Evolution at the Molecular Level

1. Evolution at the Molecular Level

2. Outline • Evolution of genomes • Review of various types and effects of mutations • How larger genomes evolve through duplication and divergence • Molecular archeology based on gene duplication, diversification, and selection • globin gene family: an example of molecular evolution

3. Speculations on how the first cell arose • The first step to life must have been a replicator molecule • The original replicator may have been RNA • Ribozymes? • More complex cells and multicellular organisms appeared > 2 billion years after cellular evolution

4. Earliest cells evolved into three kingdoms of living organisms • Archaea and bacteria now contain no introns • Introns late evolutionary elaboration Fig. 21.3

5. Basic body plans of some Burgess shale organisms Many species resulting from metazoan explosion have disappeared Fig. 21.4

6. Evolution of humans • 35 mya – primates • 6 mya – humans diverged from chimpanzees Fig. 21.5

7. Evolution of Humans • Human and chimpanzee genomes 99% similar • Karyotypes almost same • No significant difference in gene function • Divergence may be due to a few thousand isolated genetic changes not yet identified • Probably regulatory sequences

8. DNA alterations form the basis of genomic evolution • Mutations arise in several ways • Replacement of individual nucleotides • Deletions / Insertions: 1bp to several Mb • Single base substitutions • Missense mutations: replace one amino acid codon with another • Nonsense mutations: replace amino acid codon with stop codon • Splice site mutations: create or remove exon-intron boundaries • Frameshift mutations: alter the ORF due to base substitutions • Dynamic mutations: changes in the length of tandem repeat elements

9. Effect of mutations on population • Neutral mutations are unaffected by agents of selection • Deleterious mutations will disappear from a population by selection against the allele • Rare mutations increase fitness

10. Genomes grow in size through repeated duplications • Some duplications result from transposition • Other duplications arise from unequal crossing over

11. Genetic drift and mutations can turn duplications into pseudogenes • Diversification of a duplicated gene followed by selection can produce a new gene

12. Genome size increases through duplication of exons, genes, gene families and entire genomes Fig. 21.10

13. Basic structure of a gene Fig. 21.11

16. Genes may elongate by duplication of exons to generate tandem exons that determine tandem functional domainse.g., antibody molecule Fig. 21.12a

17. Exon shuffling may give rise to new genese.g., tissue plasminogen activator (TPA) Fig. 21.12b

18. Duplications of entire genes can create multigene families Fig. 21.13a

19. Unequal crossing over can expand and contract gene numbers in multigene families Fig. 21.13b

20. Intergenic gene conversion can increase variation among members of a multigene family • One gene is changed, the other is not Fig. 21.14a

21. Concerted evolution can lead to gene homogeneity Fig. 21.15 Unequal crossing over Gene conversion

22. Evolution of gene superfamilies • Large set of genes divisible into smaller sets, or families • Genes in each family more closely rated to each other than to other members of the family • Arise by duplication and divergence

23. Evolution of globin superfamily Fig. 21.16

24. Organisation of globin genes Fig. 21.16

25. Evolution of mouse globin superfamily Fig. 21.16

26. Evolution of mouse globin superfamily Fig. 21.16