Ch. 16- Genes and Variation

What you will learn today… How do we measure genetic variation in a population? What are the sources of genetic variation in a population? Why is genetic variation in a population important?

What determines a heritable trait? DNA (gene) Observed trait mRNA protein translation transcription protein function (enzyme activity) Therefore, if traits vary in a population, then the genes (alleles) must vary in the population!

How do we measure genetic variation in a population? Gene Pool- Total genetic information available in a population (all the alleles that are present). Allele (Relative) Frequency- The percentage of an allele in the gene pool. Tells you whether a given allele is common or rare (%)

New Vocab Population – group of individuals of the same species that interbreed Gene Pool – all genes (including all alleles) present in a population Relative Frequency – number of times an allele occurs in a gene pool Because members of a population interbreed, they share a common group of genes called a gene pool

A population with variation in traits… Grey White Tall ears Short ears

...is caused by variation in alleles Grey allele = G White allele = g Tall ear allele = T Short ear allele = t T t G G t t G g T T G g t t g g t t G g t t G G T t g g t t G g

How do we measure genetic variation in a population? Grey allele = G White allele = g Tall ear allele = T Short ear allele = t 8 / 16 = 50% G T t G G t t G g T T G g t t g g 8 / 16 = 50% g t t G g t t G G 4 / 16 = 25% T T t g g t t G g 12 / 16 = 75% t “Gene Pool”

Why is genetic variation in a population important? A gene pool without much variation limits a species’ ability to further evolve. Evolution- change over time in the gene pools of a species If populations do not change (adapt) to their environment, they may become extinct.

Sources of genetic variation SEXUAL REPRODUCTION Meiosis – one allele is passed on from each parent (recall that sperm and eggs are haploid cells, each containing half the necessary genetic information). Random fertilization – only one of the millions of sperm involved in mating will fertilize the egg. The randomness of sexual reproduction explains why siblings can look so different.

Crossing over during meiosis http://www.sciencegeek.net/Biology/review/graphics/Unit3/meiosis.jpg

Sources of genetic variation MUTATION A change in DNA sequence. New DNA sequence = new allele of a gene. Many mutations produce genes that are harmful (e.g. Huntington’s disease) Some mutations produce genes that are neutral (neither helpful nor harmful) Very, very few mutations produce genes that are advantageous, beneficial

Mutations add new alleles to the gene pool Mutations add new alleles to the gene pool. That is, they increase the variety of alleles in the population.

Deck of Cards Analogy Deck is Gene Pool – It contains all possible alleles for the next generation. Drawing cards picks the alleles that are inherited by the next generation. Shuffling of the deck is sexual reproduction. Adding new cards to the deck is mutation. (Mutation is rare, but shuffling happens each time a new generation is produced)

Natural Selection Individuals with advantageous genes survive to reproduce and pass on these genes to their offspring. Individuals without advantages genes do not survive to reproduce, and these genes do not get passed on in the population.

Kerosene Karl- Changes in Allelic Frequency

Allele Frequency example Warmup: Allele Frequency example Figure 16-2, pg. 394- work out the frequency of each allele. Sample Population Frequency of Alleles allele for brown fur, b allele for black fur, B 48% heterozygous black, Bb 16% homozygous black, BB 36% homozygous brown, bb Population = mice Gene Pool – all possible genes/alleles for coat color Relative frequency of each allele 20 alleles are black (40%), 30 alleles are brown (60%).

Sources of Genetic Variation Mutations Mistakes in replication Radiation or chemicals in environment Gene Shuffling Assortment of chromosomes Crossing over Mutation – change in DNA sequence don’t always affect phenotype some affect fitness; other’s don’t Gene Shuffling – during meiosis independent assortment of chromosomes from each parent 23 pairs of chromosomes can produce 8.4 million different combinations of genes sexual reproduction is major source of variation within population – can produce many different phenotypes but does NOT change relative frequency of alleles

Single-Gene Trait Variation in single gene traits lead to only two distinct phenotypes Frequency of phenotype determined by frequency of alleles 100 80 60 40 20 Frequency of Phenotype (%) The number of phenotypes produced for a given trait depends on how many genes control the trait Widow’s peak No widow’s peak Phenotype

Frequency of Phenotype Polygenic Trait Trait controlled by 2 or more genes Many possible genotype and phenotype possibilities Bell shaped curve typical of polygenic traits Frequency of Phenotype Phenotype (height)

CHECK POINT How do we measure genetic variation in a population? What are the sources of genetic variation in a population? Why is genetic variation important in a population?

Homework Read section 16-1 in textbook (pages 393-396) Complete worksheet 16-1: Genes and Variations

Natural Selection on Polygenic Traits Directional Selection Low mortality, high fitness High mortality, low fitness Food becomes scarce Individuals at one end of curve have higher fitness Range of phenotypes shifts

Natural Selection on Polygenic Traits Individuals near center of curve have highest fitness Keeps center of curve at same position and narrows graph Stabilizing Selection Low mortality, high fitness High mortality, low fitness Selection against both extremes keeps curve narrow and in same place Percentage of Population Birth Weight

Natural Selection on Polygenic Traits Disruptive Selection Largest and smallest seeds become more common Population splits into two subgroups specializing in different seeds. Low mortality, high fitness Number of Birds in Population Number of Birds in Population High mortality, Low fitness Beak Size Beak Size Individuals at upper and lower ends of the curve have higher fitness than individuals in the middle Selection acts strongly against individuals of the intermediate type

Genetic Drift Random change in allele frequency by chance Occurs in small populations Founder effect – allele frequencies change as a result of migration of subgroup of population A a 1st (26) .62 .38 3rd (28) .30 .70 4th (30) .37 .63 5th (32) .52 .48 6th (28) .34 .66 In small populations, an allele can become more or less common simply by chance Genetics controlled by laws of probability – works in large population not always in small population In small populations, individuals that carry a particular allele may leave more descendents than others just by chance. Over time a series of chance occurrences of this type can cause an allele to become common in a population May occur when a small group of individuals colonize new habitat

Genetic Drift Section 16-2 Sample of Original Population Descendants Founding Population A Founding Population B

Genetic Drift Section 16-2 Sample of Original Population Descendants Founding Population A Founding Population B

Genetic Drift Section 16-2 Sample of Original Population Descendants Founding Population A Founding Population B

Hardy-Weinberg Principle- Are there any conditions when evolution DOES NOT happen? States that allele frequencies remain constant (genetic equilibrium) unless one or more factors cause them to change No change in allele frequency of population = no evolution in population To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place. Are there any conditions under which evolution will not occur?

Mechanisms for Evolution Random genetic drift Gene flow Non-random mating Mutation Natural selection

5 Conditions of Genetic Equilibrium- 1 or more of these must happen in order for evolution to occur Random Mating Equal chance of passing on alleles to offspring Large Populations Genetic drift less likely to occur No Movement In or Out of Population New members might bring new alleles Random mating ensures that everyone has an equal chance of passing on their alleles. In natural populations, mating is rarely completely random- species select mates with traits they want. Nonrandom mating means genes for those selected traits are NOT in equilibrium, but are under strong selection pressure. Genes drifting “in and out” of a large population will have less of an effect on the gene pool of a small population. In new individuals come n, new alleles will too- the gene pool will change.

5 Conditions of Genetic Equilibrium No Mutations New alleles may be introduced No Natural Selection All genotypes must have equal chance of survival and reproduction 4. Genes mutate = allele frequencies will change. 5. No phenotype must have selective advantage over the others.

Speciation Speciation = formation of new species Species = group of organisms that breed with one another and produce fertile offspring As new species evolve, populations become reproductively isolated from each other Members of same species share same gene pool Genetic change that occurs in one individual can spread through the population Gene pools must become separated for speciation to occur When two population cannot interbreed and produce fertile offspring, reproductive isolation has occurred

Isolating Mechanisms (lacewing interactive) Behavioral Isolation Differences in courtship rituals or other reproductive strategies Geographic Isolation Two populations separated by geographic barrier such as rivers, mountains or bodies of water Temporal Isolation Two or more species reproduce at different times of the day or year

Reproductive Isolation results from Isolating mechanisms which include Behavioral isolation Temporal isolation Geographic isolation produced by produced by produced by Behavioral differences Different mating times Physical separation which result in Independently evolving populations which result in Formation of new species

Speciation of Darwin’s Finches Founders Arrive Separation of Populations Changes in the Gene Pool Reproductive Isolation Ecological Competition Continued Evolution

Speciation in the Andes (Ecuador) Hummingbird video Explain the hypothesis presented by the scientists profiled in this segment to explain the process of speciation in hummingbirds and possibly other species. How does this hypothesis differ from the traditional view that speciation often requires geographic separation of populations? Why were the researchers collecting blood from the populations they studied? Discuss at least two possible analyses that could be performed on those samples and, identify at least two different questions that might be answered with sufficient data.