Evolution

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Lamarck’s theory• Believed that the extensive use of a certain body part would make

that body part stronger or longer, while not using that body part would make it shorter or weaker. Traits acquired during lifetime can be passed on to the next generation.• This is called the Theory of Use and Disuse• While this part of the theory was wrong, Lamarck did believe in

descent with modification (traits being passed on from generation to generation)

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Darwin’s Theory• Went on a 5 year trip on the HMS Beagle around the globe to develop

his theory.• Theory based on Natural Selection: there are variations of traits

among the member of a population. Individuals with more favorable traits will survive and reproduce, passing on their traits to the next generation, and thus, in time, making their traits more common among the population.

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Natural selection• Darwin stated that overproduction of offspring leads to competition

for resources among the members of a population. The individuals that have the traits necessary to outcompete the other individuals will therefore survive, reproduce, and pass on their traits.

In this example, beetles with green color are being prayed on, so the orange individuals have an advantageous trait. Orange beetles, then, are more likely to survive, reproduce, and pass on their traits of orange coloring. In time, the advantageous orange color trait will appear more often in the population because more orange individuals have reproduced. This is how change occurs. Populations evolve, not individuals! evolution.berkeley.edu

Evidence of Evolution• Direct observation: insect

populations can become immune to pesticides in short periods of time. Also, viruses and bacteria can do the same thing.

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• Fossil record: Fossils are remains or traces of organisms from the past, showing that changes have occurred over time and the origin of major groups or modern organisms.

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Homology and Convergent Evolution: Characteristics shared by two different (but related) species that serve a different function. Example: forelimbs of mammals

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Homologous structures!

• Embryonic homologies: At early stages of animal development, there are many anatomical similarities among species. They suggest a common ancestor.

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• Vestigial Organs: Anatomical structures that are of little, if any, importance to the organism. They’re there because those structures served an important function in the organism’s ancestor. Suggests change over time.• Some snakes have a femurand leg bones.

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Molecular Homologies• All known life forms use the same genetic code ( DNA & RNA). Amino

acid sequence similarities among species show common ancestry.

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Less amino acid differences show a closer common ancestor (therefore they are more closely related).

Convergent Evolution• When two different species develop similar adaptations to similar

challenges that the environment presents. These similarities are considered analogous and do not reflect common ancestry. The similarities are the result of a similar solution to a similar problem.

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Biogeography• The geographic distribution of species: species in a desert in South

America are more closely related to species in other habitats of South America than to species in other geographic areas.

Genetic Variations Cause Evolution• Microevolution: change in allele frequencies of a population over

generations• Mutations are the only source of “new genes” and “new alleles” (point

mutations, chromosomal mutations).• Most variations, however, are due to the recombination of alleles during

reproduction (increase variations in genes already in population):1. Crossing over- in prophase I of meiosis, homologous chromosomes exchange

genetic information.2. Independent assortment- chromosomes are passed on independently 3. Fertilization- ( 2^23 x 2^23 possible combinations for human sperm & egg)

Hardy Weinberg Principle• States that frequencies of alleles in a population will remain the same

over generations unless it is acted upon by evolutionary forces (natural selection, genetic drift, and gene flow). Populations in Hardy Weinberg equilibrium are not evolving (however, populations are always evolving)• Equation: p^2 + 2pq + q^2 =1 , where p^2 is the frequency of the

homozygous dominant allele, 2pq is the frequency of the heterozygous allele, and q^2 is the frequency of the homozygous recessive allele, and:• p + q=1, where p is the dominant allele and q is the recessive allele.

Evolutionary Forces• Natural selection: individuals with variations that are advantageous in

an environment are able to reproduce more efficiently (and pass their genes on) than those that are less suited to the environment.• Genetic drift: unpredictable fluctuation in allele frequencies from one

generation to the next. The smaller the population is, the more prone it is to genetic drift. Ex- founder effect, bottleneck effect.• Gene flow: when allele frequencies in a population change because of

emigration or immigration of individuals to or from a population.

• Types of selection:- Directional selection: when individuals with a traits at one extreme

variant are favored.- Stabilizing selection: when individuals with traits that are intermediates are favoredrather than the extremes.- Disruptive selection: when individuals with traits that are at the two extremes are favored.

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• Sexual selection: when individuals of a species choose mates for reproduction based on a specific trait. This trait is therefore favored, and over time, it will be more frequent in the population.• Example:

Female peacocks are attracted to males with bigger tails

Populations Evolve• Species: A population or group of populations whose members can

reproduce in nature and produce viable, fertile offspring, but cannot produce viable, fertile offspring when mating with members of other populations. Species are isolated from each other (reproductively)• Prezygotic isolation:- Habitat isolation: species don’t live in the same habitat- Behavioral isolation: different species use different signals or behaviors to attract mates.- Mechanical isolation: species may be anatomically incompatible- Gametic isolation: gametes may be unable to from a zygote.

• Postzygotic isolation:- Reduced hybrid viability: zygote can’t develop into an organism.- Reduced hybrid fertility: the offspring produced is sterile and can’t

reproduce.- Hybrid breakdown: when the hybrid mates, it cant produce healthy

offspring, or the offspring are sterile.

• Adaptive radiation: when many species arise from a single common ancestor.• Speciation can occur rapidly or slowly:• Gradualism: states that species descended from a common ancestor

and gradually diverged more and more (slower pace)• Punctuated equilibrium: characterized by periods of time where there

is no change followed by a sudden change (quick).

Origin of Life• Earth was formed about 4.6 billion years ago, and life on earth

emerged 3.8 to 3.9 billion years ago.• The current theory states that1. Small organic molecules were synthesized2. These small molecules joined into macromolecules, such as proteins

and nucleic acids.3. All these molecules were packaged into protocells (ancient cell

ancestor).4. Self replicating molecules emerged that made inheritance possible

• Miller and Urey tested the hypothesis that organic compounds could’ve been synthesized in primitive earth conditions (Oparin & Haldane stated this hypothesis)• It is hypothesized that RNA was the first genetic material. RNA can

also carry out catalytic functions (called ribozymes).

• Earliest living organisms were prokaryotes.• 2.7 billion years ago: oxygen started to accumulate in the atmosphere• 2.1 billion years ago: first eukaryotes appeared.

Endosymbiotic hypothesis • Proposes that mitochondria and chloroplasts were small prokaryotes

that began a symbiotic relationship with other cells.- Both have enzymes and transport systems similar to those of

prokaryotes- Replication process similar to that of prokaryotes- Both have a single, circular DNA with no histone proteins (histone

proteins are associated with eukaryotes)- Both have their own ribosomes which can translate their own DNA

into proteins