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The genetic basis of evolutionary change in morphology, phenotypic adaptations, and behavior

Transcript of Part 1: Introduction

00:00:00.18	Hi, my name is Hopi Hoekstra and I'm a professor
00:00:03.10	at Harvard University. Today what I'm excited to do is to
00:00:06.23	tell you about the field of evolutionary genetics, and in particular,
00:00:09.26	the genetic basis of evolutionary change. I'm going to tell you two
00:00:13.29	stories, one about morphology and one about behavior.
00:00:16.23	So, here's the outline of the 3 segments of my presentation.
00:00:21.12	So what I'm going to do now is give you an introduction
00:00:24.10	and introduce you to some of the longstanding questions
00:00:27.17	in the genetics of adaptation, and give you a sense of how
00:00:30.14	we're addressing these questions. And then the second segment,
00:00:33.21	in particular, I'll tell you a story about how we're tracking down
00:00:38.09	the genes and developmental mechanisms involved in camouflaging
00:00:42.11	and color differences between two species of wild mice.
00:00:45.08	And then in the third segment, I'll tell you about how we're using
00:00:47.25	very similar approaches to track down genes involved in burrowing behavior
00:00:51.25	differences in these same mice.
00:00:55.26	So I want to start today by talking about Darwin. Because in
00:01:00.06	2009, we had a number of celebrations celebrating everything that
00:01:05.07	Darwin knew on his 200th birthday and on the 150th anniversary
00:01:09.12	of his magnum opus, On The Origin of Species. Now Darwin
00:01:13.19	certainly knew a lot about evolutionary change, but there is one thing
00:01:19.00	that he didn't get quite right. And that is the mechanism
00:01:22.09	of evolutionary change, or the genetic nuts and bolts about how
00:01:26.02	organisms adapt to their environment. Now Darwin knew the traits were
00:01:29.04	inherited, he knew that offspring resembled their parents, but he didn't
00:01:32.14	know how. And this maybe isn't surprising because during this
00:01:36.02	time, of course we didn't know about DNA or genes, much less
00:01:39.08	the whole genome. And that's really what I want to focus on
00:01:43.21	today, is this mechanism of how changes in genes actually produce
00:01:47.26	variation in phenotypes on which natural selection can act.
00:01:52.08	So I'm going to start by telling you a brief anecdote that links
00:01:55.22	Darwin to a second great discovery, and that is the discovery of
00:01:59.06	DNA. So what I'm showing you in this next slide is Darwin's
00:02:04.24	last publication. Now I don't expect you to read it, but i just want
00:02:08.05	you to appreciate the fact that you're looking at his last publication.
00:02:12.08	It was published in 1882, just two weeks before he died
00:02:15.03	in a prestigious journal called Nature. The title of this article
00:02:18.23	is called, "On the Dispersal of Freshwater Bivalves."
00:02:21.28	And what this really is, is a report of the finding of a freshwater
00:02:26.10	beetle clamped to its leg was a freshwater clam, or a cockle.
00:02:30.15	So why you may be wondering was this published in such a
00:02:34.14	prestigious journal even 100 years ago? Well, this actually
00:02:40.06	resolved this great debate about why freshwater cockles were so similar
00:02:44.10	among disjunct lakes in the British midlands. One hypothesis
00:02:49.09	was that these cockles could migrate from lake to lake,
00:02:52.17	thereby homogenizing the populations and thereby, making them
00:02:56.23	very similar in size and shape. But the big question always was,
00:03:00.15	well how do they get from lake to lake if they can't cross
00:03:03.07	terrestrial habitats? Well here was a mechanism. They could
00:03:06.15	hitchhike by attaching to things that could fly or traverse
00:03:11.29	this terrestrial habitat -- in this case by clamping to the leg of a
00:03:15.22	beetle. But that actually isn't the point of telling you this story.
00:03:19.16	The point of telling you this is to mention that Darwin was sent this
00:03:24.08	beetle with a cockle clamped to its leg by a shoemaker who
00:03:28.22	was working in the British midlands, who was an amateur naturalist.
00:03:31.21	And his name was Walter Drawbridge Crick. Now this name should ring a
00:03:36.12	bell, because his grandson was the one with his colleague, Jim Watson,
00:03:41.04	that made the second great discovery. That is the discovery of the 3-dimensional
00:03:45.09	structure of DNA. And it's in this DNA text that we find even more
00:03:52.14	evidence for Darwin's great theory, that is our 3 billion year
00:03:56.20	existence, the shared evolutionary history of all living organisms,
00:04:00.18	and the subject of what I want to talk about today. And that is the
00:04:03.20	mechanistic basis for evolutionary change.
00:04:07.17	So like Darwin, one of the big questions in evolutionary biology
00:04:11.12	today is what gives rise to this amazing diversity? How is variation
00:04:16.02	generated and maintained in natural populations?
00:04:19.10	But thanks to Watson and Crick, we can look for that answer
00:04:22.11	in the genetic code. So the big question that we're focusing
00:04:26.02	on is what is the genetic basis of fitness-related traits?
00:04:30.14	By fitness-related traits, I mean traits that improve the probability
00:04:35.10	of survival or reproduction of organisms in natural populations.
00:04:39.07	So finding the genetic changes or the precise DNA changes
00:04:44.04	that contribute to variations either between populations or between
00:04:47.15	species, is a fun endeavor. And we certainly can learn things about the mechanistic
00:04:53.14	aspects of evolutionary change. Like how do changes in genes
00:04:57.15	actually produce changes in phenotype? But I'd like to argue that
00:05:00.21	we can actually learn even more about the evolutionary process.
00:05:04.11	So what can finding genes tell us about how evolution works?
00:05:09.03	Well there are a number of longstanding questions that I think we're
00:05:12.13	just now starting to be able to answer, because we're armed
00:05:16.06	with molecular biology and the ability to link genotype and phenotype.
00:05:20.17	So I'm just going to list a few of these big questions.
00:05:24.03	So for example, how does evolution proceed? Is it through
00:05:28.27	many small changes? Many small mutations, each that have a small
00:05:33.13	effect on the trait, or can evolution take big leaps?
00:05:37.08	That is, can mutations have large effects that are beneficial?
00:05:40.08	We also want to know about the dominance of these mutations.
00:05:45.07	So, do adaptive alleles or mutations that appear, do they tend to be
00:05:49.21	dominant or recessive? So J.B.S. Haldane, one of the founders
00:05:53.28	of population genetics, argued that adaptive mutations tend to be
00:05:57.19	dominant. Because when they first appear, they're visible to selection
00:06:01.03	and then can quickly spread through the population.
00:06:03.01	Compared to a recessive mutation, which would have to build up enough
00:06:07.14	number in a population to be contained in the same individual,
00:06:12.12	and that recessive trait then expresses. We also want to know, how
00:06:16.23	many -- how do these mutations interact? So if multiple mutations
00:06:19.26	are responsible for changing the phenotype, do they interact
00:06:24.03	in a complex way? Or does each mutation additively affect
00:06:28.07	that trait? We also want to know where these mutations, these beneficial
00:06:35.08	mutations are. Do they occur in the protein itself? For example,
00:06:39.17	amino acid changes that affect that structure and function of that
00:06:42.15	protein. Or do they occur in what we call non-coding DNA,
00:06:46.22	which affects the regulation, let's say the timing or place of expression
00:06:51.21	of that protein. And then we want to know where these mutations come
00:06:55.25	from. So for example, if there's a change in the environment, do we have to
00:07:00.13	wait around for new mutations to appear in that population?
00:07:03.08	Or are there these mutations maybe at a low frequency in the
00:07:07.18	population already that are pre-existing that can be selected
00:07:10.27	on almost immediately? And then finally, if we find mutations in one
00:07:16.03	population that are responsible for an adaptive trait, and we
00:07:19.01	have a similar trait involved in another population, is it the same
00:07:22.08	mutations and same genes that are responsible for those
00:07:25.16	convergent traits? Now importantly, all of these questions that I've listed
00:07:29.17	don't have simple yes or no answers. And in fact, we're more
00:07:33.08	interested in the frequency, whether for example, more often
00:07:37.13	beneficial mutations occur in regulatory regions versus structural
00:07:41.17	regions. But even more importantly than that, we want to know
00:07:44.26	why. Why in some cases do we see protein changes and in other
00:07:49.09	cases we see regulatory changes. Now these I would argue are
00:07:55.01	still largely open questions, but questions we can start to answer
00:07:58.25	by making the connection between genotype and phenotype.
00:08:01.20	So the context in which we're studying the genetic basis of
00:08:05.15	adaptation looks like this. That is, we're trying to make the connection between
00:08:09.16	environment and phenotype. In other words, trying to implicate
00:08:12.21	a role for natural selection in driving that phenotypic variation.
00:08:16.00	That is, suggesting that the phenotypic differences affect fitness.
00:08:20.02	But we also want to understand the genes underlying that phenotypic
00:08:23.29	variation, and not just what those genes are, but how those genes
00:08:27.12	through let's say development, actually produce the differences
00:08:30.28	in variation. And then once we make those links, we'll have a much
00:08:34.18	more complete picture of the adaptive process. I think this is where things
00:08:38.09	can get really fun, because we can go back out in the wild and ask how traits
00:08:41.21	evolved in nature. So, to make these links between environment
00:08:47.07	and phenotype and genotype, my lab group is studying one particular
00:08:51.26	group of wild mice, commonly referred to as deer mice.
00:08:55.05	Or mice in the genus peromyscus. These are the most abundant
00:08:59.10	mammal in North America. And the reason we study them is because
00:09:04.04	first, they're found in almost every habitat type. So from the top of the Rocky
00:09:09.03	Mountains out to the shores of Maine, to the plains of Kansas, to the deserts
00:09:15.21	of Arizona. So they're very widespread in their distribution
00:09:19.12	and because they live in all sorts of different habitat types,
00:09:22.06	there's a lot of opportunity for local adaptation.
00:09:24.23	So in addition to all the variation that we find in the wild,
00:09:28.14	they also can be treated much like laboratory mice. That is
00:09:32.14	we can bring them into a controlled laboratory environments. They
00:09:35.24	breed in the lab just like laboratory mice, and we can do controlled
00:09:39.22	experiments. And finally, while we're still behind traditional
00:09:45.05	model organisms, my group, as well as others, is building a series of
00:09:49.27	genetic and genomic tools that are going to be useful in trying to
00:09:54.17	make these connections between genotype and phenotype. But I would argue
00:09:58.00	one of the main reasons for studying these mice is because
00:10:01.11	we have this amazing literature of natural history studies on their
00:10:08.02	ecology. That is, these mice have been studied for nearly a century
00:10:11.13	by natural historians who have described morphological, physiological,
00:10:15.05	and reproductive behavioral variation in natural populations.
00:10:19.21	Just to give you a sense of how these mice vary, here are just
00:10:25.01	a number of traits that I picked out of the literature that describe
00:10:29.01	traits that have been studied and traits vary either between
00:10:32.14	populations or between species of peromyscus species.
00:10:36.12	So they vary in body size, tail length, foot size, color patterning,
00:10:39.27	testis size, sperm morphology, et cetera. They vary in morphological
00:10:43.05	traits, physiological traits, and behavioral traits. So, using these
00:10:48.20	mice, we're trying to make those connections between genotype and
00:10:52.01	phenotype. And the next two segments of my presentation, I'm
00:10:54.23	going to focus on two of these traits. One morphological trait,
00:10:58.04	color patterning, and a second trait, burrowing behavior.
00:11:01.18	So, the second part of my presentation, what I'd like to do
00:11:07.05	is focus on the morphological trait. And in particular, camouflaging
00:11:10.28	and color differences between subspecies of peromyscus polionotus.
00:11:16.02	Both to understand the ultimate reasons why these color differences
00:11:20.10	evolved, as well as the mechanisms or the underlying genetics
00:11:24.21	contributing to these differences in camouflage and color.
00:11:28.08	And for the third part, we'll switch gears and focus now using
00:11:32.24	very similar approaches. But instead of studying a morphological
00:11:34.26	trait, we've substituted in a behavior where we're taking advantage of these dramatic
00:11:39.18	differences in burrowing behavior; there are species that build these
00:11:42.24	large burrows versus those that build small burrows. To try and
00:11:47.11	understand how genes can affect behavioral variations in natural
00:11:50.21	populations. So I've hoped to have gotten you excited about
00:11:54.28	biology, in the sense that we're at this amazing time where
00:11:59.11	we can use approaches like Darwin first did, that is studies of
00:12:03.29	natural history, observation and experiment in the wild,
00:12:07.14	but combine that with studies of modern day molecular
00:12:10.15	genetics. To try to understand the genetic basis of what Darwin
00:12:14.26	referred to as that perfection of structure and coadaptation which
00:12:19.01	most justly excites our admiration. So thank you very much
00:12:23.00	for your attention, and I hope you'll join me for the next two
00:12:26.24	segments, where I'll tell you about more detailed studies from
00:12:29.19	laboratory group, trying to connect genes and phenotypes for both
00:12:33.11	morphological and behavioral traits. Thank you.

This material is based upon work supported by the National Science Foundation and the National Institute of General Medical Sciences under Grant No. 2122350 and 1 R25 GM139147. Any opinion, finding, conclusion, or recommendation expressed in these videos are solely those of the speakers and do not necessarily represent the views of the Science Communication Lab/iBiology, the National Science Foundation, the National Institutes of Health, or other Science Communication Lab funders.

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