Evolution and Adaptation by Natural

Selection

A Puzzle• Unity of Life

– Life arises only from preexisting life (vs. spontaneous generation)

– All living organisms are composed of one or more cells, and cells come only from other cells

– All organisms use the same DNA code (A T G C), which gets passed from cell to cell and generation to generation

• Diversity of Life– Earth has been host to a staggering variety of life forms

…penguins to petunias, dandelions to dinosaurs, ostriches to octopi, lobsters to lions, banana trees to bacteria, mushrooms to monkeys, sharks to saber-toothed cats

How can life be so diverse yet still unified???

Interpretation: All life is related, sharing a common ancestry – that is, a single DNA lineage. BUT… DNA sequences gradually change over time. Organisms past and present are connected by the flow of genes along branches on the “Tree of Life.” Living organisms are like leaves at the tips of twigs. Ancient species (now extinct)

are back down the branches.

Solution: Life changes and diversifies over time

Tree of Life Web Project

Curtis & Barnes Biology

Horse evolution (“equids”)• Superb 50 million year fossil

record• Hyracotherium: dog-sized,

adapted for browsing on leaves and fruit in tropical forests• Later species: increasingly

adapted for life of grazing in grasslands• Molars became bigger and

more durable, with broader grinding surface for tough grass• Larger size, longer legs, and

reduction in toes to one big hoof for escaping predators in the open

Evolution= a change in a population’s genes and traits over time,

with increasing adaptation to the prevailing environment

Campbell et al. BiologyBranching, not Linear

Speciation rates usually exceed extinction rates, so diversity of life increases (exception: mass extinction events; e.g., dinosaurs)

“Speciation”(branching off of new species and

lineages)-VS-

“Extinction”(dead ends)

• Galapagos: young volcanic islands off Ecuador• 13 species of finch, with distinct varieties on

different islands• Darwin later proposed that these finches all

shared a common ancestor, a colonist from South America, but the lineages had “evolved” into different forms – a process he called “descent with modification” …BUT HOW???• Later biologists would show that Darwin’s

finches were adapted for the distinct foods on each island (even a “vampire” finch that parasitizes seabirds!)

(This was but one of many observations of plants, animals, fossils, and geology that Darwin made during the 5-year

expedition)

Charles DarwinVoyage around South America

aboard HMS Beagle, 1831-1836

Lewis Life

Wikimedia Commons

• Darwin – a pigeon breeder himself – noticed that no two members of the same species are perfectly identical. Each organism is born slightly different from its parents, from its brothers and sisters, and from other members of that species

• (Today we know this stems from random reshuffling of genes during sexual reproduction, plus occasional mutations …things Darwin had no knowledge of)Darwin reasoned that over many generations, a lot of

little differences could add up to some enormous changes!

“Artificial”

SelectionSimilarly, pigeon

fanciers can create new varieties by “selective breeding”

Photos courtesy of Wikimedia Commons

Try a little “artificial selection” yourself by creating your own “biomorphs” at

www.annanardella.it/biomorph.html• Click Quick Start button (browser must be Java enabled)

• Upper left: The “parent” biomorph, resembling a bush, whose shape is controlled by 9 “genes” (see table of genes on main page)

• The other 18 biomorphs are the parent’s “mutant” offspring (9 genes mutated in the positive direction, 9 in the negative direction)

• Simply click whichever mutant offspring you find most interesting. That now becomes the parent, and 18 new mutants appear. Repeat.• Advice: Be PATIENT and CHOOSY. Always review all 18 options,

select one that you like best, and weed out the less interesting ones. Can you “evolve” the original bush into a bird-, bug-, or frog-like critter?

Evolution by “Natural Selection”(Charles Darwin, The Origin of Species,

1859)

1) In every population, more offspring are produced than can survive (they breed well beyond the environment’s “carrying capacity”)

(ex) Not all goldfish will survive when released into the wild

2) Those offspring will compete with one another for survival and limited resources

(ex) Goldfish race to escape predators, capture food, etc.

(cont’d…)

Wikimedia Commons

The Far Sideby Gary Larson

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Length Class (Caudal Height as a % of Fork Length)

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Mean Tail-to-Length Ra-tio: 31.9%

3) A random variety of traits will appear among those offspring

4) Some traits are more favorable (or “adaptive”) than others for competing and surviving in that environment• Other traits may be “weeded out” (“Survival of the

Fittest”)

5) Those offspring that happen to be born with more adaptive traits will have a better chance of surviving long enough to reproduce, thereby passing on their adaptive traits

6) In the next generation there will be a higher frequency of adaptive traits

7) Over many generations – aided by an occasional lucky mutation – the distribution of traits may shift dramatically (= evolution)

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Tail-to-Body Ratio, Present Day

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Tail-to-Body Ratio in 500 Generations

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Tail-to-Body Ratio in 1,000 Generations

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Galapagos ground finches: Catching evolution in the act

(a study by Rosemary & Peter Grant and Peter Boag)

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The Grants and colleagues collected data on Galapagos ground finches for many years. Here is the “beak depth” distribution for

the 1976 population. Which important condition for natural selection does it reveal?

Phenotypic

Variation

They also compared the beak depth of

parents to that of their offspring. What important aspect of natural

selection does this demonstrate?

The differences are GENETIC (genotypic variation in the DNA):Parents with deep beaks tend to have offspring with equally

deep beaks.

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Correlation of Ground Finch Beak Depthsbetween Parents and Offspring

Parental Beak Depth (mm)

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In 1977 a brutal drought struck the Galapagos. Vegetation was decimated and seeds became terribly scarce. Over 80% of the finch population starved to death. The birds quickly exhausted their supply of small, soft seeds. That left only the much larger,

harder seeds of the Tribulus plant. Normally a finch would ignore Tribulus seeds, but now that was the only food available.

Birds with deeper beaks outcompeted birds with shallow beaks, probably because they could crack Tribulus seeds. And since

the trait is genetic, this means the population EVOLVED a deeper beak!

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1976 Survey1978 Survey

Beak Depth (mm)

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the Grants again

surveyed beak depths.

Interpretatio

n?

Back to the crew of the Zostera on planet XK-714…who had the best plan?

(one last chance to vote)

• True: Plants and animals do have a limited ability to “adapt” to certain changes in their environments, like adjusting to falling temperatures or stronger sunlight. Some can develop resistance to disease, poisons, etc.• BUT: The ABILITY to “adapt” to certain

changes is itself an adaptation, produced by evolution through natural selection. Omicron radiation is a completely new and novel challenge. Thus there is no way taro’s ancestors could have evolved an ability to adjust to omicron.

Cooper: Expose living plants to low but increasing doses of omicron radiation in the laboratory. This will allow them to adapt as needed by building an immunity or

resistance to omicron radiation. Then transplant to the greenhouse. In this way, the greenhouse can be slowly but steadily stocked with omicron-tolerant plants.

• Probably the SECOND best strategy …but unlikely to work• Flaw: She’s confusing short-term INDIVIDUAL “adaptation” with

long-term EVOLUTIONARY adaptation …an easy mistake to makeThe Far Side

by Gary Larson

An excellentEVOLUTIONARY

adaptation

• Seems quite reasonable …but impossible• Flaw: He believes that changes acquired DURING an organism’s

lifetime can be passed to its offspring …another common misunderstanding (even Darwin was prone to this error)• True: Organisms sometimes experience bodily changes during

their own lifetimes• BUT: These changes CANNOT be inherited by their offspring

(because DNA and genes were not changed)

Delano: Same as Cooper, but once an omicron-tolerant line of plants has been developed, collect their seeds and plant in the greenhouse. This will be a much

faster way to populate the greenhouse.

Bloom County

by Berke Breathed

• True: She has a good point that genetic alterations will not be helpful – nor inherited by offspring – unless they occur BEFORE development (though even the seed stage is usually too late)• BUT: (a) Omicron radiation doesn’t

mutate DNA, and (b) even if it did, the mutations would be utterly RANDOM, unlikely to provide radiation resistance. Omicron radiation does not directly cause omicron tolerance.

Eriksson: Expose taro seeds to low but increasing doses of omicron radiation. This will fundamentally alter them prior to development. Check growth rates, and once

an optimal dosage is found, plant the now omicron-resistant seeds in the greenhouse.

• Attractive idea …but incorrect• Flaw: She thinks it is the environmental conditions – namely

omicron radiation – that alter DNA/genes, and only in a helpful, adaptive way. This is yet another common misunderstanding.The Far Side by Gary Larson

Mutations are RANDOM.A few are lucky, most

aren’t.

• True: Many populations become better adapted to changing environments• BUT: Many populations also go

extinct; there are no guarantees. Adaptation requires VARIATION plus SELECTION: Some varieties survive and reproduce, others don’t. Evolution is not a change in the WHOLE population; rather, it’s a change in a population’s MEMBERSHIP.

Alvarez: Keep the crops under ideal growing conditions in the greenhouse, but otherwise let the species adapt naturally to the new radiation on its own. It will

slowly improve itself, eventually becoming omicron-tolerant.

• Tempting …perhaps the most common misunderstanding of evolution• Flaw: He believes that species automatically progress and

improve as needed, with offspring always a bit better adapted than their own parents

Extinction – not progress – is the fate of most

species

The Far Sideby Gary Larson

• True: Plants that happen to have some natural omicron tolerance – even a little bit – should be allowed to pass that trait on, while plants that seem especially vulnerable to omicron should be “weeded out,” thereby removing their DNA/genes from the population• BUT: It still might not work; there are no

guarantees. There may not be enough variety to select from …or lucky mutations might never arise …or there may simply not be enough time (too few generations). Sometimes natural and artificial selection can yield quick results, but other times it is very slow and sluggish.

O’Brien: Raise plants in the greenhouse, but monitor their growth. If any grow faster than others, re-plant their seeds or stem cuttings. Remove slower-growing

plants to make room. Repeat until plants are omicron-tolerant enough to sustain a mining colony.

• The BEST strategy. She understands that to evolve an omicron-tolerant population, the crew will have to mimic natural selection.

The Far Sideby Gary Larson

Three Pitfalls to Avoid1) Evolution as effort-based or behavior-based:

“Lamarckism”Jean Lamarck (1809) – a different model of evolution (now rejected):• Individuals can change their own bodies through use (or

disuse) of body parts …thereby acquiring new traits during their lifetimes

• Their offspring would then inherit these acquired traits

NO! Even if an organism changes its body, its offspring will NOT inherit those changes. Organisms cannot alter

their genes or DNA through behavior!

Classic Lamarckian example: Lamarck suggested that early

giraffes acquired longer necks by stretching for distant leaves. Their offspring inherited the additional length. They then stretched even more, and so

on…

(Pitfalls cont’d)2) Evolution as directional: Lamarck’s “ladder” of life

Lamarck also thought life had a natural, “progressive” tendency to evolve from simple to complex, driven by unseen alchemical forces

NO! Natural selection has no “foresight” into the future. It works only on genetic variations in the here-and-now, and

direction depends only on immediate consequences. In fact, simpler forms are often more adaptive (e.g., bacteria still rule

the Earth).

3) Evolution as need-based or environment-induced (not Lamarck)

Organisms can sometimes “adapt” as needed to new circumstances through bodily reactions, behavioral

responses, and even learning, but this is NOT the same as true genetic, evolutionary adaptation. Remember, evolution

occurs not WITHIN individual lifetimes, but BETWEEN and ACROSS generations.

Calvin & Hobbesby Bill

Watterson

Naughty words to avoid(They’ll lead you into those 3 pitfalls)

“Need”“Had To”

“Try”“Want”“Learn”

How Darwin would explain giraffe necks:• The ancestors of

modern giraffes had short necks, but not equally short. Some happened to be born with slightly longer necks than others.

Calvin & Hobbesby Bill

Watterson

• In times of drought or famine, those with slightly longer necks could reach higher leaves, while those with shorter necks could not. Longer-necked giraffes thus outcompeted shorter-necked giraffes for food.

• Giraffes with slightly longer necks survived longer and so left more offspring behind. Those offspring inherited the genes for longer necks.

• Meanwhile, shorter-necked giraffes died young and did not reproduce, so genes for shorter necks died out with them.

• Once in a while an offspring of two longer-necked giraffes would develop an even longer neck than either parent (perhaps due to a lucky mutation). This gave that giraffe a competitive edge over its neighbors, etc., etc. After many, many generations of weeding out shorter necks from longer ones, the exceptionally long neck of the modern day giraffe evolved.

Practice

Modern day flying fish have broad, wing-like pectoral fins that enable them to glide just above the water’s

surface. Like many small fish, when a predator strikes from below, a flying fish leaps out of the water. Unlike most fish, however, it then spreads its fins (or “wings”) and glides as far as 100 yards, escaping its attacker!

Suppose modern flying fish evolved from ancestors with “normal” pectoral fins. Explain how a flying fish’s wings might have gradually evolved through natural selection.

C Q Jessens