What are good analogies for chromosomes

Homology and analogy

The hand bones of different mammals are homologous organs; the front legs of the mole and the mole cricket, on the other hand, are analogous organs. What Homology and analogy mean, we explain to you here using examples.

Homology and analogy simply explained

How do you actually know that we humans are related to birds and whales? At first glance, we don't have that much in common!

When biologists want to find out if two species are related, an important approach is to examine the similarities between the two species. Here, for example, they look at whether the living beings match in their body structure (anatomy), in their metabolism, in their behavior or in their genetic material (DNA). You then differentiate between two forms of similarity:Homology and analogy

Characteristics that have the same basic construction plan due to a common ancestor are called Homologies / homologous organs. An example are the limbs of various vertebrates such as humans, whales or birds.

If there is a similarity of certain characteristics without a common origin, you name it Analogies / analog organs. They therefore differ in the basic construction plan. Analog organs, however, have the same function due to similar environmental influences.An example are the front legs of the mole cricket and the mole.

Homology

By homology you understand similarities of biological structures in different living beings due to a common descent. This means that the species descend from a common ancestor and are related to each other. You call the similar features homologous organs. They all have the same basic blueprint.

You should definitely remember that only homologous characteristics are a criterion for a relationship.

A example for homologous organs are the forelimbs of vertebrates like the arm of humans, the leg of dogs, the wings of birds or the fin of whales. They all have the same basic anatomical structure, as you can see from the illustration.

However, the homologous organs can definitely differ in their functions. We grab with our arms, dogs run with their legs, birds fly with their wings, and whales swim with their fins. The homologous organs can develop from a basic form / starting form in different directions. You call that in biology divergent development / divergence or divergent evolution. Depending on the way of life, there may be deviations in the basic construction plan or the function.

The homologous organs are therefore evidence of evolution. The anatomical similarities in recent species - that is, species that live today - can be traced back to a common ancestor in evolution.

By knowing homologies and using other methods such as genetic analysis (DNA sequencing), family trees can be created. In them you can then understand the tribal history and the relationships of the species.

Homology (Greek: homologia for agreement / English homology) is a similarity between two structures, which can be traced back to a descent from common ancestors. The structures are called homologous organs.

Homology criteria

Often homology cannot be recognized at first glance, as many structures have developed different functions in the course of evolution. There are three criteria here - that Homology criteria - of the biologist Adolf Remane, which should help to distinguish:

  • Location criterion
  • Specific quality / structure criterion
  • Criterion of continuity (continuity)

If one of the three homology criteria applies, then a characteristic is homologous. Let's look at the three homology criteria below with examples.

Location criterion

After this Location criterion organs are homologous if they are in the same position in a comparable structure.

A example for this are again the front extremities of the various vertebrates. The order of the individual bones is always the same for every animal: upper arm, forearm, wrist, metacarpal and finger. But you can see here that the number of bones does not always have to match. For example, a bird's wing or a horse's foot have fewer finger bones than the five bones in the basic blueprint.

The regression or fusion of individual organs can arise due to different development or specialization.

By the way: The criterion of location does not only apply to bones, of course, but also to organs. An example are the digestive organs of vertebrates, which are always arranged in the same order: mouth - esophagus - stomach - intestines - anus.

Specific quality criterion

According to the Specific quality criterion structures / organs are homologous regardless of their location. The prerequisite here is that they match in many individual features.

At first glance, a human tooth has absolutely nothing in common with a shark scale, does it? However, they are the same in many structural features and materials. For example, both structures are hollow on the inside. In addition, the first layer of our tooth and that of the shark scale are made of the same material - namely (tooth) enamel.

The swim bladders of the bony fish are also homologous to the lungs of the mammals, since they match in many individual features.

Criterion of continuity

After this Criterion of continuity (Continuity) organs are also homologous if they can be connected to one another through intermediate forms / transitional forms. This means that dissimilar organs with different locations are also defined as homologous here.

A example is the bloodstream of fish, amphibians, and mammals:

If you compared the bloodstream of mammals to that of fish, you would probably hardly see any similarity. Mammals have a separation between body and pulmonary circulation and, accordingly, no “mixed blood”. Here, oxygen-poor and oxygen-rich blood is transported separately. With fish, on the other hand, there is only one cycle. But if you now look at the amphibian cycle, you can see an intermediate form between fish and mammals. In the course of evolution, there was a separation of the lungs and body circulation.

analogy

Analogies arise when living things with different origins adapt to the same habitat. The similarity of the structures resp. analog organs is therefore not due to a relationship between the species. Therefore, in contrast to homologies, analogies do not have a common basic plan. The similarity in tasks and functions is only due to adaptation to the same environmental conditions or lifestyles. This is what you also call in biology convergence / convergent evolution or convergent evolution.

A example for analog organs, the digging legs of the mole - a mammal - and those of the mole cricket - an insect. They differ completely in their basic anatomical plan and material. However, both live underground and have therefore developed similarities. In both the legs are shortened and thickened and have claws at their ends.

Analogy (Greek: analogia for "correspondence") is a correspondence of two structures in their function and often also in their appearance (= analog organs). The similarity is not due to a common ancestor, but to convergent evolution.

Further Examples for analog organs are:

  • Lungs of the vertebrates / trachea of ​​the insects: Both organs are responsible for the gas exchange, but have completely different structures.
  • wing of insects and birds: Both structures are used for flying, but are also constructed completely differently.
  • Thorns (e.g. Rose) and Spines (e.g. barberry) in plants: Both structures are used for defense, but have a different blueprint.

So you can see that not all similar traits are signs of a relationship. In tasks in your examination you often have to justify / assign whether a characteristic is homologous or analog. tip: So take a good look at our examples!

Homology in genetics

You also use the term “homology” in genetics. For every two chromosomes that occur in our body cells. These pairs of chromosomes, one from the father and one from the mother, are what you call homologous chromosomes. They are similar (with the exception of the male sex chromosomes) in shape, size and sequence of the individual genes. If you want to learn more about homologous chromosomes, watch our video now!