ZOOHCC - 602: Evolutionary Biology (Theory)
Neutral theory of molecular evolution
The neutral theory of molecular evolution is a scientific theory that
suggests that most of the genetic variations and mutations that occur in a
population do not affect the fitness or function of the organism. These
neutral mutations are thought to be governed by random processes like
genetic drift and mutation, rather than natural selection. According to this
theory, the rate of molecular evolution is expected to be constant over time
and across different species.
Explanation
The neutral theory of molecular evolution is a theory that proposes that
most genetic mutations and variations that arise in a population are
neutral, meaning they do not affect the fitness of the organism. According
to this theory, genetic drift, or the random fluctuation of gene frequencies
in a population, plays a larger role in evolution than natural
selection.
The neutral theory of molecular evolution was first proposed by Japanese
biologist Motoo Kimura in the 1960s. Kimura suggested that most genetic
variation arises from the accumulation of neutral mutations that do not
affect the function of the protein they encode. These neutral mutations are
not subject to natural selection and are instead governed by the stochastic
processes of genetic drift and mutation.
Under the neutral theory, the rate of molecular evolution is expected to be
constant over time and across different species, as the rate of neutral
mutations is not influenced by the fitness of the organism. The neutral
theory has been supported by studies of genetic variation within
populations, as well as comparisons of genetic sequences between different
species.
However, the neutral theory of molecular evolution has been challenged by
some scientists, who argue that natural selection plays a larger role in
molecular evolution than previously thought. Some studies have shown that
many mutations previously thought to be neutral are actually slightly
deleterious, meaning they have a small negative effect on fitness.
Despite these debates, the neutral theory of molecular evolution remains an
important concept in evolutionary biology, and has contributed to our
understanding of the forces that drive genetic variation and
evolution.
Different terms involved in Neutral theory of molecular evolution are
Neutral mutations:-
The genetic changes or variations that occur in a DNA sequence but do not
affect the function or fitness of the organism. These mutations are not
subject to natural selection and are instead governed by random processes
like genetic drift and mutation. Neutral mutations can accumulate in a
population over time, and they can serve as a marker for estimating the
divergence time between two species or the evolutionary history of a
particular genetic sequence. In the context of the Neutral Theory of
Molecular Evolution, neutral mutations are thought to be the predominant
type of mutation that occurs in a population.
Genetic drift:-
Genetic drift is a random process that can cause changes in the frequency
of different versions of a gene, called alleles, in a population over time.
These changes occur due to chance events, like mutations, migrations, or
random deaths, and can happen more frequently in smaller populations.
Genetic drift can lead to the loss of certain alleles, or the fixation of
others, which can decrease genetic diversity in a population. It is
different from natural selection, which favors alleles that provide an
advantage for survival or reproduction. However, genetic drift can still
influence the evolutionary trajectory of a population, especially when
selection pressures are weak.
Molecular clock:-
The molecular clock is a tool used in evolutionary biology to estimate the
time of divergence between two species based on the rate of genetic
mutations that occur in their DNA sequences. The basic idea behind the
molecular clock is that mutations accumulate in a species' DNA at a roughly
constant rate over time, like the ticks of a clock.
By comparing the number of genetic differences between two species in a
particular DNA sequence, researchers can estimate how long ago the two
species shared a common ancestor. This information can be used to construct
evolutionary timelines and infer the history of life on Earth.
The molecular clock is based on the assumption that mutations occur at a
regular rate, but this rate can vary depending on factors like the type of
mutation, the location of the gene in the genome, and the evolutionary
history of the species. As such, the molecular clock is a useful but
imperfect tool that can provide insights into evolutionary relationships
between organisms.
Silent mutations:-
A type of mutation that occurs in the DNA sequence, but does not alter the
amino acid sequence of the protein it codes for, and therefore has no effect
on the phenotype.
Synonymous mutations:-
Another term for silent mutations, as they produce a synonymous codon that
codes for the same amino acid as the original codon.
Non-synonymous:-
Mutations: A type of mutation that alters the amino acid sequence of the
protein, and therefore has the potential to affect the phenotype.
Evolutionary rate:-
Evolutionary rate refers to the speed at which genetic changes, such as
mutations, accumulate in a population over time. It can be measured as the
number of changes in a DNA sequence that occur over a certain period or as
the amount of genetic divergence between two species.
The evolutionary rate is influenced by various factors, including the
generation time of the species, the intensity of natural selection, and the
rate of genetic drift. It can vary widely between different organisms and
can also vary within a single genome or gene depending on its location and
function.
By studying the evolutionary rate of different genes or genomes,
researchers can gain insights into the mechanisms of evolution and the
evolutionary history of different species. The evolutionary rate can also be
used to estimate the time of divergence between different populations or
species and to understand the patterns and processes of evolution on a
broader scale.