Chapter 22: Evolution by Natural Selection (Exam 1)

Species

A group of populations that're genetically similar

Why do speicies diverge?

if gene flow stops or a result of mutation, NS, and genetic drift

Biological Species - Advantages and Disadvantages

Reproductive isolation between interbreeding populations (don't breed & no viable/fertile babies)

-reproductive isolation = evolutionary independence

- not applicable to asexual or fossil species; difficult to assess if populations don't overlap geographically

Morphospecies -- Advantages & Disadvantages

morphologically distinct populations

-widely applicable

-misidentifies polymorphic species; misses cryptic; subjective

Phylogentic Species -- Advantages & Disadvantages

Identifies species based on evolutionary history: all species are related

by common ancestry (clade/lineage)

-widely applicable; based on testable criteria

-relatively few well-estimated phylogenies are available

Cryptic Species

species that look identical

Polymorphic Species

species with more than one form

Monophyletic Group

Homologous traits found in common ancestor and descendants but

missing in more distant ancestor

Elephant Example :3

DNA sequences in elephant populations within monophyletic group not distinguishable from each

other

Sequences of pairs of elephants in different groups are distinguishable

Strong evidence that African forest elephants are sufficiently distinct from African savanna elephants

and Asian elephants

Speciation

A splitting event that creates two or mroe distinct species from an ancestral species

Reproductive Isolation

Results in lack of gene flow between populations

Members of populations do not interbreed or fail to produce

viable, fertile offspring after mating

Prezygotic vs Postzygotic isolation

Individuals of different species are prevented

from mating successfully vs The hybrid offspring do not survive or reproduce

Allopatric

Speciation that begins with geographic isolation

How does Allopatry occur ? (2 ways)

Dispersal & Vicariance

Sympatric Speciation

Speciation w/o geographical isolation (lives in same geographical area)

Extrinsic Events

Disruptive selection based on different ecological niches or mate preferences

Intrinsic Events

Chromosomal mutations

Polyploidy

condition of posessing more than 2 complete sets of chromosomes caused by errors in mei/mitosis

Autopolyploid vs Allopolyploid (more common)

Individuals produced when a

mutation results in a doubling of the

chromosome number

Chromosomes are all from the same

species

vs

ndividuals are created when

parents of different species mate

and an error in mitosis occurs,

resulting in viable, nonsterile

offspring

Produces offspring with two

different sets of chromosomes

What happens if populations are NOT reproductively isolated

Populations fusing over time coming

into contact: gene flow erases any distinctions between them

If divergence HAS occurred & prezygotic isolation exists:

Mating between populations is rare,

Gene flow is minimal, Populations continue to diverge

Reinforcement (divergence)

NS for traits that prevent interbreeding among populations

Hybrid Zone & when does it typically form?

a geographic area where interbreeding between two

populations occurs and hybrid offspring are common -- occurs in well-defined geographical areas