Welcome back to Nature League!
It’s a brand new month, and that means it’s time for a brand new theme.
This month’s theme is evolution and speciation.
[CHEERY INTRO MUSIC]
Evolution is one of the central concepts in biology, and it’s no wonder why.
Everything alive today is a product of many changes over time.
While evolution is a relatively straight-forward topic, and most people know about the basics,
there are some fun facts and nuances to be explored.
Charles Darwin is considered to be the founding father of the theory of evolution, namely
because of publishing the seminal work on the topic.
However, in the first edition of his book, “On the Origin of Species by Means of Natural
Selection”, Darwin never once actually used the word “evolution”.
For Darwin, it made more sense to speak of this change over time as “descent with modification”;
that is, the existence of new and different species over time that showed numerous changes.
But where did this idea come from?
And what led to these observations?
It turns out that the theory of evolution came to us from some of my favorite places
on Earth: islands.
Darwin and other naturalists in the mid-19th century like Alfred Russel Wallace made observations
on islands that led to several different inferences.
These inferences were then combined into the single working theory of natural selection.
Here’s a breakdown of these observations:
Observation 1: All species have great potential fertility.
Yep.
Life on Earth is pretty good at making more of itself.
Rabbits, case and point.
Observation 2: Populations are generally stable.
Fair enough.
In general, populations don’t boom overnight or go extinct in a few weeks - there are regulating
forces at play that create a general sense of stability, at least over short time periods.
Observation 3: Resources are limited.
Indeed.
There’s only so much available food, water, shelter, and habitat for all living members
of a certain species.
These observations led to:
Inference 1: There is a struggle for existence
And inference 2: This struggle isn’t random, and must depend on some heritable trait being
passed on from the parents
And that process of unequal survival ability is the process of natural selection.
So that’s natural selection.
However, natural selection isn’t the same thing as evolution: natural selection is one
mechanism of evolution.
Evolution depends on four main mechanisms: mutation, gene flow, genetic drift, and yep-
natural selection.
Let’s briefly check out these mechanisms.
Genetic mutation is the point source for almost all variations of life on Earth.
Mutations are errors in processes like DNA replication that cause a change to the DNA itself.
This means that the mutation can be passed on to the next generation.
Gene flow is exactly what it sounds like- it’s the flow of genes between populations.
This happens when individuals move into and reproduce with a new population.
Genetic drift is an odd concept, but it can be defined as the mechanism of change in population
gene frequencies due to random sampling of genes.
Let’s look at an example:
Let’s say there’s a population of sharks, and at a certain gene location, 75% of genes
in that population are a “blue” version, and 25% are “red”.
When individuals in this population begin to mate, there are all kinds of things that
can happen.
By chance, it’s possible that one individual doesn’t mate at all, or that a shark with
two blue copies mates with a shark with two red copies.
This means that in the next generation, the frequencies of those blue and red genes won’t
be 75% and 25% anymore.
They could be something like 71% and 29%.
And that change in gene frequencies over time due to random sampling chance is what we call
genetic drift.
The fourth and final mechanism of evolution is natural selection, which we’ve already
talked about.
Natural selection is the process of change over time due to unequal reproductive success.
That uneven success has to do with differences in traits, and whether or not those traits
are well-suited to the current environment.
So how are these processes related to speciation, or the appearance of new species over time?
It turns out Darwin had an additional inference in mind, and this third inference states that
the process of gradual change by natural selection could lead to new species.
When we talk about speciation, there needs to be some real talk up front.
The definition for a species can be...blurry at best.
In fact, there are several different working definitions for what a species is even within
the same fields of science!
That being said, the various definitions for what a species is can all agree that different
species arise over time.
This formation of new species due to continual evolution is called speciation, and there
are two main types:
The first type is allopatric speciation.
This happens when two populations of a species are geographically isolated.
Over enough time, genetic changes can occur for both populations, and if enough changes
happen and the two populations never mate with each other, a new species can evolve.
What about a situation where the populations don’t get separated in space?
This is called sympatric speciation, and it’s when two populations become reproductively
isolated even though they share the same space.
This isolation can happen because of things like food preference, filling a certain ecological
niche, or any number of other behaviors.
So, we’ve got natural selection which can lead to speciation and we’ve figured it
all out.
Evolution, check!
Or...maybe not.
Are there ways that organisms, populations, and species can change over time that aren’t
part of Darwin’s theory of evolution?
For the last part of this Lesson Plan, we’re going to touch on the concept of epigenetics-
a newer topic that potentially explains a separate, not-quite-Darwinian mechanism of evolution.
Epigenetics can be broadly defined as heritable changes to an organism’s genome that affect
things like gene expression or packaging, but don’t change the actual DNA sequence itself.
These changes can be caused by the environment...
basically, gene expression can be altered chemically
because of something like diet, or exposure to a chemical, or trauma.
This idea of the environment directly changing an organism’s traits and those traits being
passed on wasn’t/quite on Darwin’s radar when he was thinking through heritable changes.
However, it was on someone else’s mind... sort of.
Before Darwin came on the scene, there were several scientists and naturalists who noticed
life on Earth changing gradually over time.
One of the most famous, or infamous, was Jean-Baptiste de Lamarck, who published a hypothetical mechanism
for how evolution occurs in 1809, the very same year that Darwin was born.
In his mechanism he mentioned two principles.
First, that used traits become more pronounced while unused traits deteriorate, and second,
that these acquired traits could be inherited.
That second part is what made myself and others in my 9th grade biology class laugh out loud
when our teacher mentioned Lamarck’s ideas.
We were all like, “yeah, cool Lamarck, because if you chop someone’s arm off, they’re
totally going to have babies with only one arm”.
And yet, here we are now discovering that acquired characteristics, especially chemical
changes caused by the environment, can in fact be inherited.
Not in terms of the DNA sequence itself, but in terms of gene expression- i.e., epigenetics.
Lamarck’s revenge?
Not quite...it’s true that traits acquired during an organism’s lifetime can be passed
on, but we’re still trying to figure out the basics, like how many generations those
changes can persist across.
At the very least, we should consider his work as an example of why keeping an open
mind is beneficial in science, especially when it comes to life on Earth.
Thanks for watching this episode of Nature League.
Come back next week for an evolution and speciation themed Field Trip!
Nature League is a Complexly production.
Check out this episode by our sister channel SciShow for more on evolution.
It clarifies some common misconceptions on the topic.