when organisms are buried in sediment and their bones become mineralized You can tell how old the fossils are by the layer of rocks where the fossils are found. Transitional forms link past and present Comparative anatomy Did whales evolve from land-dwelling ancestors? A series of fossilized whales connect them to four-legged ancestors This is a picture of fossilized leg bones of Basilosaurus, an ancient whale Homology among vertebrate limbs the same basic bones are present in each forelimb Homologous structures The forelimbs of all mammals are constructed from the same skeletal elements Anatomical evidence of descent with modification Comparative Embryology Comparative embryology is the comparison of structures that appear during the development of different organisms. All vertebrate embryos share a basic set of developmental stages Molecular Biology Evolutionary relationships are determined by comparing Genomes Genes Proteins Organisms that are more distantly related show more differences in their DNA 3

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 based on rrna Molecular biology is being used to understand the relationships among different groups of organisms Ribosomes are found in nearly all organisms RNA sequence data is used to compare rrna among species How do organisms evolve? Variation in Populations 4 mechanisms of evolution Mutations Natural selection Genetic drift 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 of this 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 Deletion, duplication or translocation of 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 4

5 Mechanisms of evolution: #2 Natural Selection Natural selection occurs when some individuals leave more offspring than others Of all mechanisms of evolution, only natural selection leads to adaptation. Natural selection: a closer look It s not always survival of the fittest Reproductive success is generally more subtle For example, some plants attract more pollinators Mechanisms of evolution #3 Genetic drift Natural Selection More successful in reproducing Certain moths survive and produce more offspring because they are better camouflaged Genetic drift a change in the gene pool of a small population due to chance Example: small wildflower population Generation 1: only 5 plants produce fertile offspring Generation 2: only 2 plants leave offspring Generation 3: only plants with red flowers remain Two forms of the peppered moth Genetic drift: the bottleneck effect Disasters (floods, fires, etc) kill large numbers of individuals Results in a drastic reduction in population size. Reduces genetic variation Due to chance The white allele has been lost from the population Genetic Drift and Hereditary Disorders in Human Populations The Founder Effect When a few individuals colonize a new place Gene pool differs from the parent population Explains the relatively high frequency of inherited disorders in human populations established by small numbers of colonists Residents of an isolated island in the Atlantic Ocean have a high rate of hereditary blindness 5

6 Mechanisms of evolution: #4 Gene flow Gene Flow Movement of individuals between populations migration Introduces new genes Here the genes for brown beetles become more frequent in the green beetle population Gene flow genetic exchange with another population Tends to reduce genetic differences between populations. The migration of people throughout the world increases gene flow and introduces new alleles An important agent of evolutionary change Computer-generated image blending features from several races Evolution by natural selection 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 Evolution DVD Vol 3 Why Sex? The peacock s tail (5 min) 6