Chapter 17: Evolution of Populations

1. Chapter 17:Evolution of Populations By: Tyler Kong Skyler Johnson

2. 17.1 Genes and Variations • Alleles: specific forms of a gene contributed by both parents, often varies from individual to individual • Genotype: is the particular combination of alleles it carries • Phenotype: includes the physical, physiological, and behavioral characteristics of an organism, such as eye color or height

3. In the environment, some Genotypes of Phenotypes are better suited for environment • Gene Pools: all genes, including all different alleles for each gene, that are present in a population, they are studied in populations • Allele Frequency: the number of times an allele occurs in a gene pool, compared to the total number of alleles in that pool for the same gene *Evolution, in genetic terms, involves a change in frequency of alleles in a population over time.

4. Sources of Genetic Variation • Mutations: any change in genetic material • Ex. different colored corn kernel • Genetic Recombination in Sexual Production • Crossing-over further increases the number of new genotypes created in each generation. • Lateral Gene Transfer • This can occur between organisms of the same species or organisms of different species.

5. Single Gene Traits and Polygenic Traits • Single Gene Traits: traits controlled by only one gene • Ex. Shell Banding- either have no bands or bands • Polygenic Traits: traits controlled by two or more genes • Ex. height in humans, varies from very short to very tall • Phenotypic Determination: a bell shaped curve determining the average of individuals genes

6. 17.2 Evolution as Genetic Change in Population Natural Selection Works on: • Single-Gene Traits: can lead to changes in allele frequencies and phenotype frequencies • Polygenic Traits: can effect the relative fitness of phenotypes and thereby produce one of the three types of selection Three types of selection- Directional Stabilizing Disruptive

7. Directional Selection- individuals at one end of curve have higher fitness than middle or at other end Stabilizing Selection- individuals near center have higher fitness than individuals at either end Disruptive Selection- individuals at outer ends of curve have higher fitness than individuals in middle of curve

8. Genetic Drift *In small populations, individuals that carry a particular allele may leave more descendants than other individuals leave, just by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population. Bottleneck effect- is a change in allele frequency following a dramatic reduction in the size of a population Founder effect- change in allele frequency due to migration of a small subgroup of a population

9. 17.3 THE PROCESS OF SPECIATION • Isolating Mechanisms • Speciation-the formation of a new species • Interbreeding links a population together genetically • Some members of a population stop breeding with other members • The gene pool splits and genetic changes no longer pass through an entire population • Reproductive isolation occurs because they no longer interbreed

10. Behavioral Isolation • -capable of interbreeding • -differences in mating rituals • -leads to differences in species • Temporal Isolation • -happens when two or more species reproduce at the same time • -No interbreeding occurs

11. Geographic Isolation • Geographic isolation- occurs when two populations are separated by geographic barriers such as rivers and mountains • -Barriers do not always insure isolation • -Birds are not isolated by rivers and hills but squirrels and rodents cannot cross over these barriers • -If populations still interbreed then they remain a single species

12. Speciation in Darwin’s Finches • -Different species of animals develop distinct characteristics to help them adapt and thrive in their environment • Founders Arrive • -A new species of finches appears, possibly lost or blown off course • -They live and reproduce • -The allele frequencies of the new differ from the original population of finches already there

13. Speciation in Darwin’s Finches (cont.) • Geographic Isolation • -This new species of finches, once having reproduces for a while, does not usually cross open water. • -By chance some of them do and because they do not usually cross open water, they develop differently and no longer share a common gene pool • Changes in Gene Pools • -Populations on each island adapt to its environment • -Natural selection caused the ones who survived to reproduce because they were best suited to their environment

14. Speciation in Darwin’s Finches (cont.) • Behavioral and Reproductive Isolation • -Even if some birds crossed back over there would be differences in mating behavior • -Even if they lived in the same place again, they would not become one species • Competition and Continued Evolution • -These two types compete for seeds • -If one is more adapted than the other, it will reproduce and the other will eventually die out

15. 17.3 Molecular Evolution • Timing Lineage Splits: Molecular Clocks • Molecular Clock-comparing stretches of DNA to mark the passage of evolutionary time • -Molecular clocks rely on mutations as markers of evolution • -Many mutations have no effect on phenotype, when DNA is compared this can show how many mutations have occurred independently in a species • -The more differences, the longer it’s been since they shared a common ancestor

16. Calibrating the Clock • -There are many different clocks that “tick” or mutate at different rates • -Scientists can check the accuracy of these clocks by estimating how often mutations take place • -They compare the number of gene mutations in a particular gene in a species

17. Gene Duplication • Copying Genes • -Organisms carry multiple copies of a gene • -Remember that homologous chromosomes exchange DNA during meiosis • -Sometimes this crossing over involves unequal swapping of DNA • -These DNA parts are then duplicated

18. Developmental Genes and Body Plans • Hox Genes and Evolution • -Hox genes determine which parts of an embryo develop • -Humans and insects share the same homologous Hox genes • Timing is everything • -Each part of an embryo starts and stops growing at a different time • -Even small changes in this timing can make a big difference in an organism