1. Lecture 28 Evolution

2. Variation Without variation (which arises from mutations of DNA molecules to produce new alleles) natural selection would have nothing on which to act.DNAalleles A population is a group of individuals living in the same geographical area and sharing a common gene pool.population The gene pool is the sum of all genetic information carried by the members of a population.gene pool All genetic variation in a population is generated by mutation. Mutation is any heritable change in DNA. Mutations can be changes of a single nucleotide base or may involve changes in chromosome number. Whether a mutation is good, neutral, or harmful depends on how it affects survival and reproductive success.Mutationnucleotide

3. Population Genetics A population is a group of potentially interbreeding organisms of the same species occupying a certain area. Members of a population vary from one another. This variation is the raw material on which natural selection operates.natural selection

4. There are several types of mutations, both at the gene-level and the chromosome-level. Gene mutations provide new alleles, making these mutations the ultimate source of variation. A gene mutation is an alteration in the DNA nucleotide sequence, producing an alternate sequence, termed an allele.nucleotide sequence Mutations occur at random, and can be beneficial, neutral, or harmful. Some chromosomal mutations are changes in the number of chromosomes inherited, while others are alterations in arrangement of alleles on chromosomes due to inversions and translocations.inversionstranslocations

5. In sexually reproducing organisms, genetic recombination is the reallocation of alleles and chromosomes. Recombination results from crossing-over during meiosis, the random segregation of chromosomes to gametes during meiotic division, and the random combination of gametes during fertilization.meiosisfertilization The entire genotype is subject to natural selection since new combinations of alleles may have improve the reproductive success of the organism.genotype For polygenic traits, the most favorable combination may occur when the right alleles group by recombination.polygenic

6. Mitochondrial Genomes and Human Evolution* Within an evolutionary tree one expects that the divergence of species will be mirrored by parallel divergence of DNA sequences. Conversely, study of DNA divergence patterns should help to establish patterns of evolution. This principle has been used on many problems, including the relationship between prokaryotes and eukaryotes, and human evolution. A recent study of the latter took advantage of the more rapid accumulation of mutations in mtDNA (2 - 4% divergence per 106 years) compared to nuclear DNA. Since mtDNA is inherited maternally it is passed from generation to generation as a clone, without mixing with mtDNA from the male parent or any other source.

7. Divergence was measured via the restriction map - if mutations lie in a restriction site they may abolish endo-nuclease action. Samples from 147 people were screened with 12 REs which identified 467 cleavage sites, of which 195 were absent (mutated) in at least some samples. The people represented African, Asian, Caucasian, aboriginal Australian and aboriginal New Guinea ethnic groups. A computer programme produced the most parsimonious evolutionary tree - that is the pattern of divergence that would give the observed variation with the least number of mutational events. Then using the estimate of the rate of divergence above, the number of years back to a particular branch point could be calculated. The results showed that only Africans appear in both branches of the tree arising from the oldest branch point - so the starting point is claimed to be about 200,000 years ago in Africa. One branch remains pure African, the other gives all races but in many cases divergence within a race occurs later than divergence between races. The maternal inheritance leads the authors to connect the tree to 'one woman........ in Africa' and others have linked this to Eve. More realistically it shows that the human race evolved once, in a restricted group, in Africa.

8. Origins of Life on Earth Special creation – life created by a supernatural being Spontaneous generation Steady state Cosmozoan Biochemical evolution