Chapter 11.4 Flashcards Preview

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Flashcards in Chapter 11.4 Deck (12)
1

The most important mechanism of evolution

NATURAL SELECTION

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How does Natural selection change the genetic makeup of a population

by favoring the alleles that contribute to the reproductive success and selecting against those that are harmful

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Natural selection does not eliminate alleles directly, Instead....

Individuals with the best phenotypes are more likely to survive long enough to reproduce, those with poorly suited phenotypes are  are less likely to survive to reproduce 

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What are the 3 modes of Natural Selection

are distinguished by their effects on the phenotypes ina population

1. Directional selection

2. Disruptive selection

3. Stabilizing

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How are the 3 modes of natural selection distinguished

by their effects on the phenotypes in a population

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DIRECTIONAL SELECTION

one extreme phenotype is the fittest and the environment selects against the others

eg. dark tree trunk with white wing moths

mrsa

 

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DISRUPTIVE SELECTION

( DIVERSIFYING SELECTION)

2 or more extreme phenotypes are fitter than the intermediate phenotype

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STABILIZING SELECTION

( NORMALIZING SELECTION)

Extreme phenotypes are less fit than the intermidiate

It tends to reduce variation in apopulation and is therefore more common in stable unchanging environment

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BALANCED POLYMORPHISM

multiple alleles of a gene persists indefinately in the population at more or less constant frequencies. ( even harmful ones)

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Circumstances where natural selection maintains BALANCED POLYMORPHISM

Heterozygous advantage

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HETEROZYGOUS ADVANTAGE

A circumstance of balanced polymorphism when  an individual with 2 different alleles for a gene ( heterozygote) has geater fitness than those  whose alleles are identical.

Heterozygotes maintain harmful recessive allele in apopulation evenif homozygous recessive individuals have greatly reduced fitness

Heterozygotes with disease have a reproductive edge than homozygotes (why natural selection does not remove an harmful allele in heterozygotes)

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