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Identifying examples of directional, stabilising and disruptive selection

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Convergent Evolution vs Divergent Evolution | Shared Traits Explained

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There are many types of evidence that support the Theory of Evolution. These pieces of evidence range from the minute molecular level of DNA similarities all the way up through similarities within the anatomical structure of organisms. When Charles Darwin first proposed his idea of natural selection, he used mostly evidence based on anatomical features of organisms he studied.

Two different ways these similarities in anatomical structures can be classified is as either analogous structures or homologous structures.

While both of these categories have to do with how similar body parts of different organisms are used and structured, only one is actually an indication of a common ancestor somewhere in the past.
Analogy
Analogy, or analogous structures, is actually the one that does not indicate there is a recent common ancestor between two organisms. Even though the anatomical structures being studied look similar and maybe even perform the same functions, they are actually a product of convergent evolution. Just because they look and act alike does not mean they are related closely on the tree of life.

Convergent evolution is when two unrelated species undergo several changes and adaptations to become more similar. Usually, these two species live in similar climates and environments in different parts of the world that favor the same adaptations. The analogous features then help that species survive in the environment.

One example of analogous structures is the wings of bats, flying insects, and birds. All three organisms use their wings to fly, but bats are actually mammals and not related to birds or flying insects. In fact, birds are more closely related to dinosaurs than they are to bats or flying insects. Birds, flying insects, and bats all adapted to their niches in their environments by developing wings.

However, their wings are not indicative of a close evolutionary relationship.

Another example is the fins on a shark and a dolphin. Sharks are classified within the fish family while dolphins are mammals. However, both live in similar environments in the ocean where fins are favorable adaptations for animals that need to swim and move in the water. If they are traced back far enough on the tree of life, eventually there will be a common ancestor for the two, but it would not be considered a recent common ancestor and therefore the fins of a shark and a dolphin are considered to be analogous structures.
Homology
The other classification of similar anatomical structures is called homology. In homology, the homologous structures did in fact evolve from a recent common ancestor. Organisms with homologous structures are more closely related to each other on the tree of life than those with analogous structures.

However, they are still closely related to a recent common ancestor and have most likely undergone divergent evolution.

Divergent evolution is where closely related species become less similar in structure and function due to the adaptations they acquire during the natural selection process.

Migration to new climates, competition for niches with other species, and even microevolutionary changes like DNA mutations can contribute to divergent evolution.

An example of homology is the tailbone in humans with the tails of cats and dogs. While our coccyx or tailbone has become a vestigial structure, cats and dogs still have their tails intact. We may no longer have a visible tail, but the structure of the coccyx and the supporting bones is very similar to the tailbones of our household pets.

Plants can also have homology. The prickly spines on a cactus and the leaves on an oak tree look very dissimilar, but they are actually homologous structures. They even have very different functions. While cactus spines are primarily for protection and to prevent water loss in its hot and dry environment, the oak tree does not have those adaptations.

Both structures do contribute to photosynthesis of their respective plants, however, so not all of the most recent common ancestor's functions have been lost. Oftentimes, organisms with homologous structures actually look very different from each other when compared to how close some species with analogous structures look to each other.

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What is the difference between analogy and homology in evolution?

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There are many types of evidence that support the Theory of Evolution. These pieces of evidence range from the minute molecular level of DNA similarities all the way up through similarities within the anatomical structure of organisms. When Charles Darwin first proposed his idea of natural selection, he used mostly evidence based on anatomical features of organisms he studied.

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Descent with modification

We've defined evolution as descent with modification from a common ancestor, but exactly what has been modified? Evolution only occurs when there is a change in gene frequency within a population over time. These genetic differences are heritable and can be passed on to the next generation — which is what really matters in evolution: long term change.

Compare these two examples of change in beetle populations. Which one is an example of evolution?

1. Beetles on a diet
Imagine a year or two of drought in which there are few plants that these beetles can eat.

All the beetles have the same chances of survival and reproduction, but because of food restrictions, the beetles in the population are a little smaller than the preceding generation of beetles.

2. Beetles of a different color
Most of the beetles in the population (say 90%) have the genes for bright green coloration and a few of them (10%) have a gene that makes them more brown.

Some number of generations later, things have changed: brown beetles are more common than they used to be and make up 70% of the population.

Which example illustrates descent with modification — a change in gene frequency over time?

The difference in weight in example 1 came about because of environmental influences — the low food supply — not because of a change in the frequency of genes. Therefore, example 1 is not evolution. Because the small body size in this population was not genetically determined, this generation of small-bodied beetles will produce beetles that will grow to normal size if they have a normal food supply.

The changing color in example 2 is definitely evolution: these two generations of the same population are genetically different. But how did it happen?

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Descent with modification We've defined evolution as descent with modification from a common ancestor, but exactly what has been modified? Evolution only occurs when there is a change in gene frequency within a population over time. These genetic differences are heritable and can be passed on to the next generation - which is what really matters in evolution: long term change.

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Early Concepts of Evolution: Jean Baptiste Lamarck

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Darwin was not the first naturalist to propose that species changed over time into new species-that life, as we would say now, evolves. In the eighteenth century, Buffon and other naturalists began to introduce the idea that life might not have been fixed since creation.

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Watch video and be able explain it

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How is this an example of behavioral isolation?

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What could you possibly have in common with a mushroom, or a dinosaur, or even a bacterium? More than you might think. In this Lab, you'll puzzle out the evolutionary relationships linking together a spectacular array of species. Explore the tree of life and get a front row seat to what some have called the greatest show on Earth. That show is evolution.

BUILD A TREE & DEEP TREE ACTIVITIES
DEVELOPED BY THE LIFE ON EARTH PROJECT

Video Intro About this Lab Educator Guide

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What could you possibly have in common with a mushroom, or a dinosaur, or even a bacterium? More than you might think. In this Lab, you'll puzzle out the evolutionary relationships linking together a spectacular array of species. Explore the tree of life and get a front row seat to what some have called the greatest show on Earth.

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A fully armored stickleback from the ocean near Loberg Lake (top), and a low-armored stickleback taken from Loberg Lake in 1994 (bottom). The fish have been stained with a dye called Alizarin Red S, which stains bones, in order to highlight their differences. Photos courtesy Michael A.

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Today there are two forms of pocket mice: light-colored mice that live on sandy soil, and dark-colored mice that live on black lava rock. The dark mice came about through the process of evolution. Naturally occurring mutations to coat-color genes produced mice with dark fur.

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Read about Darwin's voyage

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The Complete Work of Charles Darwin Online

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Citation: John van Wyhe , ed. 2002-. The Complete Work of Charles Darwin Online (http://darwin-online.org.uk/)

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Mechanisms of Evolution Quiz #1

Topics Covered: Mechanisms of evolution, natural selection, genetic drift, gene flow, immigration, emigration, mutation, reproductive isolation, geographic isolation, speciation, allele frequency, adaptive radiation, convergent evolution, divergent evolution, non-random mating, etc.

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Mechanisms of Evolution Quiz #1 Topics Covered: Mechanisms of evolution, natural selection, genetic drift, gene flow, immigration, emigration, mutation, reproductive isolation, geographic isolation, speciation, allele frequency, adaptive radiation, convergent evolution, divergent evolution, non-random mating, etc.

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How did the Rinkidinks evolve?

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Snurfle Islands!

Topics Covered: Natural selection, variation, adaptation, selective pressures, evolution, directional selection, stabilizing selection, disruptive/diversifying selection, fitness, etc.

Check out the worksheet that goes along with the game (courtesy of Mrs. Theresa Hartz)

Check out another worksheet that goes along with the game, courtesy of Ms. Julie Olson!

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Video computer games, virtual labs and activities for learning and reviewing biology content. Great for students and teachers.

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Edpuzzle assignment. Make video delivery an interactive experience. Students watch the lesson while answering the quizzes. The teacher gets the powerful hassle-free analytics.