Species & Speciation Chapter 24 Campbell

Allele frequency and gene pools Gene pool: all the genetic information present in the reproducing members of a population at a given time  Reservoir of traits What can we say about a large gene pool? A small gene pool? Allele frequency: a measure of the proportion of a specific variation of a gene in a population  Expressed as a proportion or a percent

Evolution and alleles Gene pools are relatively stable over time New alleles can be introduced due to mutation  How else might new alleles appear?  How might alleles disappear?  No change in allele frequencies = no evolution

Defining species A basic unit for classifying organisms A species is made of organisms which:  Have similar morphological and physiological characteristics (easily observed & measured)  Have the ability to interbreed to produce fertile offspring  Are genetically distinct from other species  Have a common phylogeny

There are challenges to this definition … Sometimes members of separate but similar species reproduce and succeed in producing offspring  Horse and zebra = zebroid or zorse However, different number of chromosomes, so zebroids are infertile  Other challenges: Potentially interbreed but live in different niches Asexual organisms Infertile individuals  are they a species?

Barriers between gene pools In some situations, populations of members of the same species (same gene pool) can be stopped from reproducing because of some insurmountable barrier Barriers can be pre-zygotic and post-zygotic  Examples of pre-zygotic barriers: Geographical (habitat), temporal, behavioral, mechanical, gametic  Examples of post-zygotic barriers: Reduced hybrid viability, reduced hybrid fertility, hybrid break down

Discussion of pre-zygotic barriers Geographic/habitat  Physical barrier like water, mountains  Even “micro-barriers”  doesn’t have to be large scale Temporal  Incompatible time frames prevent gametes from encountering each other Behavioral  Lifestyle and habits are not compatible Songs, courtship displays Mechanical  Anatomically incompatible Gametic isolation  Internal fertilization vs. external fertilization

Discussion of post-zygotic barriers Reduced hybrid viability  Genetic incompatibility between two species may cause self- abortion at some embryonic stage  If development does occur, hybrids may be frail Reduced hybrid fertility  Most (almost all) hybrids are sterile (infertile)  Hybrids cannot back-breed with a parental specie  One cause is a failure of meiosis to produce normal gametes b/c of different number of chromosomes Hybrid breakdown  In some cases, the first generation of a hybrid is fertile  But when these hybrids mate, future offspring my be infertile and/or very weak

Homework Read Hardy Weinberg packet Read pages in Campbell Answer IB assessment statements D.4.1 & D.4.3 (first page of packet)

Exit Slip (5 points) Name, date, period Title: Speciation Exit Slip 1. Define species 2. List 1 challenge with the definition species 3. List and define 2 pre-zygotic barriers and 1 post-zygotic barrier