Mechanisms that Produced Change in Populations
Most Essential Learning Competency (MELC)
Explain the mechanisms that produce change in populations from generation to
generation (e.g., artificial selection, natural selection, genetic drift, mutation, recombination)
(STEM_BIO11/12/-IIIc-g-9)
Key Concepts
Evolution, the gradual process of change, is naturally occurring among populations at
negligible rate. In nature, populations such as grasses, birds, dolphins and other organisms, and
even the Covid-19 virus, are usually evolving.
What are the mechanisms that produce change in populations from generation to
generation?
1.
Natural Selection. Charles Darwin introduced
natural selection as evolution process which
hypothesize that all forms of life came from a
common ancestry which developed into various
forms as they adapted to their environment.
Based on the theory, the population is controlled
by several factors in the environment. Organisms
that possessed favorable traits and who can better
adapt to their environment will survive, leaving
behind many offspring; while those that failed to
adapt, will leave fewer offspring and will die and
may totally disappear. Natural selection results in
the accumulation of new variations and the loss of
unfavorable ones, giving rise to new species. This
theory summarizes the famous evolutionary context
“survival of the fittest” (see Figure 1).
Source: pinterest.ph/pin/478929741621/
Figure 1 Natural Selection of Nature
2. Artificial selection. Farmers and genetic
engineers intentionally prefer "selective breeding”,
where they select desirable traits in agricultural
products or animals, rather than leaving the
species to evolve and change gradually without
human interference. Over the course of decades,
selection was used by farmers and breeders to
cause major changes in the characteristics of their
plants or animals. Selectively using only, the
plants and animals with desirable characteristics
to reproduce, causing the evolution of farm stock.
As shown in Figure 2, farmers have cultivated
numerous popular
crops
from
the
wild
mustard, by artificially selecting their preferred
attributes.
Source: evolution.berkeley.edu/evolibrary/article/evo_30
Figure 2 These common vegetables were cultivated
from forms of wild mustard. This is evolution through
artificial selection.
3. Genetic Drift. It describes random fluctuations in allele frequencies in a population.
Eventually, genetic drift can cause a subpopulation to become genetically distinct from its
original population. Over a long period of time, genetic drift and the accumulation of other
genetic changes can result in speciation, which is the evolution of a new species.
The cheetah shown in Figure 3 is a
species whose evolution has been
seriously affected by genetic drift
which caused their population to
decline over the last 5,000 years. As a
result,
most
of
the
cheetah’s
descendants are almost genetically
uniform
with
each
other.
One
consequence of this genetic uniformity
is reduced disease resistance- cheetah
cubs are more likely to die from
disease than the cubs of lions or
leopards. The main reason of their
hasten extinction than the other
species. One example of genetic drift is Source: cheetahlearning.com/wp/further faster/
a
Bottleneck effect which
may
Figure 3 Cheetahs are endangered. Cheetahs have gone
happen when the size of a population through at least two drastic declines in population size.
is severely reduced. Also, natural
disasters like earthquakes, floods, fires can lessen a population, leaving behind a small,
random assortment of survivors.
4. Mutation. It is a permanent change in the DNA sequence of a gene. It may sometimes useful
but mostly mutation is harmful because it changes the way a cell behaves. That is, when the genes
which contain instructions necessary for a cell to work is changed or mutated, then the cell
may not know what it is supposed to do.
Mutations are divided into
two general types based on the
ranges in size from a single DNA
base to a large segment of a
chromosome. These two general
types are DNA mutations and
chromosomal mutations. DNA
mutations are mutations that
happen when there are changes
in the nucleotide sequence of
the DNA
while
chromosomal
mutations are mutations that
occur when there are changes
or
abnormalities
in
the
Source: shutterstock.com/search/translocation
Figure 4: Types of Chromosomal Mutations
structure and number of chromosomes. Chromosomal mutations are also called chromosomal
abnormalities or chromosomal aberrations (See Figure 4).
5. Recombination. It is a process by
which pieces of DNA are broken
and recombined to produce new
combinations
of
alleles.
This
recombination
process
creates
genetic diversity at the level of
genes that reflects differences in
the DNA sequences of different
organisms.
Recombination typically occurs
during meiosis in eukaryotic cells.
It is during this type of cell division
that gametes – sperm and egg cells Source: quora.com/Which-phase-of-the-prophase-of-Meiosis-1-doesare produced. During the first
Figure 5 Recombination in Meiosis
phase of meiosis, the homologous
pairs of maternal and paternal chromosomes align. Then, the arms of the chromosomes
temporarily fuse and overlap, causing a crossover. Crossovers result in recombination (see
Fig.5) and the exchange of genetic material between the maternal and paternal chromosomes.
This led the offspring to have different combinations of genes than their parents. Those genes
that are located farther apart on the same chromosome have a greater likelihood of
undergoing recombination, likely having a greater recombination frequency.
By which means or conditions which evolution will not occur?
The answer to this question is provided in the Hardy- Weinberg Principle which states that
the allele frequencies in population will remain constant unless one or more factors caused these
frequencies to change. When allele frequencies remain, constant there is genetic equilibrium, thus,
the population will not evolve.
Hardy-Weinberg Principle hold on to the following conditions:
1. No mutation. No new alleles are generated by mutation nor are genes duplicated or deleted.
2. Random mating. Organisms mate randomly with each other with no preference for particular
genotypes.
3. No gene flow. Neither individuals nor their gametes will enter or exit the population.
4. Very large population size. The population should be effectively infinite in size.
5. No natural selection. All alleles that confer equal fitness tend to make organisms likely to
survive and reproduce and those alleles with reduced fitness tend to have a dropping
frequency from one generation to the next.
When these conditions are met, the genetic equilibrium is maintained from generations to
generations. The gene pool in a population remains pretty stable. All the five mechanisms of
evolution mentioned above may act to some extent in any natural population. In fact, the
evolutionary route of a given gene that is, how its alleles change in frequency in the population
across generations may result from several evolutionary mechanisms acting at once. However,
when the genetic equilibrium is not achieved or when at a stable state is disrupted, the population
will continuously evolve.