Although most species on the planet reproduce sexually,
why do some species still reproduce asexually?
Why is it evolutionarily beneficial
for some species to require two parents to produce offspring
and others only one?
Let's explore why different forms of reproduction exist
and what makes each one beneficial
in specific circumstances.
The two types of reproduction
are asexual and sexual.
Asexual reproduction is when an organism
makes a genetically identical clone of itself.
Most of the simplest life forms,
such as bacteria,
reproduce in this manner.
Sexual reproduction is much more complex
and involves two members of a species
coming together to produce genetically distinct offspring.
Most eukaryotic multicellular organisms,
such as humans, birds, and insects,
undergo sexual reproduction.
The biggest advantage of sexual reproduction
is that offspring are genetically distinct
from their parents and their siblings.
This allows a species to evolve,
since natural selection will favor the offspring carrying genes
that improve their survival in diverse environmental conditions.
For example, humans who carry two abnormal copies
of the hemoglobin gene
develop sickle cell anemia,
a devastating disease that results in
abnormally low levels of oxygen in the blood.
Individuals who have only one copy of the abnormal gene
have a much less severe form of the disease.
Interestingly, heterozygous individuals
are much less prone to infection by malaria,
making them more likely to survive
and have higher reproductive success
in malaria-endemic areas.
This example shows how sexual reproduction
can produce offspring
who are genetically distinct from their parents
and are better suited to survive
under certain environmental conditions.
We have been talking about genetic diversity,
but how does sexual reproduction
generate this diversity in the first place?
During meiosis, which produces the sperm and the egg
needed for sexual reproduction,
there are two key steps that produce genetic diversity.
First is recombination between homologous chromosomes.
Recombination mixes maternal and paternal alleles,
and results in novel gene combinations
on each chromosome in each generation.
The second is the independent assortment of chromosomes,
which generates unique combinations of chromosomes
in each gamete.
During fertilization,
one genetically unique sperm
and one genetically unique egg
randomly combine to form an offspring
that is genetically distinct from its parents.
While genetic diversity arising from sexual reproduction
has been critical for the survival of many species,
mating is an energy consuming, slow, and risky process.
To improve the chance of selecting the best partner,
and therefore the best alleles,
some species have evolved highly sophisticated courtship behaviors.
Male peacocks, for example,
grow long, iridescent tail feathers
and perform an elaborate dance to attract a mate.
But this process requires energy
and can attract predators.
In addition, the generation of gametes for sexual reproduction,
and the subsequent gestation and incubation,
take time and energy.
For example,
emperor penguins must sit on their eggs for over two months,
and elephants can be pregnant for almost two years.
While asexual reproduction
does not produce genetically distinct offspring,
it has important advantages
that allow for species survival.
One, it is energy efficient
and does not require attracting a mate.
Two, it is usually fast.
Some bacteria can reproduce
in as little as twenty minutes.
And three, it produces offspring that are genetically identical,
an advantage in stable environmental conditions.
For example, bacteria that carry
an ampicillin antibiotic resistance gene
will continue to grow in patients prescribed this antibiotic.
Conversely, because asexual reproduction
results in genetic clones,
these bacteria would all die if the patient
was treated with a different antibiotic.
Rare, random mutations in the genome
do allow asexually reproducing organisms
to evolve diversity.
However, this is a slow process,
and there is no mechanism to separate harmful and beneficial mutations
for propagation to future generations.
Both sexual and asexual reproduction
provide important methods
for species survival and adaptation,
allowing the millions of species on Earth today
to evolve and survive in their current forms.
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