Transcript 17.1 – Genes and Variation

17.1 – Genes and Variation
Chapter 17 – Evolution of
Populations
17.1 Genes and Variation
17.1 – Genes and Variation
THINK ABOUT IT
 Darwin developed his theory of evolution
without knowing how heritable traits passed
from one generation to the next.
 What role does genetics play in understanding
evolution?
17.1 – Genes and Variation
Genetics Joins Evolutionary Theory
• How is evolution defined in genetic terms?
• Evolution is any change in the relative frequency of
alleles in the gene pool of a population over time.
17.1 – Genes and Variation
Genetics Joins Evolutionary Theory
• Heritable traits are controlled by genes.
• Changes in genes and chromosomes create variation.
• Example = Children receive their genes from the same
parents all look different. (Exception Twins)
17.1 – Genes and Variation
Genotype and Phenotype in Evolution
• Genotype – an individuals particular combination of alleles.
• Genotype with environmental conditions produces =
phenotype.
• Phenotype - all physical, functional, and behavioral
characteristics of an organism.
17.1 – Genes and Variation
Genotype and Phenotype in Evolution
• Natural selection acts directly on phenotype.
• Some phenotypes are better suited to their
environment than others.
• **Better suited individuals produce more
offspring and pass on their genes to the
next generation.**
17.1 – Genes and Variation
Populations & Their Gene Pools
Gene Pool: consists of all the alleles in all the
individuals that make up a population
Frequency of alleles: how often certain alleles
occur in the gene pool (expressed as a decimal
or a %)
Figure 14-24
Each plant in this hypothetical
population of wildflowers has 2
alleles for flower color. In all,
there are 14 red-flower alleles
(R) and 6 white-flower alleles
(r). The frequency of each allele
is calculated as a ratio based
on the total of 20.
17.1 – Genes and Variation
Allele Frequency - Example
Example - this diagram shows the gene pool for fur color
in a population of mice.
17.1 – Genes and Variation
Populations and Gene Pools
• Natural selection operates on individuals, but
resulting changes in allele frequencies show up in
populations.
• Populations, rather than individuals, evolve!!!!
17.1 – Genes and Variation
Sources of Genetic Variation
Three sources of genetic variation:
1. Mutation
2. Genetic recombination during sexual reproduction
3. Lateral gene transfer
17.1 – Genes and Variation
Mutations
• Change in the genetic material of a cell!
• Mutations that impact phenotype may or may not
affect fitness.
• Can be:
• Lethal
• Beneficial
• Detrimental
• Mutations matter in evolution only if they can be
passed from generation to generation.
17.1 – Genes and Variation
Genetic Recombination in Sexual
Reproduction
• Combination of genes from different parents
• Half of the DNA comes from each parent.
• Each offspring receives a unique set of genetic
information. (Exception = Twins)
• This is the result of crossing over and independent
assortment during meiosis!
17.1 – Genes and Variation
Lateral Gene Transfer
• When organisms pass genes from one individual to
another that is not its offspring.
• Increases genetic variation in the individual who acquires
the new genes.
• Example = Formation of antibiotic resistant bacteria.
17.1 – Genes and Variation
Single-Gene and Polygenic Traits
 KEY: What determines the number of phenotypes for a
given trait?
 ANSWER: The number of phenotypes produced for a trait
depends on how many genes control the trait.
17.1 – Genes and Variation
Single-Gene and Polygenic Traits
• The number of phenotypes produced for a trait depends
on how many genes control the trait.
• Single-Gene Traits
• Polygenic Traits
17.1 – Genes and Variation
Single-Gene Traits
• Single-gene trait = is a trait controlled by only one gene.
• May have just two or three distinct phenotypes.
• The most common form of the allele can be dominant or
recessive.
17.1 – Genes and Variation
Single-Gene Trait - Example
• Dominant allele isn’t always the most common in a population!
• In a snail population some have dark bands on their shells while
others don’t.
• Allele for no bands = dominant
• Allele for bands = recessive
17.1 – Genes and Variation
Polygenic Traits
• Polygenic traits = traits controlled by two or more genes.
• Each gene of a polygenic trait often has two or more
alleles.
• Results in many different genotypes and phenotypes!
Examples: Height and skin color
17.1 – Genes and Variation
Polygenic Trait - Example
• Human height, which varies from very short to very tall, is an
example of a polygenic trait.
• The bell-shaped curve in the graph is typical of polygenic
traits.