Evolution and Natural Selection

Evolution vs. Natural Selection Evolution- Process by which modern organisms have descended from ancient organisms Natural Selection- Process where organisms better adapted to their environment tend to survive and produce more offspring

Evolution - Early Ideas of Jean-Baptiste Lamarck Jean-Baptiste Lamarck developed the first attempt at a theory of evolution in the late 1700s (Theory of Transformation) Believed that organisms could change during their lifetimes by selectively using or not using various parts of their bodies Example: believed that if a giraffe stretches its neck reaching for leaves, its neck will grow longer and pass that feature down to future generations Flaws in theory Did not hold up when tested with experiments Couldn’t explain how certain features developed Important ideas which resulted from his theory Evolution takes place by small gradual steps Proposed that simple organisms could develop into more complex organisms over many generations

Natural Selection- Observations and Ideas of Charles Darwin Charles Darwin proposed that all species of life have descended over time from common ancestors Many of his ideas came from observations made during a 5 year trip around the world on the HMS Beagle Compared organisms found on different Galapagos Islands to organisms that lived elsewhere Noticed similarities and differences of among the species he studied Realized that island species had become different from their mainland relatives over time

Darwin’s Research Darwin collected birds (finches) from several of the Galapagos Islands Noticed that the birds were different from one island to the next Concluded that the finch species were all related to a common ancestor species that came from the mainland Overtime, different finches developed different beak shapes and sizes suited to the food they ate The beak shape is an example of adaption (an inherited behavior or physical characteristic that helps an organism survive and reproduce in its environment) Based on his observations, Darwin hypothesized that species change over many generations and become better adapted to new conditions

Darwin’s Search to Determine Evolution’s Mechanism for Change Mechanism- The natural process by which something takes place Artificial Selection- Process by which humans use animal and plant breeding to selectively develop particular traits by choosing which animas or plants reproduce and have offspring Natural Selection- Process that results in the adaption of an organism to its environment by means of selectively reproducing changes in it genotype Organisms better adapted to their environment tend to survive and produce more offspring Organisms that are not well adapted to their environment may die without reproducing Darwin published his theory on natural selection in his book “The Origin of Species”

How Natural Selection Works Darwin identified three factors that affect the process of natural selection: Overproduction, variation, and competition Overproduction- When species reproduce more offspring than can survive due to the available resources Variation- Members of a population differ from one another in many of their traits Such variations are hereditary, passed from parents to offspring through genetic material Competition- The struggle among living things to get the necessary amount of food, water, and shelter In many species, so many offspring are produced that there are not enough resources for all of them- this is why natural selection is often referred to as “the survival of the fittest”

Environmental Changes and Natural Selection A change in the environment can affect an organism’s ability to survive and may lead to natural selection Darwin proposed that, over a long time, natural selection could lead to change, where helpful variations accumulate in a population and unfavorable ones disappear Examples: Plants that are drought resistant have replaced those that are not in areas that have little precipitation Trees that are better able to withstand strong winds (shorter and have thick trunks) out number others in areas with frequent wind storms Dark colored moths, rather than lighter ones, survived in an area in the UK where soot darkened the trees

Genes and Natural Selection Only traits controlled by genes can be acted upon by natural selection Sexual reproduction causes existing gene variations to be recombined in each member of a population Genetic variations contribute to the diversity of organisms To get a new variation, there must be a gene mutation Only mutations to reproductive cells can be passed on to an offspring If offspring are born with a mutation, natural selection will determine whether that mutation gets passed onto the next generation Epigenetic changes- small changes to DNA that turn genes on or off but do not change the genetic code itself Small chemical changes to DNA affect how genes get expressed Offspring can inherit these changes from their parents or grandparents

Mutations and Evolution Over time, as individual organisms successfully respond to changing conditions in the environment, the population evolves and its fitness (how well it can survive and reproduce in its environment) increases Mutations are necessary for evolution to occur Not all mutations are harmful Mutations that increase fitness tend to grow more common in a population Mutations that decrease fitness tend to disappear as those individuals die or reproduce less successfully Mutations create variations among members of a species and account for the diversity of organisms on Earth

How Gene Flow and Genetic Drift Affect Evolution Gene Flow- Occurs when individuals with new alleles physically move from one population to another Introduces new alleles into a population Can increase the genetic variation of a population Genetic Drift- A random, directionless mechanism of evolution where some alleles are lost to a population As an example, if the only organism carrying a specific allele dies, that genetic information would be lost to future generations of the population Through random chance, the genetic variation of the population shrinks

Sexual Selection and Evolution Sexual Selection- A type of natural selection that acts on an organism’s ability to get the best possible mate Sexually reproducing organisms try to choose mates with specific traits that have higher fitness (will result in offspring with greater fitness) Female choice- In species where females choose their mates, natural selection favors traits that help females choose mates with higher fitness Example: If male ducks with brighter feathers have higher fitness, females will evolve a trait that causes them to choose males with bright feathers and males will evolve to have even brighter and fancier feathers Male competition- In species where males compete to control a territory with access to females, natural selection will favor traits that give males an advantage in competition This may cause males to evolve exaggerated characteristics like extra-large horns, pinchers, or bodies Sometimes the evolutionary adaptions end up having a negative impact on the population Even after the bright feathers start to hurt the males survival, by make them more of a target for predators, the females may continue to select the males with bright feathers The need to reproduce may cause males to evolve exaggerated characteristics that make them less likely to survive

Coevolution Coevolution- the process by which two species evolve in response to changes in each other over time Occurs when species with close interactions affect each others evolution Can happen when two species cooperate with each other (ex: flowering plants and pollinators like birds or bees) Can occur when species compete for resources and in species involved in prey-predator or host-parasite relationships

Evidence for Evolution Fossil record- (all the fossils that have been discovered) provides evidence about how organisms of the past have evolved into the forms seen today Comparisons of anatomy- Similarities in the structure of the bodies of different organisms provide clues that organisms evolved from a common ancestor Example: Similarities inn the wing of a pterosaur, the wing of a bat, the flipper of a dolphin, and a human hand are evidence of common ancestry Genetic evidence- Every living thing uses DNA for its genetic material This shared use of DNA is evidence that every organism on Earth has a common ancestor This common ancestor , called LUCA (last common ancestor), was most likely a single celled organism similar to modern bacteria Branching tree- A diagram that shows how groups of organisms may be related A species is more closely related to a species on the same branch than to one on another branch