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Objective: Understand how allele frequencies can show evolution in a population.

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Presentation on theme: "Objective: Understand how allele frequencies can show evolution in a population."— Presentation transcript:

1 Objective: Understand how allele frequencies can show evolution in a population.

2  Individuals have variations but they do not evolve.  The smallest scale (microevolution) shows change in allele frequencies in a population over time.  Causes of evolution:  Natural selection (fittest organisms)  Genetic drift (random catastrophes change allele frequencies)  Gene flow (movement of alleles between populations)

3  Genetic Variation  Diverse, inheritable traits set the stage for evolution Variation within a population Variation between populations  Sources of Genetic Variation  Formation of new alleles by mutation  Chromosomal alterations (deletion, duplication, translocation, etc.)  Sexual reproduction (crossing over, independent assortment, and fertilization).

4  A gene pool is a summative of a population’s genes.  Allele frequency is the number of times one allele appears in the gene pool.  # of time allele is present/total # of alleles Ex: 500 flowers = 320 Red (RR) pink (RW) + 20 White (WW)  Total alleles = 1,000 (each individual has 2 alleles)  = 800 red alleles  = 200 white alleles  Frequency of red = 800/1000 = 80%  Frequency of white = 200/1000 = 20%

5  A control to compare evolving populations to.  H-W Equilibrium 1. Large population 2. No movement into/out of population 3. No mutations 4. Random mating 5. No natural selection (no beneficial/lethal alleles)

6  p = dominant allele frequency  q = recessive allele frequency  p + q = 1  p 2 + 2pq + q 2 = 1  AA + Aa + aA + aa = 1  Applying the H-W Equation  See if evolution is happening (allele frequencies change.)  This can be used to calculate the number of heterozygotes vs homozygous dominant individuals

7 Natural Selection  Nonrandom mating  Traits allow you to have more offspring.  Ex: sexual dimorphism: males are elaborately decorated to attract mates.

8 © 2011 Pearson Education, Inc. Causes of Evolutionary Change Animation: Causes of Evolutionary Change Right-click slide / select “Play”

9 Genetic Drift  Random changes in allele frequencies over time, reducing variation.  Bottleneck Effect  An event causes a loss of the majority of a population.  Founder Effect  A few individuals leave to start a new population

10 Gene Flow  Reduces differences between populations by sharing of gametes across them.

11 Modes of Selection  At any moment, populations show a normal curve for most traits.  This curve can change in 3 ways depending on how the environment selects for a trait. Phenotypes (fur color) Original population Frequency of individuals

12 Directional Selection  Common when environment changes.  One of two extremes is favored.  EX: Increase size of bears during ice ages Original population Evolved population

13 Disruptive Selection  Both extremes are favored while average disappears.  EX: beak size in finches (large for hard seeds and small for soft)

14 Stabilizing Selection  Extremes disappear increasing the intermediate.  EX: birth weight of babies

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