Population Evolution

Biology Chapter 161

Genetic Variation

Populations always show variation in traits – no two individuals are EXACTLY the same

Variations are caused by two main sources:MutationsGene shuffling during sexual reproduction

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Gene Pools

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Gene pools consist of all genes (AND all alleles) within a population

Gene frequency is the number of times that an allele occurs in a gene pool

EXAMPLE of gene pools - A homozygous black mouse has two alleles for black fur. A heterozygous black mouse has one allele for black fur and one allele for brown fur. A homozygous brown mouse has two alleles for brown fur.

12 Heterozygous Black

9 Homozygous Brown

4 Homozygous

Black

Gene Pool:•Homozygous Black = •Homozygous Brown = •Heterozygous Black =

How many total genes?How many total alleles?How many B alleles?How many b alleles?

BBbbBb

25502030

Allele B frequency = 20/50 = 40% = 0.4Allele b frequency = 30/50 = 60% = 0.6

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Changes in allele frequency indicate that a change in the allele populations has occurred through evolution

Sources of Genetic Variation

Mutations• ANY change in the DNA sequence due to mistakes,

environmental sources (radiation, etc.)• Can have no effect or drastic effects

Gene shuffling as a result of sexual reproduction• Crossing over during meiosis I• Random contributions from mother & father

Most genetic variation results in little or no observable changes in organisms

Trait distribution in populations

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Polygenic traits

Single-gene traits

Natural Selection effects on single- gene traits

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CAN lead to allele frequency changes = evolution!

What happened?Environmental changes allowed black lizards to be less conspicuous to predators than the previous environment. Therefore, red and light colored lizards stood out more and were more easily eaten by predators than black lizards causing a change in the allele frequency.

Natural Selection effects on polygenic traits

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More complex than single-gene trait resultsDirectional selection

Stabilizing selection

Disruptive selection

Directional Selection Example

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BEFORE Industrial Revolution (and soot)

AFTER Industrial Revolution (and soot)

Stabilizing Selection Example

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Healthy birth weight leads to better survival rates

Disruptive Selection Example

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Genetic Drift

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In a population, an allele can become more or less common solely due to chance = genetic “drift”. (Individuals can immigrate., emigrate, etc.)

Greatest effect in small populations

Founder Effect –Individuals leave a population to initiate a new population, or change another population

Genetic Equilibrium

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No change in allele frequency = genetic equilibrium

Hardy-Weinberg Principle = a mathematical way to quantitatively determine whether a population is showing evolution by studying allele frequencies

The determination only works when there is no evolution (genetic equilibrium). Need to have these conditions:

Random matingVery large populationNo immigration nor emigrationNo mutationsNo natural selection

= Genetic Equilibrium

Speciation

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The process of forming new species

If an allele mutation allows better “fitness” for a population, then that allele will become more common

WHAT causes a species to form two (or more) new species?

Isolating Mechanisms – causing “reproductive isolation”

Behavioral

Geographic

Temporal (time)

Behavioral Isolation

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A form of reproductive isolation in which two populations that may be able to mate have differences in courtship rituals or other types of behavior that prevent them from interbreeding

Geographic Isolation

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A form of reproductive isolation where two populations are separated by a geographical border – keeping them from interbreeding.

Northern Spotted owl video

Mexican Spotted owl video

Temporal Isolation

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A form of reproductive isolation in which the two species reproduce at different times of the day, season, or year. Wood and leopard frogs are an example of two similar species whose ranges overlap.

Darwin’s Finches Speciation Example

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Founders arrive Geographic isolation Gene pool changes

Speciation – the formation of new species – takes place over many years and is influenced by many factors.Eventually, changes in the gene pool happens as a result of the birds adapting to their local environment. If some birds from island B fly back to island A, they may have changed enough that they can no longer interbreed (reproductive isolation). If they both exist in island A, they will compete for local resources, perhaps causing further evolutionary changes.

Unanswered Questions

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How does evolution add information? Mutation explains how existing genetic information is changed, but it doesn't explain where new genetic information comes from. How can evolution be so quick? Millions of years can seem like a long time to the average person, but in terms of evolution, it's quite quick. Current evolutionary theory hasn't explained how a one-celled organism can evolve into an organism as complex as a human being in the time available in Earth's history. Where did the first living cell come from? In order for mutation and natural selection, processes essential to evolution, to operate, life must already exist. The current hypothesis is that life formed spontaneously from chemical reactions about 4 billion years ago, but is this really Possible?

What’s next?

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