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Biology Ch.16, Sec's1-3: Genes & Variation; Evolution as Genetic Changes;The Process of Speciation (By:Genesis & Jose)

Miller Levine Bio Book

Genesis Marquina

on 26 June 2014

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Transcript of Biology Ch.16, Sec's1-3: Genes & Variation; Evolution as Genetic Changes;The Process of Speciation (By:Genesis & Jose)

Genes and Variation
Many traits are controlled by two or more genes and are called
polygenic traits

One polygenic trait can have many possible genotypes and phenotypes.
Single-Gene and Polygenic Trait
A widow's peak in humans is an example of a(an)
invariable trait.
single-gene trait.
polygenic trait.
The main sources of inherited variation are
gene shuffling and mutations.
gene pools and frequencies.
single-gene and polygenic traits.
genotypes and phenotypes.
Most inheritable differences are a result of
gene shuffling.
frequency of alleles.
DNA replication.
Which of the following statements is TRUE?
The relative frequency of an allele is not related to whether the allele is dominant or recessive.
Mutations always affect an organism's phenotype.
Crossing over decreases the number of different genotypes that appear in an offspring.
Evolution does not affect the frequency of genes in a gene pool.
The number of phenotypes produced for a given trait depends on how many genes control the trait.
Single-Gene and Polygenic Traits
Gene Shuffling

Most heritable differences are due to gene shuffling.

Crossing-over during meiosis
increases the number of genotypes that can appear in offspring.
Sources of Genetic Variation
Sources of Genetic Variation

In genetic terms, evolution is any change in the relative frequency of alleles in a population.
Variation and Gene Pools
relative frequency
of an allele is the number of times the allele occurs in a gene pool.

Relative frequency is often expressed as a percentage, and it is not related to whether an allele is dominant or recessive.
Variation and Gene Pools
is a group of individuals of the same species that interbreed.

gene pool
consists of all genes,
including all the different alleles
, that are present in a population.
Variation and Gene Pools
Many genes have at least two forms, or

All organisms have genetic variation that is “invisible.”

An individual organism is
for many genes.
How Common Is Genetic Variation?
A graph of the length of the little finger on the left hand versus the number of people having fingers of a particular length is a bell-shaped curve. This indicates that finger length is a
single-gene trait.
polygenic trait.
randomly inherited trait.
strongly selected trait.
bell-shaped curve
is typical of polygenic traits.
Single-Gene and Polygenic Trait
single-gene trait
is controlled by one gene that has two alleles.

Variation in this gene leads to usually two, sometimes three possible phenotypes.
Single-Gene and Polygenic Traits

The two main sources of genetic variation:


Genetic shuffling from sexual reproduction
Sources of Genetic Variation
Genes and Variation
Gene Pool for Fur Color in Mice
Variation and Gene Pools
is any change in a sequence of DNA.

Causes of mutations:
-mistakes in DNA replication
-damage from radiation or chemicals

Mutations do not always affect an organism’s phenotype.
Single gene trait
= 1 gene controlling
Polygenic trait
= more than 1 gene controlling
Evolution as Genetic Change
Evolution as Genetic Change

Five conditions are required to maintain genetic equilibrium from generation to generation:

mating must be random
population must be very large
no gene flow (migration)
can be no mutations
no natural selection (all with equal fitness)
Evolution Versus Genetic Equilibrium
Random mating
ensures that each individual has an equal chance of passing on its alleles to offspring.

In natural populations, mating is rarely completely random. Many species select mates based on particular heritable traits.
Evolution Versus Genetic Equilibrium
Because individuals may bring new alleles into a population, there must be no movement of individuals into or out of a population (migration).

The population's gene pool must be kept together and kept separate from the gene pools of other populations.
Evolution Versus Genetic Equilibrium
Genetic drift
has less effect on large populations than on small ones.

Allele frequencies of large populations are less likely to be changed through the process of genetic drift.
Evolution Versus Genetic Equilibrium
If genes
, new alleles may be introduced into the population, and allele frequencies will change.
Evolution Versus Genetic Equilibrium
All genotypes in the population must have equal probabilities of survival and reproduction.
Evolution Versus Genetic Equilibrium
There can be no natural selection operating on the population.

A random change in allele frequency in a
small group
What is
genetic drift
Genetic Drift
is any change over time in the relative frequencies of alleles in a population.

Populations, not individual organisms, can evolve over time.
Evolution as Genetic Change
Natural Selection on Single-Gene Traits

Natural selection can affect the distributions of phenotypes in any of three ways:

directional selection

stabilizing selection

disruptive selection
Natural Selection on Polygenic Traits
Directional Selection 
When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end.
Natural Selection on Polygenic Traits
What scenario might cause this to happen?
Stabilizing Selection 
Happens when individuals near the center of the curve have higher fitness than individuals at either end of the curve.
Natural Selection on Polygenic Traits
Disruptive Selection
Happens when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.
Natural Selection on Polygenic Traits
Genetic Drift
Genetic drift may occur when a small group of individuals colonizes a new habitat.
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