Bellwork: What indicates that a population is evolving Bellwork: What indicates that a population is evolving? What are some of the key sources of genetic variation?

Genes and Variation Section 17.1

How are genetics and evolution linked? When Mendel’s work was rediscovered around 1900, genetic research took off! Discovered that heritable traits are controlled by genes found on chromosomes Changes to genes and chromosome generate variation Variation is key to natural selection Modern genetics allows us to understand evolution better than Darwin ever could

Genotype and Phenotype in Evolution Plants and animals typically have one gene from each parent Specific forms of this gene (alleles) will vary from individual to individual Genotype = combination of alleles Genotype and environmental conditions give an organism it’s phenotype Natural selection acts on phenotype not genotype Some phenotypes are better suited to the environment than others Organisms with this phenotype will be fitter, and produce more offspring Natural selection refers to an entire organism, not an individual gene

What is a population? A group of individuals of the same species that mate and produce offspring Members of a population interbreed - they contain a common group of genes – a gene pool A gene pool is all of the genes, including every allele present for each gene, in a population Researches study gene pools by investigating the numbers of different alleles they contain Allele frequency is the number of times an allele occurs in a gene pool Has nothing to do with whether an allele is dominant or recessive

Evolution in genetic terms Evolution is the change in the frequency of alleles over time It is populations not individuals the evolve Natural selection operates on individual organisms, BUT the changes it causes on allele frequency will show up in the population as a whole

What are sources of genetic variation? Heritable variation can be produces in one of three ways: Mutations Genetic recombination in sexual reproduction Lateral gene transfer

Mutation A mutation is any change in the genetic material of a cell Some can involved changes of an individual gene, others large pieces of a chromosome Not all mutations will change an organisms phenotype (neutral mutations) Not all mutations that affect phenotype will affect fitness Some can have positive impacts, some very negative and serious Mutations are pretty common Each of us is thought to have 300 million mutations that make our DNA different from our parents Most of these are neutral One or two may potentially be harmful Mutations only matter if the can be passed from generation to generation – Germ line cells. Cells that go on to produce egg or sperm cells

Genetic Recombination in Sexual reproduction Most heritable differences are not due to mutations, but due to genetic recombination during sexual reproduction During Meiosis, each chromosome will move independently Give rise to 8.4 million possible gene combinations Crossing over during meiosis produces genetic variation Explains why siblings are not identical (apart from identical twins) Explains why individual members of a species differ from one another

Lateral gene transfer In Eukaryotic organisms, genes are passed from parent to offspring In some organisms however, this is not the case Bacteria can swap genes on plasmids as if they are trading cards Passing of genes from one organism to another that is not it’s offspring is lateral gene transfer Can occur between organisms of the same or different species Important for antibiotic resistance in bacteria Most applicable to single celled organisms

Single-gene and polygenic traits Sometime one genes controls a trait, other times, multiple genes can control a trait The number of phenotypes produced for a trait depends on how many genes control the trait A single gene trait, is a trait controlled by only one gene Phenotypes are distnict In this situation, in a population a recessive allele can be more dominant Polygenic traits are controlled by two or more genes Multiple possible genotypes and phenotypes Phenotypes are not distinct Form a normal distribution – example of height

Single gene and polygenic traits

Evolution as genetic change in population Section 17.2

Why don’t pesticides always work? At first virtually all of the targeted insects are killed off BUT, a few individuals will survive These will survive and reproduce Gives rise to a population of insects with a natural resistance to this insecticide

How does natural selection work? Evolutionary fitness is success in passing on genes to it’s offspring But, there is a difference between single trait and polygenic genes Single trait genes Natural selection can lead to changes in allele frequencies and changes in phenotype frequency

Natural selection of polygenic traits Natural selection on polygenic traits can affect the relative fitness of phenotypes and thereby produce one of three types of selection Directional selection Stabilizing selection Disruptive Selection

Directional selection If individuals at one end of the curve are fitter than the other end directional selection occurs Phenotypes shift, because some individuals are better at surviving and reproducing than others Example – Darwin’s finches beak size, peppered moths

Stabilizing selection When individuals near the center of the curve have a higher fitness, stabilizing selection occurs Curve stays in the same place, but the overall curve will narrow Example – birth weight of infant babies – those with an average mass are more likely to survive

Disruptive selection When individuals at the two extremes of the curve have a higher fitness, disruptive selection occurs Acts against individuals of an intermediate type This can eventually lead to two distinct phenotypes Example – are where medium size seeds become less common

What is genetic drift? In small populations, individuals that carry a particular allele may leave more descendants than other individuals leave, just by chance A series of chance occurrences can cause an allele to become less common in a population This random change in allele frequency is called genetic drift

Genetic bottlenecks Sometimes a disaster can kill off many individuals in a population By chance, this may change the allele frequency in the gene pool Bottleneck effect : change in allele frequency following a dramatic reduction in the size of the population A severe bottleneck can sharply reduce a population’s genetic diversity

The founder effect Genetic drift may occur when a few individuals colonize a new habitat The alleles in these founders may differ in relative frequencies from those of the main population Again this would occur by chance When allele frequencies change as a result of migration of a small subgroup is called the founder effect

Evolution vs genetic equilibrium If a population is not evolving then, allele frequencies in it’s gene pool will not change, and it is in genetic equilibrium Sexual reproduction and allele frequency Gene shuffling during sexual reproduction produces many gene combinations, but meiosis and fertilization by themselves do not change allele frequencies Hardy-Weinberg Principle Allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change Like making a Punnett square for a population, not an individual

Hardy-Weinberg principle Using these equations, it is possible to predict phenotype frequencies If a population doesn’t show these predicted frequencies, the evolution is taking place

Exception to Hardy – Weinberg principle There are five predicted conditions that can disturb genetic equilibrium, and result in evolution occurring Normally at least one of them is happening and evolution is occurring Non random mating In practice not common – organisms choose a partner on the basis of heritable traits – size, strength, color. Forms basis of sexual selection Small population size Evolutionary change due to genetic drift is more common in small populations Immigration or emigration New individuals may bring in new alleles, people who leave may remove alleles Mutations New alleles can be introduced into the gene pool Natural selection If genotypes result in different fitness, genetic equilibrium will be disrupted and evolution may occur