microevolution and speciation (14.4, 15.1). microevolution evolution on the smallest scale- a...
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Microevolution
Evolution on the smallest scale- a generation to generation change
Comes from a change in a population’s gene pool Gene pool- consists of all the alleles in all of
the individuals that make up a population Remember a population is a local group of
individuals of the same species A population is the smallest level at which
evolution can occur
Gene Pools
A gene pool is like a reservoir from which the next generation of individuals gets their genes It is the raw material for evolution
Gene pools reflect the variation among individuals that is largely a result of sexual recombination Meiosis and fertilization shuffle alleles
and deal out fresh combinations to the offspring
The many different colors of wild mustangs are a reflection of genetic variation in their gene pool
Humans have a variety of possible genetic combinations available in the gene pool
Changes in the Gene Pool
Processes that lead to genetic variation are random, but natural selection is not)
The environment favors genetic combinations that increase the chance of survival and reproductive success of an individual Some alleles become more common than
others; frequency of alleles- how often certain alleles pop up in the gene pool
Hardy-Weinberg In contrast to microevolution
populations that do not undergo a change in their gene pool are at Hardy-Weinberg equilibrium
This means the frequency of alleles in the gene pool are constant over time
This rarely happens in nature but is useful because it allows a “no change” baseline for comparison when looking at a population that is changing A controlled variable
Hardy-Weinberg fails to apply when there is: Mutation Gene flow Genetic drift Nonrandom mating Natural selection
All of which occur naturally in populations
Genetic Drift
A change in the gene pool of a population due to chance Only the alleles of organisms that
successfully reproduce in one generation appear in the gene pool of the next generation
All populations are subject to drift, the smaller the population, the more impact genetic drift will have
Genetic drift affects the genetic makeup of the population but, unlike natural selection, through an entirely random process. So although genetic drift is a mechanism of evolution, it doesn’t work to produce adaptations.
The genes of the next generation will be the genes of the “lucky” individuals, not necessarily the healthier or “better” individuals. That, in a nutshell, is genetic drift. It happens to ALL populations—there’s no avoiding the erratic nature of chance.
Only the alleles of organisms that successfully reproduce in on generation appear in the gene pool of the next. In this population of ten plants, the frequency of white-flower alleles was reduced to zero due to genetic drift.
Bottleneck Effect
Reducing the size of a population also reduces the size of its gene pool Due to natural disaster, predation or habitat
reduction By chance, certain alleles may occur more
frequently than others among survivors, some alleles may be eliminated all together- this is the bottleneck effect
Some endangered species, like the cheetah, are believed to have undergone a bottleneck effect due to the fact they have very little genetic variation in the remaining populations
http://www.youtube.com/watch?v=AcuQbaid0SY
Founder Effect
Genetic drift also occurs when a few individuals colonize a new habitat Isolated island, lake etc
The smaller colony has very little genetic representation of the larger population it came from
Known as founder effect because the change relates to the genetic makeup of the founders of the colony Finches on Galapagos Islands
Gene Flow
Genetic drift and natural selection are the main causes of changes in gene pools, gene flow and mutation also have a role
Gene flow- the exchange of genes with another population Ex: neighboring wildflower populations
Gene flow tends to reduce the genetic difference between populations If it is extensive, it can eventually mix
neighboring populations into one large population with a single gene pool
Mutation
A change in an organism’s DNA
Natural selection and/or genetic drift can influence the frequency of a new mutation Ex- albino deer- light
colored fur would prove hazardous but gene gets passed down in heterozygous individuals
Mutations play a key role in evolution as raw material for natural selection Important in
asexually producing organisms such as bacteria
Natural Selection and Fitness
Genetic drift, gene flow and mutation can all lead to microevolution but do not necessarily lead to adaptation
Only natural selection usually leads to adaptation Chance comes from mutations and
sexual recombination of alleles which produce random genetic variation in a population
Survival of the fittest Misconception that there are direct contests
between individuals Fitness- the contribution that an individual
makes to the gene pool of the next generation Who can reproduce
Production of healthy, fertile offspring is all that counts in natural selection
Speciation
How do biologists identify species? Biological species concept- a population
or group of populations whose member can breed and produce viable (fertile) offspring
This concept can not be applied to all life Asexual reproducers, fossils But is still useful in classifying and
identifying new organisms
Micro to Macroevolution
Microevolution and adaptation explain how populations evolve
Macroevolution looks at the major biological changes in the fossil record These changes occur with the origin of
new species, extinction and evolution of major features of living things such as wings or flowers
Speciation- the origin of new species Leads to biological diversity
Speciation can lead to an increase in the number of species
Barriers to Speciation
Reproductive isolation - A condition in which a reproductive barrier keeps species from breeding
Some barriers that contribute to this are timing, behavior and habitat
Timing- 2 closely related species have different breeding seasons
Behavior- 2 similar species have different courtship or mating behaviors
Habitat- some species are adapted to different habitats
Other barriers include infertility, reproductive structures are incompatible
The eastern and western spotted skunk are very closely related but are kept in reproductive isolation due to different breeding seasons
The eastern and western meadowlark remain separate because their courtship rituals differ
Geographic Isolation
Separation of populations as a result of geographic change or dispersal to geographically isolated locations
Can occur when mountain ranges form, glaciers move, small groups of population colonizing an island
These two species of antelope squirrels live on opposite sides of the Grand Canyon- this geographic barrier keeps these populations separated
For each small, isolated population that becomes a new species, many more fail to survive in their new environment
Adaptive Radiation
Evolution from a common ancestor that results in diverse species adapted to different environments
Studied on islands which are used as living laboratories for speciation
Separated populations have different gene pools and frequency of alleles.
Darwin’s Finches adapted different beaks on different parts of the Galapagos Islands
Species A comes from the mainland and undergo genetic changes to become Species B. Species B can migrate to a neighboring island and become Species C – could probably coexist with B but geographic isolation keeps that from occurring
Punctuated Equilibrium
A model that suggests species often diverge (separate) in spurts of relatively rapid change Seen in the fossil record
In just a few hundred to a few thousand generations, genetic drift and natural selection can cause significant changes in a population
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