Processes of Evolution - Del Mar...

Processes of Evolution

Chapter 18 Part 2

18.6 Maintaining Variation

Natural selection theory helps explain diverse

aspects of nature, including differences between

males and females, and the relationship

between sickle-cell anemia and malaria

Sexual Selection

With sexual selection, some version of a trait gives an individual an advantage over others in attracting mates

Distinct male and female phenotypes (sexual dimorphism) is one outcome of sexual selection

Sexual Selection

Balanced Polymorphism

Balanced polymorphism

• A state in which natural selection maintains two or more alleles at relatively high frequencies

• Occurs when environmental conditions favor heterozygotes

Example: Sickle cell anemia and malaria

• HbA/HbS heterozygotes survive malaria more often than people who make only normal hemoglobin

Sickle Cell Anemia and Malaria

Fig. 18-13a, p. 287

Fig. 18-13b, p. 287

Fig. 18-13c, p. 287

18.7 Genetic Drift—

The Chance Changes

Genetic drift

• A random change in allele frequencies over time

• Can lead to a loss of genetic diversity, especially in small populations

Fixation has occurred when all individuals in a population are homozygous for one allele

Genetic Drift and Population Size

Genetic Drift and Population Size

Bottlenecks

Bottleneck

• A drastic reduction in population size brought about by severe pressure

• After a bottleneck, genetic drift is pronounced when a few individuals rebuild a population

• Example: Northern elephant seals

The Founder Effect

Founder effect

• Genetic drift is pronounced when a few individuals start a new population

Inbreeding

• Breeding or mating between close relatives who share a large number of alleles

• Example: Old Order Amish in Lancaster County, Pennsylvania (Ellis-van Creveld syndrome)

18.3-18.6 Key Concepts

Patterns of Natural Selection

Natural selection is a microevolutionary process

Depending on the population and its

environment, natural selection can shift or

maintain the range of variation in heritable traits

18.8 Gene Flow

Gene flow

• Physical movement of alleles caused by individuals moving into and away from populations

• Tends to counter the evolutionary effects of mutation, natural selection, and genetic drift on a population

• Example: Movement of acorns by blue jays

Gene Flow Between Oak Populations

18.7-18.8 Key Concepts

Other Microevolutionary Processes

With genetic drift, change can occur in a line of

descent by chance alone

Gene flow counters the evolutionary effects of

mutation, natural selection, and genetic drift

18.9 Reproductive Isolation

Speciation

• Evolutionary process by which new species form

• Reproductive isolating mechanisms are always part of the process

Reproductive isolation

• The end of gene exchange between populations

• Beginning of speciation

Four Butterflies, Two Species

Reproductive Isolating Mechanisms

Reproductive isolating mechanisms prevent interbreeding among species

• Heritable aspects of body form, function, or behavior that arise as populations diverge

• Prezygotic isolating mechanisms prevent pollination or mating

• Postzygotic isolating mechanisms result in weak or infertile hybrids

Prezygotic Isolating Mechanisms

Temporal isolation

Mechanical isolation

Behavioral isolation

Ecological isolation

Gamete incompatibility

Mechanical Isolation

Behavioral Isolation

Animation: Albatross courtship

Postzygotic Isolation Mechanisms

Reduced hybrid viability (ligers, tigons)

• Extra or missing genes

Reduced hybrid fertility (mules)

• Robust but sterile offspring

Hybrid breakdown

• Lower fitness with successive generations

Reproductive Isolating Mechanisms

Fig. 18-17, p. 290

Different

species! Prezygotic isolating mechanisms

Temporal isolation: Individuals of

different species reproduce at different

times.

Mechanical isolation: Individuals cannot

mate or pollinate because of physical

incompatibilities.

Behavioral isolation: Individuals of

different species ignore or do not get

the required cues for sex.

Ecological isolation: Individuals of

different species live in different places

and never do meet up.They interbreed

anyway.

Gamete incompatibility: Reproductive

cells meet up, but no fertilization occurs.

Zygotes form,

but . . .

Postzygotic isolating mechanisms

Hybrid inviability: Hybrid embryos die

early, or new individuals die before they

can reproduce.

Hybrid sterility: Hybrid individuals or

their offspring do not make functional

gametes.

No offspring, sterile offspring, or weak offspring that die before reproducing

Animation: Reproductive isolating

mechanisms

18.10 Allopatric Speciation

Allopatric speciation

• A physical barrier arises and ends gene flow between populations

• Genetic divergence results in speciation

• Example: llamas, vicunas, and camels

Allopatric Speciation

The Inviting Archipelagos

Winds or ocean currents carry a few individuals of mainland species to remote, isolated islands chains (archipelagos) such as Hawaii

Habitats and selection pressures that differ within and between the islands foster divergences that result in allopatric speciation

Allopatric Speciation on an Isolated Archipelago

Fig. 18-21a, p. 293

B Later, a few

individuals of a new

species colonize

nearby island 2.

Speciation follows

genetic divergence

in the new habitat.

C Genetically different

descendants of the

ancestral species may

colonize islands 3 and 4

or even invade island 1.

Genetic divergence and

speciation may follow.

A A few individuals of a

mainland species reach

isolated island 1. In the new

habitat, populations of their

descendants diverge, and

speciation occurs.

Fig. 18-21b, p. 293

Insects, spiders

from buds

twisted apart

by bill, some

nectar; high

mountain rain

forest

Akekee (L.

caeruleirostris)

Insects,

spiders, some

nectar; high

mountain rain

forest

Nihoa finch

(Telespiza

ultima)

Insects, buds,

seeds, flowers,

seabird eggs;

rocky or

shrubby slopes

Mamane

seeds ripped

from pods;

buds,

flowers,

some berries,

insects; high

mountain dry

forests

Maui parrotbill

(Pseudonestor

xanthophrys)

Rips dry branches

for insect larvae,

pupae, caterpillars;

mountain forest

with open canopy,

dense underbrush

Apapane

(Himatione

sanguinea)

Nectar, especially

of ohialehua flowers;

caterpillars and other

insects; spiders;

high mountain

forests

Akepa

(Loxops

coccineus)

Palila(Loxioides

bailleui)

Fig. 18-21c, p. 293

Tree snails,

insects in

understory;

last known

male died in

2004

Bark or leaf

insects, some

nectar; high

mountain rain

forest

Kauai Amakihi

(Hemignathus

kauaiensis)

Bark-picker;

insects, spiders,

nectar; high

mountain rain

forest

Akiapolaau

(Hemignathus

munroi)

Akohekohe

(Palmeria

dolei)

Iiwi (Vestiaria

coccinea)

Probes, digs

insects from

big trees; high

mountain rain

forest

Mostly nectar

from flowering

trees, some

insects, pollen;

high mountain

rain forest

Mostly nectar

(ohia flowers,

lobelias, mints),

some insects;

high mountain

rain forest

Maui Alauahio

(Paroreomyza

montana)

Poouli

(Melamprosops

phaeosoma)

Animation: Allopatric speciation on an

archipelago

18.11 Other Speciation Models

Populations sometimes speciate even without a

physical barrier that blocks gene flow

• Sympatric speciation

• Parapatric speciation

Sympatric Speciation

In sympatric speciation, new species form within a home range of an existing species, in the absence of a physical barrier

A change in chromosome number (polyploidy) can cause instant speciation

On Lord Howe Island, species of palms are reproductively isolated

Sympatric Speciation in Wheat

Fig. 18-22, p. 294

T. aestivum

(one of the

common

bread

wheats)

Triticum

monococcum

(einkorn)

Unknown

species of

Triticum

spontaneous

chromosome

doubling

T. tauschii

(a wild

relative)

T. turgidum

(wild emmer)

14AA X 14BB 14AB 28AABB X 14DD 42AABBDD

A By 11,000 years ago,

humans were cultivating

wild wheats. Einkorn has

a diploid chromosome

number of 14 (two sets of

7). It probably hybridized

with another wild wheat

species having the same

number of chromosomes.

B About 8,000 years ago,

the allopolyploid wild emmer

originated from an AB hybrid

wheat plant in which the

chromosome number

doubled. Wild emmer is

tetraploid, or AABB; it has

two sets of 14 chromosomes.

There is recently renewed

culinary interest in emmer,

also called farro.

C AABB emmer probably

hybridized with T.

tauschii, a wild relative

of wheat. Its diploid

chromosome number

s 14 (two sets of 7 DD).

Common bread wheats

have a chromosome

number of 42 (six sets

of 7 AABBDD).

Animation: Sympatric speciation in

wheat

Sympatric Speciation in Palms

Parapatric Speciation

In parapatric speciation, populations in contact along a common border evolve into distinct species

Hybrids in the contact zone are less fit than individuals on either side

Parapatric Speciation

Fig. 18-24c, p. 295

T. barretti

hybrid zone

T. anophthalmus

Different Speciation Models

18.9-18.11 Key Concepts

How Species Arise

Speciation varies in its details, but it typically

starts after gene flow ends

Microevolutionary events that occur

independently lead to genetic divergences,

which are reinforced as reproductive isolation

mechanisms evolve

18.12 Macroevolution

Macroevolution

• Large-scale patterns of evolutioary change

• Includes patterns of change such as one species giving rise to multiple species, the origin of major groups, and major extinction events

Coevolution

Two species in close ecological contact act as agents of selection on each other (coevolution)

• Predator and prey

• Host and parasite

• Pollinator and flower

Over time, the two species may come to depend on each other

Coevolution

Fig. 18-25, p. 296

proboscis

nectar tube

10 cm

Stasis and Exaptation

Stasis

• A lineage exists for millions of years with little or no change (e.g. coelacanth)

Exaptation (preadaptation)

• Some complex traits in modern species held different adaptive value in ancestral lineages (e.g.feathers in birds and dinosaurs)

Adaptive Radiation

Adaptive radiation

• A burst of speciation that occurs when a lineage encounters a new set of niches

Key innovation

• A structural or functional adaptation that allows individuals to exploit their habitat in a new way

Extinction

Extinction

• The irrevocable loss of a species from Earth

Mass extinctions

• Extinctions of many lineages, followed by adaptive radiations

• Five catastrophic events in which the majority of species on Earth disappeared

Adaptive Radiation of Mammals Following the K-T Extinction

Fig. 18-26a, p. 297

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Evolutionary Theory

Evolutionary biologists try to explain how all species are related by descent from common ancestors

Genetic change is the basis of evolution, but many biologists disagree about how it occurs

18.12 Key Concepts

Macroevolutionary Patterns

Patterns of genetic change that involve more

than one species are called macroevolution

Recurring patterns of macroevolution include

exaptation, adaptive radiation, and extinction

Animation: Models of speciation

Animation: Morphological differences

within a species

Animation: Simulation of genetic drift

Animation: Temporal isolation among

cicadas

Video: Rise of the super rats

Video: Humpback whales

Video: Salamander gills