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In other cases, similar phenotypes evolve independently in distantly related species. For example, flight has evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight. However, the wings of bats and insects have evolved from very different original structures. This phenomenon is called convergent evolution    , where similar traits evolve independently in species that do not share a common ancestry. The two species came to the same function, flying, but did so separately from each other.

These physical changes occur over enormous spans of time and help explain how evolution occurs. Natural selection acts on individual organisms, which in turn can shape an entire species. Although natural selection may work in a single generation on an individual, it can take thousands or even millions of years for the genotype of an entire species to evolve. It is over these large time spans that life on earth has changed and continues to change.

Evidence of evolution

The evidence for evolution is compelling and extensive. Looking at every level of organization in living systems, biologists see the signature of past and present evolution. Darwin dedicated a large portion of his book, On the Origin of Species , to identifying patterns in nature that were consistent with evolution, and since Darwin, our understanding has become clearer and broader.

Fossils

Fossils provide solid evidence that organisms from the past are not the same as those found today, and fossils show a progression of evolution. Scientists determine the age of fossils and categorize them from all over the world to determine when the organisms lived relative to each other. The resulting fossil record tells the story of the past and shows the evolution of form over millions of years ( [link] ). For example, scientists have recovered highly detailed records showing the evolution of humans and horses ( [link] ). The whale flipper shares a similar morphology to appendages of birds and mammals ( [link] ) indicating that these species share a common ancestor.

Photo A shows a museum display of hominid skulls that vary in size and shape. Illustration B shows five extinct species related and similar in appearance to the modern horse. The species vary in size from that of a modern horse to that of a medium-sized dog.
In this (a) display, fossil hominids are arranged from oldest (bottom) to newest (top). As hominids evolved, the shape of the skull changed. An artist’s rendition of (b) extinct species of the genus Equus reveals that these ancient species resembled the modern horse ( Equus ferus ) but varied in size.

Anatomy and embryology

Another type of evidence for evolution is the presence of structures in organisms that share the same basic form. For example, the bones in the appendages of a human, dog, bird, and whale all share the same overall construction ( [link] ) resulting from their origin in the appendages of a common ancestor. Over time, evolution led to changes in the shapes and sizes of these bones in different species, but they have maintained the same overall layout. Scientists call these synonymous parts homologous structures    .

Illustration compares a human arm, dog and bird legs, and a whale flipper. All appendages have the same bones, but the size and shape of these bones vary.
The similar construction of these appendages indicates that these organisms share a common ancestor.

Some structures exist in organisms that have no apparent function at all, and appear to be residual parts from a past common ancestor. These unused structures without function are called vestigial structures . Other examples of vestigial structures are wings on flightless birds, leaves on some cacti, and hind leg bones in whales.

Questions & Answers

The phenomenon by which Protoplasm of a cell shrinks from the wall is 
test Reply
which vein connective from harm to Brian
Burton Reply
which nevel succulate blood system from harm to hear
Burton
if an organism is cartilaginous is fully capable of a protecting the internal organs
Verah Reply
what is different between cell and tissue
Rose Reply
cell is the functional and structural unit of life while tissue is a group of cells aggregate to perform a similar or particular function
Tajudeen
cell is the basic unit of life while tissue is group of related cells that perform the same function Eg. the nerve tissue is made up of nerve cells
AAGifty
please what is meant by mutation
Kyere Reply
mutation is the change in DNA which results in the evolution
Shshank
what is binominal nomenclature?
Furaha Reply
the system of naming organisms depending on their xtics of origin
Vincent
The system of naming organisms using two-words Latin names
Agyeiwaa
what is the fate of reduced NAD in cell metabolism
namirembe Reply
what is plant cell
OLUWAKEMI Reply
what is the importance of meoisis
Kamasa Reply
it also aids in formation of pollen grain in anther and formation of ovules in ovary of flowering plant
Orawo
it also aids in formation of sperm or ova in animals
Orawo
explain the function of the skeleton
Jonathan Reply
skeleton is mainly the infrastructure of the organisms
Shshank
what 's different between immunity and vaccination
Symon Reply
vaccination simply refers to as drugs usually injected,most times in a liquid form given to a person or an animal,while immunity refers to a source of prevention usually in a liquid form that help in disease prevention
Grace
In children
Grace
Golgi apparatus helps in the formation of lysosomes
Diana
what is the difference between placenta and umbilical cord?
Grace Reply
No difference
Onwane
what's golgi apparatus.
Grace
What is biology
Rita Reply
is the study of living things
Kisa
biology is the study of living and non living things
Tinta
Biology is the study of living things
Addai
the study of living organisms
Furaha
Biology is the study of living organism or things
Rose
Biology is the study of both living and non living things
Funteh
distinguish between sex linked characters and sex limited characters
Kisa Reply
what is active transport
Jessy Reply
why is diffusion important
Jessy
dissolved substance move with in the cytoplasm by diffusion
Kisa
what role does mutation play in the process of evolution
Kisa
active transport refers to the movement of certain molecules or substances from lower to higher concentration with the help of energy or ATP
Shshank
mutation occurs due to change in DNA and DNA forms our body & if DNA changes then our body also acquire some changes & these changes gradually leads to the evolution of new species
Shshank

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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