Genes and VariationSection 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 organism's 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 distinct• 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

The process of speciationSection 17.3

How does one species become two?

• It takes more than just a change in allele frequency caused by genetic drift and natural selection

Speciation• Speciation is the formation of a new species • Interbreeding links members of a species genetically

• Genetic changes can spread through a population over time

• But it is possible to split the gene pool• If members of the population stop breeding with each other• The genetic changes will not spread from one group to another• Because the two populations can no longer interbreed, reproductive isolation occurs

• When two populations become reproductively isolated, they can evolve into two separate species.

Reproductive isolation can occur in a variety of ways

• Behavioral isolation• When two population capable of interbreeding develop different courtship rituals

or other behaviors• Example – mating song of meadowlarks

• Geographic isolation• When two populations are separated by a geogrpahic barrier – such as rivers,

mountains or bodies of water• Do not apply to all organisms at any one time, may isolate one organism but link another – e.g.

flood• Example – squirrels in the grand canyon

• Temporal isolation• When two or more species reproduce at different times

• Example – orchids flowering in a forest.

Speciation and Darwin’s finches

•How have the founder effect and natural selection produced reproductive isolation that could have lead to speciation amongst Galapagos finches?

•What has been observed by studying the Galapagos finches is essentially the culmination of all aspects of speciation• Founder effect• Geographic isolation• Changes in the gene pool• Behavioral isolation• Ecological competition

Writing assignment: Explain how natural selection and behavioral isolation may have lead to reproductive isolation in Darwin’s finches

Molecular evolutionSection 17.4

Timing lineage splits•Molecular clock: Stretches of DNA are

compared to mark the passage of evolutionary time• Mutation rates in DNA are used to estimate the time

that two species have been evolving independently

•Neutral mutations have no effect on phenotype, and tend to accumulate in the DNA of different species at a similar rate

•By comparing DNA sequences, it is possible to reveal how many mutations have occurred independently in each group• The more differences, the more time has passed

since a common ancestor

How do you calibrate the clock?

•Each genome will have many molecular clocks that ‘tick’ at different rates

• Some genes accumulate mutations faster than others• Like second, minute and hour hands on a watch

•Different ’clocks’ are used to compare different common ancestors

•Researchers check the accuracy of the clock by trying to estimate how often mutations occur• Done by comparing the number of mutations in a particular gene in species

whose age has been determined by other methods

Where do new genes come from?

•Modern genes (our 25,000 working genes) probably descended from a much smaller number in the earliest life forms - How?

•One way is through the duplication and then modification of existing genes

Copying Genes

•Most organisms have several copies of various genes• Sometime two copies, sometimes thousands of copies• Where do they come from? Why are they there?

•Crossing over of Homologous chromosome sometimes involves the unequal swapping of DNA• One chromosome gets extra DNA• This can carry part of a gene, a full gene, or a longer length chromosome.

Duplicate Genes Evolve

•Extra copies of genes can undergo mutations that change their function• The original gene is still there, so the new genes

evolve without affecting the original gene function or product

•Multiple copies of duplicated genes can turn into a group of related genes or a gene family• Members will produce similar, yet slightly

different proteins• Our body produces a number of proteins that

carry oxygen – the globin family• HOX genes are another gene family

Hox genes and evolution•Hox genes determine which parts of an embryo develop arms, legs or

wings

•Groups of hox genes control the size and shape of these structures

•Homologous hox genes shape the bodies of both insects and humans• Although our last common ancestor was over 500 million years ago!

• Small changes in Hox gene activity during embryological development can produce large changes in adult animals • One small change to a Hox gene can lead to a large evolutionary difference

• Example – UBX gene changes number of legs between insects and crustaceans

Timing is everything

•Every part of an embryo starts to grow at a specific time, grows for a specific time and stops growing at a specific time• Small changes in starting and stopping times can make a big difference to

organisms

• Small timing changes can be the difference between long slender fingers and short stubby fingers

•Evolutionary development is a new and exciting area of Biology!