Chapter 19 presentation

Chapter 19: THE EVOLUTION OF POPULATIONS

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• All life on Earth is related.• Humans, beetles, plants, and bacteria all share a common ancestor.

• Natural selection acts to promote traits and behaviors that increase an organism’s chances of survival and reproduction.

• Mutations and other sources of variation among individuals, as well as the evolutionary forces that act upon them, alter populations and species.

Introduction

Figure 19.1

• Living things may be single-celled or complex, multicellular organisms. They may be plants, animals, fungi, bacteria, or archaea. This diversity results from evolution. (credit “wolf”: modification of work by Gary Kramer; credit “coral”: modification of work by William Harrigan, NOAA; credit “river”: modification of work by Vojtěch Dostál; credit “fish" modification of work by Christian Mehlfuhrer; credit “mushroom”: modification of work by Cory Zanker; credit “tree”: modification of work by Joseph Kranak; credit “bee”: modification of work by Cory Zanker)

19.1: Population EvolutionPopulation genetics

• Study of allele frequencies in a population, (i.e. studying if they are changing from generation to generation, or not)

• Evolution and Flu Vaccines• ABO blood type system

• Evolution is a significant change in allele frequencies of a population over generations

• Equilibrium is no significant change in allele frequencies of a population over generations

• Gene pool is the sum of all the alleles in a population.

Hardy–Weinberg Principle of Equilibrium

• States that population in genetic equilibrium will maintain consistent allele frequencies from one generation to the next.

• Genetic equilibrium relies on maintenance of these criteria:

1. No mutation takes place2. No genes are transferred to or from other sources

(no immigration or emigration)3. Random mating is occurring4. The population size is very large5. No selection occurs

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The HW Principle can be written as an equation: p + q = 1

• For 2 alleles, p and qp = frequency of allele B for black coat color

• Black cat is BB or Bbq = frequency of allele b for white coat color

• White cat is bband random mating in that population leads to the next generation, such that:

p2 + 2pq + q2 = 1

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Generation One

Phenotypes

Genotypes BB Bb bb

0.36 0.48

84% 16%

0.16

Frequency of gametes

Frequency ofgenotype in population

0.36 + 0.24 = 0.60B 0.24 + 0.16 = 0.40b

• If all 5 assumptions for Hardy-Weinberg equilibrium are true, allele and genotype frequencies do not change from one generation to the next

• In reality, most populations will not meet all 5 assumptions

• To determine this, look for changes in frequency from one generation to the next, and then suggest hypotheses about what process or processes are most likely at work to cause changes to the frequencies (i.e. try to figure out what is causing evolution)

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Eggs

Sperm

p2 + 2 pq + q2 = 1

bq = 0.40

Bbpq = 0.24

bbq2 = 0.16

Bp = 0.60

BBp2 = 0.36

Bbpq = 0.24

Bp = 0.60

bq = 0.40

Generation Two


Figure 19.2• When populations are in the Hardy-

Weinberg equilibrium, the allelic frequency is stable from generation to generation and the distribution of alleles can be determined from the Hardy-Weinberg equation. If the allelic frequency measured in the field differs from the predicted value, scientists can make inferences about what evolutionary forces are at play.

• https://www.youtube.com/watch?v=oG7ob-MtO8c

https://www.youtube.com/watch?v=oG7ob-MtO8c

https://www.youtube.com/watch?v=oG7ob-MtO8c

19.2: Population Genetics• Population variation is the distribution of phenotypes (physical traits)

among individuals.• Influenced by a number of factors, genetic structure and environment (Figure

19.3). • Polymorphism is when individuals of a population display different

phenotypes, or express different alleles of a particular gene (i.e. fur color).

Figure 19.3

• The distribution of phenotypes in this litter of kittens illustrates population variation. (credit: Pieter Lanser)

Genetic Variance• Natural selection and some other evolutionary forces can only act upon

heritable traits (an organism’s genetic code).• Alleles are passed from parent to offspring.• Acquired traits are not heritable.

• For example: If an athlete works out in the gym daily, building muscle strength, would the athletes offspring grow up to be a body builder?

• Heritability is the fraction of phenotype variation that can be attributed to genetic differences, or genetic variance among individuals in a population.

• Genetic variance is the diversity of alleles and genotypes within a population.• When scientist breed species, such as zoo animals, they try to increase a population’s

genetic variance to preserve that particular phenotype.• Inbreeding is mating of closely related individuals which can cause abnormalities and

susceptibility to disease.

Genetic drift• In small populations, allele

frequency may change by chance alone

• Small populations are at high risk for the loss of alleles

• Magnitude of genetic drift is negatively related to population size

• “Founder Effect” and “Bottleneck Effect” are associated with genetic drift

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Genetic Drift

d. Statistical accidents. The random fluctuation in allele frequencies increases as population size decreases.


Founder Effect

Figure 19.4• Genetic drift in a population can lead to

the elimination of an allele from a population by chance. In this example, rabbits with the brown coat color allele (B) are dominant over rabbits with the white coat color allele (b). In the first generation, the two alleles occur with equal frequency in the population, resulting in p and q values of .5. Only half of the individuals reproduce, resulting in a second generation with p and q values of .7 and .3, respectively. Only two individuals in the second generation reproduce, and by chance these individuals are homozygous dominant for brown coat color. As a result, in the third generation the recessive b allele is lost.

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Figure 19.5

• A chance event or catastrophe can reduce the genetic variability within a population.

• Genetic drift can lead to the loss of alleles in isolated populations.

• Alleles that initially are uncommon are particularly vulnerable.

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Parentpopulation

Bottleneck(drastic reduction

in population)

Survivingindividuals

Nextgeneration


Bottleneck Effect

Bottleneck Effect• Can occur when a population’s size is drastically

reduced (often due to human activity or natural disaster)

• Survivors may constitute a random genetic sample of the original population

• Results in loss of genetic variability (loss of alleles) which cannot be regained simply by increased breeding of the survivors

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• Northern Elephant Seal • Bottleneck case study• Nearly hunted to extinction in 19th century• As a result, species has lost almost all of its genetic

variation, even though the population now numbers in the tens of thousands

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UNITEDSTATES

Guadalupe

MEXICO

population in 1890,reduced to inhabitingGuadalupe onlycurrent population


Figure 19.6

• Gene flow occurs when an individual travels from one geographic location to another.

Other Evolutionary Forces• Mutations are changes to an organism’s DNA and are an important

drive of diversity in populations. • Nonrandom mating: if individuals nonrandomly mate with their peers,

the result can change the population. • i.e. female peahens may prefer peacocks with bigger, brighter tails.

• Assortative mating: an individual’s preference to mate with partners who are phenotypically similar to themselves.

• Environmental variance: have an influence on phenotypes.• Sun exposure (skin color), temperature-dependent sex determination (TSD) in

some turtles and other reptiles (Figure 19.7).

19.3: Adaptive Evolution• There are various ways in which natural selection can shape

populations.• Adaptive evolution: when natural selection only acts upon heritable traits:

selecting for beneficial alleles and therefore increasing their frequency in the population and decreasing the harmful alleles frequency.

• Evolutionary (Darwinian) fitness: selection for individuals with greater contributions to the gene pool of the next generation.

• Natural selection acts at the level of the individual.

Fitness and its measurement• Fitness

• Individuals with the highest fitness leave more surviving offspring in the next generation than individuals with less fitness

• Fitness has many components1. Survival2. Sexual selection – some individuals are more successful at attracting mates3. Number of offspring per mating

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Stabilizing selection• Acts to eliminate both

extremes• Makes intermediate

more common by eliminating extremes

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0 25 50 100 12575

Body Size (g)

0 25 50 100 12575Body Size (g)

Selection for mid-size individuals

c. Stabilizing selection

Distributiongets narrower

Num

ber o

f Ind

ivid

uals

Num

ber o

f Ind

ivid

uals


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Directional selection• Acts to eliminate one

extreme• Often occurs in nature

when the environment changes Peak shifts

0 25 50 100 12575Body Size (g)

Body Size (g)0 25 50 100 12575

Selection for larger individuals

b. Directional selectionN

umbe

r of I

ndiv

idua

lsN

umbe

r of I

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Disruptive selection• Acts to eliminate

intermediate types

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0 25 50 100 12575

Body Size (g)

Num

ber o

f Ind

ivid

uals

Num

ber o

f Ind

ivid

uals

0 25 50 100 12575

Body Size (g)

Selection for small and large individuals

a. Disruptive selection

Two peaksform


Figure 19.8• Figure 19.8 Different types of natural

selection can impact the distribution of phenotypes within a population. In (a) stabilizing selection, an average phenotype is favored. In (b) directional selection, a change in the environment shifts the spectrum of phenotypes observed. In (c) diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against.

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Disruptive, stabilizing, or directional?

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15

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203050

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2 3 4 5 6 7 8 9 10Birth Weight in Pounds

Perc

ent o

f Birt

hs in

Pop

ulat

ion

Perc

ent I

nfan

t Mor

talit

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births in populationinfant mortality


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0 2 4 6 8 10

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1011

12 18 2014 16

Light

Dark

Number of Generations

Aver

age

Tend

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to F

ly T

owar

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Scenario: Only large and small seeds are available for food.• Birds with intermediate-sized beaks are at a disadvantage with both seed types

– they are unable to open large seeds and too clumsy to efficiently process small seeds


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• Occurs when the fitness of a phenotype depends on its frequency within the population

• Positive frequency-dependent selection• Favors the most common form• Tends to eliminate variation because “oddballs” stand out

(and get eaten!)• Negative frequency-dependent selection

• Rare phenotypes are favored by selection• Rare forms may not be in “search image” of the predator,

so they are overlooked.

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“Frequency-dependent selection”

Figure 19.9• A yellow-throated side-blotched lizard is

smaller than either the blue-throated or orange-throated males and appears a bit like the females of the species, allowing it to sneak copulations. (credit: “tinyfroglet”/Flickr)

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Positive frequency-dependent selection:

• Predator notices the unusual phenotype more

• Common phenotype has the survival advantage

dark brownmedium brownlight brown

Perc

ent o

f Col

or T

ype

Take

n by

Fis

h Pr

edat

ors

20 40 60 80 100

20

40

60

80

100

Color Type Frequency in Population

Color type ofwater boatman

Negative frequency-dependent selection• Predator prefers to eat the most

common color of “water boatman” (insect)

• Common phenotype has a survival disadvantage

• Oscillating selection• Selection favors one phenotype at one time and

another phenotype at another time• The overall effect will be to maintain genetic

variation in the population• Medium ground finch of Galápagos Islands

• Birds with big bills favored during drought (only big seeds available)

• Birds with smaller bills favored in wet conditions (smaller seeds in abundance)

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Sexual Selection• Sexual dimorphism: when males and females of certain species are

often quite different from one another in ways beyond the reproductive organs.

• i.e. males are often larger, and display many elaborate colors and adornments, like the peacock’s tail, while females tend to be smaller and duller in decoration. (Figure 19.10).

Figure 19.10

• Sexual dimorphism is observed in (a) peacocks and peahens, (b) Argiope appensa spiders (the female spider is the large one), and in (c) wood ducks. (credit “spiders”: modification of work by “Sanba38”/Wikimedia Commons; credit “duck”: modification of work by Kevin Cole)

Chapter 19 presentation

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