Download Evolution and Ecology

BIO 300 Ecology
Karl Kleiner
1
LECTURE 3
The interaction between evolution and ecology
I. WHAT IS EVOLUTION?
A. Evolution is change, and biological evolution can be defined as changes in any attribute of a
population over time.
1. A more specific definition is that evolution represents a change in the allele frequency at a
locus ( i.e. a gene) in a population that persists among generations.
II. Redefinition of terms and concepts.
1. Species ≡ a natural population or group of (potentially) interbreeding natural populations that
produces fertile offspring and is reproductively isolated from other groups.
2. Population ≡ a group of individuals of a single species that interact and interbreed in a
particular area (locale).
3. Gene pool ≡ the total sum of genes available for reproduction in a given population at a given
time.
a. The number of alleles in a population is 2NG.
4. Allele frequency – if a gene has two alleles, A1 and A2, then the frequency of A1 is
(# copies of A1)/(# copies of A1 + A2).
a. Old definition of evolution – descent with modification, inherited change in population.
b. New definition – If allele frequencies describe composition of gene pool, then evolution is
the change in allele frequencies.
5. Fitness.
a. Old Darwinian def. – The ability to survive and reproduce in a particular environment.
b. New definition – A measure of the expected relative contribution of an individual to the
gene pool of the next generation.
III.
Allele and genotype frequencies.
1. If we know the numbers of all the genotypes, we can easily calculate p and q.
2. But, we cannot do the reverse. Given a p and a q, we cannot determine the number of different
genotype frequencies.
IV.
Hardy-Weinberg equilibrium. (p + q)2 = p2 + 2pq + q2
1. Can be used to calculate the frequencies of alleles in the gene pool if the genotype frequencies
are known
2. And the genotype frequencies can be determined if the allele frequencies are known.
V.
Conditions of Hardy-Weinberg equilibrium.
1. Assumptions made;
a. Random mating.
b. Population is large.
c. No differential survival or reproduction.
d. No net mutations
e. No migrations into or out of the population.
BIO 300 Ecology
Karl Kleiner
2
2. If these 5 assumptions are met, then a population will reach H-W equilibrium in one generation
of random mating.
3. A single generation of random mating establishes binomial genotype frequencies, and neither
these frequencies, nor the allele frequencies p and q, will change in subsequent generations.
The gene pool is in equilibrium – a non-evolving population.
VI.
Hardy-Weinberg in the real world.
1. What good is H-W? It provides a baseline against which we can measure effects of processes
that cause evolution.
a. That is, it provides a reference point with which to compare the frequencies of alleles and
genotypes of natural populations whose gene pools may be changing.
2. If the frequency of alleles or genotypes deviate from the expected H-W equilibrium, then the
population is evolving.
3. Chi-square test for H-W equilibrium.
4. The classic study Biston betularia –Peppered moth
VII.
Modes of Evolution
The following are based upon violations of the Hardy-Weinberg theorem that describes a gene
pool in equilibrium.
1. gene flow –a population may gain or lose alleles due to the migration of fertile
individuals or gametes between populations. (genetic exchange)
2. genetic drift – changes in the gene pool of a small population due to chance.
3. meiotic drive – ‘outlaw’ or ‘selfish’ genes that obtain disproportionate representation
in a carrier’s gametes at the expense of alternate alleles on a homologous chromosome.
These alleles do not obey the Mendelian lottery of meiosis and recombination.
VIII. Natural Selection
A. Process of natural selection.
1. The individuals that make up a population of a species are not identical: they vary.
2. Some of this variation is heritable.
3. All populations have the potential to populate the earth, and they would do so if each
individual survived and each individual produced the maximum number of descendants.
But, they don’t.
4. Different ancestors leave different numbers of descendants. Life is difficult, and not all
individuals survive.
5. Finally, the number of descendants that an individual leaves depends on the interaction
between the characteristics (or properties) of the individual and its environment.
6. Definition of Natural Selection:
Natural selection is the differential success in reproduction, and its product is adaptation
of organisms to their environment.
B. Natural selection is not synonymous with evolution.
1. Evolution refers to temporal changes of any kind, whereas
2. Natural selection specifies one particular way in which these changes are
brought about.
BIO 300 Ecology
Karl Kleiner
C. Modes of Natural Selection
Natural selection acts upon phenotypes. This selection for phenotype occurs in one of three
ways.
1. Directional selection –one extreme is favored by selection.
2. Stabilizing selection – Intermediate phenotypes are favored.
3. Disruptive or diversifying selection – selection favors both extremes, not intermediates.
IX. Proximate and ultimate explanations.
A. Natural history provides questions: ecology attempts to answer them.
B. Scientists distinguish between two kinds of explanations for ecological
phenomena, proximate and ultimate explanations (or factors, or causes).
1. A proximate explanation is the immediate cause of a phenomena.
a. It explains how a trait is regulated by an individual. Mechanistic.
2. An ultimate explanation provides a more general reason, involving evolutionary arguments
and adaptations. Ultimate causes are factors that are remote in time and space.
C. Adaptation or abaptation? A cautious distinction.
1. The phrase ‘organisms X is adapted to live in Y’ can mean many things.
2. The word adaptation implies that:
a. organisms react to present circumstances
b. prepares them for the future
c. is predictive and implies some sort of forward planning, or at the very least, of
design.
BUT, organisms are not designed for, or matched, or fitted to, the present or the future.
Their character or properties are entirely consequences of the past.
They reflect the successes and failures of ancestors.
3. Abaptation (or exaptation) would be a better word.
a. its etymology brings the implication that the aptness of organisms for their
environment is a product of their past rather than a program for the future.
X. Some other considerations on natural selection.
A. Natural Selection does not predict perfection
B. Historical Factors
3