how populations evolve - 2.files.edl.io

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Lectures by Chris C. Romero

PowerPoint® Lectures for

Essential Biology, Third Edition

– Neil Campbell, Jane Reece, and Eric Simon

Essential Biology with Physiology, Second Edition

– Neil Campbell, Jane Reece, and Eric Simon

CHAPTER 13

How Populations Evolve

Figure 13.1


Charles Darwin and The Origin of Species

• Charles Darwin’s On the Origin of Species by

Means of Natural

• Two concepts: “descent with modification” and

natural selection.


• The basic idea of natural selection is that:

- Success in reproduction leads to an accumulation

of favored traits in the population over generations

- Darwin based this off of:

- Species tend to produce excessive numbers of

offspring (only a small percentage will survive)

- The variation among individuals of a population

Example: The evolution of pesticide resistance in

hundreds of insect species

Figure 13.2


• The Darwinian revolution in its historical context

1831-36: Darwin travels around the world on the

HMS Beagle

1837: Darwin begins his notebooks on the origin of

species

1844: Darwin writes his essay on the origin of species

1858: Wallace sends his theory to Darwin

1858: The Origin of Species is published

Grand Canyon

13_08GrandCanyon_SV.mpg


• Jean Baptiste Lamarck was a French naturalist who

suggested that the best explanation for the

relationship of fossils to current organisms was that

life evolves.

• He explained evolution as a process of adaption.

(evolutionary adaption)

• He also proposed that acquired characteristics are

inherited.


The Voyage of the Beagle

• Darwin sailed from Great Britain on the Beagle in

December 1831.

• Main mission: chart poorly known stretches of the

South American coastline.

Figure 13.4


• On his journey on the Beagle, Darwin

– Collected thousands of specimens.

– Observed various adaptations in organisms.

– The plants and animals living in temperate

regions of South America seemed more closely

related to species living in tropical regions of that

continent than to species living in temperate

regions of Europe.


• Darwin was intrigued by the geographic

distribution of organisms on the Galapagos Islands.

Figure 13.5


Descent with Modification

• Darwin made two main points in The Origin of

Species:

• Descent of diverse species from common

ancestors.

• Natural Selection as the mechanism of Evolution

Figure 13.6


Evidence of Evolution

• Fossils

• Biogeography

• Comparative Anatomy

• Comparative Embryology

• Molecular Biology


The Fossil Record

• Over millions of years, deposit piles up and

compress older sediments.

• If organisms that have been swept into the water

die, could be compressed too and may leave

imprints into the rock.

Figure 13.7

Figure 13.8


Comparative Anatomy

• Comparison of body structures between different

species.

• Certain anatomical similarities show signs of

evolutionary history.


• Homology: similarity due to common ancestry.

• Arms, forelegs, flippers, and wings of different

mammals are variations on a anatomical themes

that has become adapted to different functions.

Figure 13.11


Comparative Embryology

• Comparative Embryology- The comparison of

structures that appear during the development of

different organisms

Figure 13.12


Molecular Biology

Molecular Biology says:

• If two species have genes with nucleotide

sequences that match closely, they might have been

inherited from a common ancestor.

• The greater the number of sequence differences,

the less likely the share a common ancestor.

Figure 13.13


Natural Selection

• Darwin studied finches that lived on different

Galapagos islands and concluded that:

• The finches began to differ from one another

because they adapted to their environment.

o Different beak shapes (adapted to specific foods).

o Might become dissimilar enough to be classified

as different species.

Figure 13.14


Natural Selection in Action

• Examples of natural selection include

– The evolution of pesticide resistance in hundreds

of insect species: a pesticide does not create

resistant individuals, but selects for resistant

insects that were already present in the

population.

– The development of antibiotic-resistant bacteria:

doctors have recently documented an increase in

drug-resistant strains of HIV.


Three General Outcomes of Natural Selection

• Directional selection-occurs when the environment

changes in a particular way, there is therefore a

selective pressure for species to change in response

to the environmental change.

– Populations do not decide to adapt to the change

in environment, the adaptations, mutations and

combination of alleles must already be present.

– species shifts the phenotypic curve of a

population by selecting in favor of extreme

phenotype.


• Disruptive (diverging) selection- occurs when the

environmental changes produces selection

pressures which favor the two extremes of a

characteristic.

• Example: sickle cell anemia – in areas where

malaria does not exist, it is a disadvantage but in

parts of the world where malaria is common,

sickle cell anemia is advantageous. Therefore

the population in malaria affected areas have a

higher proportion of the sickle cell anemia allele


• Stabilizing selection – this occurs when the environment doesn’t

change, natural selection doesn’t have to cause change. There

are many species which are thought to have remained the same

over the course of millions of years

• Maintains variation for a particular trait within a narrow

range

• Occurs in a relatively stable environment to which

populations are already well adapted

• This evolutionary conservatism works by selecting against

the more extreme phenotypes.

Example: Stabilizing selection keeps the majority of human birth

weights between 3 and 4 kg


Sources of Genetic Variation

• Mutations: a mutation in a gene may substitute one

nucleotide for another, but the change will be

harmless if it does not affect the function of the

protein the DNA encodes.

• Sexual recombination: plants and animals depend

on sexual recombination for the genetic variation

that makes adaption possible. The two sexual

processes of meiosis and random fertilization

shuffle alleles and deal them out to offspring in

fresh combinations.


Mechanisms of Microevolution

• The changes in allele frequencies that occur over time within a

population

• The main causes of microevolution are:

– Genetic drift- a change in the gene pool of a small

population due to chance

– Gene flow- A population may gain or lose alleles when

fertile individuals move into or out of a population or

when gametes are transferred between populations

– Mutations- Changes in an organism’s DNA.

– Natural selection- The offspring with the best traits will

survive and reproduce


Genetic Drift

• Can cause a population to shrink down. A change

in the gene pool of a population due to change.

• where gene drift occurs happens in 2 ways.

– Bottleneck Effect

– Founder Effect


The Bottleneck Effect

• Genetic drift resulting from a drastic reduction in

population size.

• Ex: disasters killing people, producing a small

surviving population with different genetic makeup

as the original population.


The Founder Effect

• Random change in the gene pool that occurs in a

small colony of a population.

• Effect contributed to evolutionary divergence of

the finches on the Galapagos Islands.


Genetic Drift and Hereditary Disorders in Human Populations

• 1854 – 15 people found a British colony on Tristan

da Cunha.

• One of the colonists carried a recessive allele called

retinitis pigmentosa

• Of the 240 descendants who still lived on the

islands in 1960’s, 4 had retinitis pigmentosa and 9

were known to be carriers



Gene Flow

• The gain or loss of alleles from a population by the

movement of individuals or gametes into or out of

the population.

• Another source of evolutionary change.

• Gene flow tends to reduce differences between

populations.

• Has become an important agent of evolutionary

change in previously isolated human populations.


Mutation

• Changes in an organism’s DNA.

• A mutation transmitted in gametes can immediately

change the gene pool of a population by

substituting one allele for another.

• For one gene locus, however, mutation does not

have much effect on the population.

• Over the long term, mutation is, in itself, essential

to evolution because it’s the original cause of

genetic variation.


Natural Selection: A Closer Look

• Of all causes of microevolution, only natural

selection promotes adaptation.

• All individuals in a population is supposed to

equal in their ability to survive and reproduce


Darwinian Fitness

• It’s not about “survival of the fittest” but survival

to sexual maturity. Production of fertile offspring is

the only score that counts in natural selection

Example: The biggest, fastest, toughest frog in the

pond has a fitness of zero if it’s sterile.

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