The Theory of Evolution by Natural Selection

Objectives 3.4.1- Explain how fossil, biochemical, and anatomical evidence support the theory of evolution. 3.4.2- Explain how natural selection influences the changes in species over time. 3.4.3- Explain how various disease agents (bacteria, viruses, chemicals) can influence natural selection.

The Theory of Evolution by Natural Selection Changes in species occur over time, because organisms with favorable variations in traits survive, reproduce, and pass those variations on to the next generation. The basis of the theory is that all organisms share a common ancestor. Life’s history is not linear, but looks more like a tree.

Studying Life’s History Fossil evidence is clear, tangible evidence that the types of living organisms on Earth have changed over the course of time. Fossils reveal organisms that no longer exist, fossils that are similar to present day organisms, and fossils that look the same as some present day organisms. New fossils are constantly being found. Relative and absolute dating techniques are used to create a timeline of the appearance of organisms on Earth. Generically speaking organisms evolved simplest to most complex and a variety of life forms all exist together today. The first organisms to exist on Earth were anaerobic prokaryotes.

Studying Evolution in Real Time Unicellular organisms such as bacteria multiply so fast that in just a few years 1000s of generations can be studied. Bacteria evolve to tolerate diverse environments and feed off of unlikely food sources. Some have a hard time understanding evolution because with humans and other organisms with long life-spans changes are not necessarily noticeable after a generation or two.

Changes and Variations Mutations in DNA coding generate genetic variation. Natural selection increases the number of beneficial mutations in organisms. Species fit or suited for their environments do not experience as much evolution as species that are not. New species can form over time.

Factors Affecting Evolution Long-term and short-term changes in geography. Catastrophic events Human influence Geography changes include islands, mountains, valleys, lakes, oceans, rivers, etc. Catastrophic events include wildfires, volcanoes, asteroids etc. Human influence include urbanization, pollution, climate change, introduction of species into environments where they are not native, genetic engineering, selective breeding, hunting, etc.

Common Ancestry Shared anatomical structures may reflect common ancestry although some cases of shared anatomy reflect similarities in habitat or life strategies. Vestigial structures are structures that are smaller, underdeveloped, or have no purpose in an organism that did have a purpose in ancestors. Examples of shared anatomy: The forelimbs of mammals including humans, dogs, cats and even whales and bats are similar reflecting a common ancestor. Wings in birds and bats do not reflect a common ancestor. They have similarities because they have both evolved to fly over time. Examples of vestigial structures: Dew claw in dogs, wings on flightless birds, appendix in humans

The Endosymbiosis Theory Eukaryotes evolved through a symbiotic relationship between ancient prokaryotes. The primary evidence supporting this theory is that mitochondria and chloroplasts have their own DNA which is similar to the DNA found in prokaryotes.

Biochemical Similarities The genetic code is universal amongst all living organisms. Different species share sequences of DNA in common- some coding for functional proteins and some just there. The more similar two organisms are in their biochemistry the more closely they are related on the evolutionary tree of life.

Examples of Evolution 1. Darwin’s Finches There are 13 types of finches found in the Galapagos Islands. The beak shape is one of the major differences which reflect the types of food found on the islands. As environmental conditions change such as amount of rainfall, the number of finches with certain beak shapes change accordingly. Birds with larger beak shapes have been noted to be in higher numbers in times of extreme drought. A large beak allows birds to crush large seeds that are not a typical part of a finch’s diet.

2. Rat Snakes The United States is covered with rat snakes of various colors. The colors tend to reflect the local environments that the snakes are found. This is an example of camouflage as an adaptation for survival.

3. Three-toed Skinks In Australia, skinks of the same species are giving birth to young in two different ways depending on their location. Skinks near the coast are laying eggs while skinks in mountainous regions are giving live birth. Perhaps in time these two reproductive strategies will result in enough change that one will form a new species (speciation).

4. Lactose Tolerance in Humans Some humans can digest lactose found in milk on into adulthood thanks to a mutation that occurred when cattle became domesticated in Europe and Africa centuries ago. This was an advantage for obtaining nutrition and water during times of drought.

5. HIV The virus HIV which can cause AIDS is weakening in human populations that it has been common in for long periods of time. Although currently deadly, HIV may weaken to a point that it is not deadly. Strong immune systems and antiretroviral therapy are causing mutations in the virus so that it becomes milder.

6. Chemical Resistance Numerous pests that humans have killed with chemicals are developing a resistance or immunity to the chemical after just a few generations. The ones that survive are naturally immune and create offspring who are also naturally immune. New chemicals are constantly being engineered to target resistant strains of pests. In addition, scientists are genetically modifying crops so that they are pest resistant.

7. Antibiotic Resistance Overuse and misuse of antibiotics is causing many strains of bacteria to become resistant or immune to antibiotics which are drugs used to kill bacteria. Strains of MRSA, TB, and gonorrhea have been found to be resistant to antibiotics.