Lecture 23 10 29 2012 Requirements for evolution by natural selection Variation in phenotypes Variation has a genetic basis Unequal reproductive success Natural selection occurs over thousands of generations Can be experimentally observed in short generation lineages such as viruses and bacteria EXPERIMENT changed increased population have changed Decent with Modification After drought of the birds that survived the mean beak depth Conclusion natural selection occurred The characteristics of the Life on earth had a single origin and all the diverse organisms around today are descendants modified by evolutionary processes of this common ancestor Mechanism to explain evolution Natural Selection A process by which individuals with certain heritable traits produce more offspring than individuals lacking those traits alleles associated with favored traits increase in frequency from one generation to the next Homology in biology refers to similarities due to common descent Structural homology similarity of morphological traits such as the same general limb structure in vertebrates Darwin interpreted structural homologies as a product of descent with modification Developmental homology similarity in embryo morphology and or pattern of tissue differentiation embryonic development For example all vertebrates have gill pouches and tails early in Structural homologies result from genetic homology a similarity in the DNA sequences of genes from different species Anatomical similarities among species are generally reflected in their molecules in their genes DNA and gene products proteins Evolutionary perspective Darwin and Wallace encompasses all life forms One common origin for all life on this planet Ex almost all organisms use the same 64 mRNA codons to specify the same amino acids copy DNA using DNA polymerase use ribosomes to make proteins etc is consistent with the hypothesis that all organisms on earth are descended from a single common ancestor Modern Evolutionary Synthesis represents the combination of Darwin and Wallace s theory of the evolution of species by natural selection Mendel s theory of genetics as the basis for biological inheritance and mathematical population genetics Genetic variation arises spontaneously through random mutation and is constantly reshuffled into new allelic combinations by meiosis and random fertilization Evolution consists primarily of changes in the frequencies of alleles between one generation and another as a result of genetic drift gene flow natural selection and other processes Speciation is usually due to the gradual accumulation of small genetic changes and most easily occurs when populations are reproductively isolated e g by geographic barriers DARWIN S BIG IDEA A PARADIGM SHIFT The victorian world view changed o The was ancient o Species could and did change over generations o New species evolved from ancestral species o Some species went extinct o Human evolution was part of the same natural process Darwin s theory is among the most important and enduring contributions in all of science Has withstood repeated experimental and observational testing and stimulated many research programs Humans are not the pinnacle of evolution but rather one tiny twig among many twigs on the tree of life Natural selection in Action Most biologists think that small evolutionary changes accumulate over long time periods to generate large evolutionary changes like speciation Even if its hard to watch speciation taking place natural selection works fast enough that we can often observe rapid phenotypic and genotypic change in populations in nature and lab Evolution by Natural Selection Evolution by natural selection does not push organisms forward toward some ideal form it is not progressive Reduces the representation of those traits that are less advantageous in the current environment As environment changes the traits that lead to increased reproductive success in the environment can also change Natural selection can move populations away from Hardy Weinberg Equilibrium Of the processes that can change allele frequencies natural and sexual selection are the only processes that produces adaptations heritable traits that increase survival and or reproductive success o Alleles can increase or decrease in frequency based upon their influence on an individual s fitness Biologists often compare relative fitness the fitness of an individual relative to other members of the population Timescale for evolution by natural selection Depends on generation time of organism fast for bacteria and viruses slower for many eukaryotes HOW DID MYCOBACTERIUM TUBERCULOSIS BECOME RESISTANT TO ANTIBIOTICS 1 A chance mutation occurs 2 Drug therapy kills most bacteria without the mutation 3 Mutant cells proliferate 4 Drug therapy is ineffective against mutant cells Types of selection Directional selection changes the average value of a trait o One tail of phenotypic distribution or 1 allele is favored If directional selection continues long enough the favored allele s eventually reach a frequency of 1 0 and are said to be fixed Those alleles that are no longer found in the population are los frequency 0 Over time genetic diversity is reduced o selection can act on discrete traits and polygenic traits o Allele frequencies change in one direction one allele or one tail of phenotypic distribution is favored Directional artificial selection o Drives phenotypic divergence in dogs Ex directional selection caused average body size to increase in a cliff swallow production Balancing Selection 10 29 2012 10 29 2012