Population Genetics Overview

Populations

• A species is a group of individuals with a common ancestry, able to breed and produce fertile offspring
• A population is a group of the same species in the same area, at the same time, which can interbreed
• A gene pool is all the alleles of all the genes in a population
• Allele frequency is the proportion of individuals with a single copy of an allele
• Allele frequencies change due to natural selection within and between populations

Population Genetics

• Populations can be seen as gene pools consisting of all alleles of all genes
• Populations evolve as allele frequencies change across generations
• Allele frequencies can be determined using the Hardy-Weinberg equation

Variation

• Within populations, there's phenotypic variation
• Continuous variation is polygenic (influenced by many genes), while discontinuous variation is monogenic (influenced by a single gene)
• Genetic variation comes from mutations, meiosis, and random fertilisation
• The environment affects how genes are expressed

The Effect of Selection on Allele Frequencies

• Predation, disease, and competition lead to differential survival and reproduction
• Individuals with advantageous traits are more likely to reproduce, passing on their favorable alleles
• This causes allele frequency changes within a population, leading to evolution
• Directional selection favors one extreme trait
• Stabilizing selection favors the average trait
• Disruptive selection favors both extreme traits, potentially leading to speciation.

Speciation

• Speciation is the formation of new species from existing ones
• Reproductive isolation, followed by genetic changes, results in the formation of new species
• Genetic differences between populations can arise from geographic isolation (allopatric speciation) or reproductive isolation within the same geographic area (sympatric speciation)
• Reproductive isolation can occur due to different breeding seasons, courtship behaviors, anatomical differences or genetic incompatibility
• Hybrid sterility occurs if the resulting offspring cannot produce viable gametes

Genetic Drift

• Genetic drift occurs when allele frequencies change due to chance events in a population (not selection pressures)
• It's more prominent in smaller populations, allowing chance fluctuations to greatly affect the gene pool
• Random gamete production and fertilization can lead to allele fluctuations

Ecosystems and Population Size

• A community consists of all the populations of various species interacting in a given area
• An ecosystem encompasses all living (biotic) and non-living (abiotic) factors within a defined area
• Every organism occupies a specific ecological niche, fitting its needs with abiotic and biotic factors
• Carrying capacity is the greatest population an area can sustain
• Limiting factors (abiotic and biotic) affect population size
• Population size can be estimated using methods like quadrats and mark-recapture

Succession

• Succession involves gradual changes in an ecosystem over time
• Primary succession starts on bare rock, with pioneer species gradually building soil, followed by other species
• Secondary succession occurs after a disturbance in an existing habitat
• Climax community is reached when a stable ecosystem is formed with a higher biodiversity

Conservation

• Conservation is maintaining biodiversity, including diversity between and within species
• Conservation efforts manage communities and ecosystems to preserve biodiversity from extinction risks
• The increasing human population and demand for resources create tension between human needs and conservation efforts