Genetic Drift, Gene Flow, and Natural Selection Overview

Overview of Genetic Drift

Genetic drift refers to random changes in allele frequencies within a population.
These changes can have significant impacts on smaller populations due to their limited gene pool.

The founder effect occurs when a small number of individuals establish a new population.
This leads to a reduced genetic diversity as only the alleles present in those founding individuals are carried into the new population.
Example: The Pennsylvania Amish population has observable traits such as shorter stature and polydactyly (six fingers) that originated from a small founding population.

The bottleneck effect describes a sharp reduction in population size due to environmental events (e.g., natural disasters).
This reduction significantly shrinks the gene pool, leading to changes in allele frequencies.
Example: Northern Elephant Seals experienced a drastic decline in numbers due to hunting in the early 20th century, leading to low genetic variability.
- Increased genetic variability was artificially achieved by introducing individuals from the Southern population.
Cheetahs also exhibit low genetic diversity resulting from past bottlenecks and current threats to their population.

North American and European cheetahs went extinct during historical climatic events, leaving only African cheetahs.
Notably, cheetahs can undergo skin grafts without rejection due to their genetic similarity.

Genetic drift can lead to significant changes in allele frequencies in small populations, potentially leading to fixation of harmful alleles.
Drift can lead to decreased genetic variation, impacting the population's adaptability to environmental changes.

Gene flow is the transfer of alleles from one population to another, reducing genetic differences.
Example: In a study of banded water snakes in Lake Erie, gene flow among populations leads to variations in banding based on environmental backgrounds (rocky vs. grassy substrates).

Populations with high gene flow can maintain variation that aids in adaptation to environmental shifts.

Discussion mentions exposing student interest and parallels historical frameworks of evolution (Lamarck’s hypothesis).
Lamarck proposed that characteristics acquired during an organism's lifetime could be inherited (e.g., giraffes stretching for food).

Adaptive evolution leads to species better suited to environments via natural selection, where advantageous traits increase in frequency.
Adaptive evolution is ongoing, as environments can change rapidly or gradually, impacting the relevance of certain traits.

Defining fitness: primarily related to reproductive success rather than compatibility or direct competition between individuals.
Relative fitness measures how one allele compares to another in terms of gene transmission to the next generation.

Directional Selection: Favors one phenotype over others, shifting the traits of the population in that direction (e.g., peppered moths during the Industrial Revolution).
Disruptive Selection: Favors extreme phenotypes by selecting against intermediate forms (e.g., habitat changes favoring very light or very dark individuals).
Stabilizing Selection: Favors intermediate phenotypes and selects against extreme traits (e.g., human birth weights).

Intrasexual selection involves direct competition between males for female attention, while intersexual selection involves female choice.
Secondary sex characteristics (e.g., peacock tails, human traits) evolve through these selective pressures to enhance reproductive success.

Balancing selection maintains variation through stabilizing influences that preserve multiple phenotypes within a population.
Frequency dependent selection occurs when prey targeted by predators alter their awareness, affecting allele frequencies based on observed patterns (e.g., left vs. right mouth fish).

Heterozygous individuals (carriers of different alleles) may have fitness advantages over homozygous individuals under certain conditions (e.g., sickle cell trait offering malaria resistance).

Adaptations evolve as organisms develop characteristics that enhance their survival in changing environments.
Climate change affects species such as Arctic foxes, which depend on seasonal cues for coat color changes critical for camouflage.

Understanding variation, genetic drift, gene flow, and selection is crucial for comprehending evolutionary processes.
Evolution is ongoing and does not produce perfect organisms; rather it configures populations in flux with changing environments.