Evolution Part 2

By the end of this class you should understand:

• How genetic variation leads to natural selection and adaptive radiation

• The different effects that environmental factors can have on populations

• The nature of sexual selection and sexual competition

• How genes can flow from one population to another and how speciation occurs

Darwin’s Finches

• The famous example of natural selection that led Darwin to make sense of the diversity of life was the finches on the Galapagos islands– They were clearly very similar

but had different beaks on different islands

– The beaks allowed them to eat different things

Struggle for Existence• In an ordinary population, there is

a “struggle for existence” (Darwin’s own words) that keeps populations at about the same size from year to year despite rapid reproduction– Darwin calculated that two breeding

elephants with no death except from old age would sire a population of 15 million in 500 years

• When the first finches arrived at this island, they found no predators and no competition and so they didn’t have the same struggle

Genetic Variation• Genetic variation is normal in

all populations thanks to mutations that make recessive and codominant alleles– This includes for important

things like beak shape• Normally, the only food the

bird has a chance at getting is the food that the bird’s beak is already specialized to get– There’s a fierce competition for

all the foods!

Brave New World• When a population arrives at a new, untapped area, the

population will grow quickly, which means many unusual mutations that normally get weeded out quickly will instead become part of the population– Slightly thicker and slightly more curved beaks

• These finches will happen to find other foods are easier to get with their unusual beak!– All random chance!

Adaptive Radiation

• Anytime a population suddenly finds a lack of competition, the variations that are normally weeded out expand into new species

• This is known as adaptive radiation– Very different from

regular radiation• Can you think of another

example of this?...

Permian Extinction!• The end of the Permian era saw a

mass die-off of most large animals and almost all marine life– Lystrosaurus went extinct ->

• In the late Permian and early Triassic, dinosaurs evolved, at first represented by only a few species– Coelophysis ->

• By the Jurassic era, dinosaurs had undergone adaptive radiation to a multitude of species!– Behold! ->

Adaptive Radiation in Action

Danger and Opportunity

• It seems that the most species changes happen when the environment changes the most

• When the environment is stable, the most successful organisms resemble their successful parents, and the species becomes more specialized

• Catastrophes are also opportunities for new species to emerge and dominate– Mammals did this too!

Quantifying Opportunity

• A species cannot grow a new feature simply by trying, the mutations must exist in the gene pool already

• Many mutations are lethal mutations or neutral mutations but occasionally one appears that makes a different phenotype– Whether it’s beneficial or not

depends on the environment!

“Beneficial” Mutation

• One organism’s beneficial mutation is another organism’s harmful mutation– It all depends on the environment!

• In the 1800s, when the Industrial Revolution hit England, a lot of the trees with light-colored lichen became soot-covered and the lichen died– The light-colored Peppered Moth

that camouflaged on the tree had a new problem

Brain Check!

• What environmental pressures were acting on this moth?

• Which of these pressures might be altered by having the trees change color?...

The Birds!• Birds are monstrous creatures

to insects, and they hunt primarily by sight

• Being visible to birds is a serious threat for these moths

• Thanks to genetic variation, there was already a rare recessive allele that caused the moth to be dark instead of light– Guess who suddenly had the

advantage!

Change in Allele Frequency

Gene Pool Changes• When organisms with one allele become more common

than those with another allele, this is called a change in allele frequency

• If these changes are in response to the environment (which they usually are) it is referred to as microevolution

• If the changes are random, it is referred to as genetic drift

More on the Peppered Moth

• The light-dark moth saga is an example of directional selection, when one attribute (being lighter or darker) is strictly better than the other in a given environment

• Once pollution controls went into effect, the light moths became more prevalent again

• Evolution happens at the speed of reproduction, so bacteria evolve super fast, insects evolve fast, and mammals evolve slowly

Directional Selection• Directional Selection is what

happens when one particular environmental factor changes that benefits one part of the population more than another– Camouflage changes and

one particular predator is a serious threat

• In reality, there are usually many threats with many possible solutions– Not all predators hunt by

sight!

Disruptive Selection

• Another type of selection is disruptive selection, when having more or less of a particular trait is favored– One example is the African

Seedcracker, where large or small bills are helpful when food is scarce

• Currently these two phenotypes interbreed freely

Speciation

• The African Seedcrackers are liable to ultimately undergo speciation (one species becoming two)– Why? The large-bills that mate with the small-bills will

mostly have intermediate-bill babies that will die off when food is scarce

• If any large-bill birds develop a genetic preference for mating with other large-bills, their offspring will have two traits:– Large bills– A preference for mating with other large-bills

Sexual Selection• Animals do not typically mate

randomly but instead choose their mates intentionally– This choice can be driven by

“preference” which can be genetic in nature

• This means if some trait will result in more viable offspring, ultimately more animals will prefer to mate with animals with this trait– This is because they actively chose to

mate with animals that had this trait– This is known as sexual selection

Brain Check!

• What are things that are considered “attractive” in humans?

• Why might we humans be undergoing sexual selection for these traits?

Additional Sexual Selection• Peacocks are well-known to

have massive tails which are a hindrance to surviving in the wild– The ladies LOVE a survivor– Also requires good health to

maintain all those feathers symmetrically

• This is an example of sexual dimorphism– Many animals exhibit traits

found only in one gender, almost always sexual selection

Sexual Dimorphism!

Fun Fact!• The larger owl is in fact the female!• Larger males seem to be selected

for with larger harems• Birds of prey are one of the few

species that are naturally monogamous, and the females are larger in most of these cases– No, humans are NOT naturally

monogamous• Chimpanzees have a similar gender size

ratio and the dominant male mates with all the ladies in the group– No I am NOT encouraging you to cheat on

your significant other

Sexual Priorities in Mammals

• Consider that males can pass on their genes to as many offspring as they can father– Male goal: mate as much as possible

• Females, on the other hand, grow the next generation inside their bodies and also literally provide food for the babies out of their own body– Female goal: mate with a successful and healthy male– Additional goal: mate with a male who will actually

stick around and help

This explains a lot of cultural problems:

Bottlenecks and Founder Effect• Additional force in

evolution is a bottleneck– A bottleneck is when a

population loses many individuals in a catastrophe of some kind and alleles are lost in the process

• Another version is the founder effect: only a few organisms make it to a new island– The resulting population

only has as much diversity as those founders

Gene Flow• In large populations, genetic drift (random variation

of allele frequency) and bottlenecks are not really a problem

• In smaller populations they can be an issue, but often organisms travel from one area to another and mate in the new region– Referred to as gene flow– This maintains genetic diversity

Lack of Gene Flow

• If two populations become isolated (no more gene flow), the allele frequencies will begin to diverge from each other– Especially if the environments are different!

• This ultimately will result again in speciation– In particular this is known as allopatric speciation,

where the organisms are physically separated– The previous example of African Seedcrackers

would be a case of sympatric speciation, where the organisms are still in close proximity

See you in lab!