Homology and Analogy

Objective

Our objective is to identify and study the homologous and analogous organs in plants and animals.

The Theory

The central idea of biological evolution is that all life on earth shares a common ancestry and some similarities have evolved in other ways. These are called homologies and analogies. We will look at the different characteristics and identify the homologous and analogous organs in the plants and animals we have selected.

Homology

Homology refers to the traits inherited by two different organisms from a common ancestry.

Homology in Animals

Organs such as bat's wing, wings of birds, seal’s flipper, forelimb of a horse, and human arm have a common underlying anatomy that was present in their last common ancestors; therefore their forelimbs are homologous organs.

We can observe that though the shape and the size of the bones are not similar, there is a similarity in their structure that is, they have the same set of bones - humerus, radius, ulna, carpals, metacarpals and phalanges.

Man uses his hands to grasp and perform tasks, whales use their flippers for swimming, bats and birds use their wings for flight and horses use their forelimbs for running.

Frogs, birds, rabbits and lizards all have differently shaped forelimbs, reflecting their different lifestyles. But those different forelimbs all share the same set of homologous bones: the humerus, radius, and ulna. Such homologies reveal the common ancestry of all these animals.

 

Homology in Plants

Plants too have homologous structures like those seen in animals. In some plants like the pitcher, venus fly trap, poinsettia and cactus, the leaves show different functions and shapes from the ‘normal’ leaves we think about. Each of these leaves is a homologous structure, derived from a common ancestral form.

The pitcher plant and the venus flytrap uses leaves to trap and digest insects. The bright red leaves of the poinsettia though leaves, look like flower petals. The cactus leaves are modified into small spines that help reduce water loss and protect the cactus from plant-eaters.

The tendril of a pea plant and spines of a barberry plant are homologous organs showing similar functions. The tendril of a pea plant is a modified leaf used for providing mechanical support to the plant. But in the case of barberry plants, the spines are modified leaves meant for protection. The tendril of a pea plant and spines of a barberry though originated from leaves, are homologous organs performing different functions.

 

Analogy

Analogy refers to the similarity in function of two different organisms due to convergent evolution and not common ancestry.

Analogy in Animals

Analogous organs are the opposite of homologous organs, which have similar functions but different origins. An example of an analogous trait would be the wings of insects, bats and birds that evolved independently in each lineage separately after diverging from an ancestor without wings. The wings of insects originate from the inner or outer surface of the insect’s body. Feathers of birds originate from their forelimbs, and the wings of bats originate from both the fore limb and the membranous skin of the abdomen.

Another example of analogous animals is sugar gliders and flying squirrels. These two animals can glide in air using their gliding wings. Both species are different from each other in many ways. Flying squirrel are placental mammals, where as sugar gliders are marsupial mammals like kangaroos. To adapt a common function, the flying squirrel and sugar glider evolved similar gliding wings.

Analogy in plants

The leaves of opuntia and peepal are analogous organs in plants. In opuntia the stem is modified into a broad succulent leaf like structure that performs photosynthesis like leaves. Peepal leaf is a normal leaf that performs photosynthesis. So both the opuntia and peepal leaves perform common function through photosynthesis, so they are analogous.

Many of the cacti and African euphorbias are similar in appearance, being succulent, spiny, water-storing, and adapted to desert conditions generally. But these two plants belong to different families though they share traits according to the similar environmental conditions they are placed in.

Potato and sweet potato also show similar characteristics, but have different origins. Potato is a modified stem meant for storage of food. Sweet potato is a modified root also meant for the storage of food, so they are analogous.

 

 

Learning Outcomes

  1. Students understand the terms homology and analogy.
  2. Students will be able identify organisms that show homological similarities.
  3. Students will be able identify organisms that show analogical similarities. gvgv

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