Darwin and His Theory of Evolution by Natural Selection Evolution- changes in the characteristics of groups of organisms over time Charles Darwin was the first to propose a feasible mechanism for evolution. It is called natural selection. *

Evolution by Natural Selection is a theory. What is a scientific theory? Scientific Theory- an idea that is strongly supported by evidence. It is generally accepted and used to explain many observations. *Remember: The word “theory” in everyday language and in scientific language mean very different things. In everyday language, people use the word theory to mean a hypothesis or an educated guess.

Natural Selection- differential success in survival and reproduction of individuals with different phenotypes (physical characteristics) resulting from interactions with their environments

Conditions Needed for Natural Selection: VISTA Variation There are a variety of forms of traits or phenotypes. *

Conditions Needed for Natural Selection: VISTA 2. Inheritance Only traits that are determined by an organism’s genes will be passed to the next generation. Some variation is caused by the environment, such as the loss of a limb or the building of muscle. These traits are not heritable, so they can’t be selected for in the next generation. *

What causes genetic variation? 1) Mutations- changes in the DNA sequence can result in new traits  2) Sexual Reproduction- New combinations of traits can be created in gametes during meiosis. The wide variety of gametes created will join together to make diverse offspring.  

Remember natural selection can only act on the heritable variation that EXISTS in a population. *

Conditions Needed for Natural Selection: VISTA 4. Survival and Reproduction Individuals compete for limited resources, such as food, shelter, and mates. Individuals with traits best adapted for the environment are able to out compete other individuals for resources, allowing them to survive and reproduce more. *

“Survival of the Fittest” Evolutionary fitness isn’t a measure of physical fitness but of reproductive fitness. Fitness- Relative ability to survive and produce offspring in an environment *

Conditions Needed for Natural Selection: VISTA 4) Time Evolution occurs over generations.

Conditions Needed for Natural Selection: VISTA 5) Adaptations- an inherited characteristic that increases an organism’s chance of surviving and reproducing Over time, natural selections causes adaptations to become more common in a population. *

Drug Resistant Bacteria Practice Variation existed in a population of bacteria. Some were resistant to antibiotics, while some weren't. The resistance was an inherited trait caused by a gene. When treated with antibiotic, the resistant bacteria survived and reproduced more than the nonresistant ones, which were killed by the antibiotic. Over time, the adaptation of drug resistance became more common in each generation of bacteria.

Natural selection can lead to a variety of changes within a population Natural selection can lead to a variety of changes within a population. These changes can be roughly grouped into three categories: Stabilizing Selection: natural selection that favors an intermediate phenotype Example: Birth weight, Human height

Natural selection can lead to a variety of changes within a population Natural selection can lead to a variety of changes within a population. These changes can be roughly grouped into three categories: 2. Directional Selection: natural selection that favors an extreme phenotype and causes a shift in the distribution curve Example: Birth weight, Human height

Natural selection can lead to a variety of changes within a population Natural selection can lead to a variety of changes within a population. These changes can be roughly grouped into three categories: 3. Disruptive Selection: natural selection that favors two or more extreme phenotypes are favored over any intermediate phenotype

Since Darwin’s time, several other forces of selection have been explained: 1. Sexual Selection: adaptive changes in males and females that lead to an increased ability to secure a mate.

Since Darwin’s time, several other forces of selection have been explained: 2. Artificial Selection: adaptive changes in organisms that occur because of human forces.