Microevolution and Speciation

(14.4, 15.1)

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

Close populations can have gene flow between them. Eventually may lead to a single population

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

Why would this color fur be hazardous for the deer?

Albino deer and normal deer

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

Check:

What is a gene pool? What are the two main forces of

evolutionary change in gene pools?

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