Download Recombination, Lateral Gene Transfer, and Gene Duplication Can

Recombination, Lateral Gene Transfer,
and Gene Duplication Can Result in New
Features
15.6
Sexual Recombination Amplifies the Number of
Genotypes
Several evolutionary processes can result in the acquisition of major new characteristics in
populations, as seen in previous sections of chapter 15.
Each one of these processes results in a larger and more rapid evolutionary changes than do a
single point mutation.
Sexual reproduction results in new gene combinations and produces genetic variety that
increases evolutionary potential.
Sexual Recombination Amplifies the
Number of Genotypes
Asexual reproducing organisms are genetically identical unless there is a mutation but when
organisms reproduce sexually, offspring differ from their parents because of crossing over,
independent assortment, and fusion of different gametes.
Sexual recombination produces an endless variety of genotypes which increases the
evolutionary potential of populations
This would be considered a long-term advantage of sex
In the short term, sexual reproduction has disadvantages:
• Recombination can break up adaptive combinations of genes
• Reduced rate at which females pass genes to offspring
• Dividing offspring into genders reduces the overall reproductive rate
Sexual Recombination Amplifies the
Number of Genotypes
Exactly how these process of genetic recombination came about is puzzling, in the short term,
sexual reproduction has three distinct disadvantages:
• Recombination can break up adaptive combinations of genes
• Reduced rate at which females pass genes to offspring
• Dividing offspring into genders reduces the overall reproductive rate
Sexual Recombination Amplifies the
Number of Genotypes
To see why separating into different genders reduces
overall reproductive rate consider
• A asexual mother who has two offspring
• A sexual mother has two offspring
•Assume both females produce two offspring, but that half
of the sexual females are males
•The asexual offspring will produce two females
•In the next generation, there are two females for asexual
and only one in the sexual offspring
•Thus, effective reproductive rate in the asexual lineage is
twice that of the sexual lineage
Sexual Recombination Amplifies the
Number of Genotypes
So then why did sexual reproduction evolve?
Possible advantages:
• Facilitates repair of damaged DNA— damage on one chromosome can be repaired by copying intact
sequences on the other chromosome
• Elimination of deleterious mutations through recombination followed by selection
• The variety of genetic combinations in each generation can be advantageous (e.g., as defense against
pathogens and parasites)
Sexual recombination does not directly influence the frequencies of alleles, it generates new
combinations of alleles on which natural selection can act.
Sexual Recombination Amplifies the
Number of Genotypes
• In asexually reproducing species, deleterious mutations can accumulate; only death of the lineage can
eliminate them
◦ Muller called this the genetic ratchet—mutations accumulate or “ratchet up” at each replication; known as Muller’s ratchet.
Lateral Gene Transfer Can Result In the
Gain of New Functions
We have see the tree of life branching as new adaptations and specialization occurs within
individual organisms, however there are processes that can result in lateral gene transfer
Lateral gene transfer—individual genes, organelles, or genome fragments move horizontally
from one lineage to another
• Species may pick up DNA fragments directly from the environment.
• Genes may be transferred to a new host in a viral genome.
• Hybridization results in the transfer of many genes.
Lateral Gene Transfer Can Result In the
Gain of New Functions
Lateral gene transfer can be advantageous; it increases genetic variation.
◦ Most common in bacteria; genes that confer antibiotic resistance are often transferred
among species
◦ Relatively uncommon in eukaryotes, but hybridization in plants leads to gene exchange
The endosymbiosis events that gave rise to mitochondria and chloroplasts were lateral
gene transfers.
Lateral Gene Transfer Explained
Lateral Gene Transfer: Rethinking Evolution
Bozeman – Mechanisms that Increase Genetic Variation
Many New Functions Arise Following
Gene Duplication
Gene duplication is another way genomes can gain new
functions
When a gene is duplicated, one copy of that gene is potentially
freed from having to perform its original function
The identical copies have one of four different fates
◦ Both copies retain original function, which can increase the
amount of gene product.
◦ Gene expression may diverge in different tissues or at different
times in development.
◦ One copy may accumulate deleterious mutations and become a
functionless pseudogene.
◦ One copy retains original function, the other changes and evolves
a new function.
Many New Functions Arise Following
Gene Duplication
Sometimes entire genomes may be duplicated,
providing massive opportunities for new functions
to evolve.
In vertebrate evolution, genomes of the jawed
vertebrates have four diploid sets of many genes.
Two genome-wide duplication events occurred in
the ancestor of these species. This allowed
specialization of individual vertebrate genes.
These duplications allowed for considerable
specialization of individual vertebrate genes, which
are now highly specific
Transfer,
and Gene Duplication Can Result in New
Features
Successive rounds of duplication and sequence evolution may result in a gene family, a group of
homologous genes with related functions.
The globin gene family probably arose via gene duplications.
A duplication event
led to the α and β
gene clusters
Numbers indicate the estimated
number of DNA sequence changes
along that branch of a tree
Evolutionary Theory Has
Practical Applications
15.7
Knowledge of gene evolution is used to
study protein function
Evolutionary theory has many practical applications across biology, and new ones are being
developed every day
A few applications can be applied to fields such as agriculture, industry and medicine.
Molecular evolutionary principles can be used to understand protein structure and function.
Knowledge of gene evolution is used to
study protein function
Puffer fish produce a toxin (TTX) that blocks
Na+ channels and prevents nerve and muscle
function.
People that eat the toxin of puffer fish can
become paralyzed and die because the toxin
will block their nerves and muscles from
functioning properly.
But Na+ channels in the puffer fish itself are
not blocked by the toxin.
Nucleotide substitutions in puffer fish genes
result in changes in the channel proteins that
prevent TTX from binding.
TTX Video
TTX Video #2
Knowledge of gene evolution is used to
study protein function
Mutations in human Na+ channel genes cause several neurological diseases.
Study of these gene substitutions aids in understanding how Na+ channels function.
Biologists compare rates of synonymous and nonsynonymous substitutions across Na+ channel
genes in various animals that have evolved TTX resistance.
In vitro evolution produces new
molecules
Living organisms produce many compounds useful to
humans. The search for such compounds is called
“bioprospecting.”
These molecules result from millions of years of
evolution.
But biologists can imagine molecules that have not yet
evolved.
In vitro evolution—new molecules are produced in the
laboratory to perform novel functions
2. Select the RNA
molecules with the
highest ligase
activity
1. Start with a
random pool of
RNA sequences
5. Transcribe back
into RNA, and then
repeat the cycle for
10 rounds
3. Reverse
transcribe the RNA
into DNA
4. Use Polymerase
Chain Reaction
(PCR) amplification
to introduce new
mutations into the
DNA population
6. After several
rounds, an
effective
ribozyme has
evolved from
the pool of
random RNA
sequences
Evolutionary theory provides multiple
benefits to agriculture
In agriculture, breeding programs have benefited from evolutionary principles, including
incorporation of beneficial genes from wild species.
An understanding of how pest species evolve resistance to pesticides has resulted in more
effective pesticide application and rotation schemes.
Resistance video
Knowledge of molecular evolution is
used to combat diseases
Molecular evolution is also used to study disease
organisms.
All new viral diseases have been identified by
evolutionary comparison of their genomes with
those of known viruses.
Studies of the origins, timing of emergence, and
global diversity of human pathogens (including HIV &
SARS) depend on evolutionary principles and
methods, as do efforts to develop effective vaccines.
Once biologists have collected genome data for
enough infectious organisms, it will be possible to
identify an infection by sequencing a portion of the
pathogens genome and comparing this sequence
with other sequences on an evolutionary tree.