Darwin and Natural Selection

Evolution is a Theory – Just like Gravity! Evolution is a well supported explanation of phenomena that have occurred in the natural world A theory in science is a well tested hypothesis, not just a guess

Charles Darwin (1809-1882) Sailed around the world 1831-1836 

What did Darwin’s travels reveal  What did Darwin’s travels reveal The diversity of living species was far greater than anyone had previously known!! These observations led him to develop the theory of evolution!!

Diversity on the Galapagos  Diversity on the Galapagos Each island had its own type of tortoises and birds that were clearly different from other islands

Galapagos Turtles

Influences on Darwin’s Theories  Influences on Darwin’s Theories Religion stated that the earth was around 6000 years old Lyell argued that the earth is many millions of years layers of rock take time to form processes such as volcanoes and earthquakes shaped the earth and still occur today

Influences on Darwin’s Theories  Influences on Darwin’s Theories Jean-Baptiste Lamarck: Theory of acquired characteristics Organisms acquired traits by using their bodies in new ways These new characteristics were passed to offspring Lamarck was wrong!

Influences on Darwin’s Theories  Influences on Darwin’s Theories Thomas Malthus If the human population continued to grow unchecked, sooner or later there would be insufficient living space and food for everyone

Darwin’s The Origin of Species (1859)  Darwin’s The Origin of Species (1859) Other naturalists, like Alfred Wallace, were developing the same theory as Darwin’s Even though Darwin was afraid of the Church’s reaction to his book he wanted to get credit for his work.

Summary of Darwin’s Theory of Natural Selection  Summary of Darwin’s Theory of Natural Selection 1. Organisms differ; variation is inherited 2. Organisms produce more offspring than survive 3. Organisms compete for resources 4. Organisms with advantages survive to pass those advantages to their children (“survival of the fittest”) 5. Species alive today are descended with modifications from common ancestors

Summary of Darwin’s Theory POINT 1 of 5 Organisms differ; variation is inherited OBSERVED OBSERVED THEORY ? HYPOTHESIS Discuss …

Summary of Darwin’s Theory POINT 2 of 5 Organisms produce more offspring than survive OBSERVED OBSERVED THEORY ? HYPOTHESIS Discuss …

Summary of Darwin’s Theory POINT 3 of 5 Organisms compete for resources OBSERVED OBSERVED THEORY ? HYPOTHESIS Discuss …

Summary of Darwin’s Theory POINT 4 of 5 Organisms with advantages survive to pass those advantages to their children OBSERVED OBSERVED THEORY ? HYPOTHESIS Discuss …

Summary of Darwin’s Theory POINT 5 of 5 Species alive today are descended with modifications from common ancestors. THEORY, much evidence exists OBSERVED THEORY ? HYPOTHESIS

What is evolution?  changes in living organisms over time explains how modern organisms have descended from ancient organisms

Fitness and Adaptation  Fitness and Adaptation Organisms compete for limited resources and some organisms are more “fit” than others Fitness – the ability of an individual to survive and reproduce in its specific environment Adaptation – any inherited characteristic that increases an organism’s chance of survival, including reproduction

What determines survival?  What determines survival? Traits that help individuals survive survive predators survive disease compete for food compete for territory Traits that help individuals reproduce attracting a mate compete for nesting sites successfully raise young

 Natural Selection There is variation in traits. For example, some beetles are green and some are brown. There is differential reproduction. Since the environment can’t support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.

 Natural Selection There is heredity. The surviving brown beetles have brown baby beetles because this trait has a genetic basis. End result: Natural Selection The more advantageous trait, brown coloration, which allows the beetle to have more offspring, becomes more common in the population. If this process continues, eventually, all individuals in the population will be brown.

 Natural Selection Over time, natural selection results in changes in the inherited characteristics of a population. These changes increase a species’ fitness in its environment.

Sources of Genetic Variation (page 394)  Sources of Genetic Variation (page 394) Gene flow – movement of genes from one population to another (migration) Mutation – any change in a sequence of DNA Some mutations can affect an organism’s fitness while others have no effect on fitness. Gene Shuffling Independent assortment Crossing over Sexual reproduction

Evolution as Genetic Change (page 397) Natural selection determines which alleles are passed from one generation to the next. As a result, it can change the relative frequencies of alleles in a population over time. Evolution is any change in the relative frequencies of alleles in a population’s gene pool. Evolution acts on populations, not on individuals.

Genetic Drift (page 400) Natural selection is not the only source of evolutionary change Genetic drift – random change in allele frequency In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population.

Founder Effect (page 400) Occurs when a small group of individuals colonizes a new habitat. Members of the colony may carry alleles in different relative frequencies than the larger population from which they came. If so, the population that they found will be genetically different from the parent population.

Genetic Equilibrium (pages 401-402) What happens when no change takes place? Genetic equilibrium – evolution does not occur because the allele frequencies remain constant Five conditions to maintain genetic equilibrium Random mating Large population No movement into or out of the population No mutations No natural selection