CH3: Evolution by Natural Selection
BIOL-2142 (F25): Evolution by Natural Selection
Evolution by Natural Selection: Core Principles
Variation: Individuals within a population exhibit differences in their displayed traits.
Inheritance: Offspring typically inherit traits from their parents, resembling them.
Differential Reproductive Success: Individuals with specific traits have a greater likelihood of survival and reproduction.
Example: Brighter (redder) beetles are bitter, causing predators to learn avoidance. This leads to brighter beetles having higher survival and reproductive rates compared to dull-colored ones.
Result: The proportion of different trait variants within a population changes over successive generations, demonstrating evolution by natural selection.
Semantics of Natural Selection
Fitness: Defined as an individual's capacity to survive and reproduce effectively within its particular environment.
Adaptation: Refers to a trait or characteristic of an organism that, when present, enhances its fitness relative to individuals lacking that trait.
Four Conditions for Evolution by Natural Selection
Individuals within populations are variable: There must be observable differences among members of a population.
The variations among individuals are, at least in part, passed from parents to offspring: These variations must be heritable.
Differential reproduction: Individuals possessing certain traits are more successful at surviving and reproducing in their environment compared to others.
These three conditions are necessary and, when present together, inevitably lead to evolution via natural selection.
Four Key Points to Keep in Mind Regarding Natural Selection
Mutations generate variation:
Mutation is a primary source of the genetic variation upon which natural selection acts.
While some mutations may be favored by natural selection, mutations themselves occur randomly concerning the organism's needs.
This means mutations arise independently of whether they would be beneficial or detrimental.
Traits are the object of explanation:
The focus is on how a specific trait changes or remains constant over time.
Traits can be:
Physical: e.g., feather color.
Behavioral: e.g., boldness or shyness.
Genetic.
Physiological.
Populations change over time, not individuals:
Natural selection is a process that alters the characteristics of a population, not those of individual organisms.
Therefore, studies of natural selection are always conducted with reference to a specific population.
Selection acts on individuals, but the evolutionary outcome is observed and measured at the population level.
Genotype interacts with environment to produce the phenotype (G x E):
Natural selection does not act directly on genotypic differences but rather on phenotypic differences.
Understanding the interplay between an organism's genotype (G) and its environment (E) is crucial for comprehending how phenotype is shaped.
A gene alone does not code for a trait; instead, it codes for how a trait is expressed in the context of particular environmental conditions.
Example: Elevation and a plant's genotype interact to determine the height of individuals in different plant populations.
In most cases, a single genotype does not produce just one phenotype but rather a reaction norm.
A reaction norm illustrates how flexible or sensitive an organism's traits are to changes in environmental conditions.
Example (Plant Height and Altitude):
Same seeds (representing identical genotypes) were grown at three different altitudes: high, medium, and low.
Plants from Genotype 1 grew tall at high and low elevations but short at medium elevations.
In contrast, Genotype 4 exhibited the opposite response to elevation, demonstrating different reaction norms based on genotype.
Model Systems for Studying Natural Selection
Darwin Finches
Oldfield mouse
Trinidadian guppies
Sticklebacks
Drosophila (fruit flies)
E. coli (bacteria)
Case Study: Oldfield Mouse Coat Coloration
Context: Oldfield mice experience significant mortality due to visual predators (e.g., owls).
Across most of their range, mice have dark coloration.
However, on Santa Rosa (FLA) beaches, they possess a much lighter coat color.
Hypothesis: Natural selection favors a match between an individual's coat color and its environmental background.
Predictions:
Light coat color will be favored in coastal dune populations living on light sand.
Dark coat color will be favored in inland populations residing in more vegetated environments.
Evidence of Variation:
Coat color varies both among and within sampled populations (e.g., N = 20 for Alabama beach mouse, N = 40 for Santa Rosa Island beach mouse).
Shading in diagrams represents the proportion of individuals with different pigmentation levels (darker shading = darker pigmentation).
Evidence of Heritability:
Coat color is a heritable trait, with a heritability (h^2) ranging from 0.7 to 0.9.
Molecular Basis: Pale coat coloration on beaches versus dark mainland coats is largely due to two pigmentation genes:
Melanocortin-1 (Mc1r) receptor and agouti signaling protein (Agouti).
A single amino acid mutation, along with interaction between Mc1r and Agouti, creates the different colorations (identified through QTL studies).
Evidence of Differential Survival:
Experiments used light and dark-colored silicone mouse models placed in different environments.
Light background (beach environment): Light models experienced a significantly lower proportion of attacks compared to dark models.
Dark background (inland environment): Dark models experienced a significantly lower proportion of attacks compared to light models.
Evidence of Population Evolution:
Semi-natural enclosures (50 \text{ m} \times 50 \text{ m}) were built in both sand dunes (light environment) and dark vegetation (dark environment).
Researchers examined the probability of survival as a function of fur coloration.
Light site: Individuals with lighter dorsal fur brightness had a significantly higher probability of survival.
Dark site: Individuals with darker dorsal fur brightness had a significantly higher probability of survival.
This demonstrates that natural selection operates very strongly in both experimental populations, favoring camouflage.
Case Study: Trinidadian Guppies
High-predation sites: Presence of predators like Crenicichla alta leads to females producing many small offspring.
Low-predation sites: Presence of predators like Rivulus hartii leads to females producing fewer but larger offspring.
Life History Trait Differences: Guppies from low-predation sites have larger average offspring size and fewer offspring compared to those from high-predation sites.
Transplant Experiment: When guppies from a high-predation site were transferred to a low-predation site, their life history traits evolved over time to resemble those of the native low-predation site populations (i.e., producing fewer, larger offspring).
Natural Selection in the Lab: E. coli Evolution Experiment
Experiment Setup (Long-Term Evolution Experiment - LTEE):
Step 1: A single bacterial clone was used to create 12 genetically identical lines.
Step 2: A daily protocol was carried out for over 15,500 days (or generations):
Grow overnight.
Dilute 100-fold into fresh medium.
Periodically store samples in a -80^ ext{o}\text{C} freezer.
Step 3: Evolved strains and their frozen ancestors are available for comparative studies.
Results:
Cell Volume: The average cell volume (in femtoliters, fl) in E. coli populations increased steadily over 10,000 generations, from approximately 0.3 fl to about 0.7 fl.
Relative Fitness: Relative fitness increased rapidly in the initial generations and continued to increase, though at a slower rate, indicating ongoing adaptation over the 10,000 generations.
Constraints on What Natural Selection Can Achieve
Short-term limitations due to genetic variation:
The rate of adaptation is often proportional to the supply of new genetic variation available.
An influx of too many genes from a neighboring population can limit local adaptation if those genes are from a population facing different selective pressures.
Physical laws:
Natural selection operates on physical structures, thus being constrained by the same physical laws that govern the material world.
Example: Eye Placement (Ostrich vs. Owl):
Ostrich (vigilant for predators): Eyes are set on the sides of the head, providing nearly a 360^ ext{o} field of view, but with almost no stereoscopic vision due to minimal overlap of visual fields.
Owl (visual predator): Eyes are set on the front of the head, allowing for a full stereoscopic view of the environment (crucial for hunting prey) but presenting a much more limited total field of view compared to the ostrich.
Overcoming Constraints:
Owls: Compensate for limited field of view by being able to turn their heads up to 180^ ext{o} to look behind themselves.
Spiders: Some species have 8 eyes, allowing for both stereoscopic forward vision for visual hunting and a near 360^ ext{o} field of view.
Natural Selection Lacks Foresight
No anticipation: Natural selection cannot anticipate the future; it only reacts to past and present environmental conditions.
Immediate favorability: Selection favors changes that are immediately advantageous, not those that might be useful in the distant future.
No predetermined goal: Natural selection has no
Evolution by Natural Selection: Core Principles are centered around several key concepts. First, individuals within a population display variation in their traits. Second, offspring typically inherit these traits from their parents, leading to resemblances. Third, differential reproductive success means that individuals with particular traits are more likely to survive and reproduce effectively. For example, brighter or redder beetles are often bitter, which predators learn to avoid. This results in brighter beetles having higher survival and reproductive rates compared to dull-colored ones. Consequently, the proportion of different trait variants within a population changes over successive generations, demonstrating evolution by natural selection.
Semantics of Natural Selection are crucial for understanding the process. Fitness is defined as an individual's capacity to survive and reproduce effectively within its specific environment. An adaptation refers to a trait or characteristic of an organism that, when present, enhances its fitness relative to individuals lacking that particular trait.
Four Conditions for Evolution by Natural Selection must be met for the process to occur. First, individuals within populations must be variable, meaning there are observable differences among members of a population. Second, the variations among individuals must be, at least in part, passed from parents to offspring, indicating these variations are heritable. Third, differential reproduction occurs when individuals possessing certain traits are more successful at surviving and reproducing in their environment compared to others. These three conditions are necessary and, when present together, inevitably lead to evolution via natural selection.
Four Key Points to Keep in Mind Regarding Natural Selection help further clarify the concept. Firstly, mutations generate variation, serving as a primary source of the genetic variation upon which natural selection acts. While some mutations may be favored by natural selection, mutations themselves occur randomly concerning the organism's needs, meaning they arise independently of whether they would be beneficial or detrimental. Secondly, traits are the object of explanation; the focus is on how a specific trait, which can be physical (e.g., feather color), behavioral (e.g., boldness or shyness), genetic, or physiological, changes or remains constant over time. Thirdly, populations change over time, not individuals. Natural selection is a process that alters the characteristics of a population, not those of individual organisms. Therefore, studies of natural selection are always conducted with reference to a specific population, as selection acts on individuals, but the evolutionary outcome is observed and measured at the population level. Fourthly, genotype interacts with environment to produce the phenotype (G x E). Natural selection acts not directly on genotypic differences but rather on phenotypic differences. Understanding the interplay between an organism's genotype (G) and its environment (E) is crucial for comprehending how phenotype is shaped. A gene alone does not code for a trait; instead, it codes for how a trait is expressed in the context of particular environmental conditions. For example, elevation and a plant's genotype interact to determine the height of individuals in different plant populations. In most cases, a single genotype does not produce just one phenotype but rather a reaction norm. A reaction norm illustrates how flexible or sensitive an organism's traits are to changes in environmental conditions. For instance, in an example concerning plant height and altitude, identical genotypes (same seeds) grown at three different altitudes (high, medium, and low) showed varied responses. Plants from Genotype ext{ }1 grew tall at high and low elevations but short at medium elevations, while Genotype ext{ }4 exhibited the opposite response to elevation, demonstrating different reaction norms based on genotype.
Model Systems for Studying Natural Selection include Darwin Finches, Oldfield mouse, Trinidadian guppies, Sticklebacks, Drosophila (fruit flies), and E. coli (bacteria).
Case Study: Oldfield Mouse Coat Coloration provides a compelling example. Oldfield mice face significant mortality due to visual predators like owls. While most mice in their range have dark coloration, those on Santa Rosa (FLA) beaches possess a much lighter coat color. The hypothesis is that natural selection favors a match between an individual's coat color and its environmental background. Predictions included that light coat color would be favored in coastal dune populations living on light sand, and dark coat color would be favored in inland populations residing in more vegetated environments. Evidence of variation was found as coat color varies both among and within sampled populations (e.g., N = 20 for Alabama beach mouse, N = 40 for Santa Rosa Island beach mouse), with diagrams illustrating the proportion of individuals with different pigmentation levels. Evidence of heritability shows that coat color is a heritable trait, with a heritability (h^2) ranging from 0.7 to 0.9. The molecular basis for pale coat coloration on beaches versus dark mainland coats is largely due to two pigmentation genes: Melanocortin-1 (Mc1r) receptor and agouti signaling protein (Agouti). A single amino acid mutation, along with interaction between Mc1r and Agouti, creates the different colorations, as identified through QTL studies. Evidence of differential survival was demonstrated using light and dark-colored silicone mouse models placed in different environments. In a light background (beach environment), light models experienced a significantly lower proportion of attacks compared to dark models; conversely, in a dark background (inland environment), dark models experienced a significantly lower proportion of attacks compared to light models. Evidence of population evolution involved semi-natural enclosures (50 ext{ m} imes 50 ext{ m}) built in both sand dunes (light environment) and dark vegetation (dark environment). Researchers examined the probability of survival as a function of fur coloration. In the light site, individuals with lighter dorsal fur brightness had a significantly higher probability of survival, while in the dark site, individuals with darker dorsal fur brightness had a significantly higher probability of survival. This demonstrates that natural selection operates very strongly in both experimental populations, favoring camouflage.
Case Study: Trinidadian Guppies illustrates how predation pressure influences life history traits. In high-predation sites, the presence of predators like Crenicichla alta leads females to produce many small offspring. In contrast, at low-predation sites, with predators like Rivulus hartii, females produce fewer but larger offspring. Consequently, guppies from low-predation sites have larger average offspring size and fewer offspring compared to those from high-predation sites. A transplant experiment further confirmed this: when guppies from a high-predation site were transferred to a low-predation site, their life history traits evolved over time to resemble those of the native low-predation site populations, meaning they began producing fewer, larger offspring.
Natural Selection in the Lab: The E. coli Evolution Experiment (Long-Term Evolution Experiment - LTEE) provides insights into evolution in controlled conditions. The experiment setup involved first using a single bacterial clone to create 12 genetically identical lines. Second, a daily protocol was carried out for over 15,500 days (or generations), involving growing overnight, diluting 100-fold into fresh medium, and periodically storing samples in a -80^\text{o}\text{C} freezer. Third, evolved strains and their frozen ancestors were made available for comparative studies. Results showed that the average cell volume (in femtoliters, fl) in E. coli populations increased steadily over 10,000 generations, from approximately 0.3 fl to about 0.7 fl. Relative fitness also increased rapidly in the initial generations and continued to increase, though at a slower rate, indicating ongoing adaptation over the 10,000 generations.
Constraints on What Natural Selection Can Achieve highlight the limitations of the process. Short-term limitations are often due to genetic variation; the rate of adaptation is typically proportional to the supply of new genetic variation available. An influx of too many genes from a neighboring population can limit local adaptation if those genes are from a population facing different selective pressures. Additionally, physical laws constrain natural selection because it operates on physical structures, thus being governed by the same physical laws that apply to the material world. For example, eye placement in an ostrich versus an owl illustrates this. An ostrich, vigilant for predators, has eyes set on the sides of its head, providing nearly a 360^\text{o} field of view, but with almost no stereoscopic vision due to minimal overlap of visual fields. An owl, a visual predator, has eyes set on the front of its head, allowing for a full stereoscopic view of the environment, which is crucial for hunting prey, but this setup presents a much more limited total field of view compared to the ostrich. However, organisms can overcome some constraints. Owls compensate for their limited field of view by being able to turn their heads up to 180^\text{o} to look behind themselves. Some spider species have eight eyes, allowing for both stereoscopic forward vision for visual hunting and a near 360^\text{o} field of view.
Natural Selection Lacks Foresight; it cannot anticipate the future but only reacts to past and present environmental conditions. Selection favors changes that are immediately advantageous, not those that