how to fosssil provide evidence for evolution....
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Fossils are the impression of dead animals or plants that have lived millions of years ago on tge earth.
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If two or more species share a unique physical feature, such as a complex bone structure or a body plan, they may all have inherited this feature from a common ancestor. Physical features shared due to evolutionary history (a common ancestor) are said to be homologous.
To give one classic example, the forelimbs of whales, humans, birds, and dogs look pretty different on the outside. That's because they're adapted to function in different environments. However, if you look at the bone structure of the forelimbs, you'll find that the pattern of bones is very similar across species. It's unlikely that such similar structures would have evolved independently in each species, and more likely that the basic layout of bones was already present in a common ancestor of whales, humans, dogs, and birds.
The similar bone arrangement of the human, bird, and whale forelimb is a structural homology. Structural homologies indicate a shared common ancestor.
Image credit: "Understanding evolution: Figure 7," by OpenStax College, Biology, CC BY 4.0.
Some homologous structures can be seen only in embryos. For instance, all vertebrate embryos (including humans) have gill slits and a tail during early development. The developmental patterns of these species become more different later on (which is why your embryonic tail is now your tailbone, and your gill slits have turned into your jaw and inner ear)^2
2
start superscript, 2, end superscript. Homologous embryonic structures reflect that the developmental programs of vertebrates are variations on a similar plan that existed in their last common ancestor.
The small leg-like structures of some snakes species, like the Boa constrictor , are vestigial structures. These remnant features serve no present purpose in snakes, but did serve a purpose in the snakes' tetrapod ancestor (which walked on four limbs).
Image modified from "Rudimentary hindlegs spurs in Boa constrictor snake," by Stefan3345, CC BY-SA 4.0. The modified image is licensed under a CC BY-SA 4.0 license.
Sometimes, organisms have structures that serve no apparent function but are homologous to useful structures in other organisms. These reduced or nonfunctional structures, which appear to be evolutionary “leftovers," are called vestigial structures. Examples of vestigial structures include the tailbone of humans (a vestigial tail), the hind leg bones of whales, and the underdeveloped legs found in some snakes (see picture at right)^3
3
start superscript, 3, end superscript.
Analogous features
To make things a little more interesting and complicated, not all physical features that look alike are marks of common ancestry. Instead, some physical similarities are analogous: they evolved independently in different organisms because the organisms lived in similar environments or experienced similar selective pressures. This process is called convergent evolution. (To converge means to come together, like two lines meeting at a point.)
For example, two distantly related species that live in the Arctic, the arctic fox and the ptarmigan (a bird), both undergo seasonal changes of color from dark to snowy white. This shared feature doesn’t reflect common ancestry – i.e., it's unlikely that the last common ancestor of the fox and ptarmigan changed color with the seasons^4
4
start superscript, 4, end superscript. Instead, this feature was favored separately in both species due to similar selective pressures. That is, the genetically determined ability to switch to light coloration in winter helped both foxes and ptarmigans survive and reproduce in a place with snowy winters and sharp-eyed predators.
Arctic fox and ptarmigan. Both are white-colored and shown in snowy winter landscapes.
Image credit: "Understanding evolution: Figure 6," by OpenStax College, Biology, CC BY 4.0.
How can we tell if features are homologous or analogous?
Determining relationships from similar features
In general, biologists don't draw conclusions about how spec
To give one classic example, the forelimbs of whales, humans, birds, and dogs look pretty different on the outside. That's because they're adapted to function in different environments. However, if you look at the bone structure of the forelimbs, you'll find that the pattern of bones is very similar across species. It's unlikely that such similar structures would have evolved independently in each species, and more likely that the basic layout of bones was already present in a common ancestor of whales, humans, dogs, and birds.
The similar bone arrangement of the human, bird, and whale forelimb is a structural homology. Structural homologies indicate a shared common ancestor.
Image credit: "Understanding evolution: Figure 7," by OpenStax College, Biology, CC BY 4.0.
Some homologous structures can be seen only in embryos. For instance, all vertebrate embryos (including humans) have gill slits and a tail during early development. The developmental patterns of these species become more different later on (which is why your embryonic tail is now your tailbone, and your gill slits have turned into your jaw and inner ear)^2
2
start superscript, 2, end superscript. Homologous embryonic structures reflect that the developmental programs of vertebrates are variations on a similar plan that existed in their last common ancestor.
The small leg-like structures of some snakes species, like the Boa constrictor , are vestigial structures. These remnant features serve no present purpose in snakes, but did serve a purpose in the snakes' tetrapod ancestor (which walked on four limbs).
Image modified from "Rudimentary hindlegs spurs in Boa constrictor snake," by Stefan3345, CC BY-SA 4.0. The modified image is licensed under a CC BY-SA 4.0 license.
Sometimes, organisms have structures that serve no apparent function but are homologous to useful structures in other organisms. These reduced or nonfunctional structures, which appear to be evolutionary “leftovers," are called vestigial structures. Examples of vestigial structures include the tailbone of humans (a vestigial tail), the hind leg bones of whales, and the underdeveloped legs found in some snakes (see picture at right)^3
3
start superscript, 3, end superscript.
Analogous features
To make things a little more interesting and complicated, not all physical features that look alike are marks of common ancestry. Instead, some physical similarities are analogous: they evolved independently in different organisms because the organisms lived in similar environments or experienced similar selective pressures. This process is called convergent evolution. (To converge means to come together, like two lines meeting at a point.)
For example, two distantly related species that live in the Arctic, the arctic fox and the ptarmigan (a bird), both undergo seasonal changes of color from dark to snowy white. This shared feature doesn’t reflect common ancestry – i.e., it's unlikely that the last common ancestor of the fox and ptarmigan changed color with the seasons^4
4
start superscript, 4, end superscript. Instead, this feature was favored separately in both species due to similar selective pressures. That is, the genetically determined ability to switch to light coloration in winter helped both foxes and ptarmigans survive and reproduce in a place with snowy winters and sharp-eyed predators.
Arctic fox and ptarmigan. Both are white-colored and shown in snowy winter landscapes.
Image credit: "Understanding evolution: Figure 6," by OpenStax College, Biology, CC BY 4.0.
How can we tell if features are homologous or analogous?
Determining relationships from similar features
In general, biologists don't draw conclusions about how spec
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