How Populations Evolve Charles Darwin and the Origin of Species Evidence of Evolution Natural...

How Populations Evolve Charles Darwin and the Origin of Species

Evidence of Evolution

Natural Selection

The Modern Synthesis: Darwinism Meets Genetics

Mechanisms of Evolution

• Darwin made three observations from these facts.

1. Life shows rich diversity.

2. There are similarities in life that allow the classification of organisms into groups nested within broader groups.

3. Organisms display many striking ways in which they are suited for their environments.

CHARLES DARWIN AND THE ORIGIN OF SPECIES

© 2013 Pearson Education, Inc.

• In The Origin of Species, Darwin

– proposed a hypothesis, a scientific explanation for his observations,

– used hundreds of pages in his book to describe the evidence supporting his hypothesis,

– made testable predictions, and

– reported the results of numerous experiments he had performed.



• Darwin hypothesized that

– present-day species are the descendents of ancient ancestors that they still resemble in some ways and

– change occurs as a result of “descent with modification,” with natural selection as the mechanism.

– Found that plants and animals living on a continent seemed more closely related to each other than to plants and animals living in similar regions on other continents



• Natural selection is a process in which organisms with certain inherited characteristics are more likely to survive and reproduce than are individuals with other characteristics.

• As a result of natural selection, a population, a group of individuals of the same species living in the same place at the same time, changes over generations.



• Natural selection leads to evolutionary adaptation, a population’s increase in the frequency of traits suited to the environment.

• Natural selection thus leads to evolution, seen either as

– a change in the genetic composition of a population over time or

– on a grander scale, the entire biological history, from the earliest microbes to the enormous diversity of organisms that live on Earth today.



Descent with Modification

• Darwin made two main points in The Origin of Species.

1. Organisms inhabiting Earth today descended from ancestral species.

2. Natural selection is the mechanism for descent with modification.


EVIDENCE OF EVOLUTION

• Evolution leaves observable signs.

• We will examine five of the many lines of evidence in support of evolution:

1. the fossil record,

2. biogeography,

3. comparative anatomy,

4. comparative embryology, and

5. molecular biology.


Figure 13.7-3

Bones of extinct whales is a component of the fossil record

Figure 13.8

Commonringtailpossum

Red kangaroo

Common wombat

Australia

Koala

Human Cat Whale Bat

Homology is evidence of common ancestry. Homologous appendages are seen here. It means that the human arm descended from a bird arm.Similar embryonic stages also evidence common ancestry.

Comparative embryology – common ancestor evidence

• Vestigial structures

– are remnants of features that served important functions in an organism’s ancestors and

– now have only marginal, if any, importance.

Comparative Anatomy


Molecular Biology

• The hereditary background of an organism is documented in

– its DNA and

– the proteins encoded by the DNA.

• Evolutionary relationships among species can be determined by comparing

– genes and

– proteins of different organisms.


Figure 13.11

Percent of selected DNA sequences that match a chimpanzee’s DNA

Chimpanzee

100%96%92%

Human

Gibbon

Orangutan

Gorilla

Primate

Old Worldmonkey

NATURAL SELECTION

• Darwin noted the close relationship between adaptation to the environment and the origin of new species.

• The evolution of finches on the Galápagos Islands is an excellent example.


Figure 13.12

(a) The large ground finch

(b) The warbler finch (c) The woodpecker finch

Darwin’s Theory of Natural Selection

• Darwin based his theory of natural selection on two key observations.

1. All species tend to produce excessive numbers of offspring.

2. Organisms vary, and much of this variation is heritable. Natural expression of genotype


Figure 13.14

• Inference: Unequal reproductive success(natural selection)

– Those individuals with traits best suited to the local environment generally leave a larger share of surviving, fertile offspring.

– Sexual Selection: EX: peahens choose to mate with peacocks that have the most beautiful tails.

Darwin’s Theory of Natural Selection


Natural Selection in Action

• Examples of natural selection include

– pesticide-resistant insects,

– antibiotic-resistant bacteria, and

– drug-resistant strains of HIV.

– STABILIZING SELECTION – natural selection that favors intermediate variants by acting against extreme phenotypes. EX: the birth weight at which newborn humans are most likely to survive is about the average weight of newborns!


Figure 13.15-3

Chromosome with geneconferring resistanceto pesticide

Reproduction

Survivors

Insecticide application

Evolutionary Trees

• Darwin saw the history of life as analogous to a tree.

– The first forms of life on Earth form the common trunk.

– At each fork is the last common ancestor to all the branches extending from that fork.

– The tips of millions of twigs represent the species living today.


Figure 13.17

Tetrapodlimbs

Amnion

Feathers

Lungfishes

Mammals

Amphibians

Lizardsand snakes

Crocodiles

Hawks and other birds

Ostriches

Am

nio

tes

Tetrap

od

s

Bird

s

Common ancestor oflineages to the right

Homologous traitshared by all groupsto the right

2

1

3

4

6

5

THE MODERN SYNTHESIS: DARWINISM MEETS GENETICS• The modern synthesis is the fusion of

– genetics with

– evolutionary biology.


Populations as the Units of Evolution

• A population is

– a group of individuals of the same species, living in the same place at the same time and

– the smallest biological unit that can evolve.


Figure 13.18

(a) Two dense populations oftrees separated by a lake

(b) A nighttime satellite view of North America

• The total collection of alleles in a population at any one time is the gene pool.

• When the relative frequency of alleles changes over a number of generations, evolution is occurring on its smallest scale.

Populations as the Units of Evolution


Sources of Genetic Variation

• Genetic variation results from processes that both involve randomness:

1. mutations, changes in the nucleotide sequence of DNA, and

2. sexual recombination, the shuffling of alleles during meiosis.


Sources of Genetic Variation

• For any given gene locus, mutation alone has little effect on a large population in a single generation.

• Organisms with very short generation spans, such as bacteria, can evolve rapidly with mutation as the only source of genetic variation.


Analyzing Gene Pools

• A gene pool

– consists of all the alleles in a population at any one time and

– is a reservoir from which the next generation draws its alleles.

• Alleles in a gene pool occur in certain frequencies.

• New alleles come from mutations

• Human height is a polygenic trait (influenced by more than one gene)


Figure 13.20

Figure 13.21

Allele frequencies

Genotype frequencies

Sperm

Eggs

p 0.8(R)

q 0.2(r)

p 0.8

R

q 0.2

r

RR

p 0.8

R

q 0.2

r

p2 0.64

rR pq 0.16 q2 0.04

rr

pq 0.16 Rr

(RR) p2 0.64 q2 0.04

(rr) 2pq 0.32

(Rr)

Population Genetics and Health Science

• The Hardy-Weinberg formula can be used to calculate the percentage of a human population that carries the allele for a particular inherited disease.


Microevolution as Change in a Gene Pool

• How can we tell if a population is evolving?

• A non-evolving population is in genetic equilibrium, also known as Hardy-Weinberg equilibrium, meaning the population’s gene pool is constant over time.

• From a genetic perspective, evolution can be defined as a generation-to-generation change in a population’s frequencies of alleles, sometimes called microevolution.


MECHANISMS OF EVOLUTION

• The main causes of evolutionary change are

– genetic drift – result of chance

– gene flow – gain or loss of alleles from individuals entering or leaving a population

– natural selection

• Natural selection is the most important, because it is the only process that promotes adaptation.


Figure 13.23-3

Only 5 of 10plants leave

offspring

RR

rr

Rr

RR

RR

RR

RR Rr

RR

Rr

Rr

Only 2 of 10plants leave

offspring

RR

rr RR

Rr

rr

RR

Rr

Rr Rr

rr

RR

RR

RR

RR

RR

RR

RR

RR RR

Generation 1p 0.7q 0.3



The Bottleneck Effect

• The bottleneck effect

– is an example of genetic drift and

– results from a drastic reduction in population size.

• Passing through a “bottleneck,” a severe reduction in population size,

– decreases the overall genetic variability in a population because at least some alleles are lost from the gene pool, and

– results in a loss of individual variation and hence adaptability.


• Cheetahs appear to have experienced at least two genetic bottlenecks:

1. during the last ice age, about 10,000 years ago, and

2. during the 1800s, when farmers hunted the animals to near extinction.

• With so little variability, cheetahs today have a reduced capacity to adapt to environmental challenges.

The Bottleneck Effect


Figure 13.25

The Founder Effect

• The founder effect is likely when a few individuals colonize an isolated habitat.

• This represents genetic drift in a new colony.

• The founder effect explains the relatively high frequency of certain inherited disorders in some small human populations.


Figure 13.26

SouthAmerica

Tristan da Cunha

Africa

Gene Flow

• Gene flow

– is another source of evolutionary change,

– is separate from genetic drift,

– is genetic exchange with another population,

– may result in the gain or loss of alleles, and

– tends to reduce genetic differences between populations.


Natural Selection: A Closer Look

• Of all causes of microevolution, only natural selection promotes adaptation.

• Evolutionary adaptation results from

– chance, in the random generation of genetic variability, and

– sorting, in the unequal reproductive success among the varying individuals.


Evolutionary Fitness

• Relative fitness is

– the contribution an individual makes to the gene pool of the next generation

– relative to the contributions of other individuals.


Figure 13.28

1. Directional selection shifts the overall makeup of a population by selecting in favor of one

extreme phenotype.

2. Disruptive selection can lead to a balance between two or more contrasting phenotypic

forms in a population.

3. Stabilizing selection favors intermediate phenotypes, occurs in relatively stable environments, and is the most common.

Three General Outcomes of Natural Selection


Figure 13.29

(a) Directional selection (b) Disruptive selection (c) Stabilizing selection

Originalpopulation

Evolvedpopulation

Originalpopulation

Phenotypes (fur color)F

req

uen

cy o

fin

div

idu

als

Sexual Selection

• Sexual selection is a form of natural selection in which individuals with certain traits are more likely than other individuals to obtain mates.

• Sexual dimorphism is a distinction in appearance between males and females not directly associated with reproduction or survival.


Figure 13.30

(a) Sexual dimorphism in a finch species (b) Competing for mates

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How Populations Evolve Charles Darwin and the Origin of Species Evidence of Evolution Natural...

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