Evolution: Fact and TheoryEvolution: Fact and Theory

Fact: Species change over time.Fact: Species change over time. Theory: Species arise from common descent Theory: Species arise from common descent

through natural selectionthrough natural selection Random mutations lead to changes in genes.Random mutations lead to changes in genes. Changes in genes lead to changes in physical Changes in genes lead to changes in physical

form (phenotype)form (phenotype) Physical form best adapted to the environment at Physical form best adapted to the environment at

the time produce more offspring and thus survive.the time produce more offspring and thus survive. Evolution the theory addresses the origin of Evolution the theory addresses the origin of

species NOT the origin of life.species NOT the origin of life.

Fossil Record:Evolutionary transition from Fish to Amphibians

– 1991 – A fish-like tetrapod (Acanthostega gunnari )

• one specimen is so well preserved it has internal gills!

– 1998 – A Devonian fish is discovered in Pennsylvania

• eight fingers common to the earliest of tetrapods!

– 2004 –Tiktaalik roseae found in Canada

• has the thin, fish-like bones AND the tetrapod arm bones in the same structure!

Evolutionary transitions: Dinosaurs to birds

Evolutionary transitions: Horses

• Over 50 million years, horses adapted from woods to plains.

Evolutionary transitions: Elephants

• There have been 22 new species in the last 6 million years.

• 20 of them are extinct.

• In the last 4 million years there have been 10 successive species of Indian elephants alone. From J. Shoshani (1997)

Natural History, 106, 38

Evolutionary transitions: Artiodactyls to Whales• 29+ Intermediate forms

Evolutionary transitions: Humans• Human and great ape fossil record leads to an estimate for the last common

ancestor date as approximately 5-6 million years ago• Note that this image only goes back approximately 1.2 million years and the

upper left image is the modern chimpanzee for comparison• Image: Smithsonian Institute

Genetic Evidence: Humans and Great ApesGenetic Evidence: Humans and Great Apes Mutation RatesMutation Rates Humans: 3.75 mutations per yearHumans: 3.75 mutations per year Chimps: 5 mutations per yearChimps: 5 mutations per year 8.75 differences per year between the two populations8.75 differences per year between the two populations Genome sequences for the two species indicate a Genome sequences for the two species indicate a

total of 40 million differences between the two total of 40 million differences between the two genomesgenomes

Leads to a last common ancestor date of app. 5 Leads to a last common ancestor date of app. 5 million years agomillion years ago

Note this is a crude estimate a (much) more careful Note this is a crude estimate a (much) more careful analysis indicates a range of 5-6 million years agoanalysis indicates a range of 5-6 million years ago

Kondrashov A. S. (2002) Kondrashov A. S. (2002) Direct Estimates of Human Per Nucleotide Mutation Rates at 20 Loci Causing Mendelain Diseases;Direct Estimates of Human Per Nucleotide Mutation Rates at 20 Loci Causing Mendelain Diseases; Human Mutation 21-12:27Human Mutation 21-12:27

Nachman M. W., and Crowell S. L. (2000) Nachman M. W., and Crowell S. L. (2000) Estimate of the Mutation Rate per Nucleotide in HumansEstimate of the Mutation Rate per Nucleotide in Humans; Genetics 156-297:304; Genetics 156-297:304

Large scale Chromosome changes • Genome sequencing of

chimp and human chromosomes reveal that all 9 chromosome differences (white regions) – result from simple

inversions (shown by arrows)

– and one fusion.• “A Spectacular

confirmation”• Figure from RELICS OF EDEN

by Daniel J. Fairbanks, Prometheus Books, New York, 2007

Chromosome Numbers in the great apes (Hominidae):

human (Homo) 23 pairschimpanzee (Pan) 24 pairsgorilla (Gorilla) 24 pairsorangutan (Pogo) 24 pairs

Testable prediction:Testable prediction: The marks of that fusion must appear in one of the human chromosomes.

Centromere

Telomere

Ancestral Chromosomes

FusionHomo sapiens

Centromere #1

Telomere sequences

Centromere #2

“Chromosome 2 is unique to the human lineage of evolution, having emerged as a result of head-to-head fusion of two acrocentric chromosomes that remained separate in other primates. The precise fusion site has been located in 2q13–2q14.1 (ref. 2; hg 16:114455823 – 114455838), where our analysis confirmed the presence of multiple subtelomeric duplications to chromosomes 1, 5, 8, 9, 10, 12, 19, 21 and 22 (Fig. 3; Supplementary Fig. 3a, region A). During the formation of human chromosome 2, one of the two centromeres became inactivated (2q21, which corresponds to the centromere from chimp chromosome 13) and the centromeric structure quickly deterioriated (42).”

“Chromosome 2 is unique to the human lineage of evolution, having emerged as a result of head-to-head fusion of two acrocentric chromosomes that remained separate in other primates. The precise fusion site has been located in 2q13–2q14.1 (ref. 2; hg 16:114455823 – 114455838), where our analysis confirmed the presence of multiple subtelomeric duplications to chromosomes 1, 5, 8, 9, 10, 12, 19, 21 and 22 (Fig. 3; Supplementary Fig. 3a, region A). During the formation of human chromosome 2, one of the two centromeres became inactivated (2q21, which corresponds to the centromere from chimp chromosome 13) and the centromeric structure quickly deterioriated (42).”

Homo sapiens

centromere #13 (inactive)

Telomere sequences

Hillier et al (2005) “Generation and Annotation of the DNA sequences of human chromosomes 2 and 4,” Nature 434: 724-

731.

Hillier et al (2005) “Generation and Annotation of the DNA sequences of human chromosomes 2 and 4,” Nature 434: 724-

731.

Human Chromosome #2 shows the exact point at which this fusion took place

Human Chromosome #2 shows the exact point at which this fusion took place

centromere #12 (active)