The Theory of Evolution

Ch. 13

Biology

Ms. Haut

Lamarck’s Theory of Acquired Inheritance

(early 1800s)

• Jean Baptiste Lamarck observed fossil records and the current diversity of life and formulated his theory

• Suggested giraffes acquired long necks because ancestors stretched higher and higher into the trees to reach leaves– Lengthened neck was passed to offspring

Charles Darwin• Compared South American fossils

with living species there and elsewhere

• Observed organisms and their distributions on Galápagos Islands

Darwin’s Theory of Natural Selection

• Observations:– Overproduction of offspring leads to competition

of limited resources (food, space, breeding partners)

– Individuals of a population vary in characteristics, and many such traits are passed on to offspring

• Conclusions:– Individuals with inherited characteristics make

them best adapted to survive in their environment and reproduce and leave more offspring than less fit individuals

Natural Selection

• Prominent force in nature

• Support in the results of artificial selection—selective breeding of domesticated plants/animals

• Populations tend to evolve in response to environmental conditions

Populations are the Units of Evolution

• Population=group of individual organisms living in the same place at the same time

• Evolution is measured as the change in frequency of a given characteristic within a population over a succession of generations

What is a species?

• Biological Species Concept– Species—group of organisms that have the

potential to interbreed and produce fertile offspring

– Reproductively isolated by various factors preventing mixing with other species

Geographic Isolation can lead to Speciation

Subspecies of Deer mice

Islands are Living Examples of Speciation

• Adaptive Radiation– Evolution of many diversely adapted species

from a common ancestor– Example: Darwin’s Finches

• Finches with different beak shapes fit different ecological niches

1. Seeds blown over from mainland and form small colony

2. Gene pool isolated—evolves into new species B

3. Storms/other agents blow seeds to nearby island and evolve into species C

4. Some of species C recolonize the first island and cohabit with species B and some populate a new island

5. Speciation continues between new areas and previously colonized areas

Overview: Theory of Evolution

1. Variation exists within the genes of every population or species (due to random mutation)

2. In a particular environmnet, some individuals of a population/species are better suited and have more offspring (natural selection)

3. Over time, the traits that make certain individuals of a population able to survive and reproduced tend to spread in that population

4. There is clear evidence from fossils and many other sources that living species evolved from organisms that are extinct

Evidence of Evolution

• Fossil Record

• Biogeography

• Comparative anatomy

• Comparative embryology

• Molecular Biology

Fossilization

• Most fossils are actually casts of animals or plants. • Animal dies and sinks to the sea floor. • Tissue begins to decay and is buried under layers of

sediment such as mud or sand. • These layers become rock. • The hard parts of the animal are

replaced with minerals such as iron pyrites or silica.

• These minerals form the fossil.

• Usually fossils show the hard parts of the animal or plant - such as shell or bones.

• Trace fossils—evidence of living plants or animals, such as worm burrows or dinosaur footprints.

• Most fossils are found in sedimentary rocks - rocks which were created when shells or small loose bits of rock are laid down in layers (limestone, sandstone, clay and chalk)

Fossil Record

Trilobite510 million years ago

Chancelloria eros543 million years ago

Keichousaurus hui250 million years ago

Knightia humilis, Diplomystus dentatus, Mioplosus

Plantanus wyomingensis (sycamore)

54 million years ago

Determining Age of Fossils

• Relative age—determined by position in sedimentary rock

• Absolute age—determined by radiometric dating (radioactive isotopes)– Based on half-life of an isotope—period it takes

for half the radioactive material to decay

Carbon-14 (C-14) Up to 50,000 yrs

Potassium-40 (K-40) Up to 1.28 billion yrs

Number of Half-Life C-14 Remaining

(atoms)

0 100

1 50

2 25

3 12.5

4 6.25

5 3.13

Half-Life of Carbon 14

0

20

40

60

80

100

120

1 2 3 4 5 6

Number of Half-Life

Nu

mb

er

of

Ato

ms o

f C

-14

Evidence: Biogeography

• Geographical distribution of species suggests organisms evolve from common ancestors

• Island forms are most similar to forms found on the closest mainland, rather than forms on ecologically similar, but more distant islands

Comparative Anatomy

• Comparison of body structures between different species– Similarities give signs of common descent

• Homologous structures—features that have similar structure but have different functions

Comparative Anatomy

• Comparison of body structures between different species– Similarities give signs of common descent

• Homologous structures—features that have similar structure but have different functions

• Vestigial structures—Small body structures that may have been functional in the ancestors of a species, but has no real function at the present time (appendix, tail bone)

Comparative embryology

• Different organisms go through similar embryonic stages

• All vertebrates have an embryonic stage in which gill pouches appear in the throat region—evidence of a common ancestor

Molecular Biology

• Study of molecular basis of genes and gene expression

• Universality of genetic code

• Conservation of amino acid sequences in proteins such as hemoglobin

Causes of Microevolution• Mutation—random change in organism’s DNA

that creates a new allele– Rare events– Ultimate source of the genetic variation that

initiates evolution

Insecticide-resistant

Populations

Modes of Natural Selection

• Original population demonstrates the continuum of shell color (light to dark)

Modes of Natural Selection

• Divergence– Accumulation of

differences between groups

– Leads to Speciation—process by which new species form

Reproductive Barriers Keep Species Separate

• Prezygotic barriers—prevent mating or fertilization between species– Habitat isolation—species live in same general

area but not the same places– Behavioral isolation—special signals

recognized– Temporal isolation—breeding occurs at

different times– Mechanical isolation—anatomically

incompatible– Gametic isolation—gamete recognition

Reproductive Barriers Keep Species Separate

• Postzygotic barriers—Prevent the hybrid zygote from developing into a viable, fertile adult

– Reduced hybrid viability—embryo aborted

– Reduced hybrid fertility—offspring sterile

– Hybrid breakdown—offspring of hybrids sterile

Overview: Natural Selection

1. All species have genetic variation2. The environment presents many different challenges to an

individual’s ability to reproduce3. Organisms tend to produce more offspring than their

environment can support; thus, individuals of a species often compete with one another to survive

4. Individuals within a population that are better able to cope with the challenges of their environment tend to leave more offspring than those less suited to the environment

5. The traits of the individuals best suited to a particular environment tend to increase in a population over time