lecture 2 a brief introduction to evolutionary thinking

Lecture 2Lecture 2

A brief introduction to A brief introduction to evolutionary thinkingevolutionary thinking

Today:Today:

• brief history of evolutionary theory• natural selection• evolutionary thinking and some important

evolutionary themes (following the paper by Stephen Stearns)

Adaptation, Constraints, Trade-offs, Conflict

A very brief history of evolutionA very brief history of evolution

• Evolutionary ideas go back long before Darwin (Lamarck, Erasmus Darwin)

• Darwin was the first to present an overwhelming case for “descent with modification”

• Critically, he also articulated a mechanism for evolution: natural selection

“I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection”.

Charles Darwin (1809-1882)

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Charles Darwin in 1854, five years before publishing The Origin of Species.

http://en.wikipedia.org/wiki/The_Origin_of_Species

evolution by natural selectionevolution by natural selection

• Alfred Russel Wallace (while suffering from a bout of malaria) hit upon the same insight before Darwin had published

• On the Tendency of Varieties to Depart Indefinitely From the Original Type


• Alfred Russel Wallace (while suffering from a bout of malaria) hit upon the same insight before Darwin had published

• They co-published a paper in 1858 but it wasn’t until the publication of The Origin of Species (1859) that the idea caught on


• Natural selection was eclipsed for several decades because of misunderstandings about inheritance.

• Darwin, unaware of Gregor Mendel’s discoveries about genetics, adopted incorrect ideas about genetics: pangenesis, blending inheritance

• In fact, genetics depends on particulate inheritance


• In 1900, Mendel’s findings were rediscovered.

• But instead of leading directly to a positive re-appraisal of Darwin’s ideas on natural selection, early geneticists were originally opposed

• The main problem was their focus on mutations of large effect

• JBS Haldane, RA Fisher, Sewall Wright were population geneticists who synthesized genetics and evolution: the modern synthesis


• Darwin started his argument for natural selection with insights from pigeon-breeding (he spent a lot of time drinking with animal breeders and became a pigeon fancier himself)

• “But man can and does select the variations given to him by nature, and thus accumulate them in any desired manner. He thus adapts animals and plants for his own benefit or pleasure. He may do this methodically, or he may do it unconsciously by preserving the individuals most useful to him at the time, without any thought of altering the breed.”


• “It is certain that he can largely influence the character of a breed by selecting, in each successive generation, individual differences so slight as to be quite inappreciable by an uneducated eye. This process of selection has been the great agency in the production of the most distinct and useful domestic breeds. That many of the breeds produced by man have to a large extent the character of natural species, is shown by the inextricable doubts whether very many of them are varieties or aboriginal species.”


• A process much like artificial selection, used by breeders of domesticated plants and animals to select for desirable traits,also happens in nature:

“Why, if man can by patience select variations most useful to himself, should nature fail in selecting variations useful, under changing conditions of life, to her living products?”


• A process much like artificial selection, used by breeders of domesticated plants and animals to select for desirable traits,also happens in nature:

1. Individuals within populations are variable2. The variations among individuals are, at least in

part, passed on from parents to offspring.3. In every generation, some individuals are more

successful at surviving and reproducing than others

4. The survival and reproduction of individuals are not random: those with the most favorable variations are naturally selected


“That many and grave objections may be advanced against the theory of descent with modification through natural selection, I do not deny. I have endeavoured to give to them their full force. Nothing at first can appear more difficult to believe than that the more complex organs and instincts should have been perfected not by means superior to, though analogous with, human reason, but by the accumulation of innumerable slight variations, each good for the individual possessor.”


“Nevertheless, this difficulty…cannot be considered real if we admit the following propositions, namely, -- that gradations in the perfection of any organ or instinct, which we may consider, either do now exist or could have existed, each good of its kind, -- that all organs and instincts are, in ever so slight a degree, variable, -- and, lastly, that there is a struggle for existence leading to the preservation of each profitable deviation of structure or instinct. The truth of these propositions cannot, I think, be disputed.


• Natural selection is a “blind watchmaker”

• It’s what?


• William Paley’s argument for a designer:

“In crossing a heath, suppose I pitched my foot against a stone and were asked how the stone came to be there, I might possibly answer that for anything I knew to the contrary it had lain there forever; nor would it, perhaps, be very easy to show the absurdity of this answer.”


• William Paley:

“But suppose I had found a watch upon the ground, and it should be inquired how the watch happened to be in that place, I should hardly think of the answer which I had before given, that for anything I knew the watch might have always been there.”


• William Paley:

“Yet why should not this answer serve for the watch as well as for the stone? Why is it not as admissible in the second case as in the first? For this reason, and for no other, namely, that when we come to inspect the watch, we perceive—what we could not discover in the stone—that its several parts are framed and put together for a purpose.”


“Nothing at first can appear more difficult to believe than that the more complex organs and instincts should have been perfected not by means superior to, though analogous with, human reason, but by the accumulation of innumerable slight variations, each good for the individual possessor.”

“Let us hope that what Mr. Darwin says is not true; but, if it is true, let us hope that it will not become generally known."

Evolutionary thinking

• The Stearns paper is the introduction to a book about evolutionary medicine, hence the medical focus

• “Evolutionary biology is a rich collection of well-developed alternative approaches to the interpretation of biological diversity and organismal design.”


Evolutionary topics:• Adaptations

• Relationships and history

• Neutral versus selectively advantagous variation

• The study of conflicts and cooperation

• Maladaptation: Why am I not perfect?


Wikipedia: Evolution

• The basic mechanisms that produce evolutionary change are natural selection (which includes ecological, sexual, and kin selection) and genetic drift;

• these two mechanisms act on the genetic variation created by mutation, genetic recombination and gene flow.

• Natural selection is the process by which individual organisms with favorable traits are more likely to survive and reproduce. If those traits are heritable, they are passed to succeeding generations, with the result that beneficial heritable traits become more common in the next generation.

• Given enough time, this passive process can result in varied adaptations to changing environmental conditions.[6]

http://en.wikipedia.org/wiki/Natural_selection

http://en.wikipedia.org/wiki/Ecological_selection

http://en.wikipedia.org/wiki/Sexual_selection

http://en.wikipedia.org/wiki/Kin_selection

http://en.wikipedia.org/wiki/Genetic_drift

http://en.wikipedia.org/wiki/Mutation

http://en.wikipedia.org/wiki/Genetic_recombination

http://en.wikipedia.org/wiki/Gene_flow

http://en.wikipedia.org/wiki/Biological_reproduction

http://en.wikipedia.org/wiki/Adaptation


Different sorts of evolutionary biologists:

• Population geneticists (genes, alleles, change or maintenance in variation)

• Evolutionary ecologists (design of phenotypes for survival and reproduction, life history, sexual selection, behaviour)


Different sorts of evolutionary biologists:

• Molecular evolutionists (history stamped into genomes, patterns in DNA and the processes underlying them)

• Systematists (phylogenies, relationships among taxa

• Paleontologists (fossils, deep time, major trends)

No clear boundaries, and many evolutionists where several hats. All try to observe patterns and infer process that underly evolution


Different evolutionary approaches:

• Changes in gene frequencies (population and quantitative genetics)

• Optimization approach

• Game theory approach

• Phylogenetic approach

natural selection and adaptationnatural selection and adaptation

• In short:

Heritable variationplusDifferential survival and reproductive successLeads toNon-random survival and reproduction such that

favorable variation is naturally selected


• The ultimate source of variation is mutation (in DNA, it turns out)

• Mutation is random

• Selection is NOT!

• Natural selection filters and preserves random mutation-derived variation and is the antithesis of randomness (it is, however, still blind)


• “Selection extracts order from randomness”

• “THERE IS GRANDEUR IN THIS VIEW OF LIFE”

• 31 genes, each with 26 alleles

• This is just one of the 2631 possible “haplotypes”

• http://home.pacbell.net/s-max/scott/weasel.html


• Adaptation is both a process and a state

• The process of adaptation = what happens over successive generations of selection of heritable variation in reproductive success

• The state of adaptation = a particular trait that does a job very well, just as though it were designed by an engineer

• E.g. opposable thumb, acute hearing, vision, sex drive, camouflage, venom, crystallins in eyes

Constraints on adaptationConstraints on adaptation

• Natural selection does not produce perfection

• Rather it is a “tinkerer” that produces “good-enough” solutions to context dependant problems.

• Often this occurs through duplication and divergence


“As natural selection acts by competition, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates; so that we need feel no surprise at the inhabitants of any one country, although on the ordinary view supposed to have been specially created and adapted for that country, being beaten and supplanted by the naturalised productions from another land.”


“Nor ought we to marvel if all the contrivances in nature be not, as far as we can judge, absolutely perfect; and if some of them be abhorrent to our ideas of fitness.”

Constraints on adaptationConstraints on adaptation• Time is a major constraint on adaptation. It takes time

to generate variation and select for it

• Absorbtion of milk sugar (lactose) by human adults

• Normally lactase is effective until weaning age (about 4). If you can’t digest lactose you suffer flatulence, intestinal cramps, diarrhea, nausea, vomitting

• But in cultures where dairy has been used for thousands of years, >90% of adults can digest lactose, like giant, lumbering babies

• Thousands of years, but probably not decades, are sufficient for this selection to shape human evolution

Constraints on adaptationConstraints on adaptation• Trade-offs are major constraints too

• E.g. sex is dangerous for fruit flies. There is a trade-off between survival and reproduction

• Similarly, there is a trade-off between having a robust immune system and suffering from asthma, or lupus, or diabetes type I

• Other trade-offs?

Trade-offs

Constraints on adaptationConstraints on adaptation• Historical constraints are also important:

“How strange it is that a bird, under the form of woodpecker, should have been created to prey on insects on the ground; that upland geese, which never or rarely swim, should have been created with webbed feet”

Constraints on adaptationConstraints on adaptation• Historical constraints are also important:

• The vertebrate eye has a basic flaw: the nerves and blood vessels that feed it enter right in the middle of the region of photosensitive cells.

• You would fire any engineer who designed an optical device in this way, but because of developmental constraints deriving from much simpler, ancestral eyes, that’s what we’re stuck with

• If squid could talk, they would taunt us about our poorly designed eyes.

Release from constraint: adaptive Release from constraint: adaptive radiationradiation

• Adaptive radiations can occur when new ecological niches open up, or a new adaptation opens up new possibilities (each beetles/flowering plants)

• Natural selection can operate very rapidly in such cases to generate new adaptations

• Islands, such as the Galapagos, are classic areas for observing evidence of adaptive radiation (eg Darwin’s Finches)

• But other favorable conditions also exist.

• The Great Lakes of Africa are home to the most species rich vertebrate radiation

• Hundreds of species in each lake, with relatively recent common ancestors

• A few million years for the ~500 species in Lake Malawi

• 12,000 years for Lake Victoria

Evolutionary conflictsEvolutionary conflicts• Lots of conflicts arise in nature:

• Predators and prey• Parents and offspring• Insects and plants• Fungi and crop plants they destroy• Chromosomes competing for transmission through

gametes

Evolutionary conflictsEvolutionary conflicts• Conflicts are where much of the action is in

evolutionary biology

• “Conflicts occur when genes have different patterns of transmission but interact, directly or indirectly, in the organisms that carry them”

• Perhaps the most obvious example is genes of pathogens and genes of hosts

• Endless cycles of damage and damage control adaptations can lead to “evolutionary arm’s races”

Evolutionary conflictsEvolutionary conflicts• In one sense, these arms races lead to lots of

adaptation

• In another, they present a serious constraint on adaptation and amount to running just to stay still.

lecture 2 a brief introduction to evolutionary thinking

Documents

natural selectionin

natural selectiondarwin

natural selectionit

character of natural

natural selectiona process

charles darwin

darwin lamarck

origin of species