The process of speciation Section 17.3

How does on 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 evolution Section 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!