What does ‘Mechanism of Evolution’ mean? Something that causes change in the allelic frequencies of a population’s gene pool….therefore, driving evolution. Without looking…do you remember the three largest mechanisms of evolution? Natural Selection Genetic Drift Gene Flow

Charles Darwin ( ) An English scientist It took him years of observation and study to develop his theory of natural selection. He began in 1831 at age 22 when he took a job as a naturalist on the English ship HMS Beagle, which sailed around the world on a five-year scientific journey.

Section 15.1 Summary – pages

Darwin studied and collected biological and fossil specimens at every port along the route. He became curious about possible relationships among species because of similarities he saw.

Section 15.1 Summary – pages One of the things Darwin observed is that in nature, the traits of individuals vary in populations. Give some examples of variations in populations of one species. Darwin hypothesized that there was a force in nature that determined which traits/variations are better for the survival in a population.

Natural Selection When organisms with the most beneficial traits in a population are the ones that survive and reproduce, and pass those traits on…

Essence of Darwin’s Ideas Darwin’s argument for natural selection was based on two main observations and two inferences from those observations….. 1. In any population, individuals have variations.

Occurrences that contribute to Genetic Variation Gene mutations can create new allelesGene mutations can create new alleles Crossing over or genetic recombination are normal results of meiosis (sex cell creation).Crossing over or genetic recombination are normal results of meiosis (sex cell creation). Independent assortment of chromosomes occurs in meiosis (when sex cells are created).Independent assortment of chromosomes occurs in meiosis (when sex cells are created). Fertilization between genetically varied gametes produces “new” combinations of genesFertilization between genetically varied gametes produces “new” combinations of genes Abnormal changes in chromosome structure or number can occur.Abnormal changes in chromosome structure or number can occur.

MutationsMutations – changes in DNA that can alter gene expression. Mutations are random and the phenotypic outcome may be neutral, beneficial, harmful, or lethal.

11.3 Section Summary 6.3 – pages Mutations HAVE to be on sex cells (gametes) in order to be passed on to offspring. Some mutations are inherited from parents If a sperm or egg cell has a mutation and is fertilized, the altered gene would become part of the genetic makeup of the offspring. Every cell in that organism would contain the mutation.

The Effect of Mutations Lethal MutationLethal Mutation an expression of a gene that results in death Neutral MutationNeutral Mutation have no effect on survival and reproduction regardless of whether or not they are expressed in a phenotype Beneficial MutationsBeneficial Mutations provide some survival advantage

Mutation: Remix Mutations are the ONLY source of NEW alleles. This is the genetic foundation for biological diversity.

Crossing Over When sex cells are being made (meiosis), there is a recombination between the paired chromosomes inherited from each of one's parents.

Crossing-Over This causes the traits found in each gamete to be varied. This is why siblings from the same parents can look so different from each other.

FROM MOM FROM DAD This chromosome inherited by mom could code for straight hair, brown hair, and green eyes… This chromosome inherited by dad could code for curly hair, black hair, and brown eyes… After crossing-over Now one chromosome passed on to offspring could have traits from both homologous… curly hair, brown hair, and green eyes…

Section 10.1 Summary – pages Genes for different traits—(for example, eye shape and eye color)— are inherited independently of each other. You don’t inherit traits in pairs. The Law of Independent Assortment Bb bb rrRR Bb Rr

Section 10.2 Summary – pages Fertilization Zygote When sex cells unite, the new zygote receives a unique combination of genes, half from mom…half from dad.

Chromosomal Abnormalities Sometimes organisms inherit the wrong # of chromosomes or damaged chromosomes

2. Many more offspring are born than can possibly survive to maturity.

Section 15.1 Summary – pages The individuals whose traits give them a higher probability of surviving….tend to leave more offspring than others, contributing more to the gene pool… The individuals with those beneficial traits will be the ones that survive, reproduce, and pass those favorable variations on to the next generation. Individuals without those favorable traits are less likely to survive and reproduce.

Section 15.1 Summary – pages After generations, organisms with the favorable variation make up most of the population. -The allele for the unfavorable traits may eventually cease to exist.

Why do you think more and more bad bacteria (like the kind that cause sinus infections) are becoming resistant to anti-biotics?

How do you think natural selection will affect this species of moth? (both the same species, just have a body color variation)

Section 15.2 Summary– pages Stabilizing selection is a natural selection that favors average individuals in a population. There are three different ways natural selection can alter allelic frequencies in a gene pool… Middle sized Siberian Huskies are selected for Evolution will not occur for this allele.

Stabilizing Selection Example: human birth weight. Babies of low weight lose heat more quickly and get ill from infectious disease more easily, whereas babies of large body weight are more difficult to deliver and tend to have issues with blood sugar.

Section 15.2 Summary– pages Directional selection occurs when natural selection favors one of the extreme variations of a trait. This type of selection can lead to rapid evolution of a population.

Examples: Directional Selection Peppered Moths: as the environment changes, so do the traits that are fit for the new environment. In the case of the moths, the forests changed from light to dark and selection moved in the direction of darker moths Antibiotic Resistance Pesticide Resistance

Disruptive Selection Extreme phenotypes are favored Intermediate forms are selected against

Section 15.2 Summary– pages In disruptive selection, individuals with either extreme of a trait’s variation are selected for. This results in eventually having no intermediate form of a trait, and leading to two separate species.

Disruptive Selection Hunting and Rabbit Populations

Sexual selection Why a theory of sexual selection? Darwin needed a theory to explain the many extravagant traits that CAN reduce survival e.g. the peacock’s tail What is sexual selection? Selection based on attraction. This could be based on physical appearance or mating rituals.

Sexual selection Is sexual selection different from natural selection? Darwin saw them as distinct - only sexual selection could produce traits that could possibly compromise survival The basic principles are identical – selection favors whatever gets more genes into the next generation