Variation in a Population and Mechanisms of Evolution
Variation in Population Gene Frequencies
Variation exists within a population's gene pool, which encompasses the total set of unique alleles for every gene in a population.
Natural selection acts on all phenotypes present in the population.
Under constant conditions, a population may maintain a stable allelic frequency in its gene pool.
Changing selection pressures disrupt this stability, leading to shifts in allele frequencies such as disruptive selection.
Differential Reproductive Success (DRS)
Differential reproductive success is a fundamental component of evolution where individuals with certain traits succeed in producing more offspring than others.
The Beetle Example:
Variation in traits: A population of beetles contains both green and brown individuals.
The environment cannot support unlimited growth, creating a struggle for existence.
Differential reproduction happens because green beetles are more easily spotted by birds. Consequently, they are eaten more often and survive to reproduce less frequently than brown beetles.
Heredity: The surviving brown beetles pass on their genetic basis for brown coloration to their offspring.
End result: The advantageous brown trait increases in frequency. Over time, the process may lead to a population where all individuals are brown.
Influencing Factors of DRS:
Competition for resources.
Limited resources available in the environment.
Production of more offspring than the environment can support.
Environmental factors (climate, habitat changes).
Viability: The specific survivability of a certain genotype or trait.
Fecundity: The reproductive success or capacity of an individual.
Genetic Variation: Underlying differences in the DNA sequence.
Main Types of Selection Pressures
Directional Selection:
Natural selection favors one extreme of the phenotypic range.
This typically occurs when the environment changes in a consistent direction (e.g., a climate progressively getting colder).
The population mean shifts toward the favored extreme.
Examples:
The lengthening of the Giraffe neck.
Antibiotic resistance in bacteria populations.
Melanin levels in moth color (Industrial melanism).
Camouflage and mimicry adaptations.
Disruptive Selection:
Natural selection favors both extremes of the phenotypic range, while selecting against the average.
This causes the species to diverge and can lead to specialization for different resources in the same area.
It may result in the formation of new species (Speciation).
Example: Darwin’s Finches evolving different beak types to utilize different food sources.
Stabilizing Selection:
Natural selection favors the average or intermediate phenotype within the population.
Selection acts against both extremes, effectively reducing phenotypic variation.
It maintains the status quo and removes more severe phenotypes.
Examples:
Bird clutch size (number of eggs laid).
Human birth weight.
Human height.
External Selection Pressures
Specific factors that act as selection pressures include:
Competition for food.
Competition for a mate.
Changes in the physical environment.
Predators.
Disease and parasites.
Mechanisms of Evolution
Natural Selection:
Differential survival and reproduction. For instance, if a disease is introduced, only those with resistant traits survive to pass on their genes to many generations later.
Mutations:
The introduction of new genetic variations. An original population may experience a mutation that introduces a new trait, which may then increase in frequency over many generations.
Gene Flow:
The movement of genetic information (traits) between populations through migration.
It provides opportunities for new genetic information to enter the gene pool of a specific population.
Genetic Drift:
The change in frequency of an existing trait due to random chance rather than natural selection.
It primarily affects small populations.
Often triggered by natural disasters (e.g., a lightning strike randomly killing a subset of the population).
Artificial Selection:
Human-driven selection for specific desired traits.
Non-random Mating:
Mating patterns where individuals choose mates based on specific phenotypes or genotypes.
Genotype modeling: Blue dots representing dominant phenotypes ( or ) and red dots representing recessive phenotypes ().
Population Change Modeling
When modeling population change, dots represent percentages of individuals rather than single organisms.
Example: In a population of individuals, if half are red, a quarter are blue, and a quarter are green:
Red individuals:
Blue individuals:
Green individuals:
Estimations are preferred over exact counts when predicting outcomes many generations later.
Diversification of Species
Diversification results from the accumulation of microevolutionary changes over time, leading to macroevolutionary change.
Divergent Evolution:
Descendants become different in form from a common ancestor due to different selection pressures in different niches.
Results in reproductive isolation.
Characterized by homologous structures (similar underlying anatomy).
Convergent Evolution:
The independent development of similarities between species with different ancestors.
Caused by similar ecological roles and selection pressures in different locations.
Characterized by analogous structures (similar function but different origin).
Example: Shark and dolphin.
Parallel Evolution:
Independent evolution of similar traits in species that once shared a common ancestor.
Occurs when species evolve from the same ancestor and face similar pressures but are geographically isolated.
Example: North American flying squirrel and Australian Glider.
Coevolution:
Two or more species reciprocally affect each other's evolution.
Strong selective forces are exerted by each species on the other, often in predator/prey relationships.
Speciation Models
Speciation: The formation of new and distinct species due to microevolutionary changes.
Allopatric Speciation:
Occurs when two populations of the same species are isolated by a geographic barrier (e.g., rivers changing course, mountains, continental drift, migration).
Gene flow is halted. Unique mutations and different selection pressures cause populations to diverge until they are reproductively isolated.
Outcomes of Geographic Isolation:
Barrier breaks down before isolation is complete: Populations merge. No speciation.
Barrier breaks down with incomplete isolation: Hybridization occurs. If hybrids are less fit, selection continues the divergence.
Isolation lasts long enough: Populations become sympatric species (closely related but reproductively isolated) when the barrier breaks down.
Example: Eastern grey kangaroo and Western grey kangaroo, which now overlap in range but do not interbreed.
Parapatric Speciation:
Occurs when subpopulations evolve isolation due to habitat differences within a continuous population range.
No physical extrinsic barrier exists, but non-random mating occurs because individuals at opposite ends of a large range rarely meet.
Sympatric Speciation:
Populations become reproductively isolated while sharing the same habitat.
Often results from a mutation or polyploidy.
Character Displacement: Selection for resource partitioning leads to different gene pools. In-between characteristics are at a disadvantage (Disruptive selection).
Polyploidy and Parthenogenesis
Polyploidy:
Offspring possess more than the normal number of chromosomes due to the failure of chromosomes to separate during meiosis in eggs or sperm.
To reproduce, a polyploid individual must mate with another with the same polyploidy.
Most common in plants.
Parthenogenesis:
Reproduction from an ovum without fertilization.
Can occur due to polyploidy and commonly produces daughters.
Males are unnecessary, which can decrease genetic variability and lead to rapid divergence through genetic drift in isolated populations.