Natural Selection Flashcards
explain how environmental factors act as forces of natural
selection
(Examples such as
resistance to antibiotics,
Biston betularia
(peppered moth), the
Trinidadian guppies and
the Dominican anole.)
Resistance to Antibiotics:
Bacteria populations can develop resistance to antibiotics due to overuse or misuse of antibiotics in medicine and agriculture.
In environments with antibiotic exposure, bacteria carrying genes for antibiotic resistance have a higher chance of survival and reproduction.
Over time, antibiotic-resistant bacteria become more prevalent in the population, leading to the emergence of superbugs that are difficult to treat with antibiotics.
Biston betularia (Peppered Moth):
In industrialized areas with high pollution levels, tree trunks became darkened with soot, providing a darker background.
Initially, light-colored (typica) peppered moths were well-camouflaged against lichen-covered tree trunks.
However, with industrialization, the dark-colored (carbonaria) moths had higher survival rates because they were better camouflaged against the darker background.
As a result, the frequency of the carbonaria morph increased significantly in polluted areas due to natural selection favoring darker coloration.
Trinidadian Guppies:
In Trinidadian streams, guppy populations inhabit different sections with varying levels of predation pressure from fish and other predators.
In high-predation environments, guppies exhibit drab coloration, smaller body size, and early maturity to avoid predation.
Conversely, in low-predation environments, guppies display vibrant coloration, larger body size, and delayed maturity for increased reproductive success.
Predation pressure acts as a selective force, favoring traits that enhance survival and reproductive success in each environment.
Dominican Anole:
In the Caribbean island of Hispaniola, the introduction of invasive species such as the brown anole has led to interspecific competition with the native green anole.
In areas where brown anoles are abundant, green anoles have evolved longer limbs and larger toe pads, enabling them to exploit higher perches and escape competition from brown anoles.
Natural selection favors traits that confer a competitive advantage, allowing green anoles to coexist with brown anoles in invaded habitats.
explain how natural
selection may be an agent
of constancy or an agent of
change
(Directional, disruptive,
and stabilising selection;
knowledge of appropriate
graphs is required)
Directional Selection:
Directional selection occurs when selective pressure favors individuals with an extreme phenotype, causing the frequency of that phenotype to shift over time.
In this mode of selection, the average phenotype of the population changes in one direction.
Directional selection leads to evolutionary change by favoring traits that are advantageous under changing environmental conditions.
Example: In response to industrial pollution, darker-colored peppered moths (Biston betularia carbonaria) had a selective advantage over lighter-colored moths, leading to an increase in the frequency of the dark phenotype.
Disruptive Selection:
Disruptive selection occurs when selective pressure favors individuals at both extremes of the phenotypic range, while individuals with intermediate phenotypes are selected against.
This mode of selection can lead to increased genetic diversity within a population.
Disruptive selection may result in the formation of multiple distinct phenotypic forms or even the emergence of new species.
Example: In a population of snails, disruptive selection may favor individuals with either very small or very large shell sizes, while snails with medium-sized shells have reduced fitness due to predation pressure.
Stabilizing Selection:
Stabilizing selection occurs when selective pressure favors individuals with intermediate phenotypes and selects against individuals with extreme phenotypes.
This mode of selection tends to maintain the average phenotype of the population over time.
Stabilizing selection acts as a stabilizing force, maintaining constancy in populations adapted to stable environments.
Example: Human birth weight is subject to stabilizing selection, where infants with average birth weights have higher survival rates compared to those with extremely low or high birth weights.
discuss natural selection
as a mechanism of
evolution
(Darwin’s theory, its
observations and
conclusions)
Darwin’s Theory of Natural Selection:
Observations:
Variation: Darwin observed that individuals within populations exhibit variation in traits such as morphology, behavior, and physiology. This variation is heritable and can be passed from one generation to the next.
Overproduction: Populations tend to produce more offspring than can survive and reproduce. This leads to competition for limited resources, such as food, habitat, and mates.
Struggle for Existence: Darwin recognized that individuals within populations compete for survival and reproductive success due to environmental pressures and limited resources. This competition results in a “struggle for existence” among individuals.
Differential Survival and Reproduction: Not all individuals within a population survive and reproduce equally. Individuals with traits that confer advantages in their environment are more likely to survive and produce offspring with similar advantageous traits.
Conclusions:
Natural Selection: Darwin proposed that natural selection acts as a mechanism of evolution, whereby individuals with traits that are better adapted to their environment are more likely to survive and reproduce. Over time, these advantageous traits become more common in the population, while less favorable traits decrease in frequency.
Adaptation: Natural selection results in the accumulation of adaptive traits within populations, leading to the gradual evolution of organisms that are well-suited to their environments.
Common Descent: Darwin inferred that all species are descended from a common ancestor through a process of descent with modification. Over long periods of time and successive generations, cumulative changes due to natural selection can lead to the divergence of new species.
Evidence from the Fossil Record: Darwin drew support for his theory from the fossil record, which provided evidence of transitional forms and patterns of evolutionary change over geological time.
discuss the biological
species concept
(Discussion of the
limitations of this
concept, for example, in
breeding.)
The Biological Species Concept (BSC) is a concept in biology that defines a species as a group of organisms that can interbreed and produce fertile offspring under natural conditions. Proposed by Ernst Mayr in 1942, the BSC emphasizes reproductive isolation as the primary criterion for defining species boundaries. While the BSC has been widely used and influential in biology, it also has limitations, particularly when applied to certain situations such as inbreeding. Here’s a discussion of the BSC and its limitations:
Biological Species Concept:
Reproductive Isolation:
According to the BSC, reproductive isolation is the key criterion for defining species. Populations that are reproductively isolated from each other, meaning they cannot interbreed or produce fertile offspring, are considered distinct species.
Reproductive isolation can occur through various mechanisms, including prezygotic barriers (e.g., behavioral, ecological, temporal, and mechanical isolation) and postzygotic barriers (e.g., hybrid inviability, hybrid sterility, and hybrid breakdown).
Gene Flow:
Reproductive isolation prevents gene flow between populations, leading to the accumulation of genetic differences over time. This divergence in genetic composition contributes to the formation of distinct species.
Species Boundaries:
The BSC provides a clear and intuitive framework for defining species boundaries based on reproductive compatibility. It emphasizes the importance of reproductive traits and behaviors in species differentiation.
Limitations of the Biological Species Concept:
Asexual Organisms:
The BSC relies heavily on the criterion of interbreeding, which is not applicable to asexual organisms that reproduce by methods such as binary fission, budding, or fragmentation. As a result, the BSC cannot be easily applied to define species boundaries in asexual organisms.
Hybridization:
Hybridization, the interbreeding between different species or populations, can blur species boundaries and challenge the applicability of the BSC. Hybridization can occur in nature, particularly in zones of secondary contact or in human-altered environments, leading to the formation of hybrid individuals with intermediate characteristics.
Ring Species:
Ring species are populations that are connected by a ring-like distribution around a geographical barrier. In ring species, adjacent populations can interbreed successfully, but populations at the ends of the ring are reproductively isolated due to accumulated genetic differences. The BSC struggles to define species boundaries in ring species because adjacent populations can interbreed despite genetic differentiation.
Cryptic Species:
Cryptic species are morphologically similar but genetically distinct populations that are often overlooked using traditional morphological criteria. The BSC may fail to recognize cryptic species because they can interbreed and produce fertile offspring despite genetic differentiation.
Inbreeding:
Inbreeding within a species can reduce genetic diversity and increase the prevalence of deleterious recessive alleles. While inbreeding does not necessarily challenge the applicability of the BSC, it can impact the fitness and viability of populations by reducing genetic variation and increasing the expression of harmful traits.
explain the process of
speciation
(Isolating mechanisms –
reproductive, geographic,
behavioural and
temporal, allopatric and
sympatric speciation with
reference to two named
examples)
