Natural Selection and Evolution
Natural Selection: The Mechanism of Adaptation
20.4 Natural Selection: Fundamentals and Discovery
Definition: Natural selection is the process by which allele frequencies in a population change from generation to generation based on the impact of those alleles on the survival and reproduction of individuals.
Impact of Mutations:
Deleterious mutations: Eliminated by natural selection and have no long-term evolutionary impact.
Beneficial mutations: Can lead to adaptation to the environment over time.
Adaptations: Natural selection is the driving force behind adaptations, which are the exquisite fits of organisms to their environment.
Pre-Darwinian View: Biologists before Darwin often interpreted adaptations (e.g., desert plant's physiological coping mechanisms, fast-swimming fish's streamlined body) as evidence for the existence of a divine Creator.
Darwin's Revolution (1859):
With the publication of On the Origin of Species, Charles Darwin (Fig. 20.5) challenged two conventional biological views:
Evolution of Species: He demonstrated that species are not unchanging but have evolved over time.
Mechanism of Adaptation: He proposed natural selection as the specific mechanism responsible for bringing about adaptation.
Darwin's theory provided a brilliant explanation for how organisms achieve such a perfect fit in their environments, such as a woodpecker's powerful chisel-like bill, without foresight or intentionality.
Darwin and Wallace: For 20 years, Darwin gathered evidence for his theory. In 1858, he was spurred to publish after receiving a letter from Alfred Russel Wallace (Fig. 20.6), a naturalist who had independently conceived the theory of evolution by natural selection. A joint paper by Wallace and Darwin was published in 1858, but it was Darwin's On the Origin of Species in 1859 that brought the concepts of evolution and natural selection to widespread public attention.
Core Observations Underpinning Natural Selection
Natural selection is a simple concept with profound implications, resting on a few testable observations:
Variation Among Individuals: Members of a species differ from one another; there is genetic variation within a population.
Heritability of Variation: Some of this variation is heritable, meaning it is passed on to the next generation (e.g., tall parents tend to have tall children). Humans have utilized this principle for millennia in artificial selection of farm animals and crops.
Competition for Resources: In nature, individuals often compete for limited resources.
Malthusian Influence: Both Darwin and Wallace were influenced by Thomas Robert Malthus's An Essay on the Principle of Population (1798). Malthus noted that natural populations have the potential to grow geometrically, meaning at an ever-increasing rate.
Example: If each human couple had 4 children, a population starting with a single couple would grow to over 1 million (1,048,576) by the 20^{th} generation.
Reality: This geometric expansion does not occur in nature because resources (food, water, habitat) are limited. Population sizes are typically stable.
Consequence: Many individuals fail to survive or reproduce due to resource scarcity, leading to competition for survival.
Differential Survival and Reproduction: Individuals that are best adapted to their environment are more likely to survive and reproduce, passing their genetic material to the next generation. As a result, advantageous alleles increase in frequency in the subsequent generation.
Fitness and Adaptation
"Natural Selection" (Darwin's Term): Refers to the filtering process that acts against deleterious alleles and favors advantageous ones.
Fitness: Describes how well an individual survives and reproduces in a particular environment.
Competitive Advantage: An organism's competitive advantage is directly proportional to how well it is adapted to its environment; a better-adapted organism is more fit.
Measure of Fitness: It is the extent to which an individual's genotype is represented in the next generation.
Example: A desert plant more efficient at minimizing water loss is better adapted and, therefore, has higher fitness than one that is less efficient.
Quantifying Fitness: An individual's fitness is higher if it leaves more surviving offspring than another.
Relative Nature: Fitness is always relative; it is meaningful only when compared to other individuals of the same species (e.g., a gazelle running at 25 kph has high fitness in a herd running at 20 kph but low fitness in a herd running at 30 kph).
Environment-Dependent: Fitness is specific to the environment in which an organism lives.
Evolutionary vs. Everyday Meaning of "Fitness": In evolutionary biology, fitness refers to an individual's ability to survive and reproduce in its environment, not its physical health or athleticism in a colloquial sense.
Cumulative Effect: Assuming a trait maintains its advantage, natural selection acts over generations to increase the overall fitness of a population, leading to better adaptation (e.g., a desert plant population gradually becoming more adapted to minimize water loss).
Importance of Time: Darwin recognized, drawing from geologists, that vast timescales are crucial. Small, subtle changes (like shifts in allele frequencies) can accumulate over long periods to produce major evolutionary differences among populations.
The Modern Synthesis
Integration: The Modern Synthesis combined Darwinian evolution (change in genetic composition of populations over time) with Mendelian genetics.
Missing Link: Darwin published On the Origin of Species before the rediscovery of Gregor Mendel's work on pea plants (1866), so Darwin's theory initially lacked a genetic foundation.
Controversy and Resolution: The rediscovery of Mendel's work in 1900 was initially controversial because Mendel studied discrete traits (e.g., yellow or green peas), while most natural variation is continuous (e.g., human height).
Ronald Fisher's Insight: English theoretician Ronald Fisher resolved this by proposing that continuous variation could be explained if multiple genes contributed to a single trait, extending Mendel's theory.
Current Theory: This synthesis, forged in the mid-20^{th} century, forms our current comprehensive theory of evolution.
Natural Selection and Allele Frequencies
General Effect: Natural selection increases the frequency of advantageous alleles and decreases the frequency of deleterious alleles.
Fixation: An advantageous allele can increase in frequency until it becomes fixed, meaning 100\% of the population possesses it. A new advantageous allele initially exists once as a heterozygote.
Positive Selection: This is the process where natural selection increases the frequency of an advantageous allele.
Negative Selection: This is the process where natural selection reduces the frequency of a deleterious allele.
Most mutations are deleterious; lethal ones are eliminated rapidly.
Inefficiency against Recessive Deleterious Alleles: If a deleterious allele is recessive (even lethal), negative selection is inefficient at eliminating it. It only acts when the allele is present as a homozygote, expressing the deleterious phenotype. Since homozygotes are rare when the allele frequency is low, most copies of the allele persist unseen in heterozygotes. Many human genetic disease alleles remain in populations this way.
Balancing Selection: This form of natural selection maintains two or more alleles in a population at intermediate frequencies (between 0 and 1).
Differential Environmental Favorability: One allele might be favored in one environmental condition (e.g., a dry area), while a different allele is favored in another (e.g., a wet area), maintaining both across the species' range.
Heterozygote Advantage: Occurs when the heterozygote genotype has higher fitness than either homozygote.
Example: Malaria and Sickle-Cell Anemia: In human populations in Africa affected by malaria, mutations in the hemoglobin molecule can reduce the severity of malaria.
A allele: Codes for normal hemoglobin, resulting in round, fully functional red blood cells.
S allele: Codes for a polypeptide with a single amino acid difference, causing red blood cells to distort into a sickle shape, leading to sickle-cell anemia (which can block capillaries, causing pain and sometimes death).
Genotype Fitness:
AA homozygotes: No sickle-cell anemia, but vulnerable to malaria.
SS homozygotes: Protected against malaria, but severely burdened by sickle-cell anemia.
AS heterozygotes: Do not have severe sickling disease and have some protection from malaria.
Result: Natural selection maintains both A and S alleles at intermediate frequencies where malaria is prevalent.
Shift in Balance: In malaria-free environments (e.g., among many Black Americans descended from African populations), the S allele is no longer advantageous. Negative selection would gradually eliminate it, but this is a slow process, meaning many still suffer from sickle-cell anemia in the interim.
Types of Natural Selection Based on Trait Changes
When observing changes in an organism's trait over time, three patterns emerge:
Stabilizing Selection: (Fig. 20.7a)
Action: Maintains the status quo by favoring intermediate phenotypes and acting against both extremes.
Example: Human birth weight. (Fig. 20.7b)
Babies that are very small or very large have lower chances of survival. Optimal birth weight is in the intermediate range (around 3 kg).
Mortality is higher for low-birth-weight and high-birth-weight babies, and lower for intermediate-weight babies.
Outcome: Helps to keep a trait consistent over time.
Directional Selection: (Fig. 20.8a)
Action: Leads to a change in a trait over time by favoring one extreme phenotype and acting against the other.
Example 1: Darwin's Finches (Galápagos Islands): (Fig. 20.8b)
The 1977 drought on the Galápagos Islands killed vegetation, leading to an increase in the average size of available seeds for Geospiza fortis ground finches (larger seeds from drought-resistant plants).
Finches with larger bills were better equipped to handle these bigger seeds and thus had higher survival rates.
Since bill size is genetically determined, this resulted in rapid directional selection for increased bill size within a single year (1978). The average beak size increased significantly.
Common Occurrence: Tends to occur when the environment changes.
Example 2: Evolution of Resistance (Medical Context):
Antibiotic Resistance: Bacteria rapidly evolve resistance to antibiotics, leading to 'superbugs' resistant to multiple drugs.
Antimalarial Drug Resistance: The malaria parasite rapidly evolves resistance to drugs like artemisinin (Nobel Prize to Tu Youyou for its identification). Within years of deployment, resistance emerged.
Mechanism: Medical interventions impose strong selective pressure. Cells or parasites with mutations conferring protection survive, reproduce, and increase the prevalence of resistance mutations in subsequent generations.
Disruptive Selection: (Fig. 20.9a)
Action: Favors individuals at both extremes of the phenotypic range while acting against intermediate forms.
Example: Apple Maggot Flies (Rhagoletis pomonella):
Originally fed on hawthorn tree fruit.
About 150 years ago, the introduction of apple trees from Europe created a new host.
Apple trees produce fruit earlier than hawthorns.
Disruptive selection led to two genetically distinct groups of flies: one adapted to feeding on early-fruiting apples and another on late-fruiting hawthorns.
Selection acted against intermediate timing, as flies emerging at intermediate times would miss the fruiting peaks of both host trees.
(Fig. 20.9b) A graph shows two peaks of larval feeding activity, one in early summer (apples) and one in late summer (hawthorns), demonstrating selection for early or late feeding but not intermediate. This mechanism can contribute to the evolution of new species.
Selective Pressure and Artificial Selection
Selective Pressure: Describes the set of environmental conditions (both physical like air, soil, water; and biological like other organisms) that result in some organisms surviving and reproducing more than others.
In stable environments, selective pressures are constant; in changing environments, they shift.
Highlights the environment's role in guiding natural selection.
Important Note: Natural selection can only respond to current conditions; it is not forward-looking and cannot anticipate future environmental changes.
Artificial Selection: A form of directional selection where humans intentionally select for desirable traits in populations.
Process: Breeders observe variation in a trait (e.g., running speed in horses) and select individuals with the most desired trait to breed, leading to rapid changes over generations (e.g., faster horses, bigger corn kernels, milkier cows, larger eggs).
"How Do We Know?" - Corn Oil Content Experiment (University of Illinois, 1890s-2008): (Fig. 20.10)
Hypothesis: There is a limit to a population's response to continuous directional selection.
Experiment: Corn was artificially selected for either high or low oil content for over a century. Only the 12 kernels with the highest or lowest oil content were used for each subsequent generation.
Results: The high oil line's percentage quadrupled (from ~$5\%$ to 20%), while the low oil line's content dropped to near zero and was terminated. Both lines far exceeded the initial range of phenotypes.
Genetic Basis: Differences in oil content were attributed to at least 50 genes.
Analogy to Natural Selection: Darwin noted that artificial selection is analogous to natural selection but lacks the competitive element; breeders select phenotypes rather than competition determining survival.
Efficiency: Artificial selection is highly efficient at generating population changes due to precise control (e.g., the vast diversity in dog breeds, all descended from wolves around 30,000 years ago - Fig. 20.11).
Key Difference: Artificial selection has a specific goal (e.g., faster racehorses), whereas natural selection has no goal or endpoint; it simply favors those best adapted to the current environment.
Sexual Selection
Definition: A form of selection that increases an individual's reproductive success.
Darwin's Puzzle: Darwin was initially puzzled by traits that seemed to reduce an individual's survival chances, such as the metabolically expensive, predator-attracting, and encumbering peacock's tail. He famously said, "The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!"
Darwin's Solution: In The Descent of Man, and Selection in Relation to Sex (1871), Darwin proposed sexual selection as a counteracting force to natural selection. While natural selection might reduce peacock tail size (due to survival costs), sexual selection promotes it by increasing access to reproductive opportunities.
Two Forms of Sexual Selection (Fig. 20.12):
Intrasexual Selection: (Fig. 20.12a)
Action: Members of one sex (usually males) compete directly with each other for access to the other sex (usually females).
Traits Promoted: Leads to the evolution of physical traits like large size, horns, elaborate weaponry, and fighting ability in males (e.g., battling male elk), as competitive winners secure more mates and territories.
Intersexual Selection: (Fig. 20.12b)
Action: Males (typically) compete for the attention of females through bright colors or advertisement displays, with females choosing their mates.
Example: The peacock's tail is thought to be a product of intersexual selection, driven by female preference for showier tails (e.g., bird of paradise displays).
Trade-off: The traits evolved under sexual selection often represent a compromise between conflicting demands: attracting a mate (sexual selection) and surviving (natural selection). The peacock's tail, for instance, is a balance between being attractive and being able to escape predators.