13-1 Chapter 13 Lecture Outline See PowerPoint Image Slides for all figures and tables pre-inserted into PowerPoint without notes

13-1

Chapter 13

Lecture Outline

See PowerPoint Image Slides

for all figures and tables pre-inserted into

PowerPoint without notes.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.13-2

The Scientific Concept of Evolution

Evolution is the change in the frequency of genetically determined characteristics within a population over time.


Early Thinking About Evolution

Early 1800’s– Lamarck suggested a process by which the

changes could occur.He proposed that acquired characteristics

could be passed on to offspring.Example: Giraffes have long necks because

their ancestors stretched to reach leaves in trees.

Lamarck was wrong, but his ideas stimulated thought about evolution.


The Theory of Natural Selection

1858– Darwin and Wallace suggested the theory of

natural selection as a mechanism for evolution.– Darwin wrote Origin of Species by Means of

Natural Selection.

The theory of natural selection – The idea that some individuals have genetic

combinations that allow them to survive, reproduce and pass their genes on to the next generation

The Voyage of HMS Beagle 1831-1836


Charles Darwin set forth in 1831 at the age of 22. The voyageLasted nearly five years.The ship visited South America,The Galapagos Islands, Australia,and many Pacific Islands.

Darwin in the Galapogos

http://www.youtube.com/watch?v=KUshLAzXJm4&feature=related

http://www.youtube.com/watch?v=l25MBq8T77w



Assumptions of the Theory of Natural Selection

All organisms produce more offspring than can survive.

No two organisms are exactly alike. Among organisms, there is a constant struggle for

survival. Individuals that possess favorable characteristics for

their environment have a higher rate of survival and produce more offspring.

Favorable characteristics become more common in the species.

– Unfavorable characteristics are lost over time.


Natural Selection vs. Acquired Characteristics

What is the major difference between Lamark’s idea and Darwin’s theory of natural selection?



Modern Interpretations of Natural Selection

Darwin and Wallace’s theories were not immediately accepted because meiosis, genes and inheritance were poorly understood.

Mendel’s discovery provided an explanation for how characteristics could be transmitted from one generation to the next.

Knowledge of mutation, gene flow and reproductive isolation supported Darwin/Wallace’s theory.

Now we can integrate Darwin and Wallace’s hypothesis with what we know about meiosis, genes and inheritance.


Darwin and Wallace’s Basic Assumptions can be Updated

An organism’s ability to overproduce results in surplus organisms.

Due to mutation, new traits enter the gene pool and due to meiosis new combinations of alleles can be generated.

Resources such as food, soil nutrients, water, etc. are in short supply, so some individuals will not survive.– Disease, predators, etc. will also affect survival.– These are called selecting agents.


Darwin and Wallace’s Basic Assumptions can be Updated

Selecting agents favor individuals with the best combinations of alleles.– They will be more likely to survive and

reproduce and pass their alleles on to the next generation.

Allele combinations that favor survival and reproduction will be more common in a population.


The Role of Natural Selection

in Evolution

Natural selection will select for individuals with certain alleles.

– When allele frequencies change over time, evolution has occurred.

Natural selection works on individuals, but only populations evolve.

Three factors work together to determine how a population changes over time.

– Environmental factors that affect individuals– Sexual reproduction among the individuals– Genetic diversity within the gene pool


Reproductive Success

Individuals that have the combinations of alleles that allow them to successfully reproduce will pass on their alleles.– This success is measured as fitness.– Fitness is a relative measure.

An individual can successfully reproduce, but be less fit than another individual.

Involves the number of offspring produced and their viability


The Importance of Genetic Diversity

A large gene pool with great genetic diversity is more likely to contain genetic combinations that will allow some individuals to adapt to changing environments.

Characteristics that may allow individuals to adapt include– Structural, behavioral, biochemical or metabolic

characteristics

Common Misunderstandings About Natural Selection

What do we mean by survival of the fittest?

What do we mean by struggle for life?




Survival of the fittest– Individual survival is not as important as

reproductive success (# of descendants).

Struggle for life– Does not refer to conflict– Refers to finding and utilizing resources



The significance of acquired characteristics– Phenotypes acquired during the life of the individual

will not be passed on.– These will not be important during natural selection.

The relationship between the mechanism of natural selection and the outcomes of the selection process

– The effects of natural selection are seen in the population.– The outcome of natural selection (death, reproductive

success, mate choice) involves individuals.


What influences natural selection?

Genetic diversity within a species Genetic recombination as a result of

sexual reproduction Gene expression The ability of a species to reproduce

excess offspring


Mechanisms that Affect Genetic Diversity

Natural selection cannot occur in a population in which all of the individuals are genetically identical.

Genetic diversity is essential for natural selection.

Several mechanisms generate genetic diversity in a population.– Mutation, migration and sexual reproduction


Mutations and Migration

Spontaneous mutations are changes in DNA that cannot be tied to a specific cause.– Mutations alter existing genes and generate new

alleles.– Most mutations are harmful.

Some mutations are helpful.

– Mutations are only relevant to natural selection if they happen in cells that give rise to gametes.

Migration results in alleles entering and leaving a population.


Sexual Reproduction and Genetic Recombination

Sexual reproduction generates new combinations of alleles in individuals (genetic recombination).

– Crossing over during meiosis generates new combinations of alleles in homologous chromosomes.

– Independent assortment generates new combinations of alleles from non-homologous chromosomes.

– Random fertilization results in a genetically unique individual.

Genetic recombination can generate a new combination of alleles that would give an individual a selective advantage.


Natural Selection Works on the Total Phenotype

One good allele is not enough to give an individual greater fitness.

Natural selection acts on the total phenotype which involves a combination of characteristics.


Processes that Drive Selection; Differential Survival

“Survival of the fittest”– Survival is a prerequisite of

reproduction.– Individuals that do not survive cannot

reproduce. Darwin’s finches

– The finch’s beak size correlated with the type of seeds they ate.

Small beaks - softer seeds Larger beaks - harder seeds

– During drought, as soft seeds became scarce, only birds with larger beaks survived.


Darwin’s Finches


Differential Survival of Herbicide-resistant Weeds

When herbicides are used on weeds– Only the individual plants that are resistant will

survive.– As these reproduce, more and more individuals in

the species are resistant to the herbicide.– Over time, the herbicide is ineffective.


Differential Survival of Herbicide-resistant Weeds


Processes that Drive Selection; Differential Reproductive Rates

Survival is necessary to reproductive success, but does not guarantee reproductive success.

Reproductive success is measured by the number of offspring an individual leaves.

Clover fields– Cows eat taller plants first.– Flowers are at the top of the tall plants.– Tall plants didn’t die, but were not able to reproduce.– Therefore, shorter plants were more reproductively

successful.


Processes that Drive Selection; Differential Mate Choice

Sexual selection occurs when some individuals are chosen as mates more often than others.– Relevant only in animal populations– Those that are chosen pass on more genes than

those that are not chosen. More frequently, chosen individuals are

usually larger, more aggressive, more colorful, etc.– All of these features attract mates.


Examples of Sexual Selection

Male red-winged blackbirds establish territories in which they only allow females.– The ability to establish and maintain a territory

determines which males will mate.

Male peacocks and chickens have elaborate tail feathers.– Males with the most spectacular tails are chosen

by females more frequently.


Mate Selection


Patterns of Selection

Natural selection changes allele frequencies in a population.– This actually reduces genetic diversity.– Over time, the individuals become more alike.

Selection can favor different phenotypes in different situations.

This leads to three different forms of selection.– Stabilizing, directional and disruptive


Stabilizing Selection

Occurs when individuals at the extremes of the range of characteristic are selected against

This means that the “average” individuals are selected for.

– Brown mice selected for, white and black selected against White and black are more likely to be noticed and eaten by

predators. Over time the population will have mostly brown mice.

Stable environments favor stabilizing selection.

What would the curve for stabilizing selection look like?

Another ex. Human birth weight



Directional Selection

Occurs when individuals of one extreme of the range of characteristic are selected for

Usually occurs when the environment is changing Insecticide use will select for individuals that are

resistant.– Originally there were probably only a few resistant

individuals.– After several generations, the population will contain mostly

resistant individuals.

Directional Selection:Peppered moth (Biston betularia)


What will the curve look like?



Disruptive Selection

Occurs when both extremes of a range of characteristic is selected for– The intermediate is selected against.

Insects that live on plants with dark green or light green leaves– Medium green insects will be noticed and eaten

by predators.



Patterns of Selection


Evolution Without Selection —Genetic Drift

Genetic drift involves a significant change in allele frequency that is not a result of natural selection.– Results from chance events– More likely to impact small populations

In a population of 100 plants, 10 of them have red spots on their leaves.

If these 10 are randomly trampled by an animal or are killed by a late frost

The red spot allele would be lost, but not due to natural selection.


Genetic Drift

Cougars– Divided by human

colonization– Small populations

became isolated and experienced genetic drift.

– Now the populations are endangered because their gene pools are not diverse.


Gene-frequency Studies and Hardy-Weinberg Equilibrium

G.H. Hardy and Wilhelm Weinberg described a simple mathematical relationship that can be used to study allele frequencies.– They recognized that under certain conditions,

allele frequencies would not change over time.– In this case, alleles would mix randomly and the

same proportion of genotypes would result.

Their formula uses allele frequencies and genotypic frequencies to detect an equilibrium.


Hardy-Weinberg Equation

p2 + 2pq + q2 = 1– p = frequency of the dominant allele– q = frequency of the recessive allele– p2 = frequency of homozygous dominants– 2pq = frequency of heterozygotes– q2 = frequency of homozygous recessives

If the proportion of genotypes matches this equation, then the population is not evolving.

– If gene frequencies do not match this equation, then the population is evolving.

Certain conditions have to be met for a population to be in Hardy-Weinberg equilibrium.


The Conditions Necessary for H-W Equilibrium

If these five conditions are met, then gene frequencies will not change.

– Random mating– No mutation– No migration– Very large population size– No natural selection

Hardy-Weinberg equilibrium provides a null hypothesis for evolution in a population.

– Gives us a way to compare two populations, or two generations of a population to determine if evolution is occurring.


Determining Genotype Frequencies

Genotypic frequencies in a population can be calculated with a modified Punnett Square.– In this Punnett square, allele frequencies must be

used to determine the likelihood of particular genotypes occurring in the next generation.


Why Hardy-Weinberg Conditions Rarely Exist

Since the gene pools of most populations are changing over time, H-W equilibrium rarely exists.

Why?– Random mating rarely occurs.– Spontaneous mutations occur.– Immigration and emigration introduce new alleles.– Populations are not infinitely large.– Natural selection does occur.


Using the Hardy-Weinberg Concept to Show Allele-frequency Changes


The Effect of Changing Allele Frequencies


Summary of the Causes of Evolutionary Change

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13-1 Chapter 13 Lecture Outline See PowerPoint Image Slides for all figures and tables pre-inserted into PowerPoint without notes