chap 16: evolution of populations darwin never knew how heredity actually worked biologists...

Chap 16: Evolution of Populations

Darwin never knew how heredity actually worked

Biologists connected Mendel’s work to Darwin’s in the 1930’s

Advances in genetics since then has redefined many of Darwin’s ideas in genetic terms

Neo-Darwinism


Single-gene & Polygenic Traits

# of phenotypes produced for given trait depends on how many genes control trait

Single-gene traits controlled by a single gene with two alleles

Ex.- widow’s peak

Fre

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of

Ph

eno

typ

e(%

)

100

80

60

40

20

0

Widow’s peak No widow’s peak

Phenotype


Many traits controlled by two or more genes

Called polygenic traits

Each gene often has two or more alleles

Result: one polygenic trait can have many possible genotypes & phenotypes

Ex.- height

Distribution of phenotypes usually takes on bell shape on graph (normal distribution)


How Common is Genetic Variation?

All genes have at least two forms (alleles)

Plants & animals often have many alleles for a single trait

Some variation is “invisible” since it involves biochemical processes

Organisms may also be heterozygous for many genes


Sources of Genetic Variation

Two main sources of genetic variation:

- mutations

- genetic shuffling resulting from sexual reproduction


Mutations

Any change in a sequence of DNA

Can occur due to mistakes in replication or due to environmental factors (radiation)

Do not always affect organism’s phenotype

Those that do, may affect an organism’s fitness

Other mutations have no effect on fitness


Gene Shuffling

Even though parents provided you your genes, you don’t look exactly like them

Most heritable differences due to gene shuffling that occurs during meiosis

Each member of homologous pair moves independently:

23 chromosomes can produce 8.4 million combinations


Crossing-over also increases the # of different genotypes that can appear in offspring

Sexual reproduction major source of variation within populations

Does not change relative allele frequency on its own


Variation and Gene Pools

Genetic variation studied in populations

Population- group of individuals of the same species that interbreed

Since they interbreed, share a common group of genes called a gene pool

Gene pool- consists of all genes, including all different alleles, present in a population


Relative frequency of an allele is the # of times that allele occurs in the gene pool, compared with # of times other alleles for same gene occur

Often expressed as a percentage

In genetic terms, evolution is any change in the relative frequency of alleles in a population

Sample Population

48% heterozygous

black

36% homozygous

brown

16% homozygous

black

Frequency of Alleles

allele for brown fur

allele for black fur


Evolution versus Genetic Equilibrium

Are there any conditions under which no evolution will occur?

Is there any way to recognize when this is occurring?

Answer is provided by the Hardy-Weinberg principle


Hardy-Weinberg principle:

allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change

Situation called genetic equilibrium


Five conditions required to maintain genetic equilibrium from generation to generation:

- Must be random mating

- Population must be very large

- No movement into/out of population

- No mutations

- No natural selection


Some populations, these criteria met for long time periods

If conditions are not met, genetic equilibrium disrupted, & population evolves


Random Mating

All members of population have equal chance of producing offspring

In natural populations, mating is rarely completely random

Nonrandom mating means genes for certain traits are under strong selection pressure (sexual selection)


Large Population

Genetic drift has less effect on large populations

No Movement Into/Out of Population

New individuals could bring new alleles into population

Gene pool of population must be kept separate from others


No Mutations

If genes mutate, then new alleles might be introduced into gene pool

Gene frequencies will change


No Natural Selection

All genotypes in the population must have equal chance of survival & reproduction

No phenotype can have selective advantage over another


Evolution as Genetic Change

Each time an organism reproduces, passes along a copy of its genes to offspring

Can then view evolutionary fitness as an organism’s success in passing along genes

Can view evolutionary adaptation as any genetically controlled functional, structural, or behavioral trait that affects ability to pass along genes to next generation


Natural selection never acts directly on genes

Entire organism (not single gene) that either reproduces or does not (pass along genes)

Can only affect which organisms survive & reproduce or do not

Individual does not contribute its alleles to gene pool unless it survives & reproduces


If organism produces many offspring, its alleles stay in gene pool & may increase in frequency

Evolution is change over time in relative frequencies of alleles in a population

So, populations can evolve, not individual organisms, over time


Natural Selection on Single-gene Traits

Natural selection on single-gene traits can lead to changes in allele frequency

Ex.- population of lizards

Normally brown, experiences mutation causing red and black phenotypes

If red more visible to predators, those individuals less likely to survive & reproduce


Black lizards might warm up faster on cold days

If high body temp. allows faster movement, then black moves faster

Avoids predators & feeds easier: might produce more offspring than brown form

If color change has no effect on fitness, then no selection pressure would occur


Natural Selection on Polygenic Traits

Fitness of individuals close to one another on bell curve will not be very different

Fitness can vary great deal from one end of a curve to the other

Where fitness varies, natural selection can act


Genetic Drift

N.S. not only source of evolutionary change

In small populations, an allele can become more or less common by chance

Probability can be used to determine results of crosses in large populations

Smaller the population, farther the results might be from predicted values


This random change in allele frequency is known as genetic drift

In small populations, individuals that carry a particular allele may leave more descendants than others, just by chance

Over time, a series of chance events of this type can cause an allele to become common in a population


Genetic drift may occur when a small group of individuals colonizes a new habitat

These individuals may carry alleles in a different frequency than the parent population

Population founded will be genetically different from parent population

Cause is chance (that particular alleles were in founding individuals), not N.S.


Natural selection can affect the distributions of phenotypes in any of three ways:

- Directional selection

- Stabilizing selection

- Disruptive selection


Directional Selection

Directional selection- when individuals at one end of curve have higher fitness than individuals in middle or at other end

Range of phenotypes shifts towards one end due to some individuals failing to S&R


Ex.- Darwin’s finches (each with diff. beak size)

Food shortage causes supply of small & medium-sized seeds to run low

Birds with beaks that enable them to open large seeds will have advantage

Big-beaked birds will have higher fitness

Average beak size in population would probably increase

Directional Selection

Food becomes scarce.

Key

Low mortality, high fitness

High mortality, low fitness


Stabilizing Selection

Stabilizing selection- when individuals near center of curve have higher fitness than individuals at either end

Keeps center of curve at current position, but narrows the overall graph


Ex.- human birth weight

Babies born much smaller than average less likely to be healthy, less likely to survive

Babies much larger than average are likely to have difficulty being born

Fitness of larger or smaller individuals is lower than that of average-sized individuals

KeyP

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nta

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of

Po

pu

lati

on

Birth Weight

Selection against both extremes keep curve narrow and in same

place.

Low mortality, high fitness


Stabilizing Selection


Disruptive Selection

Disruptive selection- when individuals at upper & lower ends of curve have higher fitness than individuals near middle

Selection acts against intermediate phenotype

If selection pressure is strong and lasts long enough, two distinct phenotypes produced


Ex.- If population of birds lives in area where medium-sized seeds become less common

Large and small-sized seeds become more common

Birds with unusually small or large beaks would have higher fitness

Population might split into large seed eaters and small seed eaters

Disruptive Selection

Largest and smallest seeds become more common.N

um

ber

of

Bir

ds

in P

op

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n

Beak Size

Population splits into two subgroups specializing in different seeds.

Beak Size

Nu

mb

er o

f B

ird

sin

Po

pu

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KeyLow mortality, high fitness



Process of Speciation

How do changes caused by natural selection and genetic drift lead to formation of new species?

Gene pools of two populations must become separated for them to become new species

Biological Species Concept


Isolating Mechanisms

As new species evolve, populations become reproductively isolated from each other

When members of two populations can no longer interbreed and produce fertile offspring- reproductive isolation

Populations have separate gene pools

Respond to N.S. or genetic drift differently


Reproductive isolation can develop in variety of ways, including:

- Behavioral isolation

- Geographic isolation

- Temporal isolation


Behavioral isolation

Occurs when two populations are capable of interbreeding, but have differences in courtship rituals or other reproductive behaviors

Ex.- bird species using different songs to attract mates


Geographic isolation

Two populations are separated by geographic barriers such as rivers, mountains, or bodies of water

Ex.- Two species of squirrel (Albert & Kaibab) formed after Colorado river cut the Grand Canyon


Does not guarantee formation of new species:

Barrier might be breached/broken down shortly

If two populations still can interbreed, no new species has formed

Also, geographic barriers can exist for some organisms, but not for others (birds can fly over a large river or mountain range)


Temporal isolation

Two or more species reproduce at different times

Ex.- plants

Three plants all live in same forest, only release pollen on one day

Each releases pollen on different day

Impossible to pollinate one another

Chap 17-4: Patterns of Evolution

Punctuated Equilibrium

Darwin felt that biological change needed to be slow and steady – idea known as gradualism

Fossil record confirms that some populations of organisms change gradually over time

Also evidence that this pattern does not always hold true


Some species have changed little since they appeared in the fossil record

Much of the time these species are in a state of equilibrium

Every now and then, something upsets the equilibrium

Changes in populations then occur relatively rapidly


Rapid evolution after long periods of stability can occur for several reasons:

- Small pop. isolated from larger pop.

- Small group migrates to new environment

- Mass extinctions open up niches

Term punctuated equilibrium used to describe pattern of long, stable periods interrupted by brief periods of rapid change

chap 16: evolution of populations darwin never knew how heredity actually worked biologists...

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