Chapter 23 Populations or Species Evolution. Slide 2 of 36 Questions? What is the smallest unit of...

Chapter 23

Populations or Species Evolution

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Questions?

What is the smallest unit of natural selection?

What is the smallest unit of evolution?

What is the difference between species and populations?

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Mas Evolution Questions

What is the basis of evolution?

What are the likely sources of population variation?

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Part 1

Sources of Variation

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Mechanisms of Evolutionary Change

Mutations provide the foundation for evolution Certain mutations will be selected for

No mutations is part of Hardy-Weinberg Equilibrium (No Evolution Equilibrium)

Only cell line mutations are able to be passed to offspring

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Mutations

Mutation is any change in an organism’s DNA

2 Types of Genetic Mutations Point Mutations

Changes in 1 base of a gene Base-Pair Substitutions

Result in 1 of 3 types of Mutations Nonsense Missense Silent

Insertion & Deletions May result in a Frameshift Mutation

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Chromosomal Mutations

Mutation that affects multiple genes (multiple loci) at once

There are 4 types:1. Duplication2. Deletion3. Reciprocal Translocation4. Inversion

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Sexual Recombination

Majority of genetic variation

New combinations of already existing alleles in the genome

Every generation experiences sexual recombination

3 Mechanisms of Sexual Recombination1. Crossing over (When?)2. Independent Assortment of Chromosomes3. Fertilization

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Part 2

Mechanisms of Evolution

*** Evolution is a change in allelic frequencies in a population or gene pool.

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Mechanisms of Evolution?

Natural Selection Most important mechanism of change in allelic

frequencies in a population (Evolution)

Genetic Drift Random, nonadaptive change

Gene Flow Immigration or emigration that changes allelic frequencies

Nonrandom Mating Selective mating, Sexual Selection, Artificial selection

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Genetic Drift

If I flip a coin 1,000 times, how many times will it come up “heads”? What will be the side frequencies?

If I flip a coin 10 times, how many times will it come up “heads”? What will be the side frequencies?

So the smaller the population the greater the deviation from the expected results is possible

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Genetic Drift (Page 2)

Random Changes in allelic frequencies large changes in allelic frequencies from one generation to the next

SMALL populations When a question concerns a shrinking of a populations = Genetic drift

Unpredictable changes in allelic frequencies

Random & Nonadaptive change So it causes evolution, but does not make the population better

adapted to their environment

2 types: Bottleneck effect Founder’s effect

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Bottleneck Effect

Sudden environmental change drastically reduces the population size.

The few survivors have passed through a restrictive “bottleneck”

The few survivors have a gene pool that is different than the original population’s.

Population bottlenecks always reduce genetic variation

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Bottleneck Effect (Page 2)

In 1890s, humans reduced elephant seal population to 20

In the next 100 years, they were protected 20 30,000 elephant seals

Researchers investigated 24 loci and found NO variation Comparable species populations display significant

variation at these alleles

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Founder Effect

A small part of the population is separated from the rest of the population, and establishes a new population

The new population has a different gene pool and allelic frequencies than the original population

Colonization of a new land

Founders pass through an “isolation bottleneck”

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Founder’s Effect (Page 2)

Associated with questions on Island Populations + Reduced variation

In 1814, 15 British colonists founded settlement on Tristan de Cunha (Islands between Africa & South America) Retinitis Pigmentosa is ten times higher in this island

population than in the original population

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Gene Flow

Movement of fertile individuals or gametes into or out of the population

May gain or loose alleles

Tends to reduce differences between populations

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Nonrandom Mating

Example: In plants, insect pollination of particular types of plants in the population Speciation

Self-fertilization Remember Mendel?

Sexual Selection Changes allelic frequencies & results in adaptations, just

not to the environment

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Natural Selection

Only mechanism that causes adaptive evolution

Increases the frequency of certain genotypes

But natural selection acts on or selects for particular phenotypes Acts indirectly on genotypes

Fitness – reproductive contribution of a phenotype to subsequent generations An individual’s fitness is determined by the probability of

survival of offspring times the number of offspring produced over their lifetime

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The effect of Natural Selection

Most characters are influenced by alleles at multiple locations, hence they are considered quantitative traits Body size

Natural selection can affect quantitative traits in 3 main ways: Directional selection Stabilizing selection Disruptive selection

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Individuals toward one extreme of a phenotypic range are favored

Common response to an environmental change

Avg. size of black bears increased each glacial period

Larger bears = better insulation during cold times

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Extreme phenotypes are selected against

Reduces variation & perpetuates status quo

Birth weights of human babies

Large birth weight or small birth weight have higher infant mortalities

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Individuals with extreme phenotypes are selected for.

Small beaked birds feed successfully on soft seeds, but large billed birds feed on rigid seeds, so intermediate beak lengths are selected against.

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Maintaining Genetic Variation

Selection & genetic drift decrease variation, so variation should be decreased over time?

Diploidy Recessive alleles retain variation, but are hidden esp. in

heterozygotes Natural Selection selects phenotypes, not genotypes

Neutral alleles Some alleles have no net effect on fitness, but they

introduce additional variance into a population

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Maintaining Genetic Variation (P2)

Sexual Recombination/Reproduction Not all species reproduce sexually Crossing over, Independent Assortment, &

Recombination during fertilization

Balancing Selection Natural selection may maintain stable frequencies of 2

or more phenotypic forms in a population This is called balanced polymorphism

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Heterozygote Advantage

Sometimes heterozygous have higher fitness than either homozygote

Natural selection will maintain both alleles

Consider the case of sickle-cell anemia + Malaria

Malaria (Plasmodium vivax) successfully infects normally shaped RBCs only!!

Premature rupturing of cell membrane Abnormally formed hemoglobin is indigestible to the

plasmodium parasite

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Heterozygote Advantage (P2)

Homozygote Dominant (AA)

Heterozygote (Aa)

Homozygote Recessive (aa)

Sickle-cell RBCs

0% 50%* 100%

Normal RBCs 100% 50%* 0%

Sickle-cell Symptoms

Never Certain Circumstances

Yes

RBCs Susceptible to Malarial infection

100% 50% 0%

* I made up this number, it is actually an intermediate value and varies widely in the population, but 50% is such a nice number.

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Heterozygote Advantage (P3)

Heterozygotes are relatively unaffected by sickle-cell anemia, but also have a percentage of cells that are protected from malarial infection As long as heterozygotes stay near sea level, no sickle-

cell symptoms, and reduced severity of malarial symptoms

Q.E.D. Heterozygote advantage

So the recessive allele’s (sickle-cell anemia) harm is balanced by its benefit in reducing or preventing malarial infection

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Sickle-Cell Disease

Malarial Disease

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Why hasn’t evolution produced perfect organisms yet ?

Evolution is limited by historical constraints New structures are not completely new, rather they are

modifications of existing structures Similar to modifying an existing building or just tearing it

down and building another one

Adaptations are often compromises Animals typically perform a myriad of functions, so

adaptations are rarely designed perfectly for many functions Knees, make sense for quadrupeds, but suck for aging human

bipeds & UM running backs (Gore, McGahee, Cooper, etc.)

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Evolution ain’t perfecto

Chance and Natural Selection Interact Gene Flow or Gene Drift are created by random selective

events Alleles present in the new population are not necessarily

best suited to the environment

Selection can edit only the existing variation Natural selection favors only the fittest phenotypes

currently in the population These traits may not be ideal

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