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Chapter 22
Descent with Modification: A Darwinian View of Life

Darwin made two major points in his book:
 Many current species are descendants of ancestral species
 Natural selection is a mechanism for this evolutionary
process
 Darwin’s theory of evolution by natural selection integrates
diverse areas of biological study and stimulates many new
research questions

The study of fossils helped to lay the groundwork for Darwin’s ideas

Fossils are remains or traces of organisms from the past, usually found in
sedimentary rock, which appears in layers or strata
Video: Grand Canyon
Theories of Gradualism

Gradualism is the idea that profound change can take place through the
cumulative effect of slow but continuous processes
Lamarck’s Theory of
Evolution

Lamarck hypothesized that species evolve through use and disuse and the
inheritance of acquired traits

The mechanisms he proposed are unsupported by evidence

Rats and giraffs
LE 22-6
Cactus eater. The long,
sharp beak of the
cactus ground finch
(Geospiza scandens)
helps it tear and eat
cactus flowers and
pulp.
Seed eater. The large
ground finch (Geospiza
magnirostris) has a large
beak adapted for cracking
seeds that fall from plants
to the ground.
Insect eater. The green warbler finch
(Certhidea olivacea) used its narrow,
pointed beak to grasp insects.
The Origin of Species

Darwin developed two main ideas:

Evolution explains life’s unity and diversity

Natural selection is a cause of adaptive evolution

The phrase descent with modification summarized Darwin’s perception of the
unity of life

The phrase refers to the view that all organisms are related through descent
from an ancestor that lived in the remote past

In the Darwinian view, the history of life is like a tree with branches
representing life’s diversity
LE 22-7
Sirenia
Hyracoidea (Manatees
(Hyraxes) and relatives)
0
10,000
2
5.5
24
34
Elephas Loxodonta Loxodonta
cyclotis
maximus africana
(Africa)
(Africa)
(Asia)
Natural Selection and
Adaptation

Evolutionary biologist Ernst Mayr has dissected the logic of Darwin’s theory
into three inferences based on five observations

Observation #1: For any species, population sizes would increase
exponentially if all individuals that are born reproduced successfully

Observation #2: Populations tend to be stable in size, except for seasonal
fluctuations

Observation #3: Resources are limited

Inference #1: Production of more individuals than the environment can
support leads to a struggle for existence among individuals of a population,
with only a fraction of their offspring surviving

Observation #4: Members of a population vary extensively in their
characteristics; no two individuals are exactly alike

Observation #5: Much of this variation is heritable

Inference #2: Survival depends in part on inherited traits; individuals
whose inherited traits give them a high probability of surviving and
reproducing are likely to leave more offspring than other individuals

Inference #3: This unequal ability of individuals to survive and reproduce
will lead to a gradual change in a population, with favorable characteristics
accumulating over generations
Artificial Selection

In artificial selection, humans have modified other species over many
generations by selecting and breeding individuals with desired traits
LE 22-10
Terminal
bud
Lateral
buds
Brussels sprouts
Cabbage
Leaves
Flower
clusters
Kale
Cauliflower
Stem
Flowers
and
stems
Broccoli
Wild mustard
Kohlrabi
Summary of Natural
Selection

Natural selection is differential success in reproduction from interaction
between individuals that vary in heritable traits and their environment

Natural selection produces an increase over time in adaptation of
organisms to their environment

If an environment changes over time, natural selection may result in
adaptation to these new conditions
Video: Seahorse Camouflage
LE 22-11
A flower mantid
in Malaysia
A stick mantid
in Africa
Darwin’s theory explains a wide range
of observations

Darwin’s theory of evolution continues to be tested by how effectively it
can account for additional observations and experimental outcomes
Natural Selection in
Action

Two examples provide evidence for natural selection: the effect of
differential predation on guppy populations and the evolution of drugresistant HIV

Researchers have observed natural selection leading to adaptive evolution
in guppy populations- Differential Predation in Guppy Populations
LE 22-12a
Pools with killifish
but no guppies prior
to transplant
Predator: Killifish; preys
mainly on small guppies
Experimental
transplant of
guppies
Guppies:
Larger at
sexual maturity
than those in
“pike-cichlid pools”
Predator: Pike-cichlid; preys mainly on large guppies
Guppies: Smaller at sexual maturity than
those in “killifish pools”
200
160
120
80
40
185.6
161.5
67.5 76.1
Males
Females
Age of guppies
at maturity (days)
Mass of guppies
at maturity (mg)
LE 22-12b
100
80
60
40
20
85.792.3
58.2
48.5
Males
Females
Control population:
Guppies from pools with
pike-cichlids as predators
Experimental population:
Guppies transplanted to
pools with killifish as
predators
The Evolution of DrugResistant HIV

The use of drugs to combat HIV selects for viruses resistant to these drugs

The ability of bacteria and viruses to evolve rapidly poses a challenge to
our society
LE 22-13
Percent of HIV resistant to 3TC
100
Patient
No. 1
Patient No. 2
75
50
Patient No. 3
25
0
0
2
4
6
Weeks
8
10
12
Homology, Biogeography,
and the Fossil Record

Evolutionary theory provides a cohesive explanation for many kinds of
observations

Homology is similarity resulting from common ancestry-Homologous structures
are anatomical resemblances that represent variations on a structural theme
present in a common ancestor
LE 22-14
Human
Cat
Whale
Bat

Comparative embryology reveals anatomical homologies not visible in
adult organisms

Vestigial organs are remnants of structures that served important functions
in the organism’s ancestors
Molecular Homologies

Examples of homologies at the molecular level are genes shared among
organisms inherited from a common ancestor
Homologies and the Tree
of Life

The Darwinian concept of an evolutionary tree of life can explain homologies

Anatomical resemblances among species are generally reflected in their
molecules, genes, and gene products
LE 22-16
Species
Percent of Amino Acids That Are
Identical to the Amino Acids in a
Human Hemoglobin Polypeptide
Human
100%
Rhesus monkey
95%
87%
Mouse
69%
Chicken
54%
Frog
Lamprey
14%
Biogeography

Darwin’s observations of biogeography, the geographic distribution of
species, formed an important part of his theory of evolution

Some similar mammals that have adapted to similar environments have
evolved independently from different ancestors
LE 22-17
NORTH
AMERICA
Sugar
glider
AUSTRALIA
Flying
squirrel
The Fossil Record

The succession of forms observed in the fossil record is consistent with
other inferences about the major branches of descent in the tree of life

The Darwinian view of life predicts that evolutionary transitions should
leave signs in the fossil record

Paleontologists have discovered fossils of many such transitional forms