Forces of Evolution

Question 1

What levels does evolution work at?

Answer 1

• Evolution works at four levels:
  1. Molecular
  2. Cellular
  3. Individual
  4. Population
• The levels reflect different aspects of evolution and are integrated in a way that produces evolutionary change
• Evolution works on variation

Question 2

Describe the theory of modern synthesis

Answer 2

• Evolution is a two-stage process
  1. The production and redistribution of variation (inherited differences between individuals).

2. Environmental pressures act on this variation -ultimately affecting the ability to reproduce successfully – natural selection acting on the phenotype
3. Some combinations of individuals may have more difficulty due differences in blood type, HLA (human leukocyte antigens),and other potentially incompatible differences.

Question 3

Describe recombination as a source of variation

Answer 3

• In sexually reproducing species both parents contribute genes to offspring.
• The genetic information is reshuffled every generation.
• Recombination doesn’t change allele frequencies, however, it does produce different combinations of genes that natural selection may be able to act on.

Question 4

What is the evolutionary significance of meiosis?

Answer 4

• Meiosis and sexual reproduction are highly important evolutionary innovations.
• Meiosis increases genetic variation at a faster rate than mutation.
• Offspring in sexually reproducing species represent the combination of genetic information from two parents.

Question 5

Describe Recommination

Answer 5

• Exchange of genetic material between the strands of the homologous pairs.
• May happen as a “crossing over” when the arms touch each other and the DNA is exchanged between the chromosomes
• Creates a new mix of genetic data but not a new genetic data – like shuffling a deck of cards

Question 6

What are the issues involved with recombination?

Answer 6

• Increase crossing over occurs further away from the centromere
• There are areas called “hotspots” in our DNA where recombination is more likely
• There is sex-specific variation in the frequency and areas of recombination occurrence

Question 7

Describe the forces of evolution

Answer 7

Forces which alter allelic frequencies in populations
Mutation
Gene Flow
Genetic Drift % Founder Effect
Natural selection

Question 8

Describe Mutations:

Answer 8

-Mutation is a molecular alteration in genetic material:
-For a mutation to have evolutionary significance it must occur in a gamete (sex cell).
-Such mutations will be carried on one of the individual’s chromosomes.
-During meiosis the chromosome carrying the mutation will assort giving a 50% chance of passing the allele to an offspring.
(evolution on a molecular level)
-Source of all new alleles
-Frequency is so low, however, that allele frequencies cannot change appreciably simply by mutation
-Allele frequency of 1% in the population adequate enough to be considered 167.5 generations to reach 1% from one mutation
-In 2 million years (76,751 generations) a dominant allele could reach frequency of 99% in a population
-If allele is lethal early, it will not be passed on
-If allele is detrimental later in life, it may be passed on and not affect fertility

Question 9

Describe mutation rate

Answer 9

• Estimates of mutation per locus (unit of inheritance or gene) per generation is 2-5 per 100,000
• Estimates of mutation rate for base pair (bp) per generation is 2.5 per 100,000,000.
• Average per individual = 175 bp mutations

-Every time human DNA is passed from one generation to the next it accumulates 100–200 new mutations, according to a DNA-sequencing analysis of the Y chromosome.

Question 10

Describe the mitochondrial mutation rate

Answer 10

• The rate of mutaiton in mtDNA is 10 to 17 times higher than nuclear gene.
• several possible factors that may cause the high mutation rate in mtdna include–insufficient DNA repair system, lack of DNA proteins such as histones, or spontaneous error arising during DNA replication.

Question 11

What are the causes of mutation?

Answer 11

• Radiation: breaking the double helix strand, and the repair is not exact as previous sequences or changes in properties of cytosine and thymine
• Viruses: substitution, insertion, deletion, and frameshift
• Transposons: (ie: Alu) segments of DNA that moves around in the genome (with help of an enzyme). Serious if into promotor or enhancer regions and impede transcription
• Mutagenic chemicals: Benzene, an industrial solvent and precursor in the production of drugs, plastics, synthetic rubber and dyes

Question 12

What are the outcomes of mutation?

Answer 12

• Mutations may be lethal
• Mutations may be harmful
• Mutations may be beneficial
• Mutations may have no effect on the organism

Question 13

Define Silent mutation

Answer 13

codon produces same amino acid: may affect DNA but not the protein so phenotype unchanged

Question 14

Define missense mutation

Answer 14

-changes one amino acid for another – such as in sickle cell hemoglobin GAG changed to GUG

Question 15

Define Nonsense Mutation

Answer 15

-converts an amino acid into a stop codon shortens resulting protein or makes nonfunctional protein

Question 16

Define Gene flow:

Answer 16

• Gene flow is the exchange of genes between populations.
• If individuals move temporarily and mate in the new population (leaving a genetic contribution), they don’t necessarily remain in the population.
• Example: The offspring of U.S. soldiers and Vietnamese women represent gene flow, even though the fathers returned to their native population.

-The effect of migration or gene flow increases heterozygosity in a population

Question 17

Define Genetic drift

Answer 17

-Genetic drift is directly related to population size.
-Genetic drift occurs when some individuals contribute a disproportionate share of genes to succeeding generations.
-Drift may also occur solely because the population is small:
-Alleles with low frequencies may simply not be passed on to offspring, so they eventually disappear from the population.
-Effect of drift is to increase the individuals who are homozygous in a population
( p’s and q’s in Hardy Weinberg)
-If one allele disappears from a population, the remaining allele has reached fixation

Question 18

Describe Founder Effect:

Answer 18

• the reduced genetic diversity that results when a population is descended from a small number of colonizing ancestors.
• Pitcairn Island In Pacific Ocean (Bounty survivors)
• Crew of the Bounty sailed to Tahiti, picked up women, sailed on to Pitcairn Island, and burned their boat. 7 generations of these survivors of this founder effect

Question 19

Describe Bottleneck effect

Answer 19

• Population bottlenecks occur when a population’s size is reduced for at least one generation.
• Tristan de Cunha in the South Atlantic
• Founder effect and Bottleneck: A poor winter had left them short of food, and a boat sent to barter with a ship offshore was lost with all hands — fifteen men. Only four adult males were left on the island

Question 20

Describe Natural Selection

Answer 20

-Natural selection provides directional change in allele frequency relative to specific environmental factors.
If the environment changes, selection pressures also change.
-If there are long-term environmental changes in a consistent direction, then allele frequencies should also shift gradually each generation.

Question 21

Describe fitness

Answer 21

• Fitness: Contribution to the next generation
• Selection favoring the heterozygote produces balanced polymorphism, where both alleles retained no matter what initial frequency is

Question 22

Sickle Cell in various environments

Answer 22

1) Washington State: 
          Selection for the dominant
2) Central Africa:
          Selection for the heterozygote
          Balanced polymorphism
3) Aspen Colorado
           Selection for the dominant – and likely a more rapid loss of s allele than in Washington state

Question 23

Describe Hardy Weinberg Equilibrium

Answer 23

• Mathematical equation to establish if allele frequencies are changing
• Expresses under ideal conditions, the predicted distribution of alleles or probable genotypes in a population: The Central Theorem of Population Genetics

Question 24

What are the ideal conditions of HWE?

Answer 24

-Mutation is not occurring
-Natural selection is not occurring
-The population is infinitely large
-All members of the population breed
-All mating is totally random
-Everyone produces the same number of offspring
-There is no migration in or out of the population

By convention
p = freq of dominant (A)
q = freq of recessive (a)
pq= freq of heterozygote (Aa)

Question 25

What can HWE be used for?

Answer 25

• Can be used to determine if an assumption has been violated
• Such as nonrandom mating, or selective pressure affecting the genotype frequencies