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Course: High school biology > Unit 7

Lesson 2: Evidence of evolution
Science
High school biology
Evolution
Evidence of evolution
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Evidence of evolution review

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Key terms

TermMeaning
EvolutionThe process by which modern organisms have descended from ancient organisms over time
Common ancestorAn ancestor shared by two or more descendant species
FossilPreserved remains of ancient organisms
Homologous structureStructure that are similar in different species due to common ancestry
Vestigial structureStructure that is non-functional, or reduced in function
Analogous structureStructure that evolved independently in different organisms because the organisms lived in similar environments or experienced similar selective pressures
EmbryologyThe study of embryos and their development
BiogeographyThe study of where organisms live currently, and where their ancestors lived in the past

Evidence of evolution

Scientists who study evolution may want to know whether two present-day species are closely related. Evidence for evolution can be structural, genetic, or biogeographical.

Structural evidence for evolution

Observing anatomical features shared between organisms (including ones that are visible only during development) can indicate that they share a common ancestor.
Fossil skeletons of horse relatives dating from various time periods.
From most recent to least recent:
Equus - recent, single toe
Pliohippus - late Miocene, single toe
Merychippus - middle Miocene, three toes but with the lateral toes more reduced
Mesohippus - late Eocene, three toes
Fossils showing equine evolution. Image credit Wikimedia, CC BY-SA 3.0.
Structural evidence can be compared between extant (currently living) organisms and the fossils of extinct organisms.

Homologous structures

If two or more species share a unique physical trait they may all have inherited this trait from a common ancestor. Traits that are shared due to common ancestry are homologous structures.
The similar bone arrangement of the human, bird, and whale forelimb is a structural homology. Structural homologies indicate a shared common ancestor.
Homologous limb structure of human, bird, and whale. Image modified from Wikimedia, CC BY-SA 4.0.
For example, the forelimbs of whales, humans, and birds look different on the outside because they're adapted to function in different environments. However, if you look at the bone structure of the forelimbs, the organization of the bones is similar across species.
Compared embryological development of multiple species.
Illustration of embryo development of fish, salamander, turtle, chicken, pig, cow, rabbit, and human (left to right). Image from Wikimedia, Public Domain.
Embryology is important to understanding a species' evolution, since some homologous structures can be seen only in embryo development. For example, all vertebrate embryos, from humans to chickens to fish, have a tail during early development, even if that tail does not appear in the fully developed organism.

Vestigial structures

Vestigial structures serve little or no present purpose for an organism. The human tail, which is reduced to the tailbone during development, is one example. Vestigial structures can provide insights into an organism's ancestry. For instance, the tiny vestigial leg bones found in some snakes reflect that snakes had a four-legged 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).
Boa constrictor with vestigial legs. Image modified from Wikimedia, CC BY-SA 4.0.

Analogous structures

While similar structure can indicate relatedness, not all structures that look alike are due to common ancestry.
Analogous structures evolved independently in different organisms because the organisms lived in similar environments or experienced similar selective pressure.
The legs of a cat and a praying mantis are analogous. This means the function of the limb is the same because of similar selection pressures rather than common ancestry.
Analogous limbs of cat and praying mantis. Image modified from Wikimedia, CC BY-SA 4.0.
For example, the leg of a cat and the leg of a praying mantis are analogous. Both legs are used for walking, but they have separate evolutionary origins. On the outside, they appear similar because they have both experienced similar selection pressures that optimized them for walking. However, the actual structures that make up the leg are quite different, suggesting that the limbs are not due to a common ancestor.

DNA evidence for evolution

At the most basic level, all living organisms share the same genetic material (DNA), similar genetic codes, and the same basic process of gene expression (transcription and translation).
In order to determine which organisms in a group are most closely related, we need to use different types of molecular features, such as the nucleotide sequences of genes.
Biologists often compare the sequences of related (or homologous) genes. If two species have the "same" gene, it is because they inherited it from a common ancestor.
In general, the more DNA differences in homologous genes between two species, the more distantly the species are related.

Reading DNA gels

Segments of DNA can be analyzed using gel electrophoresis, in which fragments of DNA are separated by size.
Fragments are represented by horizontal bands. Bands that are similar in size between samples will be on the same horizontal line and indicate that DNA sequence is shared. The more fragments two samples share, the more related they are to one another.
DNA gel comparing four samples of DNA,
DNA gel comparing four species: A, B, C, and D. Species A and C are most related, as they share 3 bands with one another.

Biogeographical evidence for evolution

The notion of biogeography is what first indicated to Charles Darwin that species evolve from common ancestors. Patterns of distribution of fossils and living species may tell us how modern organisms evolved.
For example, broad groupings of organisms that had already evolved before the breakup of the supercontinent Pangaea (about 200 million years ago) tend to be distributed worldwide. In contrast, broad groupings that evolved after the breakup tend to appear uniquely in smaller regions of Earth.
Environment cannot always account for either similarity or dissimilarity. Closely related species can evolve different traits under different environmental pressures. Likewise, very distantly related species can evolve similar traits if they have similar environmental pressures.

Common mistakes and misconceptions

  • Evolution is not "just" a theory. In science, a "theory" addresses a broader question and is supported by a large amount of data from multiple sources. Evolution is a well-supported and accepted scientific theory that is supported by the evidence listed above.
  • Biologists do not draw conclusions about how species are related on the basis of structure or biogeographical evidence alone. Instead, they study both physical features and DNA sequences, and draw conclusions about relatedness based on these features as a group.
  • Not all species left fossils behind. Some people believe that all living organisms leave behind fossil evidence. Unfortunately, fossilization is fairly rare, because it requires many different conditions to occur over time in a specific order. Because these conditions do not occur all the time, we do not have fossils for all of the extinct organisms.
    Because many species that existed on earth were not fossilized, this has left gaps in our fossil record. However, that doesn’t mean these organisms didn’t exist, and the fossil record we do have contains many transitional fossils, all of which support evolution!

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