We’ll look at the interplay between population genetics and environment. Are traits individually acquired or do entire populations evolve? The flying hamsters and a few other notable experiments will provide the answers.
Population Genetics Project
Given the burgeoning success of flying hamster research and our fascination with the little rodents, you and I have decided to do our senior research thesis in Adrian’s lab.
A lot of the other researchers in Adrian’s lab are focused on studying the genetics and molecular biological function of specific genes and proteins in the flying hamsters. I think it would be interesting to study the genetics of an entire population of hamsters rather than studying just a single gene. The study of genetic variation within a population is called population genetics.Since we seem to be working together so well, let’s go ahead and collaborate on our population genetics projects. Let’s think about the mechanism of evolution before we get started, which begs the question: what is evolution? For now, let’s think about evolution as a change in a characteristic within a population over time. As we learn more about population genetics, we’ll be able to further refine this definition.
Theory of Acquired Characteristics
While having lunch with some other students in our building, we learn about their theories regarding the evolution of traits. They tell us they believe that the evolution of new anatomical factors is driven by necessity.
Similarly, if an organism doesn’t need a structure, that structure will become smaller and less developed because of disuse. The idea that characteristics an organism acquires during its lifetime can be passed on to its offspring is attributed to the scientist Jean-Baptiste Lamarck. This theory is called heritability of acquired characteristics or Lamarckian inheritance.
Under this theory, a giraffe that stretches its neck to reach leaves in higher branches of trees would pass on stronger necks to its offspring, which would enable longer reach and eventually result in the evolution of giraffes with longer necks.Similarly, the theory proposes that the disuse of the small toe in mammals, such as pigs, caused it to disappear over time. We might say that the theory of acquired inheritance is akin to the adage Necessity is the mother of invention. The need for a trait drives the evolution of that trait.
Principles of Darwinian Evolution
Now, our classmates’ theory seems pretty logical, but it’s also a good idea to consider alternative possibilities. From our previous experiments, we know that variation exists within a population. Mutations can introduce new alleles, which might or might not give improved functionality as compared to the preexisting ones. Random assortment and crossing over create new combinations of alleles of different genes.For a given situation, new combinations of a gene could provide a competitive advantage for individuals of a population.
Let’s consider an example: suppose there are variations in size and strength of wings within an island hamster population. Hamsters with stronger, larger wings can fly farther than hamsters with smaller wings. However, since food and mates are plentiful on the island, the difference isn’t really that important.
Now, suppose the giant volcano on that island erupts, and the flying hamster habitat is completely destroyed.The hamsters with larger wings are strong enough to make it to a new islands, while the weaker ones are stuck on the ruined island or simply can’t make it all the way to the other island. In the end, the significant alteration to the environment resulted in hamsters with larger, stronger wings.
These are the basic principles of the theory of evolution that was proposed by Charles Darwin.He reasoned that populations rather than individuals evolved. This evolution is possible because genetic variation already exists within a population. Individuals with different genotypes in the population survive and reproduce at different rates. The agent that determines a differential rate of reproduction is called natural selection.
Lamarckism vs. Darwinism
Note the difference between Darwin’s and Lamarck’s theories. Lamarck believed that the need for a new trait would promote the evolution of that trait. Darwin proposed that evolution occurred through the selective pressure for a preexisting trait in a population. Both theories seem kind of reasonable, so how do we determine which theory most accurately describes what is going on in nature?Well, what if we tied the wings of flying hamsters so they can’t fly? The theory of acquired characteristics predicts that the wings would shrink, since they aren’t being used, and possibly disappear over the course of several generations.
As it turns out, a scientist known as August Weismann conducted a similar experiment to disprove Lamarck’s theory. Weismann repeatedly cut off the tails of mice over several generations. He found that the tails of mice in subsequent generations neither disappeared nor grew appreciably shorter.Today, Darwin’s theory of evolution is widely accepted as the principle which governs population genetics.
As we begin our study of flying hamster population genetics, we will see how Darwinian evolution can be used to predict the allelic, genotypic and phenotypic frequency within a population.
The study of genetic variation within a population is called population genetics.Evolution is a change in a characteristic within a population over time.The theory of acquired characteristics states that characteristic acquired by an organism during its lifetime can be passed on to its offspring.
It is also sometimes called Lamarckism.The principles of Darwinian evolution include the ideas that:
- Populations, rather than individuals, evolve.
- Evolution is possible because genetic variation exists within a population.
- Individuals with different genotypes in the population reproduce at different rates.
Natural selection is the agent which determines a differential rate of reproduction.
After watching this lesson, you should be able to:
- Define the terms evolution, population genetics and natural selection
- Identify John-Baptiste Lamarck and explain his heritability of acquired characteristics theory
- Describe Darwin’s theory of evolution
- Understand which theory of evolution is more accepted and why