Forms & Patterns of Evolutionary Change

Instructor: Erika Steele

Erika has taught college Biology, Microbiology, and Environmental Science. She has a PhD in Science Education.

Since Charles Darwin introduced natural selection, scientists have come up with different ideas on how evolution proceeds, and how living things evolve over time. This lesson will focus on different patterns of evolution.

What is Evolution?

When you think of the word evolution, what comes to mind? Do you think about how one type of organism changed into another such as an apes, monkeys, or chimpanzees? Well, this is not quite accurate. Humans evolved from other humans, but they did once share common ancestors with apes, monkeys, and chimps.

A more accurate definition of evolution is the amount of genetic change in a species over time. Over time, the common ancestor of humans, chimps, apes, and monkeys were divided into separate populations that accumulated many changes in their genes that lead to the development of separate species. This lesson will discuss patterns of evolution or genetic changes that lead to new species.

Why Do Living Things Evolve?

To understand patterns of evolution, you need to understand why populations of organisms or species evolve in the first place. Natural selection acts as one of many driving forces of evolution. Species need to have the genes to make the traits that allow them to survive in the environment.

If they don't have the genes, they can't magically develop traits to survive. Natural selection happens when forces of nature cause organisms with certain traits to survive better than those without that trait. If the environment changes, different traits will be better suited for survival.

Over time, the genes and traits that a species has will change with the changing environment. When enough genetic changes happen, a new species may develop.

Rates of Change

Geologists are able to look at the fossil layers and see that some lifeforms change drastically, others change very little, and some die off all together. From these observations, scientists have created different ways in which life on Earth may have changed over time.

Catastrophism

For instance, catastrophism proposes that changes in the population of animals on Earth are due to natural disasters or catastrophes that wipe out most living things on the planet. After the catastrophe, life continues but new species develop in the aftermath. The extinction of the dinosaurs and the subsequent development of mammals is an example.

Gradualism

Gradualism is the opposite idea, proposing that life on Earth evolves at a slow and steady rate. In this theory, a single species slowly accumulates changes in their genes and gradually transforms into new species over a long time.

Punctuated Equilibrium

Finally, punctuated equilibrium states that evolution goes through periods where species do not change (equilibrium) followed by periods or points of speciation or development of new species.

Gradualism shows the butterflies gradually changing into blue or red butterflies. Punctuated equilibrium has branch points where blue or red butterflies developed.
patterns of evolution

Change in Organisms

So those are some broad ideas of evolutionary change, but what if we narrow the focus a bit? How do genetic changes lead to speciation?

Divergent Evolution

Divergent evolution occurs when two populations of the same species are separated and become genetically and phenotypically different from each other. The differences in the populations may happen because the two populations accumulate adaptations in different environments. Sometimes divergent evolution can lead to the development of separate species, called speciation.

  • Allopatric speciation is when a population is separated geographically. Over time, the separated groups develop different traits due to environmental differences and become different species altogether.
  • Sympatric speciation occurs in unseparated populations that differ in behavior. For instance, a population of fish that feeds on plants at the bottom of the lake, while the other feeds on insects near the surface. Over thousands of years of separation based on behaviors, they become different species because each population would have traits to make them better suited to their niche.

A. Allopatric speciation happens when a population of a species is separated by a geographic barrier. B. Sympatric speciation happens when a population is separated by behavior.
speciation

Coevolution

Coevolution happens when two species evolve together. This happens when the two species have a relationship with each other such as predator/prey. As prey get faster to avoid being eaten, predators also have to be faster. It can also happen between organisms with a relationship where both benefit such as the relationship between insects and flowers.

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