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MCAT TOPIC REVIEWS & RESOURCES

Biological and Biochemical Foundations of Living Systems

BIO
Evolution

Natural selection

Natural selection is a process of evolution whereby the gene pool for a population is changed over time in favor of traits that are more advantageous to the survival of the population. Less advantageous traits are therefore selected against and are reduced in the population over time. This selective pressure has a population-level effect as a result of the interaction of traits of individuals in their environment. Selection does not take place on the level of the individual for specific traits.

Fitness concept

Fitness is a conceptual measure of an individual's potential for reproducing, passing on their traits to progeny and thereby maintaining representation for their underlying genes in the population's gene pool. Many aspects can contribute to fitness, including how fit the individual is for its particular environment. For example, a bird with a mutation creating a long, pointy, awkward beak may have lower fitness than its efficient, short-beak cousins because of a decrease in ability to eat readily available insects. However, if a change in environmental conditions reduces the insect population such that the birds can now only find enough to eat by accessing deep into dirt and the bark of trees, the long-beaked mutant will now have greater fitness, more able to access the available food source.

Selection by differential reproduction

Differential reproduction refers to unequal capacities for individuals of a population to reproduce due to trait variation. Some individuals will be more likely to reproduce than others. For example, individuals that have a trait for camouflage against predators will be more likely to survive to reproduce.

Concepts of natural and group selection

Natural selection is impactful on the level of a population and does not select for the traits of an individual, but it is the survival and reproduction of a specific individual that passes their traits on to their progeny that plays a role in driving the gene pool of the population. In group selection, traits are selected for not expressly for the survivability and reproduction of an individual but for greater survivability and reproduction of the group. For example, acts of altruism and resource-sharing may not specifically benefit the providing individual but can improve the survivability of a group significant enough to outweigh loss to the individual. Human acts of agriculture and animal breeding produce an artificial selection.

Evolutionary success as increase in percent representation in the gene pool of the next generation

The products of evolution can be seen from many perspectives, from population change to the survivability of individuals to the presence of a particular gene or even its specific alleles. Alleles that produce traits that are selected for will increase in frequency in the gene pool, thereby creating another measure of evolutionary success.

Speciation

Speciation is the evolutionary process of forming new and distinct species out of a shared ancestral population.

Polymorphism

Genetic polymorphism refers to distinct genetic variations present in a population, such as genes which have multiple alleles represented at significant frequencies (e.g. eye color). These variants provide opportunity for selective forces to act on the expressed traits.

Adaptation and specialization

Selective pressures can bring about adaptation and specialization within a population. Adaptation is the improved fit of a population for its environment (e.g. mountain goats becoming more capable at traversing steep, rocky terrain). Specialization is the association with a specific ecological niche (e.g. nocturnal behavior that helps to avoid predators).

Inbreeding

Inbreeding, reproduction among closely related individuals (usually true for a small population), limits genetic diversity (raw material for selection) and increases likelihood of recessive disorders within a population.

Outbreeding

Outbreeding, reproduction among distantly or unrelated individuals, improves chances of genetic diversity and protects against the widespread increase in recessive disorders within a population.

Bottlenecks

Evolutionary bottlenecks are dramatic reductions in allelic diversity brought about by a sudden reduction in population (e.g. due to environmental catastrophe). This alteration to the gene pool completely eliminates some portion of previous genetic variation while giving opportunity for previously less frequent but surviving traits to increase in frequency. Relatedly, the founder effect, whereby a very small population moves into a new area or is separated off from a larger group, produces a chance for expressed traits based on a very small gene pool and not necessarily selected for.

Evolutionary time as measured by gradual random changes in genome

Genetic drift and other random factors will produce changes over time that are not selected for. By measuring the amount of random change or mutation rates in a genome, it is theorized to be able to estimate the amount of time that passed since the diverging of different species using this so-called molecular evolutionary clock.

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