• How Organisms Evolve

• 1. The determination of a trait in diploid individuals depends on
• A. Interaction between alleles :
– Dominant-recessive
– Codominan t
– Incomplete dominance
– Sex-linked
• B. Environment
– The changes an individual experiences as it grows and develops are not evolutionary changes

• For example, coat color in hamster is determined by 2 alleles for pigment:
– B  black, dominant
– b  brown, recessive

• Occur from generation to generation
• Cause descendants to differ from their ancestors
• Occur at the population level

• 3. Can we measure evolution?
– Yes , if we can measure allele change in the gene pool of a population.
GENE POOL :
– Sum of all genes in a population
– Allele frequency ( proportion ) = specific allele number/ Total number of possible alleles in a population
– Evolution Is the Change Over Time of Allele Frequencies
Within a Population. In order to measure change we need a point of reference

• For example, coat color in hamsters:
– A population of 25 hamsters contains 50 alleles of the coat color gene (hamsters are diploid)
– If 20 of those 50 alleles code for black coats, then the frequency of the black allele is 0.40
or 40% [20/50 = 0.40]

• 4. What would be required for a population to be in Equilibrium (without evolving)?
• Hardy-Weinberg proposed that, given the following conditions, the frequencies of alleles and genotypes in a sexually reproducing population remain constant from one generation to the next
1. No mutation
2. No gene flow between populations
3. Population must be very large
4. Mating must be random
5. No natural selection

5.
What Causes Evolution?
Answer: the opposite of equilibrium.
Violation of one or more of these five conditions may allow changes in allele frequencies
Equilibrium requires:
•a. No mutation
•b. No gene flow between populations
•c. Large population
•d. Random mating
•e. No natural selection
Evolution requires:
•a. mutation
•b. gene flow between populations
•c. Small population
•d. Selective mating (nonrandom)
•e. Natural Selection of individuals (the fittest)

• Mutations are rare changes in the base sequence of DNA in a gene
– Usually have little or no immediate effect (1 in 100,000-1,000,000 gametes)  little effect on Hardy-
Weinberg proportions of common alleles
– Are the source of new alleles .
Sources of genetic variability.
– In gamete  offspring.
– Can be beneficial, harmful, or neutral
– Not goal-directed . Arise spontaneously, not as a result of, or in anticipation of, environmental necessity

– Movement of alleles from one population to another.
• New alleles can be transferred
• Tend to homogenize alleles in separate populations (reduce the differences in the gene pools).
• Blockage of gene flow may create new species

• Genetic drift is the random change in allele frequencies over time, brought about by chance alone
– Has minor impact in very large populations
– Occurs more rapidly and has bigger effect on small populations

Genetic Drift. In small populations
(a) Population size = 10,000
1.0
(b) Population size = 4
1.0
0.9
0.8
0.9
• Frequencies of particular alleles may
0.7
0.6
0.5
0.4
0.3
0.2
0.1
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change by chance alone.
0.6
• Important in small populations.
0.5
– Bottleneck Effect - Drastic reduction in population, and gene pool size.
– Founder Effect - Few individuals found new population
0.2
(Small allelic pool).
0.3
0.1
0.0
0 1 2 3 generation
4 5 6
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0 1 2 3 generation
4 5
The effect of population size on genetic drift
6

• There are two causes of genetic drift:
• A. Population bottleneck . D rastic reduction in population size brought about by a natural catastrophe or over-hunting
– It can change allele frequencies and reduce genetic variation

• Northern elephant seal
– Hunted almost to extinction in the 1800s
– By 1890s, only 20 individuals remained
– Hunting ban allowed population to increase to 30,000
– Biochemical analysis shows that present-day northern elephant seals are almost genetically identical

• B.
It occurs when a small number of individuals leave a large population and establish a new isolated population

• By chance, allele frequencies of founders may differ from those of original population
• Over time, new population may exhibit allele frequencies that differ from original population

• Sometimes choosing the similar (snow geese)
• Sometimes choosing the strongest
• Sometimes choosing the most attractive

Natural and Sexual Selection
• Natural - Nature exerts selection.
• Variation must exist among individuals.
• Variation must result in differences in numbers of viable offspring produced.
• Variation must be genetically inherited .
– Natural Selection is a process , and
Evolution is an outcome .

• Natural selection is often associated with the phrase “survival of the fittest”
• The fittest individuals are those that not only survive, but are able to leave the most offspring behind

• Successful phenotypes are those that have the best adaptations to their present environment
– Adaptations are characteristics that help an individual survive and reproduce
• Adaptations arise from the interactions of organisms with both the nonliving and living parts of their environments

• Nonliving (abiotic) components include:
– Climate
– Availability of water
– Concentration of minerals in the soil
• Living (biotic) components include:
– Interactions with other organisms:
• Competition
• Coevolution
• Sexual selection

• Competition is an interaction among individuals who attempt to utilize a limited resource
– May be between individuals of same species or different species
– Most intense among members of the same species

• Coevolution is the evolution of adaptations in two species due to their extensive interaction
– e.g. predator-prey relationships, akin to a
“biological arms race”
• Wolf predation selects against slow, careless deer
• Alert, swift deer select against slow, clumsy wolves

• Sexual selection is a type of natural selection that favors traits that help an organism acquire a mate
– Conspicuous features (bright colors, long feathers or fins, elaborate antlers)
– Bizarre courtship behaviors
– Loud, complex courting songs
• Traits derived by sexual selection make males more vulnerable to predators

• Male-male competition for access to females
– Favors evolution of features that provide an advantage in fights or ritual displays of aggression

• Female mate choice
– Male structures, colors , and displays that do not enhance survival might provide an outward sign of a male’s health and vigor

• Natural selection and sexual selection can affect populations in three ways:
– Directional selection
– Stabilizing selection
– Disruptive selection

Forms of Natural
Selection
BEFORE
SELECTION
• Directional Selection
– Selection eliminates one extreme from a phenotypic array.
• e.g. pesticide resistance, antibiotic resistance
AFTER
SELECTION range of a particular characteristic (size, color, etc.)

Forms of Natural
Selection
• Stabilizing Selection
– Intermediate phenotype is favored
– Selection acts to eliminate both extremes.
– Phenotypic variability declines
– E nvironmental conditions are relatively constant
– e.g. body size in Aristelliger lizards
• Smallest lizards have a difficulty defending territory
• Largest lizards more likely to be eaten by owls
STABILIZING SELECTION

– Selection acts to promote both extremes and eliminate the average.
– Environment has more than one type of useful resource:
Population divides into two phenotypic groups over time.
• e.g. beak size in black-bellied seedcrackers
– Birds with large beaks eat hard seeds
– Birds with small beaks eat soft seeds

BEFORE
SELECTION
DIRECTIONAL SELECTION STABILIZING SELECTION DISRUPTIVE SELECTION
AFTER
SELECTION range of a particular characteristic (size, color, etc.)

A. Evolution is a compromise between opposing pressures …
• Not all genotypes changes are beneficial.
– i.e. giraffe’s neck:
• Larger can help on a fight
• Can make it vulnerable

B. Antibiotic Resistance
Evolves by Natural
Selection.
• Penicillin kills most
Bacteria. The ones with penicillinresistant mutation survive and reproduce.

• C. Kin selection: Altruism and Selective cannibalism . Some individuals may enhance the survival of individuals from their own species.

Notes on Selection
D. Artificial Selection
• Breeders exert selection.

E. Balanced polymorphism is the prolonged maintenance of two or more alleles in a population
• Balanced polymorphism often occurs when environmental conditions favor heterozygotes
– e.g. normal and sickle-cell hemoglobin alleles in malaria-prone regions of Africa

HH Hh hh
dies of malaria lives and reproduces dies of sickle-cell anemia
HH dies of malaria
Hh lives and reproduces hh dies of sickle-cell anemia
HH Hh hh

• Heterozygote advantage (balanced polymorphism)