Title: Evolution
1 Evolution
Satirical cartoon by Thomas Nast, from
Harpers Weekly, August 19, 1871.
2Objectives
- Define evolution.
- Outline the evidence for evolution.
- State that populations tend to produce more
offspring than the environment can support. - Explain that a consequence of over-reproduction
of offspring is a struggle for survival. - State that members of a species show variation.
- Explain how sexual reproduction promotes
variation in a species. - Explain how natural selection leads to evolution.
- Explain two examples of evolution in response to
environmental change.
3Charles Darwin
- British lived from 1809 to 1882.
- Began to develop the theory of evolution on a
trip around the world aboard the HMS Beagle.
4Charles Darwin
- Came to realize that the earth had changed
over a long period of time, which caused some
crea- tures to go extinct, opening up
space for new creatures to appear.
The history of life on earth was similar to a
branching tree, tracing back to some common
ancestor. Nature selected which creatures
survived and passed on their characteristics.
Species change!
5Evolution defined
- Evolution the change over time in the
frequency of
inherited variations in a
population over generations. - Natural selection amplifies or diminishes
inherited (not acquired)
variations.
6Evolution defined
- Evolution the change over time in the frequency
of in-herited variations in a population (not
individuals).
7Evolution defined
- Lamarckian vs. Darwinian evolution change over
time - Lamarck was a Frenchman who lived before Darwin
and first proposed creatures change over time
(evolve). - Example giraffes have a long neck because one
stretched its neck to reach high branches and
passed the trait on.
8Evolution defined
- Lamarckian vs. Darwinian evolution change over
time - Darwin would say that giraffes necks come in
various sizes. Those with a longer neck can
reach higher branches and survive better during
hard times and pass the trait on to their
offspring.
9Evolution defined
- Lamarckian vs. Darwinian evolution change over
time - Common errors
- Potato beetles evolved resistance to DDT in
order to survive. - There is no intention to evolve.
- The beetles either had or didnt have the genes
needed to survive the chemical. - Offspring of the survivors shifted the
population toward a greater likeli- hood
of resistance overall. - Peppered moths come in various colors, white
to black. Population colors shift as
colors of tree trunks change over time, and
pre- dators choose different colored moths..
10Natural selection
- Natural selection the differential success in
the reproduction of different phenotypes
resulting from the interaction of organisms with
their environment. - Charles Darwins mechanism of evolution.
- Developed from 5 observations
- 1) Species are so fertile that populations would
rise exponen- tially if all
individuals reproduced successfully. - 2) Populations tend to remain stable in size,
except for seasonal fluctuations. - 3) Environmental resources are limited.
- 4) Individuals of a population vary no two
individuals are exactly alike. - 5) Much of this variation is heritable.
11Natural selection
- How natural selection works
- Populations tend to produce more offspring than
the environment can support. - As a consequence, overproduction of offspring
leads to a struggle for existence, with only a
fraction of the offspring surviving each
generation. - A generation of mice lasts 45
days, then there are 4 7 babies. After 45
days there are perhaps 25
mice, then after another
45 days there
could be 125, then 625.
12Natural selection
- How natural selection works
- Individuals in a species vary.
13Natural selection
- How natural selection works
- Sexual reproduction promotes variation in a
species. - Genes are redistributed and mixed among
individuals. - Asexual reproduction would produce no differences.
14Natural selection
- Natural selection leads to evolution
- Survival is not random, but depends in part on
the hereditary make-up of the individuals. - Individuals more fit in their environment are
likely to leave more offspring than those less
fit. - Preferential survival reproduction leads to a
gradual change in a population, with favorable
characteristics accumulating over the generations.
15Examples of natural selection
- Evolution of resistance to in-
secticides in insect species. - Effectiveness of insecticide decreases with
time. - Each generation there are more resistant
insects. - Natural selection edits
- existing variation.
- Natural selection favors
- characteristics that
- fit the current, local
- environment.
16Examples of natural selection
- Drug-resistant strains of HIV evolve rapidly in
the viral population infecting any particular
patient. - Like pesticide resistance the drug 3TC
interferes with HIV replication in human cells. - Resistant strains become 100 of the population
in just a few weeks. - Bacterial resistance to antibiotic drugs
works in the same way.
17Genetic drift
- Genetic drift is a 2nd mechanism of evolution.
- In each generation, some individuals may, just by
chance, leave behind a few more descendents (and
their genes) than other individuals. The genes
of the next generation will be the genes of the
lucky individuals, not necessarily the
healthier or better individuals.
The percentage of green genes drops
accidentally.
18Genetic drift
- Genetic drift is a 2nd mechanism of evolution.
- The bottleneck effect A few creatures are lucky
enough to escape some disaster that kills
randomly. - Northern elephant seals were hunted nearly to
extinction (20 survived), leading to a loss of
genetic diversity.
19Genetic drift
- Genetic drift is a 2nd mechanism of evolution.
- The founder effect colonization by a small
subset of a population (little genetic variety) - Ex Polydactyly in the Amish in
Pennsylvania, a community that
was founded by a small group of
Germans in the
1700s. - Ex Huntingtons disease (nerve
cells in brain degenerate) in the
Afrikaner population of
Dutch settlers in South Africa. - Source of Asian eye characteristics
20Genetic drift
- Genetic drift is a 2nd mechanism of evolution.
- 8 of Chinese and Mongolian men descend from
Genghis Khan! His sons had many children.
Spread of Genghis Y chromosome
21Migration
- Migration is a 3rd mechanism of evolution.
- Some individuals from a population of brown
beetles might have joined a population of green
beetles. That would make the genes for brown
beetles more frequent in the green beetle
population.
22Mutation
- Mutation is a 4th mechanism of evolution.
- A mutation could cause parents with genes for
bright green coloration to have offspring with a
gene for brown coloration. That would make the
genes for brown beetles more frequent in the
population. - Ex a 6-toed cat.
Blond and red hair are human mutations from
the original brown.
23 Evolution
Satirical cartoon by Thomas Nast, from
Harpers Weekly, August 19, 1871.
24Evidence for evolution
- Evidence of evolution is everywhere in biology.
- Fossil record
- Homologous structures
- Biogeography
- Embryonic development
- Molecular biology
- Selective breeding
- Natural selection within human lifespan
25Evidence for evolution
- Fossil record
- Fossils are any traces of dead organisms
bones, tracks (foot-prints), leaf impressions,
excrement, actual organisms frozen in ice, in
amber, or in tarpits.
26Evidence for evolution
- Fossil record
- Most fossils are found in
sedimentary rock where
deeper rock is older, formed
from sand or clay deposits. - Stratigraphy dating fossils by charting the
rock layers. - Since the late 1940s, fossils are
dated by the decay of radioactive
isotopes. - This is called radiometric dating.
27Evidence for evolution
- Fossil record
- The fossil record in rocks
provides relative ages. - Radiometric dating can
determine absolute ages. - Organisms accumulate radio- active isotopes
when alive. - Isotopes decline after death - they decay
(transform) into another element.
Most carbon is 12C, but there is a small yet
constant amount of 14C in the air, and therefore
in our living bodies 1 part per trillion. This
amount declines after death.
28Evidence for evolution
- Half-life time for ½ of the isotope atoms to
decay. - Use 40K to date old rocks half-life 1.3
billion yr. - Use 235U for early vertebrates half-life 700
mil. yr. - Use 14C for recent fossils half-life 5,730
years.
29Evidence for evolution
- Using 14C dating to determine the age of organic
materials. Half-life of 14C is 5,730 years.
Work backward from the amount present today
to a time when there was maximum 14C, 1 part per
trillion.
30Evidence for evolution
- Fossil record
- Fossils give evidence about the major branches of
descent in the tree of life. - Order established
- Ex fossil fishes predate other
vertebrates amphibians are
next, followed by reptiles, then
mammals and birds.
31Evidence for evolution
- Fossil record
- Transitional forms link old
new species. - Evolution of horses hooves from 5 toes.
- Evolution of whale from horse-like animal
whale retains a pelvis where hind legs
once attached forelegs now flippers
32Evidence for evolution
- Homologous structures
- Features of new species are altered versions of
ancestral features. Similarity in
characteristics resulting from common ancestry
is known as homology.
All cats have a common ancestor.
33Evidence for evolution
- Homologous structures
- For example, the forelimbs of human, cats,
whales, and bats share the same skeletal
elements, but different functions because they
diverged from the ancestral tetrapod forelimb. - They are homologous structures.
34Evidence for evolution
- Vestigial organs are homologous structures that
have marginal, if any, importance to a current
organism, but which had important
functions in ancestors. - Skeletons of some snakes fossil whales retain
vestiges of pelvis and leg bones of walking
ancestors. - In humans - wisdom teeth, tailbone, appendix.
35Evidence for evolution
- Biogeography - the study of the distribution of
life forms over geographical areas. - If evolution is true, then we should expect to
find related species living near each other - Except in cases of great mobility (like sea
animals, birds, and animals distributed by
humans) or over long periods of time (due to
plate tectonics). - If, however, we find that species are distributed
in a random geographic manner, with closely
related species no more likely to be found close
to each other than unrelated species, then this
would be strong evidence against evolution and
common descent.
36Evidence for evolution
- Biogeography
- Plate tectonics the continents are on plates
that glide over the surface of the earth
carrying life with them.
37Evidence for evolution
- Biogeography
- Identical fossils in parts of the world now
widely separated indicate that the continents
were once joined.
The southern part of Pangaea
38Evidence for evolution
- Biogeography
- Australian example marsupials vary
widely but are more closely
relat- ed to each other than to
similarly- appearing animals on other
continents. - All have a pouch!
- Placental predators
out-competed them on
other continents, and they disappeared. Australia
(with no placentals) was isolated.
39Evidence for evolution
- Biogeography
- Marsupials ori- ginated in SA 70
million yrs ago then spread to
Australia.
40Evidence for evolution
- Biogeography
- Species tend to be more closely related to other
species from the same area than to other species
with the same way of life, but living in
different areas.
The sugar glider from Australia is more closely
related to other marsupial mammals in
Australia than to the flying squirrel, a
pla- cental mammal of North America. This is an
example of convergent evolution.
41Evidence for evolution
- Embryonic development
- Genes for embryonic development are conserved in
many different species making the embryos
similar. - All vertebrate embryos have structures called
pharyngeal pouches in their throat at some stage
in their development. These develop into
different, but still homologous, adult
structures gills of fish or Eustachian tubes in
mammals.
42Evidence for evolution
- Molecular biology corroborates
evolutionary trees. - Evolutionary relationships
among species are documented
in their DNA and proteins. - Ex the Cytochrome c protein is more similar
when crea- tures are closely related - Human chimp have the same
104 amino acids, dog
has 13 differences, rattle- snake
has 20 changes.
43Evidence for evolution
- Selective breeding
- Humans have domes-
ticated many animals,
giving them new char-
acteristics over time
(they evolved). - Dogs domesticated from wolves
about 15,000 years ago. - Sheep, cattle, horses goats, pigs, chickens
44Evidence for evolution
- Selective breeding
- Plants corn, wheat, potato, bean, cabbage, etc.