Modes of Natural Selection/Species Concepts

Modes of selection

Effects of natural selection on the distribution of a trait

3 modes of selection

Directional selection

Stabilizing selection

Disruptive selection

Directional selection

Fitness increases/decreases with value of a trait(one phenotype favored over another)

In directional selection, the mean value of a trait does what after selection

Changes

Example of directional selection

Alpine sky pilot flowers

Stabilizing selection

Selection favors intermediate traits

Does stabilizing selection change the mean value of a trait

No

In stabilizing selection, variation of continuous traits is

Reduced

Example of stabilizing selection

human birth weight

Disruptive selection

Selection favors extreme phenotypes(two at once)

In disruptive selection, individuals with extreme phenotypes have

Highest fitness

Does disruptive selection change the mean value of a trait

No

In disruptive selection, the variation of a trait does what as population divides into two groups

Increases

Disruptive selection may lead to what

Speciation

Example of disruptive selection

Bill size in black bellied seed crackers

Out of the three modes of selection, which are common and which are rare

Common: directional, and stabilizing

Rare: Disruptive

Species

Smallest independent evolutionary unit

Microevolution operates and changes allele frequencies where

Within species(not between)

Essence of speciation

Lack of gene flow

4 species concepts

Biological

Morphological

Phylogenetic

General lineage

Biological species concept

a species is a population of organisms that interbreed with each other but are reproductively isolated from other groups

prezygotic reproductive barriers

Prevent mating or fertilization from occurring

examples of prezygotic barrier

Geographic barrier, behavioral isolation

Habitat isolation

populations live in different habitats and do not meet

Temporal isolation

Species breed at different times

Behavioral isolation

different mating rituals

Mechanical isolation

Morphological differences prevent fertilization.

Gametic isolation

Sperm of one species cannot fertilize egg of another species

Postzygotic isolation mechanism

Preventing hybrid zygote from developing into fertile adult

example of postzygotic barrier

Infertile hybrid

reduced hybrid viability

Hybrids that have lower fitness and may not live to adulthood

Hybrid breakdown

First generation hybrids are fertile but offspring of this gen are sterile(low viability)

Biological species concept is not applicable to what

Not applicable to asexual organisms

The biological species concept is difficult to what

Difficult to apply in the real world

Sometimes, species only interbreed where

Hybrid zone

Biological species concept is also not applicable to what

Not applicable to extinct organisms

Morphological Species Concept

Defines species as sets of organisms that look similar to each other

First problem with morphological concept

Descriptions of characteristics are subjective

Some morphological characters may not reflect what

Evolutionary history

Convergent Evolution

Organisms evolve independently to acquire similar traits

Morphological species concept is not applicable to what

Cryptic species

Cryptic species

species that are morphologically identical but genetically distinct

phylogenetic species concept

Defines species as smallest group of individuals that share common ancestor(monophyletic group)

Monophyletic group

Contain all descendants of a common ancestor

Phylogenetic concept can be applied to fossil and living forms if what is known

Phylogeny

Phylogenies are often what

Unknown

Different genetic markers may infer what

Different evolutionary relationships

The cutoff for a species is often what

Arbitrary(subjective)

Different species concepts use different what to define species

Species criterion

General lineage concept

Species are evolving meta population lineages(each species evolves independently)

As two lineages separate, multiple species concepts come into conflict during when

Early stages of speciation(Ex: mutated flowers with different colors may be different species morphologically but not phylogenetically or biologically)

General lineage concept attempts to do what

Unify all species concepts