Evolution (Descent with Modification)

What is Evolution and Where Did the Idea Come From?

• The term “descent with modification” was a phrase Charles Darwin coined to summarize the process by which species accumulate differences from their ancestors as they adapt from different environments over time.

  • It can also be defined as a change in the genetic composition of a population from generation to generation.

• Evolution can be viewed in 2 ways: as a pattern and as a process.

  • As a pattern, evolution has been shown with data as facts and is derived from the observations about the natural world and scientific disciplines like biology, geology, physics, and chemistry.

    • The patterns of evolution are bounded by what we know in sciences.

      

  • The process of evolution consists of the natural causes of the natural phenomena we observe.

• Long before Darwin, Aristotle (384-322 BCE) viewed species as fixed (unchanging).

  • He thought that each form of life is perfect and permanent.

• The ideas on the origin of life were generally consistent and based off of the Old Testament account of creation.

  • Many 18th century scientists found it remarkable that organisms are well suited for life in their environment as evidence that the Creator has designed each species for a specific purpose.

• Carolus Linnaeus (1707-1778), was a Swedish physician/botanists that saw life’s diversity as “for the greater glory of God.”

• Darwin’s idea of evolution took inspiration from the study of fossils, called paleontology.

  • Fossils in a specific layer of rock (strata) could provide a glimpse of some of the organisms that lived on Earth at the time that layer formed.

    

• French biologist Jean-Baptiste de Lamarck (1744-1829) proposed a mechanism for how life changes over time.

  • His first principle was use and disuse, the idea that parts of the body that are used extensively become larger and stronger, while those that are not used deteriorate.

    • His example was a giraffe stretching its neck to reach leaves on high branches.

  • The second principle was inheritance of acquired characteristics, saying that an organism can pass these modifications to its offspring.

    • Lamarck reasoned that many generations of giraffes stretched their necks even higher.

    • However, this idea is rejected by modern scientists, as bonsai trees can be “trained” to grow through pruning and shaping, but its offspring would be of normal size.

      

• Lamarck also thought that evolution happened b/c organisms have an innate drive to become more complex.

  • Darwin rejected this idea.

• On Darwin’s voyage from England on the Beagle, he landed on the Galapagos near South America.

  

• He collected different kinds of mockingbirds, as saw that although they were similar to each other, each seemed to be a different species.

  • Some were unique to individual islands, while others lived on two or more adjacent islands.

  • Darwin hypothesized that the Galapagos had been colonized by organisms that had strayed from South America and then diversified, giving rise to new species on the various islands.

• At the islands, he observed examples of adaptations, inherited characteristics of organisms that enhance their survival and reproduction in specific environments.

• He also observed finches.

  • It was noted that the finches’ various beaks and behaviors are adapted to the specific foods available on their home islands.

    

  • He realized that explaining such adaptations was essential to understanding evolution.

  • His explanation of how adaptations arise centered on natural selection, a process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than do other individuals because of those traits.

• When he proposed the idea of natural selection, he was met with a lot of skepticism.

  • To draw similarities and prove his point, he pointed how humans have modified other species over many generations by selecting and breeding individuals that possess desired traits, a process called artificial selection.

    

• He had two observations on which he drew two inferences on.

• • Observation #1: Members of a population often vary in their inherited traits.

    

  • Observation #2: All species can produce more offspring than their environment can support (overpopulation) and many of these offspring fail to survive and reproduce.

  • Inference #1: Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than do other individuals.

  • Inference #2: This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations.

    

• As these inferences suggest, Darwin saw an important connection between natural selection and the capacity of organisms to “overreproduce”.

• He saw that this capacity to overreproduce exists in all species.

  • Of the many eggs laid, young born, and seeds spread, only a tiny fraction complete their development and leave offspring of their own.

  • The rest are eaten, starved, diseased, unmated, or unable to tolerate physical conditions of the environment.

    

• An organism’s heritable traits can influence not only its own performance, but also how well its offspring cope with environmental challenges.

  • An organism might have a trait that gives its offspring an advantage in escaping predators, obtaining food, or toleration physical conditions.

    

• When such advantages increase the number of offspring that survive and reproduce, the traits that are favored will likely appear at a greater frequency in the next generation.

• Thus, over time, natural selection resulting from factors such as predators, lack of food, or adverse physical conditions can lead to an increase in the proportion of favorable traits in a population.

  

Key Points:

• Natural selection is a process in which individuals that have certain heritable traits survive and reproduce at a higher rate than do other individuals because of those traits.

• Over time, natural selection can increase the frequency of adaptations that are favorable in a given environment.

• If an environment changes, or if individuals move to a new environment, natural selection may result in adaptation to those new conditions, sometimes giving rise to new species.

• Although natural selection occurs through interactions between individual organisms and their environment, individuals do not evolve.

  • It is the population that evolves over time.

    

• Natural selection can amplify or diminish only those heritable traits that differ among the individuals in a population.

  • Even if a trait is heritable, if all the individuals in a population are genetically identical for that trait, evolution by natural selection cannot occur.

• Environmental factors vary from place to place and over time.

  • A trait that is favorable in one place or time might be useless/detrimental in other places or times.

  • Natural selection is always operating, but which traits are favored depends on the context in which a species lives and mates.

How has the Idea of Evolution Changed Today?

• In the last 150 years, new discoveries have filled many of the gaps that Darwin identified.

• There are 4 types of data that document the pattern of evolution and illuminate how it occurs: direct observations, homology, the fossil record, and biogeography.

• Direct observation is observing evolution that happens in real time, like bacteria quickly evolving resistances to antibiotics.

  

• Homology is similarities in structure, genes, or development between different species that share a common ancestor.

  • Homologous structures are structures in different species that are similar b/c of common ancestry

  • Vestigial structures are structures that are “left over” from ancestors that no longer serve a purpose

  

• Fossil record is studying of past organisms’ fossils and observing their changes over time.

  

• Biogeography is the way species are distributed across the planet that shows how they spread out over time.

• Sometimes, organisms can have similar features even if they are not common ancestors and evolve independently from one another.

  • This is called convergent evolution.

    

• Structures that are similar due to convergent evolution is said to be analogous.

How Small can Evolution Be?

• There is a misconception that individual organisms evolve.

  • Although it is true that natural selection acts on individuals: each organism’s traits affect its survival/reproductive success compared to other individual.

• But the evolutionary impact of natural selection is only apparent in how a population of organisms change over time.

• Focus on evolutionary change in populations, evolution on the smallest scale is called microevolution.

  • It is the change in allele frequencies in a population over generations.

  • There are three things that causes changes in allele frequencies: natural selection, genetic drift, and gene flow.

• Natural selection is the process by which organisms with traits better suited to their environment survive and reproduce more successfully, passing those traits to the next generation.

  

• Genetic drift are chance events that can cause unpredictable fluctuations in allele frequencies.

  • There are 2 types: Bottleneck effect and Founder effect.

    • The bottleneck effect is when the size of population is reduced by a sudden event.

      

    • The founder effect is when a few individuals become isolated from a larger population and forms a new population whose gene pool differs from the original.

      

• Key points about genetic drift:

  • Genetic drift is significant in small populations.

  • Genetic drift can cause allele frequencies to change at random.

  • It can lead to a loss of genetic variation within populations.

  • Can cause harmful alleles to become fixed.

• Gene flow is the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes.

  • An example is the wind carrying a flower’s pollen to another flower far away. These two flowers have different alleles.

  • It can transfer alleles that improve the ability of populations to adapt to local conditions.

  • It increases genetic variation.

    

What Contributes to Genetic Variation?

• Genetic variation is defined as differences among individuals in the composition of their genes or other DNA sequences.

  • Genetic variation at the whole-gene level is called gene variability.

  • Genetic variation at the molecular level of DNA is called nucleotide variability.

• Many nucleotide variations occur within introns (noncoding segments of DNA) lying between exons (regions retained in mRNA after RNA processing).

• Genetic variation originates when mutation, gene duplication, or other processes produce new alleles and new genes.

  • Sexual reproduction also results in genetic variation as arranging existing genes in new ways.

    

• New alleles can arise from mutation, a change in the nucleotide sequence of an organism’s DNA.

  • Mutations can be caused by things like errors in DNA replication, exposure to radiation (like UV light), and exposure to certain chemicals.

• Although most mutations are harmful, some are not.

• Mutations in noncoding regions result in neutral variation, differences in DNA sequence that do not result in a disadvantage or advantage.

  • Even if there is a point mutation that is on a coding region but does not change the amino acid’s composition, then it has no affect on the protein’s function.

  • changes in the amino acid might not even affect the protein’s shape/function in some cases.

What is the Hardy-Weinberg Equation?

• Just because individuals within a population might be genetically different from one another, it does not guarantee that said population will evolve.

• A population is a group of individuals of the same species that live in the same area and interbred, producing fertile offspring.

• A population’s genetic makeup is its gene pool, which consists of all copies of every type of allele at every locus in all members of the population.

  

  • If only one allele exists for a particular locus in a population, that allele is fixed in the gene pool.

• Hardy-Weinberg equilibrium is when a population is not evolving and the allele & genotype frequencies remain constant from generation to generation.

  • It’s a model used to see whether natural selection or other factors are causing evolution.

• It states that the frequencies of alleles and genotypes in a population will remain constant from generation to generation.

  • Shows that only Mendelian segregation and recombination of alleles are at work.

• There are 5 conditions a population must meet to be at HW equilibrium:

  • No mutations

    • If not true, gene pool is then modified and alleles are altered, leading to genetic variation and possibly new traits/adaptations.

  • Random mating

    • If not true, certain genotypes can be favored or favored against. Can lead to some alleles becoming more common and some less common, changing allele frequencies.

  • No natural selection

    • If not true, some traits will provide an advantage and over time, advantageous traits will become more and more common.

  • Extremely large population size

    • Has to be large enough to withstand random events. If not true (small population), random events like genetic drift can dramatically alter allele frequencies.

  • No gene flow

    • If not true, individuals migrating into a population might introduce new alleles or those leaving might remove certain alleles, changing allele frequencies.

      

What are Different Types of Natural Selections?

• Natural selection can alter the frequency of distribution of heritable traits in a couple of ways.

• First way is directional selection, which is when conditions favor individuals exhibiting one extreme of a phenotypic range.

  • This shifts a population’s frequency curve for a phenotypic character in one direction or the other.

  • This is common when a population’s environment changes or when members of a population migrate to a new habitat.

    

• Disruptive selection is when conditions favor individuals at both extremes of a phenotypic range over those with intermediate phenotypes.

  • Shifts a population’s frequency to both ends a direction, leaving a dip in the middle.

    

• Stabilizing selection is when conditions favor intermediate phenotypes over extreme phenotypes.

  • This acts against both extreme phenotypes.

  • Stabilizing selection reduced variation and tends to maintain the status quo for a particular phenotypic character.

    

• Sometimes in natural selection, it may preserve variation at some loci, thus maintaining two or more phenotypic forms in a population.

  • Known as balancing selection, this includes heterozygote advantage and frequency-dependent selection.

• Heterozygote advantage is when heterozygous individuals at a particular locus have greater fitness than do both kinds of homozygotes.

  • In this case, natural selection maintains two or more alleles at that locus.

  • It could result in stabilizing or directional selection, depending on the relationship between genotype and phenotype.

    

• In frequency-dependent selection, the fitness of a phenotype depends on how common it is in the population.

  • An example is prey species that evolve to look toxic/dangerous, and some predators are more likely to avoid common colors or patterns.

    

• Sexual selection is a process in which individuals with certain inherited characteristics are more likely than other individuals of the same sex to obtain mates.

  • It can result in sexual dimorphism, a difference in secondary sexual characteristics between males and females of the same species.

  • Could differ in size, color, ornamentation, and behavior.

    

• In intrasexual selection, which is selection within the same sex, individuals of one sex compete directly for mates of opposite sex.

  • Mostly occurs among males.

    

• In intersexual selection, which is mate choice, individuals of one sex are choosy in selecting their mates from the other sex.

  • Usually females are the individuals who choose.

  • Mostly depends on the other’s appearance or behavior.

    

  • However, it might make a species like male birds more visible to predators.

  • But if such characteristics helps them gain a mate and the benefits outweighs the risk of predation, then it would increase overall reproductive success.

Why Can’t Natural Selection Make Perfect Organisms?

• Selection can only act on existing variations, meaning only fittest phenotypes that are currently within a population.

  • These may not be the most ideal traits.

  • New advantageous alleles do not arise on demand.

• It is limited by historical constraints, meaning each species has a legacy of descent with modifications from ancestral forms.

  • Evolution cannot scrap ancestral anatomy and build new complex ones from scratch.

  • Like a land animal suddenly having wings to fly is impossible.

    

  • Evolution only operates on the traits an organism already has.

• Adaptations are often compromises.

  • As organisms develop different traits, they might lose or give up old ones.

  • They face trade-offs in which the ability to perform one function may reduce the ability to perform another.

  • Like seals gaining legs to walk but losing flippers so they cannot swim nearly as well.

    

• Chance, natural selection, and the environment interact.

  • Change events can affect the evolutionary history of populations.

  • Like a founder effect can move individuals to a new environment where their traits are not best suited.

  • The environment at a particular location may change unpredictably from year to year.

• Because of these four constraints, evolution does not result in perfect organisms.

• Natural selection operates on a “better than” basis.

• Evolution will definitely result in many imperfections of organisms it produces.

Where did Species Originate From?

• When Charles Darwin came to the Galapagos Islands, there was a mystery that captivated him.

• This “mystery of mysteries” was speciation, the process by which one species splits into two species.

  • Later on, he realized that speciation explained how species share many characteristics.

    

• Speciation draws comparison between microevolution and macroevolution.

  • Microevolution was changes over time in allele frequencies in a population.

  • Macroevolution is the broad pattern of evolution above the species level.

    

• In daily life, we usually distinguish between various “kinds” of organisms based on differences in appearance.

• But biologists compare not only the morphology (body form) of different group of organisms but also their physiology, biochemistry, and DNA sequences.

  • Results show that morphologically distinct species are indeed discrete groups.

• A species is defined by a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring.

  • But cannot produce viable, fertile offspring with members of other such groups.

  • Therefore, members of a species must be reproductively compatible.

• The formation of a new species hinges on reproductive isolation.

  • The existence of biological factors (barriers) that impede members of two species from interbreeding and producing viable, fertile offspring.

    

• These barriers block gene flow between species and limit the formation of hybrids, offspring that result from an interspecific mating.

  

• These barriers can be classified according to whether they contribute to reproductive isolation before or after fertilization.

• Prezygotic barriers (“before the zygote”) block fertilization from occurring.

  • They can impede members of different species from attempting to mate.

  • Preventing an attempted mating from being completed successfully.

  • Hindering fertilization if mating is completed successfully.

• There are five types of prezygotic barriers:

  • Habitat isolation is when species live in different areas or occupy different habitats within the same area.

    

  • Temporal isolation is when species breed at different times of the day, year, or season.

    

  • Behavioral isolation is when unique behavioral patterns and rituals separate species.

    

  • Mechanical isolation is when the reproductive anatomy of one species does not fit with the anatomy of another species.

    

  • Gametic isolation is when proteins on the surface of gametes do not allow for the egg and sperm to fuse.

    

• However, if the sperm cell overcomes prezygotic barriers and fertilizes an ovum from another species, a variety of postzygotic barriers (“after the zygote”) may contribute to reproductive isolation after the hybrid zygote is formed.

• There are three types of postzygotic barriers:

  • Reduced hybrid viability is when the hybrid zygote/offspring fails to develop properly or reach maturity.

    

  • Reduced hybrid fertility is when the hybrid offspring matures but is infertile and cannot produce offspring of its own.

    

  • Hybrid breakdown is when hybrid offspring is viable and fertile in the first generation, but their offspring (next gen.) are inviable or sterile.

    

• Another way to define species is through the morphological species concept.

  • This distinguishes a species by body shape and other structural features.

  • A disadvantage of this is that it relies in subjective criteria.

• The ecological species concept defines a species in terms of its ecological niche.

  • Which is the sum of how members of the species interact with the nonliving and living parts of their environments.

• Over the years, there has been a few hypotheses about how species arise from existing species.

• In allopatric speciation (“other country”), gene flow is interrupted when a population is divided into geographically isolated subpopulations.

  

• The barriers by which species are separated in allopatric speciation depends on the ability of the organisms to move about.

  • Once geographic isolation has occurred, the separated gene pools may diverge.

  • Different mutations arise, and natural selection and genetic drift may alter allele frequencies.

  • Reproductive isolation may then evolve as a by-product.

• In sympatric speciation (“same country”), speciation occurs in populations that live in the same geographic area.

  • Although contact makes sympatric speciation uncommon, it can still occur if gene flow is reduced by different factors.

  • These factors include polyploidy, habitat differentiation, and sexual selection.

    

• A condition called polyploidy is when there is a chromosomal alteration in which the organisms possesses more than two complete chromosome sets.

  • This results from an accident of cell division.

  • Although this can happen in animals, it’s much more common in plants.

    

• There are two distinct forms of polyploidy.

  • An autopolyploid is an individual that has more than two chromosome sets that are all derived from a single species.

  • An allopolyploid is a fertile individual that has more than two chromosome sets as a result of two different species interbreeding and combining their chromosomes.

    

• Scientists have hypothesized different paces/tempos by which evolution and speciation occurs.

  • Punctuated equilibrium is when evolution occurs rapidly after a long period of stasis.

  • Gradualism is when evolution occurs slowly over hundreds, thousands, or millions of years.