Transcript Evolution: Fact and Theory

Evolution: Fact and Theory
 Fact: Species change over time.
 Theory: Species arise from common descent
through natural selection
 Random mutations lead to changes in genes.
 Changes in genes lead to changes in physical
form (phenotype)
 Physical form best adapted to the environment at
the time produce more offspring and thus survive.
 Evolution the theory addresses the origin of
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 Apes
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Mutation Rates
Humans: 3.75 mutations per year
Chimps: 5 mutations per year
8.75 differences per year between the two populations
Genome sequences for the two species indicate a
total of 40 million differences between the two
genomes
 Leads to a last common ancestor date of app. 5
million years ago
 Note this is a crude estimate a (much) more careful
analysis indicates a range of 5-6 million years ago
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Kondrashov A. S. (2002) Direct Estimates of Human Per Nucleotide Mutation Rates at 20 Loci Causing Mendelain Diseases;
Human Mutation 21-12:27
Nachman M. W., and Crowell S. L. (2000) Estimate of the Mutation Rate per Nucleotide in Humans; 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
Ancestral
Chromosomes
Fusion
Homo sapiens
Chromosome Numbers in the
great apes (Hominidae):
human (Homo)
chimpanzee (Pan)
gorilla (Gorilla)
orangutan (Pogo)
Centromere
#1
23 pairs
24 pairs
24 pairs
24 pairs
Telomere
sequences
Centromere
#2
Centromere
Telomere
Testable prediction: The marks of that
fusion must appear in one of the human
chromosomes.
Human Chromosome #2 shows the exact
point at which this fusion took place
Homo sapiens
centromere
#13 (inactive)
Telomere
sequences
centromere
#12 (active)
“Chromosome 2 is unique to the human lineage of
evolution, having emerged as a result of head-tohead 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).”
Hillier et al (2005) “Generation and Annotation of the DNA
sequences of human chromosomes 2 and 4,” Nature 434: 724731.