Heredity and Evolution

Molecular Biology

Molecular biology is the study of the smallest units of life. It is the combination of chemistry and biology because it deals with the makeup of chemicals that make up living things. It focuses on DNA and the use of proteins.


Living things inherit traits from their parents. In sexual reproduction, an organism will have traits from both of its parents.

DNA and Chromosomes

DNA is the code that contains all of the unique information, or genes, about an organism that is inherited. DNA is contained on chromosomes that are in the nucleus of every cell in the organism.


Alleles are different versions of a gene. Alleles can be either recessive or dominant. If an organism has both a recessive and dominant allele, the dominant allele is what will be shown. Punnett squares calculate the probability that an allele will be shown

Punnett Squares

Say we make a Punnett square for chicken feather color. “B” represents blue feathers, which is the dominant allele. “b” represents white feather, the recessive allele. If both parents are Bb, the both display the blue allele. Their offspring will then have a ¼ chance of being BB, a blue chicken with no chance of having a white offspring; a ¼ chance of it being bb, a white chicken; and a ½ chance of it being Bb, a blue chicken like its parents.

Genotypes and Phenotypes

Genotypes refer to the genetic makeup of an organism. In the chicken example above, Bb would be an example of a genotype. The phenotype is the observable trait, such as blue feathers.


When DNA is damaged or an error is made in replicating chromosomes, mutations can occur. Mutations can result in traits that neither parent hasEpigeneticsThe environment around a species can cause changes in the traits and genetic makeup of a species over time. The study of how a species’ genetic makeup changes over time is called epigenetics.


Changes in a species’ genetic makeup that is to suit it to the environment is called adaptation. Charles Darwin observed finches in the Galapagos Islands and noticed that they have different beaks based on what there is to eat on the island they are on. If they had to crack hard seeds, they developed thick, strong beaks. If they had to get seeds from deep in cones or husks, they developed thin beaks.

Common Ancestry

Over time, species like the finches mentioned above may become separate species. Since they started out as one species, the two new species have a common ancestor. All species have common ancestry and have developed into the species they are today. This process is called evolution


Cladograms show relationships between species. Similar species are shown on the same branches. Evolutionary trees are cladograms that show ancestors of a species

Selection Pressure

When a species is not well suited for its environment, it is pressured to adapt. In the example of the finches, if they only have hard seeds to eat, only the birds with strong beaks will survive long enough to reproduce.

Natural Selection

When the natural environment around force a species to adapt by only allowing those with desirable traits to reproduce, that is called natural selection.

Artificial Selection

When a species is bred to change genetic traits to become more desirable to others, this is called artificial selection. An example of this is livestock being bred to have larger offspring that produce more meat.



Speciation occurs when a new species is created because it had adapted and separated itself from related species. An example of this a variation of the hawthorn fly called the apple maggot fly. The apple maggot fly lives in North America and only eats apples, unlike its ancestor that only eats hawthorns. Apple maggot flies could not have always been in North America because apples were introduced by European immigrants.



Cow Photo:  Richard Webb