3 Mechanisms for selection and an introduction to HardyWeinberg Lyells Influence In attempt to explain the past in terms of present day processes Darwin went to local farmers and animal breeders and observed ID: 686043
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Slide1
Natural Selection as the Mechanism for Evolution
3 Mechanisms for selection and an introduction to Hardy-WeinbergSlide2
Lyell’s Influence
In attempt to explain the past in terms of present day processes, Darwin went to local farmers and animal breeders and observed…
Variation in the organisms that could be inheritedAfter many generations, organisms appear very different from ancestorsDarwin called this process ARTIFICIAL SELECTION Slide3
Artificial Selection in Dogs130,000 yearsSlide4
Artificial Selection in Pigeons
It was pigeons, not finches, that Darwin used to argue natural selectionThe pigeon (better termed the Rock Dove)
domesticated 5,000-10,000 years agoFood? Fun? Why pigeons??Pigeon fanciers – began breeding these pigeons for aesthetic qualities. By the time Darwin became interested in them, there were several hundred varieties of domestic pigeon available. Slide5
NATURAL SELECTION
Variation in Nature
No two organisms are exactly alike!Struggle for Existence (Malthus)
High birth rates and limited resources will cause organisms to competeSelective pressures (predation, competition, parasitism, disease, pesticides, etc)
Survival of the FittestOrganisms with the best adaptations survive and reproduce more often
Reproduction of Viable OffspringOffspring must be fertile and reproduce Slide6
Understanding Histograms
A histogram is a graphical representation of the distribution of data. It is an estimate of the probability distribution of a continuous variable (quantitative variable)
Helps to understand how data is distributed. Shape of many distributions in natural systems are bell curves.Slide7
Three mechanisms for selectionSlide8
Natural Selection on Populations
STABILIZING SELECTION
:
Individuals in the center of the curve have higher levels of fitness than the individuals at either end
Example
: Birth weight in humans or marine creatures on ocean
floorSlide9
Natural Selection on Populations
Why would stabilizing selection occur in human babies? Slide10
Natural Selection on Populations
DIRECTIONAL SELECTION
:
Individuals at one end of curve have higher levels of fitness than the individuals at the other end of the curve
Ex
: Peppered
Moths in England post Industrial RevolutionSlide11
Directional Selection
Would you expect the same amount of each phenotype in different environments?In which environment would you find the most black moths? WhySlide12
Natural Selection on Populations
DISRUPTIVE SELECTION:
Individuals on the ends of the curve have higher fitness levels than individuals in the center of the curve
Example
: Beak size in
FinchesSlide13
Natural Selection on PopulationsSlide14Slide15
Microevolution: A Population’s Gene Pool
A gene pool is all the alleles available in all of the individuals in a populationChanging allele frequencies means the population is evolving
→ microevolutionSlide16
Hardy-Weinberg Equilibrium
It is a condition in which no change in alleles occurThe equation used to determine allele frequencies:
p2
+ 2pq + q2 = 1
p = frequency of dominant allele (A)q = frequency of recessive allele (a)p2 = AA q2 = aa 2pq = AaSlide17
Hardy-Weinberg EquilibriumSlide18
Hardy-Weinberg Equilibrium
Example 1: In one hypothetical Zebra Mussel (Dreissena
polymorpha) population, most of the individuals have dark, zebra-striped shells (below left). However, solid light-colored shells (below right, caused by a homozygous recessive gene, aa) occur in 1 of every 10,000 individuals.Slide19
Hardy-Weinberg Equilibrium
Solution 1:frequency of aa = q2 = 1/10,000 = 0.0001, so q = 0.01number of aa = 0.0001 x 10,000 = 1
individualp + q = 1, so p = 0.99frequency of AA = p2 = 0.9801number of AA = 0.9801 x 10,000 = 9,801 individuals
How many heterozygous individuals?Slide20
Hardy-Weinberg Equilibrium
Example 2: The Coquina Clam (Donax
variabilis) is highly polychromic: (Polymorphism expressed as existing in several different colors. (adj. polychromic)) (with shells of many different colors.)
In a population of 2,000 clams, 1,920 are solid colored, whereas the remainder has radiating color bands. Solid color occurs in homozygous dominant (BB) and heterozygotes (Bb); color banding only occurs in homozygous recessive individuals (bb).Problem: Calculate gene frequencies
and numbers of BB and Bb.Slide21
Hardy-Weinberg Equilibrium
Solution 2: 1,920 are solid (BB and Bb), so 80 banded are recessive (bb)
frequency of bb = q2 = 80/2000 = 0.04, so q = 0.20p + q = 1, so p = 0.80number of BB: p2 = 0.64, so BB in population of 2,000 = 0.64 x 2,000 = 1,280 individualsnumber of Bb: 2pq [frequency of Bb] = 2 x 0.2 x 0.8 = 0.32, so Bb = 0.32 x 2,000 = 640 individuals