L06: Evolution and Speciation

SQ: What is evolution? What is a heritable trait?

When the traits of a population(all organisms) changes over generations

So traits passed from all parent—>all offspring

What is a heritable trait?

Heritable trait: Physical, physiological, developmental, or behavioral traits that are controlled and passed down from parent to offspring

SQ: Why must a trait be heritable for evolution to occur?

Because natural selection and genetic change require physical or behavioral characteristics to be passed down from parents to offspring across generations if you want to keep the trait alive

So if you have a trait and it cannot be transmitted via genes or hereditary mechanisms, it dies with the individual and cannot increase in frequency, meaning evolution didn’t happen.

Who discovered the principles of inheritance?

Gregor Mendel

SQ: Can individuals evolve? Meaning only one by one. What about natural selection?

NOOOO! WHOLE POPULATION EVOLVES FOR EVOLUTION TO BE VISIBLE; which individuals get to pass on their traits to the next generation leads to the evolution of the whole population,

Natural selection acts on individuals (deciding who survives and reproduces), but evolution is only visible in the population over time.

SQ: What is selection pressure? What does the selection pressure leads to in terms of traits for the next population?

Defintion: Conditions that causes individuals with certain traits to be more successful at reproduction(b/c other ones died off), survival of the fittest

Ex: predation, food type, or ability to attract males

Leads to: Because individuals with those better traits reproduced more, the next generation has MORE OF THOSE traits(bc survived to pass it on to the next generation).

The population evolves as more and more individuals possess those traits(bc more org w/ trait survive—>more reproduction)

What are the different types of ways evolution can occur? Is it just natural selection?

1. Natural selection

2. Sexual (mate) Selection

3. Genetic Drift

IT IS NOT just natural selection!

What is natural selection?

Evolution by selection

Def: Individuals that survives better or longer because of their trait that is better suited for their environment will have more opportunities to reproduce and therefore leave more offspring. In other words, the environment chooses which organisms get to live and reproduce based on their traits.

Survive longer—> reproduce more —> more offspring

Assumption is that mating is random

Proposed by Charles Darwin and Alfred Russel Wallace

What is evolution be sexual mate selection(evolution by selection)?

Def: Individuals with particular traits are preferred mating partners and therefore leave more offspring.

• Possible for preferred trait to be detrimental to the individual’s survival, so doesn’t mean the preferred survival trait is the one picked, BUT THE TRAIT THAT GETS TO MATE MORE IS THE ONE PASSED ON.

So, this is not about surviving, but about if you mate more. So if you can mate more, can steer the population more in favor of your traits.

Ex: If there are yellow and green frogs but mating with green is preferred over yellow, there will be more green frogs in the next generation bc that is the only frog getting to mate and pass on their traits while the yellow frogs just die off (keeping their traits with them).

Does sexual mate selection always lead to good traits? Do traits have to be dimorphic(exaggerated in only one sex)-social signaling? Is sexual dimorphism evidence of evolution?

NOOOOOO, only traits that increase reproductive success, not survival

Traits might be detrimental to the individual’s survival

Traits do not have to be sexually dimorphic (exaggerated in only one sex).

Sexual dimorphism is actually evidence of evolution and of sexual selection as these traits evolved due to mating prefernces.

What are some examples of sexual (mate) selection?

Ex: Peacock tails, females prefer showy tails, so trait that gets passed on is males with pretty tail feathers as they can mate. Direct Choice

• Men with tail feathers spend lots of energy to upkeep, which can hinder survival(showing how the trait is detrimental but is only there for reproductive success), but appear more bc female likes it

• Sexual dimorphism

Ex 2: Irish Elk Indirect choice

• Males with larger antlers have fewer rivals for mates as males compete with each other and the winner gets to mate. So males got the trait of larger antlers as these are the ones who win the competition. Femaels don’t choose who they mate with, males do, making this indirect.

• Sexual dimorphism

Ex 3: Crested Auklet crests

• Larger crests are preferred by all sexes, so both Male and female of species prefer the emo feathers, so BOTH get exaggerated features

• NOT sexually dimorphic.

Ex 4: Speciation via mate selection in some dinos.

• Hypothesized for some dinosaurs like hardrosaurs to explain diversity of head ornamentation in different species

• Evolved via mate selection—>led to diversity.

• body similar=doing same thing, but skull diff=implies for obtaining mates, not survival

SQ: What happens by default in the absence of selection?

GENETIC DRIFT

What is genetic drift? What is the cause of genetic drift?

How evolution can occur without selection

Def: Evolution due to chance events (literally a trait that just shows up by chance); Frequency of a gene variant(trait/allele) in a population changes over time due to random chance rather than natural selection.

Cause: Happens bc of luck (which ind happens to survive and reproduce), NOT bc traits makes an org better adpated to env. (NO SELECTION BY NATURE OR MATES—>literally something just happens randomly, making the survivors reproduce and pass their traits on)

Random events like accidents(lightning strikes, NOT pred/prey as that is nature choosing) can prevent some individuals from mating successfully.

Ex: There are 10 rabbits, 7 brown and 3 white. A tree fell and killed all the white rabbits, so since the brown ones survived BY LUCK, they get reproduce and pass on their traits, which is seen in the next generations.

Can you give me the flow of genetic drift?

Random event—>surviving individuals reproduce more bc they randomly survived—> certain trait seen in population(BY CHANCE)

Surviving organisms get to reproduce and pass on their genes simply because they were fortunate enough to survive the random event, completely regardless of their traits.

Survival by Luck, Not Fitness(ABSOLUTELY NO SELECTION)

Can be good or bad bc there is no selection occurring.

SQ: What is the difference between Evolution by Natural selection and evolution by Sexual (Mate) Selection?

Natural Selection is about Survival: The environment (predators, climate, disease, food availability) filters out organisms. Traits evolve because they help the organism stay alive long enough to reproduce.

Sexual Selection is about Reproduction: Members of the same species (usually females choosing males, or males fighting other males) filter out individuals. Traits evolve because they help the organism secure a mate, even if those traits actually harm its chances of survival.

SQ: How does evolution occur by genetic drift?

Chance events lead to some individuals reproducing more, leading to evolutionary change as that trait is seen in the next generation.

Random chance—rather than a survival advantage—changes which genes are passed down to the next generation.

SQ: How is genetic drift used in paleobiology studies?

It’s used as the control or default explanation of evolutionary change.

We aren’t looking at what was changed. but asking how did this evolution happen?

To test, make simulation of random trait changing over many generation and see results (is the trait changing random or is it due to selection(mate or natural)?

Referred to Brownian Motion(use these equations as a baseline to analzye fossil lineages).

How do you interpret Brownian Motion graphs?

Hella Random (Jagged, aimless zig-zags): This is genetic drift (Brownian motion). The line bounces up, drops down, and wanders around with no clear destination. It shows the trait is changing purely by random luck.

• For speciation, If the two lines split, and one or both of them start doing a chaotic, hella random Brownian motion walk, it means genetic drift triggered the speciation.

A Clear, Steady Trend (Straight or smooth curve): This is natural selection. The graph shows a steady, directional march toward a specific trait—such as animals consistently getting larger over time because a bigger size helps them survive.

• If the two lines on the graph immediately shoot off in completely opposite, steady directions, it means natural selection is driving the split.

SQ: Does evolution have a direction or follows a plan? Does it have an end state or “final” evolution? Why or why not?

NO. evolution has neither direction nor follows a plan.

• There is NOO final form or final evolution because the environment and other circumstances will change over time, leading to the population to change and keep evolving.

• Different traits will lead to more offspring in different situations. (Ex: for camouflage, different colors are advantageous depending on the environment, so there is no plant as the env is always changing.)

• There are many ways to leave more offspring (camouflage, poison), so there is absolutely no plan, it’s just random certain situations the organism is facing.

SQ: Must a trait be the best to be advantageous? Why or why not?

No, a trait does not have to be the "best" to be advantageous; it only needs to be better than competing traits in a specific environment

• In his words, a trait does NOT have to be the best to have an advantage; it just has to be better than another trait

The environment favors traits that are simply "good enough" for an organism to survive and reproduce in its current habitat.

Ex: You do not need to be the fastest protohorse to survive predation from a RUI, you just have to be faster than the protohorse next to you!

SQ: What is speciation? Why is preventing interbreeding important?

Def: Single ancestral population splits into 2 or more genetically distinct, reproductively isolated species.

His def: When an interbreeding population within a species evolves into a new species, separate from the original species (1 species—>2 species)

Without interbreeding: The two groups are genetically cut off from each other. They evolve independently until they become completely different species.

What are the causes of speciation?

Occurs when populations of the original species evolve separately

Populations evolve separately when they stop interbreeding

Causes: Interruption of gene flow and reprod. isolation. Since separated, each group will accumulate diff traits over time, resluting in having two different species

What is interbreeding?

Mixing individuals from different populations, breeds, varieties, or species via mating, leading to a shared gene pool and NOT different species as they are sharing their genes and can share adaptations between them.

With interbreeding: Genes flow freely between groups, mixing their traits and keeping them as a single species.

Try it out: Why is stopping mating between the evolving population and the original species necessary for speciation?

By stopping mating, the population can evolve independently of the original species, as diff. traits will only show up for each separated.

So how are dogs the same species?

All dogs have the same dog genes

Different breeds have unique variants (alleles) to some of those genes

Gene pool is the collection of all genes and gene variants that is present in the species.

BUT, because they can share gene variants through interbreeding, the genes are are mixed and matched. SO a dog individual can have any of the traits in the gene pool, regardless of breed.

What are the barriers to interbreeding(5)? What does a lack of interbreeding lead to again?

1. Geographic barrier

2. Ecological barrier

3. Behavioral barrier

4. Mechanical barrier

5. Temporal barrier

Lack of interbreeding leads to SPECIATION bc there is a lack of gene sharing, so each group will accumulate different traits over time, resulting in two different species

What is a geographic barrier? Starts speciation

Physically separated the population from breeding

Ex: Formation of a river, ocean, mountain, etc. that isolates the population

What is an ecological barrier?Starts speciation

Original species and evolving population live in the same geographic place but use different resources, leading them to not see each other and breed.

Ex: if a bird uses the top half of a tree but another bird uses the bottom half, they won’t see each other, thus not breed

What is a behavioral barrier?Starts speciation

Mating behaviors and/or appearance are misinterpreted or not recognized

So organisms in the same population don’t recognize each other—> can’t mate

A mechanism that prevents different species or populations from interbreeding due to differences in their behavior

Ex: Mating Calls and Songs: Frogs, crickets, and songbirds use distinct vocalizations to signal their availability. If a population's song changes slightly due to a genetic mutation or environmental shift, members of the original population will no longer recognize the call, creating an immediate behavioral barrier.

Ex: Courtship Rituals: Many birds rely on specific "dances," feather displays, or nest-building techniques to attract a mate. For instance, a female Blue-footed Booby will only mate with a male that performs the exact, correct foot-stepping dance. A closely related bird with a different dance will be completely ignored as don’t recognize.

What is a mechanical barrier?Starts speciation

Incompatible genitalia (important for insects)

What is a temporal barrier?Starts speciation

1. Mating occurs at different times of the year or seasons

2. Mating is impossible because individuals do not live at the same time(anagenesis) so ancestors and descendants can’t mate

Because their reproductive schedules never overlap, mating between them is impossible.

SQ: What is the difference between allopatric and sympatric speciation?

Allopatric(physical barrier)- Populations become geographically isolated; something is physically blocking them(something is physically blocking them)

• Isolated populations adapt to local environment; Geographic barrier prevents interbreeding.

• Vicariance- allopatric speciation involving the formation of large geologic barriers like oceans and mountains

Sympatric(ecological barrier)- Reduced mating in members of the same population leads to new species WITHOUT geographic isolation; Some other barrier that prevents them from mating, although living in the same place.

• The evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region.

• Fossil geographic and temporal range MUST overlap.

What is allopatric speciation?

Physical barrier- Populations become geographically isolated; something is physically blocking them(something is physically blocking them)

Geographic barrier prevents interbreeding

Ex: bugs got split into 4 regions by a river. Isolated populations adapt to local environment. Different color traits provides better camouflage in each environment. Over time, isolated populations become its own separate species due to natural selection. All evolving on their own as they are all independent and isolated from each other—> so different species (and can’t mate)

What is sympatric speciation?

Ecological, temporal, or behavioral barrier prevents interbreeding: The evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region.

Barriers other than geographic

Sympatric speciation examples (2)

1. Due to natural variation, some bugs are reddish while others are greenish. Both types can breed with each other. But, reddish bugs are more camouflaged on red trees but poorly on green trees. Greenish bugs are more camouflaged on green trees but poorly on green trees.

2. Red bugs thrive on red trees; diminish on green trees.

3. Red bugs live and reproduce predominantly around red trees (to survive) as in die if in green tree. Green bugs live and reproduce predominantly around green trees.

4. Over time, red bugs mate more and more with red bugs and green bugs mate more and more with other green bugs

5. Then, red bugs evolve-now mate in July when red trees fruit. Green bugs evolve-now mate in September when green trees fruit

6. Red bugs and green bugs have changed enough that they can no longer interbreed(as the times don’t match)

7. So over time, they change even more from each other—> leading to 2 different species. Red bugs become new species distinct from green bugs

Another example is sympatric speciation due to ecological barrier with darwin’s finches (same pop, sam place, but diff members specialize in diff things—>diff species).

SQ: What is vicariance? Is this allopatric or sympatric?

Allopatric speciation involves the formation of large geologic barriers like oceans and mountains/So allopatric speciation from large scale geographic isolation (huge time scales—>like ocean forming so huge, worldwide event)

Vicariance refers strictly to the geological or environmental action that physically splits a population. It is an earth science process, not a biological one. But this could also be like larger changes while allopatric is smaller.

Vicariance is the event that starts the process, while allopatric speciation is the final biological result.

Ex: Separation of continents and formation of new mountain ranges. Like how evolution of many different dinosaur species after Pangea broke apart and separated into continents.

What is parapatric speciation?

Interbreeding is reduced between populations on the far edges of the range; speciation due to environmental gradient

Cline or ring species (1 species before speciation) populations become completely species (speciation)

When a new species evolves within a population that is spread across a massive geographic area or a sharp environmental gradient.

SQ: What is the difference between anagenesis and cladogenesis?

Anagenesis(Left)- Lineage slowly evolves from one species into another (1 species—>1 species that replaced first species)

• So an ancestral species transforms into a new species without splitting the population

• Linear Evolution: The entire population changes together as a single interbreeding group

• No Branching

• Original ancestor differs from current a lot

Cladogenesis(Right)- Lineage splits giving rise to two new lineages (1 species —> 2 species)

• A single ancestral lineage splits into two or more genetically distinct species (such as through geographic isolation), increasing total biodiversity

• Yes Branching

Does cline species lead to parapatric speciation?

Yes

First, a single cline species spreads across a continuous geographic area where different subpopulations gradually develop distinct traits due to varying environmental pressures, yet they retain the ability to interbreed with their immediate neighbors.

Parapatric speciation is the evolutionary outcome that occurs when these environmental differences become so intense that the biological link between the far ends breaks. Even though the organisms still physically touch, natural selection favors the development of reproductive barriers—such as different mating seasons or behaviors—which completely blocks gene flow and splits the single clinal species into two distinct, separate species.

• Cline species is a single species, and once it starts to change, it becomes more than one species

What is a cline? What is a cline species?

Cline- Large geographic region with an environmental gradient

Cline species- gene flow restricted to adjacent populations (uneven), so can’t breed with further species, only ones next to them, no physical separation; single species spread across a continuous geographic area where different subpopulations gradually develop distinct traits due to varying environmental pressures, yet they retain the ability to interbreed with their immediate neighbors.

Even if cline species from different ends meet, why won’t they mate/interbreed?

If you look at the image, 1 and 4 NOT gonna mate bc too far and living in different environment (so different temporal mating times).

So since mating times are very different due to the different environments, the ones next to the pop hav overlapping mating times(so they can interbreed), so even if brought together, mating is unlikely

What are linking zones? Linking populations? What happens when there is no more link?

Linking zones: the intersection between populations directly next to each other along a continuous geographic area. Can mate and share genetic information. Ex: Green and blue can mate

Linking populations: Subpopulations that stretch across the geographic space between the two extreme ends; they are many distinct subpopulations, but they form a single, continuous intersection chain between the two adapting extremes.(to keep it one species)

Ex: Red, grey, and blue are linking populations, as they can share the green DNA with the grey mouse if a mouse in the blue region has green-region mouse DNA.

NO link= speciation, Link there=one species

What are ring species?

Cline species in which fringe populations(outer edge species) overlap, this expands all the way around the planet and circles in on itself. If break links between species—>speciation.

Fringe populations are too different to reproduce successfully; so even if they meet again, they won’t mate.

A connected series of neighboring animal or plant populations. Each population can interbreed with the populations right next to it, but the two "end" populations that meet in a circle can no longer interbreed and act as separate species.

• Circular distribution: The populations spread out in a ring around a central geographical barrier that they cannot easily cross (such as a high mountain range, a large valley, or a plateau).

• Step-by-step change: As you travel around the ring, each group looks and behaves just a little bit different from the group next to it.

• The break at the ends: By the time the two expanding branches of the ring meet back up on the other side, the genetic and physical differences have grown so large that they cannot produce offspring together—demonstrating evolution and speciation in real time.