Option D: Evolution

IB Biology

I. Origin of Life on Earth (D.1)A. Earth’s Early Beginnings

1. The Earth was formed approximately 4.6 billion years ago from a cloud of dust particles orbiting the sun

2. This compacted due to gravity and together with the decay of radioactive

elements generated heat that caused the interior to melt and form a central dense core of iron and nickel

3. Lighter materials formed the mantle around the core and the lightest silicates solidified into an outer crust, the continents and the outer floors 4. The atmosphere was formed from gases

escaping through volcanoes and consisted of H2 H2O, CH4, NH3, N2, H2S, but no O2

5. The oceans formed by the condensation of water vapour.

B. There are four processes that had to have taken place in order to provide for the spontaneous origin of life on Earth1. The non-living synthesis of simple organic molecules2. The assembly of these molecules into polymers3. The origin of self-replicating molecules that made inheritance possible4. The packaging of these molecules into membranes with an internal chemistry different from their surroundings

C. Miller and Urey

1. Wanted to test the hypothesis that organic molecules can form spontaneously under the right conditions just as they might have at the early on in earth’s history.

2. Gases used: ammonia, methane and hydrogen, which created a reducing atmosphere (could donate lots of hydrogen and electrons).

3. It worked! Amino acids and other simple organic molecules were formed by the

apparatus4. However, this experiment was conducted with sterile glass, distilled water, and

enclosed gas so it’s hard to say whether it is an accurate representation of what might have happened

D. An alternate theory to that of the spontaneous origin of organic materials on earth is that comets may have delivered the organic materials from space

1. This theory still begs the question of

how the organic compounds became living organisms, however

E. Possible locations where conditions would have allowed for the synthesis of organic compounds

1. deep-sea hydrothermal vents2. volcanoes3. extraterrestrial locations (e.g.

other planets; follows along with the comet theory)

F. Most scientists believe that RNA was the first organic molecule that allowed for life

1. Reasons for this include two important properties of RNA:

a. RNA is composed of a single helix, versus DNA’s double helix.

- The bases are exposed and ready to combine with a

complement, giving them the ability to self-replicate.

b. RNA has the ability to catalyze other reactions to occur (e.g. the synthesizing of other organic molecules such as DNA and protein)

E. Possible origin of membranes and prokaryotic cells

1. Living cells may have been proceeded by protobionts, with an internal chemical environment different from their surroundings

a. protobionts = abiotic spheres that are precursors to cells and exhibits some of the properties of life

1) Coacervates- small spheres made of hydrocarbons that contain nucleic acids in their center (forerunners of phospholipid membranes).

- can grow, shrink, and split due to having a

semi- permeable membrane2) Microspheres – spheres formed upon the cooling of thermal proteins

- considered more stable than coacervates

F. Contribution of prokaryotes to the creation of an oxygen-rich atmosphere

1. Cyanobacteria, which are photosynthetic, converted Earth’s early atmosphere from anoxia, to one containing free oxgyen

2. Allowed for heterotrophs to evolve

3. Occurred about 2.7 to 2.2 billion years ago

G. Endosymbiotic Theory1. Describes the possible formation of eukaryotes2. According to the theory,

mitochondria were originally independent prokaryotic organisms, which were engulfed by another independent prokaryotic organism

3. Instead of being dismantled for nutritional purposes, the host found it more beneficial to keep the mitochondria intact

4. Similar circumstances are believed to have occurred with chloroplasts.

I. Species and Speciation (D.2)A. Allele frequency and gene pools

1. allele frequency = the percentage with which a particular

allele is found in a population2. Gene pool = The sum total of all alleles present in all populations of a particular species3. Evolution, at the genetic level, involves a change in allele frequency

in a population’s gene pool over a number of generations

B. Speciation

1. Speciation = formation of a new species by the splitting of an existing species

2. New species result from the accumulation of many advantageous alleles over a long period of time

3. In other words, new species form as a result of macroevolution

a. macroevolution = the accumulation of multiple microevolutionary steps, combined with reproductive isolation (e.g. Darwin’s finches)

3. There are three types of reproductive isolation (barriers between gene pools)

a. Geographical Isolation – occurs when a population is physically separated, usually due to a natural disaster such as an avalanche, fire, etc.

b. Temporal Isolation – due to reproductive timing barriers (e.g. different rainforest orchid species blooming at different times of year)c. Behavioral Isolation = Courtship mating displays may only be recognized by members of the same species (e.g. bird songs)

4. Polyploidy can contribute to speciationa. Polyploidy occurs when more than two sets of homologous chromosomes are

present. b. Examples such as triploidy (3x) and

tetraploidy (4x) are often due to a disruption in the meiotic sequence. c. Chromosomes replicate, but remain together in the same cell.

d. Once polyploidy occurs, the individual is often unable to mate with the original

species, causing immediate species divergence.

5. Allopatric vs. Sympatric Speciation

a. Allopatric speciation – occurs in different areas

1) due to geographical isolation

b. Sympatric speciation – occurs in the same area

2) Due either to temporal or behavioral isolation

7. Adaptive Radiationa. As populations

drift or expand to different geographical locales, local environmental conditions will favor some traits over others, causing phenotypes in different areas to diverge

b. A classic example is the finches of the Galapagos Islands (studied by Darwin)

C. Convergent and Divergent Evolution1. Convergent Evolution

a. individuals of the same species develop similar traits in response to living in the

same habitatb. For example, many species of desert plants develop thick cuticles (waxy outer layer) to prevent water loss

2. Divergent Evolutiona. Occurs when different traits share a common evolutionary originb. For example, vertebrate limbs have

many unique shapes, but their bone patterns trace back to a common ancestral configuration

D. Pace of evolution1. Gradualism = slow changed from one form

to another2. Punctuated

equilibrium- long periods of no change and short periods of rapid evolution.

a. Some causes are volcanic eruptions

and meteor impacts on Earth.

3. Both theories are debated by scientists

E. Transient vs. Balanced Polymorphism

1. Transient polymorphism- industrial melanism and the peppered moth

a. Before 1850’s one homozygous phenotype was favored by

natural selection

b. Once industrial revolution began natural selection began to favor the other

homozygous phenotype

2.Balanced polymorphism- sickle cell anemia.

a. The heterozygous condition (carrier) gives resistance to malaria,

hence it is also favored by natural selection.

III. Human Evolution (D.3)A. Radioactive Dating of Fossils

1. Over time, a certain percentage of Carbon becomes an isotope

2. Living systems incorporate carbon, and have the same % of Carbon isotopes as the atmosphere3. Upon death, no new carbon is

incorporated into the body, and the isotopes start to decay at the half-life rate

a) half life =the amount of time it takes for half of the radioactive isotopes of a particular substance to decay.

4. The half-life of C14 is 5730 years, and can be used to date material up to 50,000 years old

5. The half-file of K40 is 1.3 billion years, and can be used to date rocks over one million years old.

B. Major physical features that defines humans as primates

1. Opposable thumbs

2. Large forward facing eyes for stereovision and distance judgment

3. Color vision to identify other primates and identify food

4. Large cranial capacity

5. Fingers with nails, not claws, and fingertips sensitive to touch

D. The evolution of apes to humans can be seen through the fossils of Ardipithecus ramidus, Austrolopithicus, and Homo

1. A. ramidusa. 5.8-5.2 million years ago b. Oldest hominidc. Large caninesd. Evidence of bipedalism is

inconclusive2. A. afarensis

a. 3.9 – 2.9 million years agob. Bipedalc. Reduced canines

3. A. africanusa. 3.3-2.5 million years agob. Similar to A. afarensis, but slightly larger brain4. H. habilisa. 2.6-1.4 million years agob. Used first simple stone toolsc. Protrusions in face starting to reduce5. H. erectusa. 1.8-1 million years agob. More advanced toolc. Probably used fire

6. H. neanderthalis

a. 500,000 – 24,000 years ago

b. Short, thick bodies adapted to cold climate

c. Largest cranial capacity

7. H. sapiens

a. 50,000 – present

b. Cranial capacity not as large as N. neanderthalis, but better able to use brains to develop agriculture and hunting skills

8. At various stages in hominid evolution, several species may have coexisted

1) for example H. sapiens

and N. neanderthalis

E. The fossil record is incomplete and can lead to some uncertainties about human evolution

1. Many fossils, from Australopithecines through the genus Homo, are incomplete

a. Usually partial skulls and just a few bones are found because only a small amount of organic matter is ever fossilizedb. due to the fact that many body parts do not fossilizec. also, the environmental circumstances needed for fossilization to take place is rare

2. The fossils between males and females differ and if only one is found it may not be an accurate representation of the species as a whole

3. Dating of fossils is approximate and not always correct

2. By arranging extinct animals and plants into some kind of geological sequence, it is possible to suggest how one group may have evolved into another

a. rarity of fossils and breaks in the fossil record can make it hard to link

groups or may lead to scientists making mistakes in assuming species are related to each evolutionarily

F. Change in diet is correlated with an increase in brain size during hominid evolution

1. As brain size increased, the ability to hunt and farm more efficiently increased

2. This leads to better nutrition, which in turn supported an even greater

cranial capacity3. In essence, an evolutionary

feedback loop

G. Genetic Evolution vs. Cultural Evolution1. Genetic Evolution- the random change of base pair sequences

a. unit is the geneb. results in changes in

anatomy/physiology2. Cultural Evolution- the change in practices and traditions, communicated in some form from

generation to generationa. unit is language or symbolsb. Examples: art /agriculture/ language technology

3. Both types of evolution help humans to rise above environmental limiting factors (e.g. food, water, disease), but cultural evolution tends to occur faster than genetic evolution

IV. The Hardy-Weinberg Principal (D.6)

A. Evolution occurs at the population level, not the individual level

1. An allele that helps members of a species to adapt best to their environment will most likely to be passed on to the next generation

2. Over time the frequency of that particular allele will increase while the other alleles for that gene will decrease

3. The change in allele frequencies of a population is what the Hardy-Weinberg principal is based on

B. Hardy-Weinberg Equation

1. Assuming a STATIC population and A=p and a =q:

–P + q = 1 (1.0 = 100%)

–Possible genotypes are: pp, pq, qp qq

–p x p = p2, etc., therefore…

–P2 + 2pq + q2 = 1

C. Example: Cystic Fibrosis is a recessive genetic disorder. In a certain population, 2 out of every 2000 individuals have cystic fibrosis. What are the values of p & q? What percentage of the population are carriers?

• q2 = 2/2000=.001• q = √.001 = .031• p + .031 = 1• p = .969• 2pq = .06• Hence, 6% of the population are carriers.

D. The Hardy-Weinberg principle can also be used to calculate allele, genotype and phenotype frequencies for genes with two alleles

E. The HW equation only works if the following assumptions hold true:

1. Large population

2. Random mating

3. No natural selection

4. No allele-specific mortality

5. No mutation

6. No immigration or emmigration

V. Phylogeny and Systematics (D.5)

A. Value of classifying organisms

1. Organization of data assists in identifying organisms

2. Suggests evolutionary links

3. Allows prediction of characteristics shared by members of a group

B. Biochemical evidence of evolution1. Universality of the genetic code

a. All amino acids are coded for by mRNA codon sequences, which are transcribed from DNA codonsb. Codons are derived from the same four bases regardless of species: A,T,C,Gc. The universality of the code points to a common evolutionary ancestry

2. Hemoglobin

a. found in most animals, but the nucleotide sequence can vary by species

b. Tracking and comparing these variations can help species relative each other on the phylogenetic tree

3. Mutations can be used as an “evolutionary clock”

a. Mutations in a genome (due to mistakes during replication) occur at

a predictable rateb. Therefore, base pair sequences in two populations can be compared, and by looking at the number of differences between the two, an inference can be made as to how

long ago the two populations diverged reproductivelyc. The greater the number of differences, the farther apart the two groups are on the phylogenetic tree

C. Cladistics

1. Clade = A group of organisms who share common

characteristics

2. Cladistics = A taxonomic system of separating clades based on the sharing of derived characteristics from common ancestors

3. Analogous vs. Homolgous characteristicsa. Analogous characteristics - show

similarity without having a common ancestor- develop due to evolving in similar habitats or facing a similar environmental challenge- e.g. the wings of bats, birds, and insects

b. Homolgous characteristics – show similarity due to having a common ancestory

- e.g. bone structure of a whale flipper and a human hand

- used for developing cladograms

4. How to create a cladograma. Cladograms start with an “in group”,

which contain certain characteristicsb. Another group is then compared to

the “in group”c. If it differs in any way it is placed in

its own claded. Clades are separated from each

other based on single differences, and are then placed in sequence

e. Note that cladograms do not make any assumptions about the time period

involved in an evolutionary change, rather they indicate that one has occurred

5. Relationship between cladograms and the classification of living

organismsa. Monophyletic – group that shares a common ancestorb. Paraphyletic – a group which contains some, but not all members associated with a common ancestorc. Polyphyletic – a group which does not share a common ancestor