Session 4: How is Evolution Measured

Transcript of Part 2: Introduction to Evolution

00:00:07.24	Hi. My name is Melina Hale.
00:00:09.08	I'm a professor at the University of Chicago.
00:00:11.14	In my lab, we study neurobiology,
00:00:13.10	biomechanics,
00:00:14.22	and evolution.
00:00:16.09	I'm going to present two different topics.
00:00:18.12	The first is an introduction to evolution.
00:00:22.12	Then we'll go on to talk about
00:00:24.10	a specific example from my lab
00:00:26.26	of how we map the nervous system
00:00:29.20	and aspects of the nervous system
00:00:31.12	onto the evolution of animals.
00:00:33.18	We work in my lab, specifically,
00:00:35.06	on vertebrate animals,
00:00:36.18	things like fish and tetrapods,
00:00:38.17	mammals, and reptiles,
00:00:40.27	and so I'm going to focus on that
00:00:43.07	part of biodiversity
00:00:45.07	in my talks.
00:00:46.22	There's a lot of other organisms out there, of course,
00:00:49.08	invertebrates, and insects, and plants,
00:00:51.18	and microbes,
00:00:52.26	that we won't touch on in these lectures.
00:00:55.26	So, we'll start with this introduction into evolution.
00:00:58.10	What is evolution?
00:01:00.05	Now, Charles Darwin
00:01:02.13	originally proposed the theory of evolution,
00:01:05.00	which can be summarized
00:01:06.27	in a very succinct phrase:
00:01:08.14	descent with modification.
00:01:10.05	Now, let's break that down a little bit, though,
00:01:12.17	to a broader definition,
00:01:14.22	which is change in the heritable characteristics
00:01:17.10	of organisms
00:01:18.29	from generation to generation.
00:01:20.29	We can break that down
00:01:22.13	even further to look at
00:01:24.11	the component parts of that sentence.
00:01:25.18	First, if we think about
00:01:27.03	this idea of generation to generation,
00:01:29.12	that means that when we look at evolution,
00:01:31.05	we're really not talking about changes
00:01:33.00	in individuals
00:01:34.17	or over short time frames.
00:01:36.01	Instead, we're talking about
00:01:38.04	changes that we see
00:01:39.28	over a long history
00:01:41.23	of the descent of an organism over time.
00:01:45.11	What about heritable characteristics?
00:01:47.04	Well, we all have lots of characteristics
00:01:49.01	to our bodies.
00:01:51.02	We may have big muscles
00:01:52.24	if we exercise a lot,
00:01:54.09	we may have had injuries in our lifetime
00:01:56.11	that have given us scars.
00:01:57.21	Those are not heritable characteristics.
00:02:00.10	Heritable characteristics
00:02:02.02	are the types of traits
00:02:03.19	that we pass on to subsequent generations,
00:02:06.11	or that we inherited from our parents
00:02:08.29	and grandparents.
00:02:10.18	Heritable characteristics
00:02:12.15	are an important part of evolution,
00:02:14.11	because it allows transmission
00:02:17.12	from one generation to the next,
00:02:18.24	and on and on through evolutionary history.
00:02:21.23	Now, the last part of this is change,
00:02:23.29	and change is also really important.
00:02:26.08	There has to be the ability in evolution
00:02:29.05	for these heritable characteristics
00:02:30.29	to vary,
00:02:32.21	to change in response to environmental factors
00:02:35.13	that might favor one type of characteristic
00:02:38.09	or another,
00:02:39.16	and we'll come back to that.
00:02:40.25	And that's what Darwin was getting at
00:02:42.21	with this idea of modification,
00:02:44.13	that there's going to be change
00:02:46.01	in how organisms are organized
00:02:47.24	and how they look over time.
00:02:52.03	So, here's an example,
00:02:53.18	a cute picture of a pair of dogs
00:02:56.07	and their puppies,
00:02:57.19	where you can really see
00:02:59.07	the variation in characteristics,
00:03:00.25	even in one generation.
00:03:02.23	If you look at the parents
00:03:04.11	and you look at the pups,
00:03:05.21	you can see some of the puppies
00:03:07.03	look like one parent,
00:03:08.21	with, you know, pure light fur,
00:03:11.03	others look like the other parent,
00:03:12.27	with very dark fur around the face,
00:03:15.03	but yet there are other puppies in the litter
00:03:17.13	that look different yet again,
00:03:19.04	that have a mix of the characteristics
00:03:21.29	of those two adults.
00:03:24.05	So, you can get a sense
00:03:25.29	of the variation in this image
00:03:27.26	that can be explored in evolution
00:03:30.29	and capitalized upon
00:03:33.06	through evolutionary time.
00:03:35.29	One example of variation
00:03:38.05	that's been really important for us
00:03:40.11	to understand how we can
00:03:43.17	change the characteristics,
00:03:45.07	the features of a species,
00:03:47.14	over time,
00:03:48.28	is the peppered moth.
00:03:50.12	So, these two moths,
00:03:51.23	that look very, very different
00:03:53.06	-- the light one on the left
00:03:54.24	and the dark one on the right --
00:03:56.01	are the same species.
00:03:57.12	They can interbreed.
00:03:58.23	Now, the dark one and the light one,
00:04:00.04	as you might expect,
00:04:02.07	do better in different types of environments.
00:04:06.03	This color characteristic
00:04:08.03	varies, of course,
00:04:10.00	and in some environments
00:04:11.29	it benefits the organisms
00:04:13.21	to be light or to be dark.
00:04:15.02	In other environments,
00:04:16.21	that same characteristic
00:04:18.16	may be detrimental to the animal.
00:04:20.14	So, these peppered moths
00:04:22.10	provided a classic example
00:04:24.01	of how characteristics can vary
00:04:27.07	with environment,
00:04:28.12	and how populations of a particular species
00:04:30.28	can vary.
00:04:32.22	So, this was noted
00:04:34.07	particularly in the industrial revolution.
00:04:36.19	At that time,
00:04:38.00	we went from manufacturing
00:04:39.22	using people
00:04:41.21	sewing or create objects
00:04:43.08	to using a lot of machines
00:04:44.26	to make products.
00:04:47.01	With the use of machines
00:04:48.21	came the use of coal,
00:04:50.25	and with coal came soot,
00:04:52.19	or pollution in the air.
00:04:54.06	Now, with that soot and pollution,
00:04:55.24	you could imagine that structures in the environment,
00:04:59.03	like trees,
00:05:00.19	would become darker,
00:05:01.28	and the peppered moth populations
00:05:04.05	changed in order to accommodate that.
00:05:06.27	And the darker morph
00:05:09.05	of the peppered moth
00:05:11.14	survived better. Right?
00:05:12.26	It was better camouflaged
00:05:14.23	against potential predators in the environment.
00:05:17.08	When the environment cleared up
00:05:19.21	and pollution decreased,
00:05:21.08	the tree barks became lighter
00:05:23.26	and the lighter version of the moth
00:05:25.25	actually survived better.
00:05:27.17	So, we can see variation
00:05:29.00	in the characteristics in a population,
00:05:31.19	even over this short amount of time,
00:05:34.20	and due to a human-induced
00:05:37.07	artifact in the environment,
00:05:38.09	this pollution from coal.
00:05:41.01	Now, just to show you how striking
00:05:43.05	this difference can be in the camouflage
00:05:45.05	of these moths on trees,
00:05:46.29	we can see some here.
00:05:48.19	So, here's our dark morph and our light morph,
00:05:50.17	and if we look at this tree,
00:05:51.28	we can see both the dark morph
00:05:53.15	and the light morph.
00:05:54.19	Here's the light one right down here,
00:05:56.20	and you can see it better camouflages
00:05:58.00	against the light bark
00:05:59.17	in this healthy tree.
00:06:01.05	The dark morph stands out against that light tree,
00:06:03.27	expect in this area over here,
00:06:05.22	where it's against this injury to the tree,
00:06:08.09	which shows up darker.
00:06:10.03	Another example in variation in populations
00:06:14.08	that we've probably all had experience with
00:06:16.14	is in bacteria
00:06:18.19	and the treatment of bacteria with antibiotics.
00:06:21.06	So, when we go to our doctor's office
00:06:22.26	with a bacterial infection,
00:06:24.08	we're prescribed antibiotics,
00:06:26.06	medicine to kill those bacteria,
00:06:28.19	and doctors are often very specific
00:06:31.09	about the need to take that medicine
00:06:34.11	over a precise time course,
00:06:36.11	and in particularly they say,
00:06:37.27	"Don't stop the medicine early.
00:06:40.12	You have to take the full course of medicine.
00:06:42.19	Even if you're feeling better,
00:06:44.21	take the full course of medicine."
00:06:46.07	It's important to do that.
00:06:47.25	Why is that?
00:06:49.02	It's because of the selection
00:06:51.00	that's acting on the variation in the population.
00:06:54.13	So, when we have a bacterial infection,
00:06:57.00	the species of bacteria
00:06:58.27	that's in our bodies
00:07:00.07	may have lots of variants to it,
00:07:02.04	and this is shown in number 1 on the left.
00:07:04.06	They might vary in aspects of their biology,
00:07:07.16	including how strong they are,
00:07:09.01	how resistant they are
00:07:11.01	to antibiotic medicines.
00:07:12.27	If we treat them,
00:07:15.06	shown in point 2 over here,
00:07:17.05	but we don't treat them long enough,
00:07:19.14	which are the bacteria
00:07:21.03	that are going to survive?
00:07:22.12	It's going to be the ones that are the strongest,
00:07:23.28	that are the most resistant
00:07:25.25	to the medication.
00:07:27.06	So, if we don't kill them
00:07:29.03	and we stop taking the medicine,
00:07:30.26	they'll be able to multiply
00:07:32.24	and will take on a larger part
00:07:34.28	of the population
00:07:36.20	of the bacteria.
00:07:37.25	It's not unless we kill them all
00:07:39.17	that we can prevent those resistant bacteria
00:07:42.06	from then multiplying
00:07:43.28	and becoming a problem
00:07:45.15	for our antibiotic medications
00:07:47.16	down the road.
00:07:49.07	So, I've shown you several examples
00:07:51.08	of how populations of a species can vary,
00:07:55.07	whether it's peppered moths or bacteria,
00:07:58.05	but how do we go from that
00:07:59.23	population-level variation
00:08:01.26	to the evolution of new species?
00:08:05.11	This is called speciation,
00:08:07.13	and in general
00:08:09.15	what happens is that populations
00:08:11.08	of a species
00:08:12.28	will be separated
00:08:14.13	and unable to interbreed,
00:08:16.03	and if they're separated
00:08:18.01	for a long enough period of time,
00:08:19.17	when they come back together
00:08:21.02	they may not be able to interbreed,
00:08:23.27	and then we would call them
00:08:25.25	different species.
00:08:27.02	One of the ways
00:08:28.25	that interbreeding is prevented
00:08:30.16	is through geographic isolation.
00:08:35.06	One of the students in my lab,
00:08:36.18	Andrew Trandai,
00:08:38.07	actually helped me out
00:08:40.13	by developing this hypothetical example
00:08:42.14	that I'm going to show you
00:08:44.23	on what a speciation event
00:08:46.13	might look like,
00:08:47.23	so I have to thank Andrew
00:08:49.19	for all of the images
00:08:50.28	that are coming up in the next series.
00:08:54.06	Okay, so in our hypothetical example,
00:08:56.29	what we're looking at is
00:08:58.29	some rodent squirrel-like animal
00:09:01.00	in an environment
00:09:02.19	-- one species --
00:09:04.06	all together as one population.
00:09:07.20	So, how do we separate them
00:09:09.16	and get new populations to evolve?
00:09:11.17	Well, in Andrew's example, here,
00:09:13.23	we have flooding
00:09:15.29	and an aquatic barrier
00:09:17.27	that these animals cannot cross,
00:09:20.01	so effectively
00:09:21.29	the population in the trees
00:09:23.12	and the population in the sand
00:09:25.19	are separated now
00:09:27.16	and will be evolving independently.
00:09:30.21	Over time, if we look at each of them,
00:09:32.23	we may see differences
00:09:34.12	being incorporated
00:09:37.17	into their biology.
00:09:38.25	Just superficially,
00:09:40.06	we might see the animals
00:09:41.29	that are in the forest
00:09:43.23	turning a different color,
00:09:45.19	other aspects of their anatomy
00:09:47.15	might change
00:09:49.17	to live in the trees.
00:09:51.04	On the opposite side of our river,
00:09:54.18	we may see the populations
00:09:56.07	that are in more of a sandy desert environment
00:09:59.18	change coat color
00:10:01.09	to match that environment,
00:10:02.20	or change size
00:10:04.15	to better adjust physiologically
00:10:06.10	to this drier environment.
00:10:08.13	Then ultimately,
00:10:10.00	once these differences have occurred
00:10:12.06	over, again, a very, very long period of time,
00:10:15.01	through evolution,
00:10:16.08	what would happen if the river dried up
00:10:18.16	and these animals
00:10:20.24	were able to come back together?
00:10:22.27	Well, they might come back together
00:10:25.07	and be able to interbreed,
00:10:27.10	but they may come back together
00:10:28.29	and not recognize each other
00:10:30.18	as the same species,
00:10:32.00	and therefore,
00:10:33.16	even though they're together
00:10:34.25	in this environment,
00:10:35.29	they would not interbreed
00:10:37.18	and their independent characteristics
00:10:39.04	would be carried on
00:10:40.25	from generation to generation
00:10:42.12	in those species.
00:10:47.14	So, that was an example
00:10:49.04	of geographic isolation,
00:10:51.04	and the biggest example of geographic isolation
00:10:53.22	happened about 200 million years ago,
00:10:56.18	when Pangaea,
00:10:58.06	which was this big super continental landmass,
00:11:00.25	broke apart to give us
00:11:03.16	the different continents that we know today.
00:11:05.28	So, South America and Africa
00:11:09.16	broke apart from North America and Europe,
00:11:13.08	and those continents
00:11:15.03	moved and separated around the globe.
00:11:17.22	With that separation,
00:11:20.00	the species that were together
00:11:22.08	prior to this breakup
00:11:23.25	then became separated,
00:11:25.16	and so if we look at species
00:11:27.10	that are in Africa versus South America,
00:11:29.20	for example,
00:11:30.28	we can see animals that
00:11:32.27	perhaps came from the same lineage,
00:11:34.14	but now are very, very different,
00:11:37.06	and are in fact different species.
00:11:43.20	Okay, so we've talked about this
00:11:46.04	process of evolution
00:11:47.16	and how it can occur.
00:11:49.28	What if we want to understand
00:11:51.16	the evolutionary history
00:11:53.11	of the animals that are
00:11:55.22	alive on earth today?
00:11:58.09	Well, we have to use a different set of techniques
00:12:00.28	to do that.
00:12:02.14	Here's just some of vertebrate diversity
00:12:04.16	and, as I said at the beginning of the lecture,
00:12:07.11	we also have lots of plants
00:12:09.16	and invertebrates and insects.
00:12:11.10	So I'm just showing you a very small part
00:12:12.14	of biodiversity here.
00:12:14.14	How do we figure out,
00:12:16.06	with animals so diverse as these,
00:12:18.17	how they're related to one another?
00:12:20.16	And how they evolved through time?
00:12:23.10	Well, we can take
00:12:25.04	a very simple example
00:12:27.03	of how we construct our own family trees
00:12:29.24	over very short time periods,
00:12:31.23	over several generations, say.
00:12:33.21	We research our genealogy,
00:12:35.11	we use birth notices and death notices,
00:12:38.27	and we recalled history
00:12:40.27	from our parents or grandparents,
00:12:42.29	and we can use that
00:12:45.01	to construct relationships
00:12:46.21	among our relatives and ourselves.
00:12:49.13	This is a really interesting family tree
00:12:51.25	that's on the wall of a Czech castle, actually,
00:12:55.01	and shows the relatedness
00:12:56.19	of this family,
00:12:58.04	going from a founder
00:12:59.17	down at the base of the tree, in the trunk,
00:13:01.20	up to the descendants at the top of the tree.
00:13:05.24	So, if we take a hypothetical example,
00:13:07.25	again,
00:13:09.01	of building a family tree,
00:13:11.05	and we start with
00:13:13.06	this family of green-ish and blue-ish,
00:13:15.13	big-eared and small-eared organisms,
00:13:18.04	and try to construct how they're related,
00:13:20.21	we can just look and see how family trees
00:13:23.05	are organized.
00:13:26.01	So, here I've taken that population
00:13:27.29	and put them onto their tree
00:13:29.25	-- that I made up --
00:13:32.01	and we can see that they're related to one another.
00:13:36.10	So, the individuals
00:13:39.12	that are connected at the first branch
00:13:41.21	are siblings.
00:13:43.11	They have the same parents.
00:13:45.21	If we move back in the tree,
00:13:48.01	we're looking at the different common ancestors
00:13:51.23	of these individuals.
00:13:54.17	So, if we go back,
00:13:56.21	these groups that are bracketed
00:13:58.23	in the orange boxes
00:14:00.14	are shared pairs of grandparents,
00:14:03.20	so they'd be cousins.
00:14:06.19	And if we look down near the base,
00:14:08.28	we can see that all of these organisms
00:14:11.07	share a pair of grandparents.
00:14:13.28	Now, because we're in recent history
00:14:16.25	and we have all sorts of ways
00:14:18.13	to record our history,
00:14:19.25	we may even know what these grandparents look like,
00:14:22.19	what our common ancestors of us,
00:14:24.15	and our sibling, and cousins, look like,
00:14:28.04	and I've reconstructed them this way.
00:14:30.01	If we look at at a group of animals
00:14:31.27	that's as broad as fish and mammals
00:14:33.27	and amphibians and reptiles, though,
00:14:36.04	we don't have that record,
00:14:38.18	to know what those common ancestors are
00:14:41.17	or what they looked like.
00:14:43.03	We have to use other types of approaches,
00:14:44.29	called phylogenetic approaches,
00:14:46.16	to basically try
00:14:48.26	to reconstruct the common ancestor
00:14:51.06	and how those species are related.
00:14:53.22	So, if we take this set of vertebrates,
00:14:56.22	this small number of animals,
00:14:58.20	and try to put them on a tree,
00:15:00.21	this is what it would look like,
00:15:02.04	and this is based on lots of peoples' research
00:15:04.08	over many, many years,
00:15:06.06	and I'll run you through it quickly.
00:15:08.20	On the far left,
00:15:10.18	we have the base of the vertebrate tree,
00:15:13.12	and these are lampreys,
00:15:14.28	these are animals that don't even have, really,
00:15:17.16	jaws.
00:15:18.20	They have these suction discs
00:15:20.02	that rasp and grip onto other species.
00:15:22.26	As we move up the tree,
00:15:24.13	we get into things like sharks,
00:15:26.00	and skates, and rays,
00:15:27.16	that have jaws,
00:15:30.02	but they have a cartilaginous skeleton.
00:15:31.29	When we move up yet again,
00:15:33.14	we get to the bony organisms
00:15:35.03	that include the fishes,
00:15:36.18	shown with these anemone fish,
00:15:38.14	the third image from the left,
00:15:40.08	and then we get up into the tetrapods,
00:15:42.26	that include amphibians,
00:15:45.17	reptiles, birds, and mammals.
00:15:48.14	Now, how do we construct
00:15:50.10	this kind of tree when
00:15:52.14	we don't have these detailed records
00:15:53.29	that we have of families?
00:15:55.12	Well, we do it by looking at
00:15:57.23	what characteristics these organisms share
00:16:00.14	and what characteristics vary between them.
00:16:02.26	There are lots of different types of characteristics
00:16:04.15	that we can use.
00:16:08.04	So, one of the features that we look for
00:16:10.20	when we're looking at shared characteristics,
00:16:12.22	or similarities and differences among organisms,
00:16:15.20	are anatomical features,
00:16:17.22	things like the shape of bones
00:16:20.06	or where sutures
00:16:21.14	-- where bones connect to one another --
00:16:23.04	or where we see holes through our skull
00:16:25.03	or other parts of our anatomy.
00:16:27.12	Bone and other structures
00:16:29.09	from the body
00:16:30.26	provide really nice characters
00:16:32.11	that we can use to try to figure
00:16:34.03	the relatedness of organisms.
00:16:36.11	In addition to using anatomical features
00:16:39.10	to try to understand the evolutionary history
00:16:41.20	of organisms and their relatedness,
00:16:43.24	DNA is now also
00:16:46.14	providing a really powerful way
00:16:48.24	of generating characters
00:16:50.17	to try to understand
00:16:52.20	how organisms have evolved.
00:16:54.08	In particular, we can compare a single gene
00:16:56.24	among different organisms,
00:16:58.14	different animals and species,
00:17:00.17	and see how it varies and how it's similar,
00:17:03.05	and look for changes in that
00:17:07.17	organization of the gene itself
00:17:09.05	that might give us signals
00:17:11.07	about how close a species is
00:17:12.27	to another species
00:17:14.20	and the relationship among them
00:17:16.28	and to different species.
00:17:18.18	Now, another set of data
00:17:20.12	that's been useful in understanding evolutionary history,
00:17:22.28	of course, is fossils.
00:17:24.22	They're really important.
00:17:26.01	Now, fossils provide information
00:17:28.16	about when and how features arose.
00:17:30.26	They won't, though,
00:17:32.20	provide the common ancestor.
00:17:34.07	It would be very unlikely
00:17:35.21	to actually dig up a fossil
00:17:37.12	that gives you the exact common ancestor of a species
00:17:39.24	but, nevertheless,
00:17:41.20	what they can provide us,
00:17:42.28	how they can ground our understanding
00:17:45.29	of when an organism
00:17:47.24	or particular elements and characteristics
00:17:49.11	of an organism arose,
00:17:50.28	is incredibly important.
00:17:53.25	So, to summarize
00:17:56.23	our introduction to evolution
00:17:58.09	and some of the major points we've talked about...
00:18:00.11	first, evolution is change
00:18:02.13	in the heritable characteristics of organisms
00:18:05.00	from generation to generation,
00:18:06.17	descent with modification
00:18:08.21	as proposed by Darwin.
00:18:11.08	Variation in characteristics
00:18:13.09	allows some subsets of populations
00:18:15.18	to be selected for or against.
00:18:18.20	And selection can cause change
00:18:20.14	in the characteristics
00:18:22.11	that persist in a population,
00:18:23.26	and this can allow for populations to diverge.
00:18:28.27	Reconstructing how the diversity of organisms
00:18:31.19	evolved
00:18:33.09	involves making trees,
00:18:34.23	or these phylogenies that I talked about,
00:18:37.09	that show different organisms
00:18:39.12	are related to one another.
00:18:41.19	And phylogenies, though,
00:18:43.08	depend on identifying characteristics
00:18:45.29	that are shared between organisms
00:18:48.06	and that can suggest their common ancestry.
00:18:50.07	And, again, we can get those characteristics
00:18:52.22	from morphology, from genes,
00:18:54.24	from all sorts of different sources.
00:18:57.19	Thank you.