BIO 1B - Midterm 1

Darwin’s argument about evolution

as there are more species than can survive (struggle for existence), it follows that any being if it vary slightly in any manner profitable to itself (under complex and varying conditions of life), will have a better chance of surviving and thus be naturally selected.

through the strong principle of inheritance, any selected variety will tend to propagate its new and modified form

5 parts of Darwin’s Syllogism

1) Struggle for survival (to reproduce)

2) Phenotype Variable

3) Existence depends on phenotype

4) Inheritance of these phenotypic differences

THEN, descent with modification occurs (also known as EVOLUTION)

Why is inheritance important to the evolutionary process?

Megan’s answer: Without inheritance, mutations that would lead to a better adaptation of a population to its environment will not be passed down to the next generation and therefore this possibly beneficial trait will be lost.

Important: Without inheritance, it does not matter what effect selection had in one generation, because it would have no impact on the phenotypes seen in the next generation

Why is inheritance important to the evolutionary process?

Inheritance allows selection to have an impact (whether positive or negative) on the next generation. It is not fair to say that inheritance gives the next generation a better chance of survival because what if the environmental pressures changed.

Profitable

full range of strategies that promote one’s reproductive success

What are the two steps of evolution?

“simple” selection and “complex selection”, consequences of selection when there are trade-offs

Important nuance of Darwin’s Theory

Darwin does not state that evolution must occur. It is about the consequences of over-reproduction, variation, selection, heredity, and the changing conditions of life

What is a driving force of evolution?

Fecundity: potential to produce offspring. Over-reproduction creates the pressure of selection

natural selection is a two step proccess

1) variation (through development of organisms) and 2) selection

• plays out in the arena of ecology

how is natural selection tied to adaptation

ns is intimately tied to adaptation, the dynamic evolutionary process by which natural selection fits organisms to their environment (it is said that ns increases fitness of a population)

fitness: ability to survive and pass on genes

example of simple selection

finches with different beaks depending on selective pressures for different food sources

what did Darwin argue about giraffes?

Darwin argued that we expect to see extreme morphologies in species that are already extreme (so the long neck of the giraffe only got longer)

• important to note that there is not a simple relationship between selective pressures and morphologies. the giraffes could have longer necks for a multitude of reasons

what are two reasons why evolution doesn’t make perfectly fit species?

1) trade-offs (good trait but also bad, think of sickle cell anemia)

2) the “appropriate” variation might not be available

balancing selection (with ex)

when a trait is both selected for and selected against (think of sickle cell anemia)

• there are trade-offs between having more offspring and having no offspring (think of the widow bird tails)

more trade-offs and complexities equals…

more solutions and morphologies

What is the relationship between natural selection and adaptation?

Species become more adapted to their environment through selection

• OR: selection occurs because of over-reproduction and we call the selected traits adaptations

how is variation random?

variation does not predict the needs of the organism (evolution is trial and error)

evolutionary hitch-hiking

hard to work out which traits were the targets of selection because traits are often correlated, thus non-selected traits can hitchhike along with the selected traits

Can there be selection without evolution?

Yes. For example, selection against extreme phenotypes there will be selection against these extreme phenotypes (without descent of modification aka evolution)

Ex: babies optimum range of birth weights, babies too small and too big will be selected against

3 types of selection

1) stabilizing: favors average phenotypes and selects against extreme variations

2) directional: a single extreme phenotype is favored over all other variations, shifts to a specific direction (ex: color of moths)

3) disruptive: extreme traits are favored over average traits (ex: beak size)

can there be evolution (descent with modification) without selection (give me specifics)

Yes!

1) Genetic Drift (random changes of allele frequencies within a population)

2) Founder Effect (small bit of one species found another colony - immigrants phenotype is due to chance)

3) Bottleneck Effect (population size greatly reduced due to environmental event/selection)

What are several mechanisms that can lead to descent with modification

• natural selection (of various kinds)

• random effects

  • genetic drift (random due to chance —> lead to fixation)

  • founder effect

  • bottleneck

• new genetic variants enter a population

  • immigration of individuals from elsewhere

  • new mutations

Causes of mutations

most mutations arise due to replication errors as DNA is duplicated during cell division

parthenogenesis

asexual reproduction

Genetic Drift

change in frequency of existing alleles in population due to random chance

• lead to fixation (all individuals have same allele)

• does not concern mutation, but new mutants may then change frequency by drift

eukaryotes vs. prokaryotes

eukaryotes have a nucleus the others don’t

Biological Species Concept

species consists of populations of organisms that can interbreed with one another in nature, but are reproductively isolated from other populations (unable to successfully mix genes)

• species consists of populations of organisms that can reproduce with one another and are reproductively isolated from other populations

Morphological Species Concept (possible issue)

species are groups of individuals that are morphologically similar but are morphologically distinct from other such groups

• species that interbreed have relatively continuous morphological variation

• discontinuity between phenotypes of different species as selection and drift cause their phenotypes to diverge

Issue: DNA can show that a species that are indistinguishable morphologically can represent different species

chronospecies

one species that grayly changes over time until it becomes distinct from its original ancestors (two species)

cryptic species

Crytpic species is one or two + morphologically indistinguishable populations where DNA data show they are distinct (implies they can’t interbreed)

• that being said, DNA is difficult to interpret, can they not interbreed or have they just not)

fixed difference

a fixed difference between species means the difference is seen between all individuals (no interbreeding)

synonymy (and importance)

when a species has multiple names, synonymy documents all of them

IMPORTANT: easy to miss older literature if you are unaware of name changes

speciation and anagenesis

speciation is emergence of new species

anagenesis is the change within a lineage

what type of evolution does Darwin focus on?

Darwin’s book is not about the origin of new lineages, rather the transformation within lineages with time (microevolution)

phylogeny

evolutionary history and relationship among a group of organisms, showing how they descended from shared common ancestors (with phylogenetic trees)

3 different types of speciation

allopatric speciation: populations become separated

parametric speciation: populations adjacent

sympatric speciation: new species within the same area

what are the two ways allopatric speciation happens

dispersal

• population disperses

• can lead to rapid evolution

vicariance

• population is split by change in environment

how can sympatric speciation happen?

• no physical barriers to prevent any member of a species from mating with another

• new species developed (could be different food sources or mating rituals)

• ALSO, by polyploidy: failure of cell division can lead to extra sets of chromosomes (leading to a new species)

zygote, pre zygotic barrier and post-zygotic

zygote - fertilized egg

pre zygotic barrier - prevents formation of an egg

post zygotic barrier - prevents zygote from successfully reproducing

temporal isolation

breeding at different times

behavioral isolation

birds with two different songs not recognized by each other

mechanical isolation

different genital morphological structures

gametic isolation

external fertilization, egg rejects sperm based on sperm’s genotype

why do we think allopatry is the most common mode of speciation?

in order for speciation to occur we need to prevent gene flow and that is the easiest with isolated populations