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THE THEORY OF EVOLUTION Natural Selection & Speciation Fossils shape ideas about evolution Scientists used fossils as a basis for evolutionary concepts When geologists provided fossil evidence of the age of Earth, biologists began to suspect that life changes slowly over time Evolution = the change in populations over time Charles Darwin English Scientist Lived from 1809 – 1882 Took a job as a naturalist on the HMS Beagle (1831) Sailed to South America & the South Pacific on a 5 yr journey Darwin studied & collected biological specimens every time the ship stopped He became curious about the possible relationships between species. The Galapagos Islands A group of small islands ~ 1000 miles off the coast of S. America Near the equator Darwin studied species unique to the islands Similar to species elsewhere Could not explain how such changes would occur Figuring out why Thomas Malthus – English economist Ideas about human population growth Said that the human population grows faster than Earth’s food supply Darwin related this to his studies: Many species produce more offspring than can survive They compete for resources like food, space, predation, mating, and shelter Only some individuals survive Two Kinds of Selection Natural Selection a mechanism for change in populations – “survival of the fittest” Organisms with certain variations survive & reproduce to pass on their variations to offspring Organisms without beneficial variations are less likely to survive and reproduce Result = each generation inherits beneficial variations and others die out Artificial Selection Organisms are purposely bred for desirable traits Examples: Horses, dog breeds Types of Natural Selection Stabilizing Selection = favors average individuals Directional Selection = one extreme is favored Spiders: Large individuals may be easier prey, small individuals may not be able to get enough food Woodpeckers: short or average beak cannot reach deep enough to get food, long beaks can feed on insects deeper within the tree trunk Disruptive Selection = either extreme is favored Dark brown & white marine limpets: white limpets have an advantage on light rocks, brown limpets have an advantage on darker/wet rocks. Tan limpets are easily seen and preyed upon by birds - tan limpets have the least favored adaptation. Alfred Russell Wallace Reached a similar conclusion Darwin & Wallace presented their ideas to the scientific community Darwin published On the Origin of Species in 1859 There have been many advances since Darwin: Any change in the gene pool of a population is now considered part of evolution Adaptations Structural Adaptations: Adaptations = any structure, behavior, or internal process that makes an organism more likely to survive develop over generations Examples: Giraffe height, mole-rats Physiological Adaptations Develop more quickly than structural adaptations Examples: antibiotics (penicillin) & pesticides Other Adaptations Mimicry = one species resembles another “warning” coloration = red, orange, and yellow Camouflage = organisms are able to blend into surroundings Insects Other Evidence of Evolution 1. Fossils – provide a record of early life and evolutionary history 2. Embryology Embryos at early stages of development are very similar in appearance 3. Biochemistry Comparisons of DNA or RNA of different species Other Evidence of Evolution 4. Anatomy Homologous structures – similar structures with different functions Example: animal forelimbs have the same bone structure even though they have different functions Analogous structures – similar functions with different structure Example: wings of a butterfly & wings of a bird have the same function but different structure 15.2 Population Genetics Darwin developed his theory without an understanding of genes & inheritance Population genetics = studies of the behavior of genes in populations of plants or animals Natural selection acts on a range of phenotypes in a population If a variation of a phenotype is beneficial, it will be more common Evolution of Populations All of the alleles of a population’s genes = its’ gene pool Genetic Equilibrium Genetic equilibrium = the frequency of alleles remains the same over generations Populations in genetic equilibrium are not “evolving” Disruptions to genetic equilibrium: Mutations Genetic drift (chance alterations to allele frequency) Movement of individuals in or out of the population Size of population – can cause recessive alleles to become more common Evolution of a species Speciation = evolution of a new species Occurs when members of similar populations no longer interbreed to produce fertile offspring Causes of speciation: Geographic isolation – physical barriers separate populations & each develops its own gene pool Reproductive isolation – formerly interbreeding organisms can no longer mate (may be caused by behavior such as breeding seasons) Chromosomal change – mistakes during cell division may result in polyploidy Speciation can occur at different rates Gradualism = species originate through gradual changes and adaptations Punctuated equilibrium = change occurs in rapid bursts with period of genetic equilibrium in between May be caused by environmental changes or competitive species Scientists agree that either method can occur, depending on the circumstances Patterns of Evolution Adaptive radiation = when a species evolves into an array of species Example: Hawaiian Honeycreepers Common on islands Divergent evolution – a diverse group of species share a common ancestor A type of adaptive radiation Convergent evolution – distantly related species develop similar traits Similar environmental
