II. Theories of EvolutionA. Development of Theories
Observations Lead to a Question• 13 species of finches unique to Galápagos islands
• ALL most closely resemble 1 finch species living on the South American mainland.
• The hypothesis: the islands were colonized by a single finch species that strayed from the mainland -- This bird adapted differently to the different habitats on each island
• key characteristic of the finches -- beaks– adapted to the foods available on the each island
• How did these different beaks arise? -- through natural selection
1. Jean-Baptiste Lamarck (1744-1829)
First person to suggest that life evolved
a) The law of Use and Disuse
b) Inheritance of acquired characteristics
Lamarck believed that giraffes stretched their necks to reach food. Their offspring and later generations inherited the resulting long necks
2. Darwin’s Theory: Natural Selection (Based on his observations)
a) Variation exists within all species
b) Populations always tend to increase
c) The amount of resources is limited
d) There will be a struggle for the available resources
e) Organisms who are most fit will live longer and have more offspring. (The offspring will inherit advantageous traits)
Most giraffes used to have short necks, but some had slightly longer necks. When the food on the lower branches was eaten, many of the giraffes with shorter necks died of starvation, leaving the ones with slightly longer necks to survive and reproduced. Through many generations, the giraffes with longer necks became the most common.
Pesticides = Natural Selection in Action
• Pesticides: poisons used to kill insects • 100’s of insects have developed pesticide resistance• Survivor bugs after the first pesticide treatment had
genes that somehow enabled them to resist the chemical attack.
• Survivor’s offspring inherited the genes for pesticide resistance.
• A pesticide does not create resistant individuals, but selects for resistant insects that are already present in the population.
Artificial Selection• selective breeding of plants / animals
produce offspring w/genetic traits that humans value
• plant breeders improve traits -- grain production, disease resistance, protein content
• animal breeders select for growth rate or temperament
• Breeders play the role of the environment, allowing only those plants or animals with desired traits to reproduce.
Artificial Selection• Darwin observed that artificial selection
could produce a great deal of change in a species in a short time.
• He reasoned that over thousands of generations, natural selection could also cause major change.
• Artificial Selection = humans choose• natural selection favors traits that benefit
the organisms in their particular environment
• environmental conditions do the "selective breeding."
3. Origin of Variation
• Genetic Recombination (Sexual reproduction. Meiosis, Crossing over and random fertilization)
• Mutation (Produces New Genes)
B. Mechanisms of Evolution
1. Species and Populations• Evolution cannot be seen in
an individual because their genes do not change.
a) Evolution is a change in the genetic make up of a population
b) Gene Pool- all the different alleles (gene forms) in a population
2. Hardy-Weinburg principle
Gene Pool
2.
Population Genetics Or Fun with Hardy-Weinburg!
The Math!
Let P = the frequency of Dominant Genes in a population
Let q = the frequency of Recessive genes in a population
Therefore:
P + q = 1
All the dominant genes + all the recessive genes = all the genes
Individuals can be:
Genotypes Hardy-Weinburg Notation
Homozygous Dominant PP or P2
Heterozygous Pq + qP or 2Pq
Homozygous Recessive qq or q2
p q
p
q
P2 Pq
qP q2
Therefore:P2 + 2Pq + q2 =1
All the Homozygous Dominant + All the Heterozygotes + All the Homozygous Recessives = All the Population
With these formulas, you can solve all Hardy-Weinburg problems
C. Factors which Change Genetic Equilibrium (ie. Cause Evolution)
1. Natural Selection: Favoring one genotype over another. Ex. H. B. Kettlewell
In England, before the industrial revolution, tree bark was a lighter color and lighter moths out numbered darker moths 9:1
Coal caused the tree bark to darken during the industrial revolution. The darker moths then numbered the lighter moths 9:1
Types of Selection
a) Directional Selection- constant genetic change of one type. Ie. Giraffe’s necks getting longer. Dinosaur size increasing
b) Stabilizing Selection- Selection that favors the average and eliminates the extremes. The population becomes more alike
Types of Selection
• Siberian Huskies – sled dogs (35-60 lbs.)• Too heavy – too slow and potentially sink in
snow• Too light – not strong enough to pull sleds• The population all becomes similar to each other
c) Disruptive selection- Selection that favors the extremes and eliminates the average. Population becomes more different. This can lead to new species
Types of Selection
• Meteor hit Earth 65 mya
• Food was depleted, dust blocked the sun, tsunamis and wildfires were plentiful.
• Larger (average) animals who need lots of oxygen and food, died out quickly.
Types of Selection
C. Factors which Change Genetic Equilibrium (ie. Cause Evolution)
2) Gene flow- Movement of organisms in or out of a population
3) Mutation- change in a gene form that is unequal. Ie. Change form gene A to a more than a to A.
4) Genetic Drift- change in gene frequency due to random chance (some genotypes may mate more than another just due to chance)
5) Isolation- separation of a population so it can’t interbreed. Either genetic or geographic
Patterns of Evolution
1. Divergent evolution-Two separate species becoming more different due to different selective pressures in different environments (human foot vs. ape foot)
a) Adaptive Radiation- process by which species adapt to a variety of habitats
b) Speciation- formation of two separate species from one
SPECIATION
2. Convergent Evolution- the process by which distantly related organisms become more similar due to similar selective pressures in similar habitats
