Transcript gene pool - Midland Park School District

Chapter 17
Evolution of
Populations
17.1 Genes and
Variation
Lesson Overview
Genes and Variation
THINK ABOUT IT
Darwin developed his theory of evolution
without knowing how inherited traits passed
from one generation to the next or where
variation came from.
What would happen when genetics answered
questions about how heredity works?
Lesson Overview
Genes and Variation
Genetics Joins Evolutionary Theory
By the 1930’s, researchers discovered that
heritable traits are controlled by genes.
They realized that changes in genes and
chromosomes generated variation.
By the 1950’s, Watson & Crick’s model of
DNA helped to demonstrate the molecular
nature of variations and mutations.
Lesson Overview
Genes and Variation
Genotype and Phenotype in Evolution
Recall that an organism’s genotype is the
particular combination of alleles it carries.
An individual’s genotype, together with
environmental conditions, produces its
phenotype.
Phenotype includes all physical, physiological,
and behavioral characteristics of an organism.
Lesson Overview
Genes and Variation
Genotype and Phenotype in Evolution
It is important for us to remember that natural
selection acts directly on phenotype, not
genotype.
Some individuals have phenotypes that are
better suited to their environment than others.
These individuals produce more offspring and
pass on more copies of their genes to the next
generation.
Lesson Overview
Genes and Variation
Populations and Gene Pools
Genetic variation is studied in populations.
Recall that a population is a group of
individuals of the same species that mate and
produce offspring. Since they interbreed, they
share a gene pool.
A gene pool consists of all the genes, including
all the different alleles for each gene that are
present in a population.
Lesson Overview
Genes and Variation
Populations and Gene Pools
Researchers study gene
pools by examining the
relative frequency of an
allele.
The relative frequency of
an allele is the number of
times a particular allele
occurs in a gene pool,
compared with the number
of times other alleles for the
same gene occur. It is
often expressed as a %.
Variation and Gene Pools
For example, the relative
frequency of the dominant
B allele is 40%.
• We get that by adding
one half of the
heterozygous population
or 24% to all of the
homozygous black
population or 16%.
What is the relative
frequency of the recessive
b allele?
Copyright Pearson Prentice Hall
Lesson Overview
Genes and Variation
Populations and Gene Pools
In genetic terms, evolution is defined as any
change in the relative frequency of alleles in the
gene pool of a population over time.
For example, if the frequency of the B allele
changed from 40% to 30% in the mouse
population, we would say the population is
evolving.
Natural selection operates on individuals, but
resulting changes in allele frequencies show
up in populations. Populations, rather than
individuals, evolve!
Lesson Overview
Genes and Variation
Sources of Genetic Variation: Mutations
Mutations that produce changes in phenotype
may or may not affect fitness. Some mutations
may be lethal or may lower fitness; others may
be beneficial.
Mutations matter in evolution only if they can be
passed from generation to generation. The
mutation must occur in the cells that produce
either eggs or sperm.
Lesson Overview
Genes and Variation
Sources of Genetic Variation
Another source of genetic variation is genetic
recombination during sexual reproduction. The
23 pairs of chromosomes in the gametes can
produce millions of different gene combinations
upon fertilization.
Crossing-over, which occurs during meiosis and
the production of gametes, increases the
number of genotypes (gene combinations) that
can appear in offspring.
Lesson Overview
Genes and Variation
Sexual reproduction can produce many
different phenotypes. However, it does not
change the relative frequency of alleles in a
population.
Since it does not change the frequencies of
alleles, It does not cause evolution. (It merely
increases variation.) Natural selection causes
evolution.
Lesson Overview
Genes and Variation
Sources of Genetic Variation:
Lateral Gene Transfer
Lateral gene transfer occurs when organisms pass
genes from one individual to another that is not its
offspring.
It can occur between organisms of the same species or
organisms of different species. Lateral gene transfer
occurs between bacteria during transformation.
Lateral gene transfer can increase genetic variation in a
species that picks up the “new” genes.
Lesson Overview
Genes and Variation
Single-Gene and Polygenic Traits
The number of phenotypes produced for a
trait depends on how many genes control
the trait.
Lesson Overview
Genes and Variation
Single-Gene Traits
A single-gene trait is a trait controlled by only
one gene. Single-gene traits may have just two
or three distinct phenotypes.
The most common form of the allele can be
dominant or recessive. Dominance of an allele
for a single-gene trait does not necessarily
mean that the dominant phenotype will always
appear with greater frequency in a given
population.
Lesson Overview
Genes and Variation
An example of a singlegene trait is the presence of
dark bands that appear on
the shells of a certain
species of snails. Even
though the allele for shells
without bands is dominant,
a population may show a
greater frequency of the
“with bands” phenotype.
Lesson Overview
Genes and Variation
Polygenic Traits
Polygenic traits are traits controlled by two or
more genes.
Each gene of a polygenic trait often has two or
more alleles.
A single polygenic trait often has many possible
genotypes and even more different phenotypes.
Lesson Overview
Genes and Variation
Polygenic Traits
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.