IB DP Biology Natural selection Study Notes

IB DP Biology Natural selection Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of

• What processes can cause changes in allele frequencies within a population?
• What is the role of reproduction in the process of natural selection?

Standard level and higher level: 2 hours
Additional higher level: 2 hours

IBDP Biology 2025 -Study Notes -All Topics

D4.1.1—Natural selection as the mechanism driving evolutionary change

The theory of evolution by natural selection can be explained with the following statements:

Organisms produce more offspring than the environment can support.
Among these offspring, there is variation.
Some variants are better suited to the environment and have a higher chance of surviving to reproductive age.
Survivors of the fittest, from less fit or less well-adapted-are sometimes have a higher risk of mortality from predation or other factors. This is sometimes called survival of the fittest.
The heritable features of the population change frequently, so the population can adapt to the environment.

While evolution can occur rapidly in response to sudden changes in the environment, the process is often gradual. Evolution by natural selection, over the billions of years that life has existed, has resulted in the immense biodiversity now present on Earth.

Lamarckism:

An older theory, Lamarckism, proposed that organisms acquire advantageous traits during their lifetime and pass those traits on to their offspring. (e.g., a giraffe stretching its neck to reach leaves would have offspring with longer necks). This theory has been disproven.

Paradigm Shift: Darwin’s theory of evolution by natural selection represented a paradigm shift in scientific understanding. It challenged the existing ideas about species evolution and provided a new framework for interpreting observations in biology.

In summary, natural selection, driven by variation, differential survival, and inheritance, is the mechanism driving evolutionary change.

D4.1.2—Roles of mutation and sexual reproduction in generating the variation on which natural selection acts

Mutation is the original source of new alleles. It introduces genetic diversity into a population by changing DNA sequences.

Meiosis shuffles existing alleles through crossing over and independent assortment of chromosomes. This creates a wide range of possible combinations of alleles in gametes, leading to genetic variation among offspring.

Sexual reproduction combines gametes from two different parents, bringing together mutations that occurred in different individuals. This increases the genetic diversity within a population and provides more raw material for natural selection to act upon.

The text highlights that species that reproduce asexually rely solely on mutation for generating genetic variation. It is generally believed that this might not be sufficient for rapid adaptation to changing environmental conditions, as compared to sexually reproducing species.

In essence, the interplay of mutation, meiosis, and sexual reproduction is crucial in generating the diverse genetic variations that fuel evolutionary processes.

D4.1.3—Overproduction of offspring and competition for resources as factors that promote natural selection

Organisms vary in the number of offspring they produce. Some species, like the southern ground hornbill, have slow breeding rates, while others, like the coconut palm and the fungus Calvatia gigantea, produce a large number of offspring. Most species produce more offspring than the environment can support. This overproduction leads to competition for resources like food and space. Not all individuals will survive and reproduce due to this competition. Only those with traits best suited to the environment will be able to thrive and pass on their genes. This competition for resources is a key factor in natural selection, as it favors individuals with advantageous traits.

D4.1.4—Abiotic factors as selection pressures

Abiotic factors, such as temperature extremes, natural disasters, and pollution, can act as selection pressures. Density-independent factors affect individuals regardless of the population density. For example, a freezing temperature will kill a portion of a plant population regardless of how many plants are present. In contrast, density-dependent factors, like competition for food or water, increase in intensity as the population density increases. Both density-dependent and density-independent factors can influence which individuals survive and reproduce, driving natural selection.

D4.1.5—Differences between individuals in adaptation, survival and reproduction as the basis for natural selection

Natural selection is driven by differences in adaptation, survival, and reproduction among individuals within a population.

Fitness: An individual’s fitness refers to how well-adapted it is to its specific niche within an ecosystem. This means having adaptations that suit the purpose or role of an organism within that environment. Different species occupy different niches and therefore require different adaptations.
Variation: Individuals in a sexually reproducing population exhibit variation in their traits. Some individuals will be better adapted than others.
Survival and Reproduction: An individual’s fitness influences its ability to survive and reproduce. Fitter individuals tend to survive longer and have more offspring. These individuals contribute more to the gene pool of the next generation.
Intraspecific Competition: This process represents a form of competition between members of the same species. Differences in survival and reproduction form the basis for natural selection, as those with greater fitness pass on their traits more effectively.

D4.1.6—Requirement that traits are heritable for evolutionary change to occur

It discusses the importance of heritability for evolutionary change.

Misconception about Acquired Traits: A common misconception is that traits acquired during an organism’s lifetime can be inherited by its offspring. For example, the idea that a professional tennis player’s children would be born with stronger arm muscles on the side used to hold the racket.
Acquired Traits are Not Heritable: Traits developed during an organism’s lifetime, such as the loss of a leg in a spider or the development of darker fur in a Himalayan rabbit due to cold exposure, are generally not heritable. These traits are caused by changes in gene expression, not by changes in the DNA sequence itself. Since these changes in gene expression don’t affect the DNA in the sex cells, they cannot be passed on to offspring.
Epigenetic Tags: While most acquired traits are not heritable, there are exceptions. Epigenetic tags, which are chemical markers added to chromosomes, can influence gene expression. A small proportion of these tags can be inherited by offspring. However, it’s important to note that only the pattern of gene expression is inherited, not changes in the DNA sequence itself. This topic of epigenetics is explored further in D2.2.

In essence, for evolutionary change to occur, the traits must be heritable, meaning they must be encoded in the DNA and passed from parent to offspring through the sex cells.

D4.1.7—Sexual selection as a selection pressure in animal species

Choosing Mates: Animals must choose mates carefully to ensure the survival and reproduction of their offspring.
Benefits of Choosing Well: If a mate is well-adapted, the offspring are likely to survive and reproduce, passing on the genes of both parents. Conversely, if the mate is poorly adapted, the offspring are less likely to survive, and the animal’s genes may not persist in the population.
Sexual Selection: The process of choosing a mate is called sexual selection. It involves various strategies, including fighting, courtship displays, and elaborate anatomical features.

Examples of Exaggerated Traits:

Birds of Paradise: Males of many bird-of-paradise species have evolved elaborate plumage, courtship dances, and even bizarre anatomical features. These traits, while seemingly excessive, are likely used to attract females and demonstrate their fitness.

Theories on Exaggerated Traits:

Darwin’s Explanation: Darwin suggested that females prefer mates with exaggerated traits because these traits indicate overall health and fitness.

In essence: Sexual selection drives the evolution of traits that enhance an individual’s attractiveness to potential mates, even if these traits seem excessive or even detrimental to survival in other ways. By choosing mates with desirable traits, animals increase the chances of their offspring’s survival and reproduction, ensuring the continuation of their genes in the population.

D4.1.8—Modelling of sexual and natural selection based on experimental control of selection pressures

John Endler’s experiments showed that predation pressure can shape the evolution of guppy coloration. In the presence of strong predators like pike-cichlids, guppies evolved to have fewer and smaller spots to avoid being detected. In the absence of strong predators, guppies evolved to have more spots, likely due to female preference for more colorful males. These experiments demonstrate how natural selection can drive rapid evolutionary change in response to environmental pressures.

D4.1.9—Concept of the gene pool

The gene pool is the total collection of all the genes and their different alleles present in a population. When members of a population reproduce sexually, they contribute to the gene pool of the next generation.

Natural selection can influence the gene pool, as individuals with higher fitness (measured by their contribution to the gene pool) are more likely to pass on their genes.

Genetic equilibrium exists when all members of a population have an equal chance of contributing to the future gene pool.¹ This state implies that allele frequencies remain constant over time.

In some cases, a single species can have multiple gene pools. This can occur when different populations within a species are geographically isolated, leading to limited gene flow between them.

D4.1.10—Allele frequencies of geographically isolated populations

Same allele might be common in one population but much rarer in another. This variation is observed even within the human population, with differences in allele frequencies seen between ethnic groups and geographical regions.

The AlFred database, housed at Yale University, is a valuable resource that allows researchers to search and compare allele frequencies across different populations. This tool can help us understand how genetic variation is distributed across the globe and how it might be influenced by factors like geography, migration, and natural selection.

D4.1.11—Changes in allele frequency in the gene pool as a consequence of natural selection between individuals according to differences in their heritable traits

It illustrates how natural selection leads to changes in allele frequencies in a gene pool. Darwin’s theory of evolution, combined with Mendel’s laws of inheritance and Weismann’s germ plasm theory, forms the basis of Neo-Darwinism. Evolution is defined as a change in the frequency of alleles within a population. Natural selection favors individuals with advantageous traits, leading to an increase in the frequency of their alleles in the next generation. The example of codominant flower color shows that changes in phenotypic frequencies alone do not necessarily indicate evolution. A change in allele frequencies is required for evolution to occur.

D4.1.12—Differences between directional, disruptive and stabilizing selection

Stabilizing Selection: This occurs when the environment favors the average phenotype. For example, in human babies, average birth weights are favored over very low or very high birth weights.
Disruptive Selection: This occurs when the environment favors the extreme phenotypes and selects against the intermediate ones. For example, in coho salmon, there are two types of males: small “jack” males that sneak fertilizations and large males that fight for access to females. Intermediate-sized males are at a disadvantage because they are less successful at both sneaking and fighting.
Directional Selection: This occurs when the environment favors one extreme of a range of phenotypes. For example, the body size of house mice introduced to Gough Island in the South Atlantic increased significantly over time. This suggests that larger body size provided an advantage in this environment, perhaps by allowing them to better compete for resources or evade predators.

These different types of selection can lead to various evolutionary outcomes, such as maintaining the status quo (stabilizing), creating new species (disruptive), or shifting the overall characteristics of a population (directional).

D4.1.13—Hardy–Weinberg equation and calculations of allele or genotype frequencies

The Hardy-Weinberg equation is a simple way to predict how often different genes will appear in a population.

Here’s the basic idea:

It uses a simple math formula to figure out the frequencies of different genes in a population.
It makes some assumptions, like that there is no mating between populations and that everyone has an equal chance of having children.

Example: Albinism

The example shows how to use the formula to calculate the frequency of albinism, a rare condition.

It starts with the known frequency of albinism in the population.
Then, it uses the formula to calculate the frequencies of different gene combinations that can cause albinism or not.

This helps scientists understand how genes are distributed in a population and how they might change over time.

In simple terms: The Hardy-Weinberg equation is a tool that helps us understand how genes are passed down from one generation to the next in a population.

D4.1.14—Hardy–Weinberg conditions that must be maintained for a population to be in genetic equilibrium

The Hardy-Weinberg equilibrium is a theoretical model that describes the conditions under which allele and genotype frequencies in a population will remain constant from generation to generation.

To maintain Hardy-Weinberg equilibrium, the following conditions must be met:

No mutation: The alleles of the genes must not change, and no new alleles should be generated.
Random mating: Individuals must mate randomly, without any preference for specific phenotypes.
No gene flow: There should be no immigration or emigration, which can alter allele frequencies.
Large population size: The population must be large enough to prevent significant random fluctuations in allele frequencies due to chance events. This is known as genetic drift.
No natural selection: Natural selection should not favor one phenotype over another.

If the observed genotypic frequencies in a population match the predictions of the Hardy-Weinberg equation, it suggests that all these conditions are being met, and the population is in genetic equilibrium.

If the observed frequencies deviate from the Hardy-Weinberg predictions, it indicates that at least one of the conditions is not being met. If we can rule out the first four conditions, then we can conclude that natural selection is likely acting on the population, causing allele frequencies to change and driving evolution.

D4.1.15—Artificial selection by deliberate choice of traits

Artificial selection, which is the process of humans deliberately choosing and breeding individuals with specific traits.

Here are the key points:

Humans have been selectively breeding animals for thousands of years. This is evident in the vast differences between modern breeds of livestock (like egg-laying hens and Belgian Blue cattle) and their wild ancestors (like junglefowl and aurochs).
Artificial selection involves repeatedly choosing and breeding individuals with the desired traits. Over time, this process can lead to significant changes in the characteristics of a species.
Effectiveness of artificial selection: The rapid changes observed in domesticated animals over relatively short periods demonstrate the effectiveness of artificial selection in driving evolutionary change.

However, it’s important to note that artificial selection does not prove that evolution occurs naturally or that natural selection is the mechanism for evolution. It simply shows that selective breeding can cause significant changes in a species within a relatively short time.

IB DP Biology Natural selection Study Notes

IB DP Biology Natural selection Study Notes