ZOOHCC - 602: Evolutionary Biology (Theory)
Hardy-Weinberg Law (statement and derivation of equation)
The Hardy-Weinberg law, also known as the Hardy-Weinberg equilibrium, is a
fundamental principle in population genetics. It describes the
relationship between allele and genotype frequencies in an idealized,
non-evolving population under certain conditions. The Hardy-Weinberg law
states that the gene frequencies in a population remain constant from
generation to generation if the following conditions are met:
Large Population Size: The population is large enough to avoid significant
genetic drift.
No Mutation: There are no new mutations introducing new alleles into the
population.
Random Mating: Individuals in the population mate randomly with respect to
the studied gene or trait.
No Migration: There is no migration of individuals into or out of the
population, preventing gene flow.
No Natural Selection: There is no differential reproductive success or
advantage associated with the studied gene or trait.
The Hardy-Weinberg equilibrium can be mathematically represented by the
Hardy-Weinberg equation. The equation describes the expected genotype
frequencies in a population based on the allele frequencies.
Let's consider a gene with two alleles, A and a. The Hardy-Weinberg
equation is as follows:
p^2 + 2pq + q^2 = 1
In this equation:
p represents the frequency of allele A in the population.
q represents the frequency of allele a in the population.
p^2 represents the expected frequency of individuals with genotype AA.
2pq represents the expected frequency of individuals with genotype Aa.
q^2 represents the expected frequency of individuals with genotype aa.
The sum of p^2, 2pq, and q^2 equals 1, representing the total genetic
variation within the population.
The Hardy-Weinberg equation can be derived using principles of Mendelian
inheritance and the laws of probability. It assumes that alleles are
transmitted to the next generation according to the rules of segregation
and independent assortment.
By comparing the observed genotype frequencies in a population to the
expected frequencies predicted by the Hardy-Weinberg equation, population
geneticists can assess whether the population is in Hardy-Weinberg
equilibrium or if evolutionary processes, such as genetic drift or natural
selection, are occurring. Any deviation from the expected frequencies
suggests that evolutionary forces are acting on the population.
Derivation of Hardy-Weinberg Law
The derivation of the Hardy-Weinberg equation involves applying the
principles of Mendelian inheritance and the laws of probability. Let's
go through the derivation step by step:
Define the allele frequencies: Let's consider a gene with two alleles, A
and a, in a population. We'll denote the frequency of allele A as p and
the frequency of allele a as q. Since there are only two alleles, p + q
= 1.
Define the genotype frequencies: In a diploid organism, individuals can
have three possible genotypes: AA, Aa, and aa. We'll denote the
frequencies of these genotypes as p^2, 2pq, and q^2, respectively.
Calculate the probabilities of each genotype: The probability of an
individual having genotype AA is p * p = p^2, as both alleles must be A.
Similarly, the probability of having genotype aa is q * q = q^2. The
probability of having genotype Aa is 2 * p * q because there are two
ways to combine the A and a alleles.
Apply the laws of probability: In a population in Hardy-Weinberg
equilibrium, the frequencies of the genotypes must add up to 1.
Therefore, the sum of the probabilities of the three genotypes must
equal 1:
p^2 + 2pq + q^2 = 1
This equation represents the Hardy-Weinberg equation, which describes
the expected genotype frequencies in a population under the conditions
of random mating, no mutation, no migration, no genetic drift, and no
natural selection.
The derivation relies on the assumptions of Mendelian inheritance,
including the random segregation of alleles during gamete formation and
the independent assortment of alleles from different genes. It also
assumes an idealized, infinitely large population.
By comparing the observed genotype frequencies in a population to the
expected frequencies predicted by the Hardy-Weinberg equation,
researchers can detect deviations from equilibrium and infer the
presence of evolutionary forces at play, such as genetic drift,
migration, mutation, or natural selection.
Natural selection (concept of fitness)
Natural selection is a fundamental mechanism of evolution that drives
the adaptation of populations to their environment. It is based on the
concept of fitness, which measures an organism's reproductive success
and ability to pass on its genes to future generations.
Fitness, in the context of natural selection, refers to the relative
reproductive success of individuals within a population. It is
determined by how well-suited an organism's traits are to its
environment and how effectively those traits enable the organism to
survive, reproduce, and produce offspring that can themselves survive
and reproduce.
Fitness can be influenced by various factors, including physical
attributes, behavioral characteristics, and physiological traits. The
key idea is that individuals with higher fitness have a higher
likelihood of surviving, reproducing, and passing on their
advantageous traits to their offspring, while individuals with lower
fitness have a reduced chance of reproductive success.
There are three main types of fitness that contribute to natural selection:
Survival Fitness (Survival of the Fittest): This type of fitness is
based on an organism's ability to survive and avoid death. Individuals
with traits that enhance their survival in a particular environment,
such as camouflage, defensive mechanisms, or efficient foraging
abilities, have a higher survival fitness.
Reproductive Fitness: Reproductive fitness is related to an organism's
ability to successfully reproduce and pass on its genetic material to
the next generation. It is influenced by factors such as mating
success, fertility, and parental care. Individuals with traits that
increase their reproductive success, such as attractive physical
features or effective mating strategies, have higher reproductive
fitness.
Fitness in the Context of Offspring Survival: This type of fitness
considers the ability of an organism's offspring to survive and
reproduce. It takes into account traits that enhance the survival and
reproductive success of the offspring, such as parental care, disease
resistance, or competitive abilities.
Through natural selection, individuals with higher fitness traits have
a better chance of surviving, reproducing, and passing on their
advantageous traits to future generations. Over time, these traits
become more prevalent in the population, while less advantageous
traits may decrease or disappear. This gradual change in the genetic
makeup of a population over generations is the basis of evolutionary
adaptation.
It's important to note that fitness is highly context-dependent and
can vary across different environments and selective pressures. Traits
that are advantageous in one environment may be disadvantageous in
another. Natural selection acts as a filter that favors traits that
increase fitness within a specific ecological niche, leading to the
diversification and specialization of organisms over time.
Density-dependent selection
Density-dependent selection is a type of natural selection in which
the fitness of individuals is influenced by the density or population
size of their species. It occurs when the fitness of individuals
depends on the interactions and competition they experience due to
crowding or limited resources within a population.
In density-dependent selection, the intensity and direction of
selection pressure can vary with population density. Here are two
common forms of density-dependent selection:
Competition for Resources: As population density increases,
individuals within the population must compete more intensively for
limited resources such as food, water, nesting sites, or mates. In
such cases, individuals with traits that enhance their competitive
abilities, such as better foraging skills or more efficient resource
utilization, have higher fitness. These individuals can outcompete
others and gain better access to essential resources, leading to
higher survival and reproductive success.
Disease and Predation: In dense populations, diseases and predation
can spread more easily due to increased contact and proximity among
individuals. As a result, individuals with traits that provide
resistance or defense against diseases or predators have higher
fitness. For example, individuals with stronger immune systems or
better camouflage may have a survival advantage, leading to higher
reproductive success in a densely populated environment.
Overall, density-dependent selection acts as a mechanism that
regulates population size and maintains a balance between individuals
and available resources. When a population reaches high density, the
selective pressure increases, favoring traits that allow individuals
to cope with competition, disease, or predation. These selective
pressures help to control population growth and maintain population
stability over time.
It's important to note that density-dependent selection does not occur
in isolation but interacts with other forms of selection and
evolutionary forces. The specific traits favored by density-dependent
selection may vary depending on the ecological context and the
particular species involved.
Heterozygous superiority
Heterozygous superiority, also known as overdominance or heterozygote
advantage, is a concept in population genetics that describes a
situation in which individuals heterozygous for a particular gene have
a higher fitness than individuals homozygous for either allele.
In a population, each individual inherits two copies of a gene, one
from each parent. In the case of heterozygous superiority, the
heterozygotes (individuals carrying two different alleles for a gene)
have a selective advantage over the homozygotes (individuals carrying
two identical alleles).
There are several mechanisms through which heterozygous superiority can provide an advantage
Genetic Complementation: Heterozygosity can compensate for the
presence of deleterious alleles or mutations. If one allele carries a
harmful mutation, the presence of a second, functional allele can
"complement" and compensate for the negative effects. As a result,
heterozygotes have a higher fitness compared to homozygotes carrying
two copies of the mutated allele.
Balanced Polymorphism: Heterozygous superiority can contribute to the
maintenance of genetic diversity within a population. When both
alleles of a gene provide a selective advantage in different
environmental conditions, the heterozygotes have an adaptive advantage
over either homozygote. This can lead to a balanced polymorphism,
where multiple alleles are maintained in the population at relatively
stable frequencies.
Resistance to Disease: Heterozygosity can confer increased resistance
to infectious diseases. Certain genetic variations that increase
susceptibility to a disease when homozygous can provide enhanced
resistance when present in a heterozygous state. This is often related
to the recognition and defense mechanisms of the immune system.
Examples of heterozygous superiority can be found in various
organisms:
Sickle cell anemia: Heterozygotes for the sickle cell allele (HbAS)
have a selective advantage over both homozygotes. While homozygotes
for the sickle cell allele (HbSS) suffer from the severe symptoms of
sickle cell disease, heterozygotes are more resistant to malaria,
which provides a survival advantage in malaria-endemic regions.
Cystic fibrosis: Heterozygotes for the cystic fibrosis allele (CFTR)
have a higher resistance to cholera, a diarrheal disease caused by the
bacteria Vibrio cholerae. This contributes to the persistence of the
cystic fibrosis allele in populations where cholera is prevalent.
Heterozygous superiority is one of the mechanisms that contribute to
the maintenance of genetic diversity within populations and can have
significant implications for the evolution and dynamics of genetic
variation in a species.
kin selection
Kin selection is a concept in evolutionary biology that explains the
evolution of behaviors that may seem altruistic but actually confer
benefits to relatives. It is based on the idea that individuals can
increase their inclusive fitness by helping close relatives, who
share a portion of their genetic material.
The key principle behind kin selection is that genes can increase
their representation in future generations not only by being passed
on through individual reproduction but also through the reproduction
of relatives who carry similar genetic material. Therefore,
behaviors that promote the survival and reproductive success of
relatives can be favored by natural selection.
Kin selection is often associated with the theory of Hamilton's
rule, which states that an altruistic behavior is likely to evolve
if the benefit to the recipient multiplied by the coefficient of
relatedness is greater than the cost to the altruistic individual.
The coefficient of relatedness quantifies the genetic relatedness
between individuals, representing the proportion of genes shared due
to common ancestry.
Kin selection can lead to the evolution of various types of
behaviors, including cooperation, parental care, and even
self-sacrifice, as long as the benefits to relatives outweigh the
costs to the individual performing the behavior.
Examples of kin-selected behaviors include:
Cooperative Breeding: In many species, individuals forego their own
reproduction and instead assist in raising offspring of close
relatives, such as siblings. This behavior enhances the survival and
reproductive success of the relatives' offspring, indirectly
increasing the representation of shared genes in future generations.
Alarm Calls: Some animals emit alarm calls to warn other individuals
in the group of approaching predators. By alerting their relatives,
they increase the chances of their kin surviving and reproducing, at
the expense of potentially attracting the predator's attention to
themselves.
Eusociality: In eusocial insect colonies, such as those of bees,
ants, and termites, individuals have specialized roles within the
colony, with most individuals being non-reproductive workers that
support the reproduction of a few highly fertile individuals
(queens). This division of labor benefits the reproductive success
of the queens, who are closely related to the workers.
Kin selection provides a framework to understand how seemingly
altruistic behaviors can evolve and be maintained in populations. By
considering the inclusive fitness gained through helping relatives,
individuals can indirectly increase their genetic representation in
future generations, even at the expense of some personal costs or
risks.
sexual selection
Sexual selection is a concept in evolutionary biology that describes
the process by which certain traits and behaviors evolve due to
their influence on an individual's mating success. It is a specific
form of natural selection that acts on traits related to
reproduction and mate choice.
Sexual selection can occur through two distinct mechanisms:
Intrasexual Selection: This form of sexual selection involves
competition between members of the same sex for access to mates. In
species where males compete for females, it often leads to the
evolution of traits that enhance male-male competition, such as
larger body size, weaponry (e.g., antlers, horns), or aggressive
behaviors. These traits allow males to outcompete rivals and gain
opportunities to mate with females. In species where females compete
for males, the traits involved may include larger body size, more
elaborate displays, or dominance hierarchies.
Intersexual Selection: Intersexual selection, also known as mate
choice, occurs when individuals of one sex (usually females) choose
their mates based on specific traits or behaviors. Individuals with
certain desirable traits are preferred as mates, leading to the
evolution of these traits over time. Examples of traits that can be
favored by intersexual selection include colorful plumage, elaborate
courtship displays, or intricate songs. These traits are often
indicators of genetic quality, health, or the ability to provide
resources or parental care.
Sexual selection can result in the evolution of exaggerated or
elaborate traits that may seem disadvantageous or costly in terms of
survival. However, if these traits enhance an individual's mating
success, they can be maintained or even become more pronounced in a
population over generations.
Some examples of sexual selection in nature include:
Peacock's Tail: Male peacocks have large, colorful tails that are
used in elaborate courtship displays. The size and vibrancy of the
tail are indicators of the male's genetic quality, and females
prefer males with more impressive tails. Despite being a handicap in
terms of survival (increased predation risk), peacock tails have
evolved through intersexual selection.
Stag Beetle Mandibles: Male stag beetles have enlarged mandibles
that are used for combat during competitive interactions with other
males. The larger the mandibles, the greater the chances of winning
fights and gaining access to mates.
Bird Songs: Many bird species have complex and melodious songs.
These songs serve as signals to attract females and establish
territory. Males with more intricate and attractive songs have a
higher chance of attracting mates.
Overall, sexual selection is a powerful evolutionary force that
drives the evolution of traits and behaviors related to
reproduction. It can result in the development of elaborate
ornaments, weapon-like structures, or complex courtship behaviors,
which play a vital role in mating success and reproductive fitness.
Genetic Drift (mechanism, founder’s effect, bottleneck phenomenon)
Genetic drift is a mechanism of evolution that occurs due to random
fluctuations in allele frequencies in a population. It can lead to
changes in the genetic makeup of a population over time, independent
of natural selection. Genetic drift is more pronounced in small
populations and can have significant effects on genetic diversity.
Mechanism of Genetic Drift:
Genetic drift occurs as a result of random sampling of alleles
during the process of reproduction. Each generation's gene pool is a
random subset of the previous generation's gene pool, leading to
fluctuations in allele frequencies. The effects of genetic drift are
more pronounced in small populations where chance events can have a
greater impact on the genetic composition of future generations.
Founder Effect:
The founder effect is a specific scenario of genetic drift that
occurs when a small group of individuals from a larger population
establishes a new population in a different geographic area or
habitat. The founding individuals carry only a subset of the genetic
variation present in the larger population, and their allele
frequencies may differ from the original population by chance. As a
result, the new population may have reduced genetic diversity
compared to the source population.
An example of the founder effect is the colonization of islands by a
small number of individuals. The allele frequencies of the founding
individuals may not be representative of the original population,
and as the isolated population grows, the genetic makeup of the
island population becomes distinct from the source population.
Bottleneck Phenomenon:
The bottleneck phenomenon occurs when a population undergoes a
dramatic and severe reduction in size, often due to a catastrophic
event like a natural disaster or disease outbreak. As a result, the
surviving individuals possess only a small fraction of the genetic
variation present in the original population. The genetic diversity
is significantly reduced, and certain alleles may be lost entirely
from the population.
The effects of genetic drift are more pronounced during and after
the bottleneck event because the small population size increases the
relative impact of random genetic fluctuations. As the population
recovers and expands, the genetic makeup of the population may
differ significantly from the pre-bottleneck state.
The founder effect and bottleneck phenomenon demonstrate how genetic
drift can lead to changes in allele frequencies and genetic
diversity within populations. These processes are important factors
in shaping the genetic composition of populations and can have
implications for their ability to adapt to changing environments or
their susceptibility to genetic disorders.
Role of Migration
Migration, also known as gene flow, is a significant factor in
population genetics and plays a crucial role in shaping the genetic
composition and diversity of populations. Migration refers to the
movement of individuals or their genetic material from one
population to another.
Here are some key roles of migration in population genetics:
Introduction of New Genetic Variation: Migration brings individuals
with different genetic backgrounds into a population, introducing
new alleles and genetic variation. This can increase the overall
genetic diversity within the receiving population, which can enhance
its adaptive potential. New alleles introduced through migration can
provide a source of variation that can be subject to natural
selection, genetic drift, or other evolutionary processes.
Counteracting Genetic Drift: Migration acts as a counterforce to
genetic drift, especially in small populations. Genetic drift, as
mentioned earlier, can lead to random fluctuations in allele
frequencies due to chance events. However, migration can introduce
new alleles into the population and replenish the gene pool,
preventing genetic drift from causing significant changes or loss of
alleles. By bringing in new genetic material, migration helps
maintain genetic diversity and mitigate the effects of genetic
drift.
Facilitating Gene Flow between Populations: Migration connects
different populations and allows for the exchange of genetic
material between them. This gene flow can equalize allele
frequencies across populations, making them more genetically
similar. It can also facilitate the transfer of beneficial alleles
or adaptations from one population to another, leading to increased
overall fitness and adaptive potential.
Homogenizing or Differentiating Populations: The extent of migration
between populations can influence the degree of genetic similarity
or differentiation among them. High rates of migration can lead to
genetic homogeneity, as alleles move freely between populations,
potentially reducing genetic differences. Conversely, limited
migration and isolation can contribute to population
differentiation, allowing distinct genetic characteristics or
adaptations to develop.
Speciation and Population Structure: Migration plays a significant
role in the formation of new species and the development of
population structure. When gene flow is limited between populations,
genetic differences can accumulate over time, leading to
reproductive isolation and the potential for speciation. Migration
patterns can shape the geographic distribution and genetic structure
of populations, contributing to the formation of subspecies or
distinct genetic clusters.
Understanding the patterns and rates of migration is crucial for
studying population dynamics, evolutionary processes, and
conservation biology. It helps elucidate the genetic relationships
among populations, assess their ability to adapt to changing
environments, and inform conservation strategies for maintaining
genetic diversity and preventing the loss of unique genetic
characteristics.
Mutation in changing allele frequencies
Mutation is one of the fundamental processes that can lead to
changes in allele frequencies within populations over time. It
introduces new genetic variation by generating novel alleles or
altering existing ones. While mutation rates are generally low, they
provide a continuous source of genetic diversity and play a
significant role in shaping the genetic composition of populations.
Here are some key points regarding the role of mutation in changing
allele frequencies:
Introduction of New Alleles: Mutations can create new alleles that
were not previously present in a population's gene pool. These new
alleles may arise through various mechanisms, such as DNA
replication errors, chemical changes in DNA, or the insertion of
genetic material from other sources (e.g., transposable elements).
When a new mutation occurs, it may be initially present in only a
few individuals. However, over time, it can spread through the
population if it confers a selective advantage or remains neutral
with respect to fitness.
Alteration of Existing Alleles: Mutations can also modify existing
alleles by introducing changes in their DNA sequence. These
alterations, known as point mutations, can result in substitutions,
insertions, or deletions of nucleotides. Such changes can influence
the functioning or expression of genes and potentially impact the
phenotype of individuals. If the altered allele provides a selective
advantage or disadvantage, it may increase or decrease in frequency
within the population over generations.
Balance between Mutation and Selection: The interplay between
mutation and natural selection is crucial in determining the fate of
new mutations. Mutations introduce genetic variation, but their
impact on allele frequencies depends on the selective forces acting
on them. If a mutation is beneficial and enhances an individual's
fitness, it is more likely to increase in frequency due to positive
selection. Conversely, deleterious mutations are likely to decrease
in frequency through negative selection. The balance between
mutation and selection shapes the evolutionary trajectory of
populations.
Neutral Mutations and Genetic Drift: Not all mutations have
immediate or significant effects on an individual's fitness. Neutral
mutations have no discernible impact on an organism's phenotype or
fitness. These mutations can still influence allele frequencies
through genetic drift, the random fluctuation of allele frequencies
in small populations. Neutral mutations can drift to higher or lower
frequencies purely by chance, leading to genetic diversity within
and between populations.
Evolutionary Innovation: Occasionally, mutations can give rise to
novel traits or adaptations that provide individuals with new
capabilities or advantages in their environment. These innovations
can potentially lead to the emergence of new species or the
expansion of ecological niches. Over long periods, beneficial
mutations can accumulate and drive evolutionary change, allowing
populations to adapt to new conditions or exploit new resources.
Mutation is a fundamental source of genetic variation, acting in
conjunction with other evolutionary forces such as natural
selection, genetic drift, and gene flow. It contributes to the
continuous reshuffling of allele frequencies within populations,
allowing for adaptation, diversity, and the potential for
speciation.
