Evolution. It’s a word that shows up in a lot
of games and cartoons – some of which one of us is quite into - but tends to be used in a
way that is often not really what evolution means in biology. Unlike what you might see in
a game where an individual character evolves, with biological evolution, individuals don’t
evolve during their lifespan. And it’s not just the misconceptions about evolution like that. Some of the vocab or terminology
can be misunderstood. For example, our video that explains how theory means something
different in science versus casual conversation. Or the word fitness – in biology, fitness is
related to how many offspring are produced, meaning genes are getting passed down---so
biological fitness is not how strong the organism may be. Or even the word “evolution”
itself - in casual conversation, the word “evolve” might refer to products getting more
complex; for example, a product changes to have more advanced features. But in biology, evolution
does not necessarily result in more complexity. Let’s talk about what this video is going to
focus on. We’re going to define biological evolution along with some of its mechanisms
such as natural selection and genetic drift. Then we will look at different lines
of evidence for biological evolution. First: let’s get a general definition for
biological evolution. Biological evolution is the change in a population’s
inherited traits over generations. Let’s talk about that word population because
it is populations, not individuals, that evolve. A population has multiple organisms of the same
species. But even though they’re the same species, there’s variety in a population, right?
Different traits. Those traits are coded for by genes. So, all together, there is
variety in the gene pool in a population. But mechanisms that cause changes in
the population’s gene pool can lead to evolution because inherited traits in
the population are coded for by genes. Consider a population of grasshoppers.
Same species of grasshopper but there can be variety in this population. In this
particular population, some are solid green, some have orange spots. Some have slightly
longer legs, some have shorter legs. Let’s illustrate some mechanisms of evolution
that could occur with this population. First mechanism we’ll mention: gene flow.
Genes that move between populations which can happen through migration. This can
impact the genetic makeup in the population. Mutations. They may be harmful,
they may be beneficial, they may be neutral. But mutations
do occur, and they are sources of changes in genetic material that can
change the genes in a population. Genetic Drift: This involves a
change in the genetic makeup of a population due to a random chance
event. In our grasshopper example: if a lawn mower happens to go through an area –
which is generally not a good thing if you’re a grasshopper- the gene pool of the remaining
grasshoppers may not represent the original population’s gene pool. This can impact
the genetic makeup in the population. Natural Selection: If, in this particular
environment, the green grasshoppers are better camouflaged than any other color
of grasshopper, they may not be seen as well by predators. So if they don’t get
eaten, they can survive and reproduce, passing on the genes that code for being green.
In this particular environment, other varieties may not reproduce as frequently and would have
lower biological fitness. The green grasshoppers, with their higher biological fitness, result
in more offspring that carry the genes for the green trait. This can impact the genetic
makeup in the population over time as more and more green grasshoppers reproduce. So these
are mechanisms of evolution as they can impact a population’s genetic makeup and the genes
passed down can code for inherited traits. Evolution doesn’t necessarily result in a
new species but it can: more about that in our speciation video. Evolution has multiple
lines of evidence: let’s explore some now. So first: homologies. Several different
homologies. When using homology in evolution, homology is referring to a similarity
due to shared common ancestry. First, molecular homologies. With molecular
homologies, many immediately think of DNA – comparing DNA relatedness – which is definitely
part of molecular homologies. But there’s also the importance of looking at homologous amino
acids and characteristics of proteins. So all animals are part of the domain Eukarya: animals
like termites and turkeys, sea slugs and snakes, emus and elephants! Molecular evidence would
support that these animals are more related to each other than they would be to a bacterium,
for example. But in this assortment of animals I just gave: molecular evidence would also support
that the turkey and emu are more closely related than the turkey and the termite. The turkey
and emu share a more recent common ancestor. Next, anatomical homologies. In this category,
we’ll focus on homologous structures and vestigial structures. Homologous structures – consider
this human arm and dog forelimb. You will find similarity in not only the general arrangement
but also the components that make up these structures. These are inherited from a shared
common ancestor. It’s important to note that they do not have the same functions. Functions
don’t indicate common ancestry. For example, a bird wing and insect wing may both be
used to fly but that’s not an indication of relatedness. A bird wing and an insect
wing are not homologous structures as they weren’t from a shared common ancestor
that had wings. And structure wise, the wings are very different- I mean, the
bird has bones for one thing. So, a bird wing and insect wing are what you call analogous
structures – same function but not homologous. Vestigial structures. To explain this one, I
first need to tell you about the algorithm that shows me videos: because I imagine it probably
shows me more chicken videos than the average person. Because I really like chickens. Among
many cool chicken facts that I could share, one is that some adult chickens actually
have a claw at the top of their wing. Yep. It can be kind of hard to see with
all the feathers but for this chicken, it’s a nonfunctional structure and
other birds can have it, not just chickens. The claw on the wing is considered a
vestigial structure. A vestigial structure is inherited from an ancestor but generally the
structure has lost all or most its function. Moving on from anatomical homologies:
Developmental homology. Embryology studies the development stages such as embryonic
stages and look for similarities in development among organisms which can support shared
common ancestry. In our animal video, we mention a phylum called Chordata. In this
phylum, all the animals have something called a notochord which they have at least in some stage
of their development; some have the notochord for their whole life. Vertebrate animals -including
humans – are all included together in Chordata and make up a large part of the phylum. During
embryonic development, organisms in this phylum have similar development structures including
pharyngeal slits (or pouches) and a postanal tail. Similarities in development can support shared
common ancestry among these organisms in Chordata. Now let’s shift from homologies and move
into another piece of evidence of evolution: the fossil record. A fossil can be remains or an
impression or a trace of an organism that once lived. Fossils aren’t just animals:
they can be plants or fungi or yes, even bacteria. Most organisms don’t actually
leave behind a fossil, because it turns out it matters the surroundings, the environment,
the type of remains that are present (because not every part fossilizes well) – but for fossils
that are discovered and continue to be discovered, there can be a lot of knowledge to gain
about the organism. Fossils can reveal how characteristics might have changed in a
population over time and build understanding about ancestral organisms that once lived.
Radiometric dating – which takes into account how long it takes radioactive isotopes to decay
– can be used to determine the age of the fossil. One more we’ll cover here: biogeography.
Biogeography combines “biology” and geography – this looks at how organisms
are distributed geographically on the planet and that way they are distributed is
supported by evolution that has occurred in the populations of organisms on the planet. For
example, populations on an island – they can be quite unique in appearance – this is expected
as the mechanisms of evolution have acted on them independently from the location where they
originally came from. However, the populations on the island tend to be the most closely related
to the populations nearest them – whether from another nearby island or mainland near them vs
somewhere much farther away. It’s also important to take into account factors like continental
drift and plate tectonics. For example, marsupials in Australia and marsupials in South America are
really far away from each other geographically, right? But, it turns out marsupials of South
America and the marsupials of Australia have shared common ancestry. Why? If you go back to
the time of Pangea, the continents were connected. As the continents separated, mechanisms of
evolution acted on these populations separately. One last thing we want to emphasize: evolution is
not done. It’s not some finished thing. Evolution continues to occur – after all, populations of
organisms continue to change over generations. Since it’s over generations, it’s easier for us
to see it in action when the generations do not take long. Such as antibiotic resistance in
bacteria - check out our natural selection video for more. Well, that’s it for the Amoeba
Sisters, and we remind you to stay curious.
