Evolution forms part of the Biological and Biochemical Foundations of Living Systems section of the MCAT exam. Therefore, it’s essential to revise this, and other topics listed in the syllabus, in order to achieve a good MCAT score. This study guide will take you through everything you need to know about evolution for the MCAT, and provide links to where you can find further guidance.
Evolution has been integral in the development of every organism in the world, with changes arising all around us. From developments at the cellular level (with the addition of mitochondria in eukaryotic cells) to the organ level (with the number of folds in the brain) and changes in the body as a whole (with people having different skin tones), it is all around us. Humans are still evolving, with an example currently being the fact that there are muscles (for instance the division of the palmaris longus, and the pyramidal muscles in the abdomen) that are present in some people but absent in others.
Natural selection is otherwise known as ‘survival of the fittest’ and is one of the concepts that can lead to evolution. In this circumstance, the word ‘fitness’ refers to the level of an individual’s ability to reproduce. Individual organisms are more likely to mate and reproduce with those that seem evolutionarily advantageous or favorable. An example of this in real life is the MHC concept, which describes how people are more likely to date and reproduce with those who have an opposite immune system to their own. This means that when the individuals reproduce, their offspring are likely to have an optimal immune system.
Differential reproduction is the more modern edition of the fitness concept. It describes how mates are not only selected due to their own evolutionary advantage but to suit the environment that they are living in. Take, for instance, the finches in the Galapagos Islands.
All the finches live in slightly different areas where the food availability varies. Over time, they have evolved from one common organism to optimize the shape of their beaks in order to be able to get their food and meet their dietary requirements.
Group selection refers to a form of natural selection that works at a group level instead of at an individual level and is otherwise known as multilevel selection. This theory suggests that alongside individual natural selection taking place to maximize reproductive success, groups of organisms compete with other groups of organisms to better increase the success of the group as a whole. Altruistic individuals (those who work for the society instead of for their own individual needs) that make up a group are more likely to increase the all-round success of the group than a group made of selfish individuals. This is because an altruistic group is more cohesive and works better together to collect materials, find food, and build homes and communities, than a selfish group. In this way, natural selection favors the level of the group as opposed to the genetic or individual level.
Gene pools describe the total genes evident within a specific population. One of the results of evolution and natural selection is to increase the gene pool in order to increase diversity and therefore be able to adapt faster to a change in the environment. Gene pools are constantly changing as desirable characteristics are more favorably chosen for reproduction, increasing their percentage in the gene pool, whereas individuals with more undesirable characteristics are less likely to be selected for reproduction and therefore over time the percentage of this gene in the gene pool will decrease.
Polymorphisms describe two or more genetic differences that exist within a population, and they often result from several different genes. An example of this is blood types: different individuals have different types of blood: either A, B, AB or O and out of these, individuals are either positive or negative. There are two main types of polymorphisms that can occur: balanced or transient.
Balanced polymorphisms are polymorphisms in which 2 polymorphisms are in equilibrium. An example of this is the gene for sickle cell anemia, HbS, in areas where malaria is present. Sickle cell anemia causes resistance to malaria as it is more difficult for a mosquito to start drinking blood. However, it also makes it more difficult for your blood to carry oxygen. In this way, several individuals in malarial areas have been found to be heterozygous for the sickle cell gene: this way they are resistant to malaria whilst also reducing complications from homozygous sickle cell anemia.
Transient polymorphisms are those where one polymorphism will take over the other over time. An example of this is skin color: in areas that receive more sun, individuals’ skin is darker, whereas in areas where there is less sun, their skin is lighter. Both populations exist, but the genes for one will overtake the other depending on the environment.
Adaptation is the process by which evolutionary mechanisms cause a species to be better suited to the environment around them. This can occur in allopatric speciation (when there is a geographical barrier between 2 different parts of a species, for instance an ocean, that prevents crossbreeding) or sympatric speciation (when two organisms develop to be better suited to different parts of their own environment until they are too genetically different to allow for reproduction). It is a process that occurs over several generations, and those organisms with more stable DNA replication mechanisms will exhibit it at a slower rate than those with more careless DNA replication mechanisms.
Inbreeding is the process of related organisms in a species reproducing together. This is often bad as, especially at a population level, it affects the genetic diversity of the population. Consider cystic fibrosis for instance. It is a recessive trait; however, it is conserved through carriers through generations. If one individual has cystic fibrosis and another has no traits, they will produce offspring that will have heterozygous cystic fibrosis genes. If these two were to reproduce, then there is a higher chance of them producing an offspring with cystic fibrosis than if they were to reproduce with someone in the general population, as the single recessive trait occurs in 1 in 25 people.
If two of these individuals were to reproduce, again there would be an increased chance of passing the cystic fibrosis trait on. Through generations, this reduces heterozygosity and increases homozygosity, especially for harmful recessive conditions, that in itself reduces the gene pool and diversity of the population.
Outbreeding is the opposite of inbreeding and is how genetic diversity increases. Individuals will mate with those that they are not related to, who have a different genotype to their own. This increases the likelihood of a more varied genotype developing in the offspring, and therefore increases the chance of advantageous mutations developing. There are, however, some negative effects that can occur if there is interspecies reproduction, as often the genetic codes are too distinct and result in detrimental effects in the offspring.
Genetic bottlenecks are cases where the majority of a population is wiped out until only a few species are remaining. This results in massively decreased genetic diversity and a much smaller gene pool. As a result, the species has to effectively start from scratch and build up a population over several generations in order to form a gene pool as large and diverse as there was before. An example of this is poaching of rhinos: a previously large population has been reduced to very, very few individuals left, to the point where in some cases rhinos are being artificially inseminated to attempt to once again increase the population and prevent full extinction.
Molecular clocks are the way in which evolutionary time is measured and they use the mutation rate of DNA and RNA within genomes to determine how old different sequences and organisms are. Mutations in DNA trigger divergence, and the level of similarity between one organism’s genome and that of its ancestor can be telling of how long ago one species diverged. Sometimes this is due to environmental changes, sometimes it’s due to genetic bottlenecks, sometimes due to evolution via the processes of speciation and natural selection amongst others and sometimes it is through a completely different mechanism.
Hopefully this has helped with your revision of evolution for the MCAT. For more study materials to support you to effectively prepare for the MCAT, you’ll find on our website blogs covering a wide selection of MCAT topics, including meiosis (and other factors affecting genetic variability), the control of gene expression in eukaryotes and nerve cells. You’ll find more information about the exam, including the MCAT format, and the MCAT test dates, fees and registration for 2023, in our comprehensive guide to the exam.