Microevolution Notes

Genetic diversity refers to the variation among individuals within a species. This diversity is crucial because it enhances a species' adaptive potential, enabling some individuals to resist diseases, survive, and pass on beneficial genes. For instance, bacteria can develop antibiotic resistance due to mutations, which they then pass on to subsequent generations.

Genotype, derived from parental alleles, determines an individual's physical characteristics or phenotype. These heritable traits are passed on to offspring. The gene pool encompasses all the alleles within a specific population.

Consider a hypothetical population of ten individuals where circles represent alleles. Black circles are dominant alleles, and white circles are recessive. An individual can be homozygous dominant, homozygous recessive, or heterozygous. If eight out of ten individuals have the dominant allele, 80% will express the dominant trait, while 20% will exhibit the recessive trait. However, when considering alleles in the gene pool, the allele frequency might be 50/50, even if the dominant trait is more prevalent in the population.

A population is a group of individuals within the same species that can interbreed. Evolution is defined in terms of populations, not individuals. Natural selection acts on individuals, influencing which traits are passed on, but it is the population that evolves over time.

Darwinian fitness refers to the contribution an individual makes to the next generation's gene pool, relative to other individuals. Those who survive and reproduce more often are considered more fit, as they pass on more of their genes. Fitness is context-dependent, varying with the environment. Adaptations that enhance survival and reproduction improve fitness.

Microevolution is the change in allele frequency within a population from one generation to the next. Macroevolution occurs over longer periods and results in the formation of new species

Five Causes of Microevolution

  1. Small Populations/Genetic Drift

  2. Non-Random Mating

  3. Mutation

  4. Gene Flow

  5. Natural Selection

Genetic drift is a random shift in allele frequencies within a population, more pronounced in smaller populations. This randomness is key. It can lead to the loss or fixation of alleles. Fixation occurs when an allele frequency reaches 100%, resulting in subsequent generations mirroring the previous one. Loss of genetic variation can happen through death or migration of individuals carrying specific alleles.

The founder effect occurs when a small number of ancestors colonize a new area, leading to a shift in allele frequencies. For instance, the higher prevalence of Huntington's disease among people of Dutch descent in South Africa is attributed to the alleles of the original colonizers.

A bottleneck effect happens when a population experiences a drastic reduction in size, nearly reaching extinction. The surviving population's gene pool has different allele frequencies compared to the original population, potentially leading to the loss of certain genes.

In small populations, there is a higher likelihood of allele fixation or loss, with wider swings in allele frequencies across generations. Larger populations tend to have more stable allele frequencies from one generation to the next.

Non-random mating occurs when individuals choose mates based on specific traits, influencing the gene pool. In the animal kingdom, this often involves females selecting males based on phenotype, with males competing for access to mates. The "sexy son hypothesis" suggests females choose males with desirable traits to ensure their sons also attract mates.

Sexual dimorphism, characterized by phenotypic differences between males and females, is often a result of non-random mating.

Mutations are permanent changes in an organism's DNA and the source of all new alleles. They can be positive, negative (deleterious), or silent. Deleterious mutations may be eliminated through purifying selection. Mutations involve changes in base pairs, potentially altering amino acid sequences and protein functions. For example, the single mutation in sickle cell anemia alters hemoglobin shape, causing health issues.

A mutation can also create a new phenotype, like a brown beetle in a population of green beetles. If the brown beetle survives and reproduces, it increases the frequency of that brown phenotype in subsequent generations.

Some mutations can provide resistance to diseases, such as the mutation that prevents the production of CCR5 protein on white blood cells, conferring resistance to HIV.

Gene flow is the movement of alleles from one population to another through immigration. It is random with respect to fitness. When individuals with certain alleles migrate to a new population, they introduce those alleles, altering the allele frequencies and increasing genetic variability.

Natural selection occurs when individuals with certain heritable traits produce more offspring than those without. It is non-random and the primary driver of adaptation within a population.

Selection pressures, such as environmental conditions or predator-prey relationships, reduce the survival or reproductive success of individuals without specific adaptations. Biological fitness is the ability to survive and produce viable offspring relative to others of the same species. Adaptations are traits that enhance an individual's fitness in a given environment.

Adaptations are based on existing genes. Natural selection determines whether these traits are passed on based on their contribution to survival and reproduction.

Predators can drive natural selection by preferentially preying on certain phenotypes, leading to a reduction of those traits in the population.

The peppered moth example illustrates natural selection in action, where the fitness of dark or light moths depends on the color of the tree bark in their environment.

Antibiotic resistance in bacteria is another example. Mutations can confer resistance to antibiotics, allowing those bacteria to survive and reproduce in the presence of the drugs, leading to an increase in the frequency of resistant bacteria.

Genetic variation in human populations can provide resistance to diseases. Historical events like the bubonic plague demonstrate how certain individuals were more likely to survive and pass on their resistance. Similarly, exposure and survival of diseases such as Ebola and HIV can lead to the passing on of resistance to offspring.

Artificial selection is the selective breeding of plants and animals by humans for desired traits. This has led to the diversification of plants like wild mustard into various cruciferous vegetables and the gray wolf into various modern dog breeds.

Postulates of Natural Selection

  1. Individuals within a population vary in their traits.

  2. Some of these trait differences are heritable.

  3. More offspring are produced than can survive.

  4. Individuals with certain heritable traits are more likely to survive and reproduce.

  5. Over generations, a larger proportion of the population possesses adaptations for the environment.

**Summary of Micro