BIOL 101: Lecture #5 Flashcards

Macroevolution – Evolution of Phenotypes

Important Focus Points

• Definitions of a species: These help us understand what makes a group of organisms distinct. Think of it like defining what makes a cat a cat, versus a dog.

  • Biological: reproductive ability. Can they breed with each other and produce fertile offspring? For example, lions and tigers can breed in captivity, but their offspring (ligers or tigons) are usually sterile, so they're considered separate species.

  • Morphological: phenotypic relationships. Do they look alike when you compare their physical traits? Different breeds of dogs, like Golden Retrievers and Bulldogs, look different but are the same species because they can interbreed.

  • Ecological: correlation to niches. Do they have similar roles and needs in their environment? Different species of woodpeckers might live in the same forest but eat different insects or nest in different types of trees, showing distinct ecological niches.

  • Phylogenetic: with relation to one another. How closely related are they in terms of their evolutionary history? A phylogenetic tree might show that wolves are more closely related to coyotes than to foxes.

  • Different branches of biology use different definitions based on available information or fundamental differences in the organism's biology. For example, paleontologists studying fossils can’t use the biological definition because they can't observe reproductive behavior.

• Speciation: process by which one species splits into two or more species. Imagine one group of birds getting divided; over time, the two groups become so different they can no longer interbreed.

  • Allopatric Speciation: geographic separation. A physical barrier like a mountain range divides a population. For example, the formation of the Grand Canyon separated populations of squirrels; over time, these became different species.

  • Sympatric Speciation: without geographic separation. A new species arises within the same area as the parent species. This is less common but can happen through things like genetic mutations or changes in behavior. An example is apple maggot flies in North America, which evolved to lay eggs on different host plants (apples vs. hawthorns), leading to reproductive isolation.

• Reproductive isolation and barriers: Mechanisms that prevent different species from interbreeding. It’s like having a lock and key – if the keys don’t match, they can’t reproduce.

  • Prevents gene flow. Keeps species distinct.

  • Maintains separate species.

  • Prezygotic: act before zygote formation. These barriers prevent mating or fertilization from occurring.

  • Postzygotic: act after zygote formation. These barriers result in infertile or unviable offspring.

• Adaptive radiation: Definition: Many diverse species evolve from a common ancestor. Think of it like a single type of seed that sprouts into many different kinds of plants in different environments.

  • Occurs: when a few organisms colonize new unexploited areas; after a mass extinction. After the dinosaur extinction, mammals underwent adaptive radiation to fill the empty niches.

Evolutionary Tree

• Darwin viewed the history of life as a tree with branches from a common ancestral trunk to descendant species. Imagine a tree where the trunk is the common ancestor and each branch is a different species.

• Biologists represent patterns of descent with an evolutionary tree. These trees show how different species are related based on shared characteristics.

• Each branch point represents the common ancestor of lineages. It’s like a family tree where each fork shows where two groups diverged from a common ancestor.

• A hatch mark represents a homologous character shared by all groups to the right of the mark. These are traits inherited from a common ancestor. For example, the presence of a backbone is a homologous character shared by all vertebrates.

What is a Species?

• Several working definitions exist. Different situations call for different definitions.

  • Biological: reproductive ability. Can they interbreed and produce fertile offspring? Horses and donkeys can breed to produce mules, but mules are sterile, so horses and donkeys are separate species.

  • Morphological: phenotypic relationships. Do they look alike when you compare their physical traits? Different breeds of dogs, like Chihuahuas and Great Danes, look very different but are still the same species because they can interbreed.

  • Ecological: correlation to niches. Do they have similar roles and needs in their environment? Two species of birds might live in the same forest but eat different foods, indicating different ecological niches.

  • Phylogenetic: with relation to one another. How closely related are they in terms of their evolutionary history? Using DNA evidence, scientists can determine how closely related different species are.

Species Definition - Historical

• Historically, species were often grouped/defined based solely on appearance (phenotype). This was before we understood genetics and DNA.

Phenotype vs. Genotype

• Phenotype: observable characteristic or trait in an organism. What you can see (e.g., eye color, height).

• Genotype: genetics underlying the phenotype; the "coding" of the characteristic or trait. The actual genetic makeup (e.g., the specific genes for eye color).

• Example:

  • PP: Dominant Homozygous - This means the individual has two copies of the dominant allele.

  • Pp: Heterozygous - This means the individual has one dominant and one recessive allele.

  • pp: Recessive Homozygous - This means the individual has two copies of the recessive allele.

Speciation

• Speciation = process by which one species splits into two or more species. Like a tree branching out.

• Speciation increases the diversity of life. More branches on the tree mean more types of living things.

• Over 3.5 billion years, an ancestral species gave rise to multiple different species, which branched to new lineages. This long history has led to the vast diversity we see today.

Reproductive Isolation & Barriers

• Prevents gene flow (interbreeding). Keeps separate species from mixing.

• Isolates the gene pools of species. Each species has its own unique set of genes.

• Maintains separate species. Ensures that different species stay distinct.

• Members of the species are distinct because they do not share the same gene pool. They have different sets of genes that define them.

• Reproductive barriers are categorized as prezygotic or postzygotic based on their function before or after zygotes form. Whether they act before or after the formation of a fertilized egg.

Prezygotic Barriers

• Habitat isolation: different habitats. Two species might live in the same region but not interact because they occupy different habitats. For example, one snake lives in the water, while another lives on land.

• Temporal isolation: breeding at different times. Two species might breed during different times of day or year. For example, one type of skunk breeds in winter, while another breeds in summer.

• Behavioral isolation: different courtship rituals. Two species might have different mating dances or songs. For example, different species of birds have distinct calls.

• Mechanical isolation: incompatible reproductive parts. Physical differences prevent mating. For example, different species of snails might have shells that don't align properly.

• Gametic isolation: incompatible gametes. The eggs and sperm of different species can't fuse. For example, the surface proteins on eggs and sperm might not match.

Postzygotic Barriers

• Reduced hybrid vitality: short-lived hybrids. The offspring (hybrids) don't survive well. For example, certain species of frogs can hybridize, but the tadpoles don't develop fully.

• Reduced hybrid fertility: sterile hybrids. The offspring (hybrids) can survive but can't reproduce. A classic example is a mule, which is the offspring of a horse and a donkey but is infertile.

• Hybrid breakdown: fertile hybrids with sterile offspring. First-generation hybrids are fertile, but later generations are infertile. This is seen in some plant species.

Hybrids

• Some distinct species occasionally interbreed, resulting in offspring called hybrids (e.g., horse and donkey producing a mule). Sometimes, different species can mate and produce offspring, but these offspring often have problems.

• The biological species concept is problematic for:

  • Fossils: no way to determine interbreeding ability. We can’t know if extinct species could interbreed.

  • Asexual organisms: reproductive isolation doesn't apply to prokaryotes or other organisms that reproduce only asexually. These species reproduce without mating, so the idea of reproductive isolation doesn't make sense.

Speciation Events

• A key event is the separation of a population from other populations of the same species. This can start the process of forming a new species.

• With its gene pool isolated, the splinter population can follow its own evolutionary course. Without interbreeding, the separated group can change independently.

• Changes in allele frequencies caused by natural selection, genetic drift, and mutation will not be diluted by alleles entering from other populations (gene flow). These forces can drive the separated population to become different more quickly.

Allopatric Speciation

• Populations are separated by a geographical barrier (e.g., mountain ranges, continental divide). A physical barrier divides a population into two or more groups.

• Reproductive isolation follows. The separated groups no longer interbreed.

• Initially isolated populations do not share changes in allele frequencies caused by evolutionary processes (natural selection, genetic drift, and mutation). Each group changes independently due to different environmental pressures and random events.

• The size of the geographic barrier needed depends on the ability of organisms to move. A small stream might isolate insects but not birds.

• Likelihood increases when a population is small and isolated; a small population may have a different gene pool due to the founder effect. Small, isolated groups are more likely to change rapidly.

Reproductive Barriers Evolve as Populations Diverge

• The environment of an isolated population may include different food sources, pollinators, or predators. Different conditions can lead to different adaptations.

• Natural selection, genetic drift, or mutation may change a population's traits in ways that establish reproductive barriers. Over time, the groups become so different that they can no longer interbreed.

• Most species are thought to have originated by allopatric speciation. A physical barrier is often the first step in forming new species.

• Isolated island chains offer some of the best evidence, with multiple speciation events likely to occur due to diverse habitats and varying distances between islands. Islands provide many opportunities for isolated populations to evolve.

The Galapagos Archipelago

• Located about 900 km (560 miles) west of Ecuador.

• A showcase of diversity with unique species found nowhere else. Famous for its unique species that Darwin studied.

• Formed from underwater volcanoes from 5 million to 1 million years ago.

• Colonized gradually from other islands and the South American mainland. Species arrived and then evolved in isolation.

Sympatric Speciation

• Populations share the same geographic area. No physical barrier separates the groups.

• Some intrinsic barrier separates them. Something within the population prevents interbreeding.

• Gene pool effects, phenotypic traits (bird calls, mating displays), etc. Differences in genes or behaviors can lead to separation.

• Some individuals are more likely to breed than others. Certain traits become more attractive, leading to reproductive isolation.

• Populations become reproductively isolated. The groups can no longer interbreed.

• Gene flow between populations may be reduced by polyploidy, habitat differentiation, or sexual selection. These mechanisms can drive sympatric speciation.

Sympatric Speciation without Geographic Isolation

• Many species have originated from sympatric speciation when accidents during cell division result in extra sets of chromosomes.

• New species formed are polyploid, with more than two complete sets of chromosomes. Polyploidy can lead to instant reproductive isolation.

• Sympatric speciation can result from polyploidy within a species (by self-fertilization) or between two species (by hybridization). Polyploidy can occur in different ways.

Timing of Speciation

• Speciation occurs faster with new niches. New opportunities accelerate the process.

• Niche: an exploitable zone or role that can be occupied by an organism in a specific habitat. Think of it like a job in the ecosystem.

• Specialist species evolve quickly to take over new niches. Species that can exploit a specific resource tend to evolve rapidly.

• Generalist species occupy diverse niches and evolve more slowly. Species that can use many different resources tend to evolve more slowly.

Varying Rates of Speciation

• Gradual Speciation: Species diverge gradually over time in small steps. Change happens slowly and steadily.

• Punctuated equilibrium: Begins with a punctuated or periodic change and then remains in balance afterwards. Change happens in bursts followed by long periods of stability.

Adaptive Radiation

• Definition: Many diverse species evolve from a common ancestor. One species diversifies into many different forms.

• Occurs: when a few organisms colonize new unexploited areas, after a mass extinction. Opportunities arise when new habitats become available or after major die-offs.

• Linked to new opportunities: lack of competitors, varying habitats and food sources, evolution of new structures. These factors drive the diversification process.

Galapagos Finches

• The Galápagos Islands have 14 species of closely related finches (Darwin’s finches). A famous example of adaptive radiation.

• These birds share finch-like traits but differ in their feeding habits and beaks, specialized for what they eat, arising through adaptive radiation. Each finch has evolved a beak suited to its particular food source.

Adaptive Radiation - Finches

• Adaptive Radiation – Many diverse species evolve from a common ancestor

Lake Victoria - Speciation

• Lake Victoria is a living laboratory for studying speciation. A great place to observe evolution in action.

• Cichlids are a family of fishes that live in tropical lakes and rivers.

• Renowned for the spectacular adaptive radiations that stocked the large lakes of East Africa. These fish have diversified into many different forms in a relatively short time.

• Sexual selection is a form of natural selection in which individuals with certain traits are more likely to obtain mates. Mate choice drives the evolution of certain traits.

• Divergence occurs as a result of differences in how males attract females or how females choose mates. This can lead to reproductive isolation and speciation.

Hybrid Zones

• Hybrid zones provide opportunities to study reproductive isolation. Areas where different species can interbreed.

• Regions in which members of different species meet and mate to produce at least some hybrid offspring.

• Reinforcement may strengthen barriers to reproduction. Natural selection favors traits that prevent hybridization.

• Fusion may reverse the speciation process as gene flow between species increases. If hybrids are fit and can interbreed, the two species may merge back into one.

• Stable hybrid zones: a limited number of hybrid offspring continue to be produced. Hybrids continue to be formed, but the two species remain distinct.

The Take Homes

• Explain why there are so many different ways to define a species. Different definitions are useful in different situations.

• Distinguish between Allopatric Speciation and Sympatric Speciation. One involves geographic separation, the other doesn't.

• Explain why reproductive isolation and barriers are so critical to speciation events (or lack there of). These prevent gene flow and allow species to diverge.

• Explain adaptive radiation and its role in speciation. Adaptive radiation leads to the diversification of species from a common ancestor.