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Chapter 22

Chapter 22


Department
Biological Sciences
Course Code
BIOA02H3
Professor
Kamini Persaud
Chapter
22

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Chapter 22 The Mechanisms of Evolution
Evolutionary War Newts evolved the ability to secrete a paralytic poison from their skin, which deters
most predators; some garter snakes have evolved resistance to it.
22.1 What Facts Form the Base of Our Understanding of Evolution
- NATURAL HISTORY = THE SCIENTIFIC STUDY OF HOW DIFFERENT ORGANISMS FUNCTION AND CARRY OUT THEIR LIVES.
- DARWIN WAS INTERESTED IN GEOLOGY AND NATURAL HISTORY, BUT WAS PLANNING ON BECOMING A DOCTOR.
- WENT ON 5 YEAR VOYAGE ON HMS BEAGLE AND FOUND THAT THE SPECIES ON THE GALAPAGOS ISLANDS WERE FOUND
NOWHERE ELSE, BUT WERE SIMILAR TO THOSE FOUND IN MAINLAND SOUTH AMERICA.
- THIS THEORY FOR EVOLUTIONARY CHANGE WAS BASED ON TWO MAJOR PROPOSITIONS:
SPECIES ARE NOT PERMANENT; THEY CHANGE OVER TIME.
THE PROCESS THAT PRODUCES THESE CHANGES IN NATURAL SELECTION.
- THOMAS MALTHUS, AN ECONOMIST, PUBLISHED AN ESSAY ON THE PRINCIPLE OF POPN ABOUT HOW
R OF HUMAN POPLN GROWTH > R OF INCREASE IN FOOD PRODUCTION
AND UNCHECKED GROWTH INEVITABLY LEADS TO FAMINE. DARWIN AND WALLACE WERE INFLUENCE BY THIS PAPER.
- DARWIN BELIEVED DEATH RATES TO BE HIGH W/O HIGH DEATH RATES, EVEN THE MOST SLOWLY REPRODUCING
SPECIES WOULD QUICKLY REACH ENORMOUS POPULATIONS.
- HE CAME UP WITH THE IDEA AND TERM NATURAL SELECTION, AND ARTIFICIAL SELECTION ALREADY EXISTED AND HE
PRACTISED IT BECAUSE HE WAS A PIGEON BREADER.
- DARWIN RECOGNISED THAT: INDIVIDUALS DO NOT EVOLVE; POPULATIONS DO.
- POPULATION = A GROUP OF INDIVIDUALS OF A SINGLE SPECIES THAT LIVE AND INTERBREED IN A PARTICULAR
GEOGRAPHIC AREA AT THE SAME TIME.
Adaptation has Two Meanings
- ADAPTATION = BOTH THE PROCESSES BY WHICH THE USEFUL CHARACTERISTICS EVOLVE TO THE BEARERS THAT IS, THE
EVOLUTIONARY MECHANISMS THAT PRODUCE THEM AND TO THE CHARACTERISTICS THEMSELVES.
- THE CHARACTERISTICS ARE THE PHENOTYPIC CHARACTERISTICS THAT HAVE HELPED ORGANISMS ADJUST TO CONDITIONS
IN THE ENVIRONMENT.
- BIOLOGISTS COMPARE THE PERFORMANCE OF INDIVIDUALS THAT DIFFER BY TRAIT (EX. DIFFERENT CONC ON TTX IN
SKIN OF NEWT) BY OBSERVING THEIR REPRODUCTION AND SURVIVAL IN THAT ENVIRONMENT.
Population Genetics Provide an Underpinning for Darwins Theory
- FOR A POPULATION TO EVOLVE, ITS MEMBERS MUST HAVE HERITABLE GENETIC VARIATION, WHICH IS THE RAW MATERIAL
ON WHICH MECHANISMS OF EVOLUTION ACT.
- PHENOTYPES = THE PHYSICAL EXPRESSIONS OF ORGANISMS GENES.
- THE FEATURES OF A PHENOTYPE ARE ITS CHARACTERS EX. EYE COLOR.
- THE SPECIFIC FORM OF A CHARACTER, SUCH AS BROWN EYES, IS A TRAIT.
- A HERITABLE TRAIT IS A CHARACTERISTIC THAT IS AT LEAST PARTLY DETERMINED BY ITS GENES.
- THE GENETIC CONSTITUTION THAT GOVERNS A CHARACTER IS CALLED ITS GENOTYPE; A POPULATION EVOLVES WHEN
INDIVIDUALS WITH DIFFERENT GENOTYPES SURVIVE/REPRODUCE AT DIFFERENT RATES.
- GREGOR MENDEL STARTED POPULATION GENETICS, WHICH HAS 3 MAIN GOALS:
TO EXPLAIN THE ORIGIN AND MAINTENANCE OF GENETIC VARIATION.
TO EXPLAIN THE PATTERNS AND ORGANIZATION OF GENETIC VARIATION.
TO UNDERSTAND THE MECHANISMS THAT CAUSE CHANGES IN ALLELE FREQUENCIES IN POPULATIONS.
- DIFFERENT FORMS OF A GENE, CALLED ALLELES, MAY EXIST AT A PARTICULAR LOCUS. An allele occupies a fixed
position or locus in the chromosome. At any particular locus, a single individual has only some of the
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alleles found in the population to which it belongs. The sum of all copies of all alleles at all loci found in a
population makes its gene pool.
- Allele frequencies measure the amount of genetic variation in a population.
- The forces of natural selection act on phenotypes, but only if there is a change in the genotypes of a
population has evolution occurred.
- The gene pool contains the genetic variation that produces the phenotypic traits on which natural
selection acts.
- Most populations are genetically variable.
- The study of the genetic basis of natural selection is difficult because:
Genotypes alone do not uniquely determine all phenotypes. Ex: AA and Aa individuals may be
phenotypically identical.
Different phenotypes can be produced by a given genotype, depending on the environment
encountered during development. Ex: the cells of all the leaves on a tree or shrub are usually
genetically identical, yet leaves of the same plant often differ in shape and size.
22.2 What Are the Mechanisms of Evolutionary Change?
- Evolutionary mechanisms = forces that change the genetic structure of a population; mutation, gene
flow, genetic drift, non-random mating, and natural selection.
Mutations Generate Genetic Variation
- ORIGIN OF GENETIC VARIATION = MUTATION; ANY CHANGE IN AN ORGANISMS DNA.
- MUTATIONS ARE RANDOM WRT TO ADAPTIVE NEEDS OF ORGANISMS.
- MUTATIONS BOTH CREATE AND HELP MAINTAIN GENETIC VARIATION IN POPULATIONS:
MUTATIONS ARE USUALLY HARMFUL OR NEUTRAL, BUT IF THERE IS A CHANGE IN THE ENVIRONMENT, THE
PREVIOUS HARMFUL/NEUTRAL MUTATIONS CAN BECOME ADVANTAGEOUS.
MUTATIONS CAN RESTORE TO POPULATIONS, ALLELES THAT OTHER EVOLUTIONARY PROCESSES REMOVED.
- MUTATIONS ARE RARE; 1 IN 1 MILLION HUMAN POPULATION OF 6.5 BILLION IS EXPECTED TO CARRY 40 BILLION NEW
MUTATIONS THAT WERE NOT PRESENT IN ONE GENERATION EARLIER.
Gene Flow May Change Allele Frequencies
- GENE FLOW = MIGRATION OF INDIVIDUALS AND MOVEMENT OF GAMETES B/W POPULATIONS VERY COMMON.
- TWO WAYS GENE FLOW CAN CHANGE ALLELE FREQUENCIES IF THE NEW ORGANISMS/GAMETES SURVIVE AND REPRODUCE
ADD NEW ALLELES TO GENE POOL OF POPULATION.
CHANGE FREQUENCIES OF ALLELES ALREADY PRESENT IF THEY COME FROM A POPN W/ DIFFERENT ALLELE
FREQUENCIES.
Genetic Drift May Cause Large Changes in Small Populations
- GENETIC DRIFT = RANDOM CHANGES IN ALLELE FREQUENCIES LARGE IMPACT IN SMALL POPULATIONS.
- HARMFUL ALLELES MAY INCREASE IN FREQUENCY AND RARE ADVANTAGEOUS ALLELES MAY BE LOST.
- IN LARGE POPULATIONS, GENETIC DRIFT CAN INFLUENCE THE FREQUENCIES OF ALLELES, BUT ONLY THOSE THAT DO
NOT INFLUENCE THE SURVIVAL AND REPRODUCTION RATES OF THEIR BEARERS.
- POPULATION BOTTLENECKS = OCCUR WHEN ONLY A FEW INDIVIDUAL FROM A LARGE
POPULATION SURVIVE A RANDOM EVENT, RESULTING IN A SHIFT IN ALLELE FREQUENCIES
WITHIN THE POPULATION - GENETIC VARIATION CAN BE REDUCED BY GENETIC DRIFT.
- FOUNDER EFFECT = Loss of genetic variation that occurs when a new
population is established by a very small number of individuals from a larger
population resulting change in genetic variation is same as that in a large population reduced by a
bottleneck.
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