CHAPTER 16 Evolution of Populations

WHAT IS A POPULATION? POPULATION – GROUP OF INDIVIDUALS OF SAME SPECIES IN THE SAME AREA THAT INTERBREED.

Gene Pool Combined genetic info. of all members Allele frequency is # of times alleles occur

Relative Frequency Relative Frequency: the number of times that the allele occurs in a gene pool compared to the other alleles. Relative Frequencies from example:  40% Dominant allele (B)  60% Recessive allele (b) Given enough time, would the allele frequency change?  Under what conditions? What would happen to the relative frequency of B if b decreased?

Variations: Differences in traits Causes  Mutations in genes  Gene Shuffling: Mixing of genes through sexual reproduction Passed on to future generations

Single-Gene vs. Polygenic Traits Single-Gene: 2 Distinct Phenotypes Polygenic: Many Phenotypes

Hardy-Weinberg Udny Yule (1902): Dominant gene will take over the population G.H. Hardy: Mathematician (1908)  Punnet brought the problem to Hardy  Proved that the gene frequencies would remain constant as long as certain conditions were met Wilhelm Weinberg (1908)  Came up with the same ideas separately

5 conditions of Hardy-Weinberg Equilibrium The original proportions of genotypes in a population remains constant IF: 1. Large Population 2. No Migration 3. No Mutation 4. Random Mating 5. No Natural Selection

5 ways a population can evolve When any of Hardy-Weinberg’s conditions are broken, the allele frequencies will change and therefore the population will evolve.  Large Population- Genetic Drift  No Migration- Gene Flow  No Mutation- Mutation  Random Mating- Sexual Selection  No Natural Selection- Natural Selection

1. Genetic Drift Random change in allele frequencies Greater effect in small populations

Founder’s Effect A type of Genetic Drift Caused by colonization of a limited number of individuals from a parent population Allele ratios are different than the parent population

Example: Nene

Bottle-Neck Effect A type of Genetic Drift Caused by mass death (normally because of a natural disaster)

Examples: Northern Elephant Seal, Cheetahs

2. Gene Flow Genetic exchange due to the migration of fertile individuals or gametes between populations Results of Gene Flow Reduces differences between populations Changes allele frequencies within a population Adds new alleles to a population

3. Mutation Change in the DNA  Normally caused by a mistake during Meiosis  Can create new alleles in a population

4. Sexual Selection Sexual Selection: Occurs when certain traits increase mating success. This is due to the preferential choice of a mate based on the presence of a specific trait  Occurs due to higher cost of reproduction for females.  males produce many sperm continuously  females are more limited in potential offspring each cycle

4. Sexual Selection There are two types of sexual selection.  Intersextual Selection- Female Choice  Example: Peacocks  Intrasextual Selection- Male/Male Competition  Example: Big Horn Sheep

While watching each video, determine if each is intrasexual or intersexual selection Birds of Paradise Neck Fighting Spiders – skip to 2:30 Big Horn Sheep

5. Natural Selection Certain traits are favored and therefore those alleles increase in the population Traits will be favored if they do any of the following: Increase ability to find or gather food Decrease predation Decrease susceptibility to disease Increase ability to reproduce Rabbit simulation

Natural Selection on Single Gene Traits Natural selection can favor one phenotype above another and thus lead to changes in the allele frequencies.

3 Types of Selection on Polygenic Traits 1. Stabilizing Selection  Shifts to middle range 2. Directional Selection  Shifts to 1 extreme 3. Disruptive Selection  Shifts to 2 extremes

Stabilizing Selection Stabilizing Selection: a type of natural selection in which the average form of a trait causes an organism to have an advantage in survival and reproduction  The extreme forms of the trait are disadvantageous  Most effective in a population that has become well adapted to its environment.  Most common type of NS.  Example: human infant size

Directional Selection Directional Selection: a type of natural selection in which the distribution of a trait is shifted toward one of the extremes Example: Anteaters

Disruptive Selection Disruptive Selection: natural selection in which individuals with either of the extreme forms of a trait have an advantage in terms of survival and reproduction  The average form of the trait confers a selective disadvantage to the organism.

Check for Understanding What are two mechanisms of evolution that cause a decrease in genetic variation? What are two mechanisms of evolution that can cause an increase in genetic variation? What are the three types of selection? Describe each.

SECTION: 3 Speciation

What is a Species Species: A group of individuals that breed with one another and produce fertile offspring.

Hybrids Hybrids are formed when two species are mated together Offspring are normally infertile Donkey + Horse= Mule (infertile)

Result of male lion and female tiger mating in captivity. Offspring are infertile. Liger

Speciation Speciation: The formation of new species. Reproductive Isolation: when members of two populations cannot interbreed and produce offspring.

Isolating Mechanisms Behavioral Isolation: species can interbreed but due to differences in behavior they do not.  Example: courtship rituals (meadowlarks have different mating songs). Geographic Isolation: two populations are separated by geographic barriers.  Example: mountains, rivers or bodies of water (squirrels in Arizona separated by the Colorado River). Temporal Isolation: two populations reproduce at different times.  Example: release of pollen at different times (orchids).

Behavioural Isolation Four species of leopard frogs: differ in their mating calls. Hybrids are inviable. Bird Dances

Geographic Isolation Separation of organisms by geographic features  Mountains  Lakes, oceans, rivers  Deserts  (May result in new  species over time)

Geographic Isolation

Temporal Isolation Rana boylii - breeds late March - May Rana aurora - breeds January - March

Speciation of Darwin’s Finches Founders Arrive: finches arrived from South America. Geographic Isolation: birds on different islands were isolated because they generally do not fly across water. Gene Pool Changes: the gene pool on each island changed over time adapting to conditions on the separate islands.

Speciation of Darwin’s Finches Reproductive Isolation: behavioral differences based on beak size and courtship rituals would keep different birds from reproducing. Ecological Competition: competition for food causes those with the biggest differences to survive and others to die off. Continued Evolution: as conditions change the finches continue to change in response to the environmental conditions.